Climate and Physiography of North America

Luc Brouillet

R. David Whetstone

Physiographic map of North America by Erwin Raisz. [Reprinted with permission from Raisz Landform Maps.]

THE area covered in this book includes North America north of Mexico plus Greenland (Kalâtdlit-Nunât). It encompasses about 21.5 million km², between latitudes 26° and 85° N, and longitudes 15° W and 173° E, and it stretches from the Florida Keys northward to Ellesmere Island, and from Greenland westward to Attu Island in the Aleutian Archipelago. Widest in the north, the continent narrows sharply at the Gulf of Mexico. South of the United States border with Mexico, it tapers gradually to the Isthmus of Panama. It is surrounded by three oceans---the Arctic, Pacific, and Atlantic, respectively to the north, west, and east---and by the Gulf of Mexico to the south. It is separated from northeast Asia by the Pacific Ocean, and by the epicontinental Bering Sea, the Chukchi Sea, and the connecting Bering Strait. The Greenland and Norwegian seas, as well as the North Atlantic Ocean, separate North America from Europe and link the Atlantic to the Arctic Ocean; the Denmark Strait divides Greenland from Iceland. The Strait of Florida divides North America from the West Indies (Cuba). 

Climate, physiography, and geology play major roles in determining the distributions of present-day soil classes, vegetation types, floras, and faunas. Biogeographers agree that climate is the primary factor in the control of these distributions (e.g., R.Good 1974; G.T. Trewartha and L.H. Horn 1980; F.I. Woodward 1987; E.O. Box 1981; M.J. Müller 1982; A.L. Takhtajan 1986; R.Hengeveld 1990; see also D.Steila, chap. 2; A.Graham, chap. 3; P.A. Delcourt and H.R. Delcourt, chap. 4; R.F. Thorne, chap. 5; M.G. Barbour and N.L. Christensen, chap. 6). Climate determines the erosional and soil-forming processes that occur, and the life forms that are able to survive at a given locale, all of which may be affected secondarily by the types of bedrock and surficial deposits encountered in the area. In turn, relief influences climatic patterns through elevation above sea level and its effects on wind patterns and rainfall (R.A. Bryson and F.K. Hare 1974b).

Geoclimatic changes that occurred throughout Earth history have affected the distribution of biotas through time. Climate has changed under cosmic influences, such as the Milankovitch cycles (discussed by P.A. Delcourt and H.R.Delcourt, chap. 4; see also F.I. Woodward 1987; H.E. Wright Jr. 1989). The climate has also been affected by the relative position of the drifting continents, because drift implies latitudinal shifts, changes in the distribution of landmasses relative to oceans and oceanic currents, and modifications in the position of mountain ranges relative to airflow patterns. For instance, the Tertiary opening of the Atlantic onto the Arctic Ocean, and the establishment of the circumantarctic current with the opening of the Drake Passage between South America and Antarctica, played a significant role in subsequent climatic cooling (e.g., M.E. Collinson 1990).

The deep oceanic conveyor belt (a bottom sea current that links all the oceans) was presumably modified by changes in continental distribution and may have affected climate (H.E. Wright Jr. 1989; J.Gribbin 1991). W.F. Ruddiman and J.E. Kutzbach (1991) proposed that the 3-km uplift of the high plateaus in Tibet and in western North America in the Pliocene-Pleistocene were instrumental in provoking the late Tertiary trend of climatic cooling. Finally, the pathways by which biotas have been able to spread between continents were also affected by the existence of bridges. Such dynamic factors influenced the evolution of life on the North American continent.

Climate

Climate may be defined in a simplified manner as the composite weather of a region (G.T. Trewartha and L.H. Horn 1980; J.Gribbin 1991). Factors that affect climate are discussed in climatology texts (e.g., G.T. Trewartha and L.H. Horn 1980; P.Pagney 1973; F.W. Cole 1980; J.R. Eagleman 1985; F.K. Lutkens and E.J. Tarbuck 1989; J.Gribbin 1991), and the effects of these factors on plant distribution are described in ecological and biogeographical treatises (e.g., S.Tuhkanen 1980; E.O. Box 1981; R.Hengeveld 1990; F.I. Woodward 1987). Changes in climate and vegetation through time are addressed by A.Graham (chap. 3) and by P.A. Delcourt and H.R. Delcourt (chap. 4) and are not discussed further here. R.A. Bryson and F.K. Hare (1974b) provided a thorough discussion of North American climates. T.W. Böcher et al. (1968) and G.Halliday (1989) described the climate of Greenland, although in very general terms.

Three major air masses affect the climate in North America (fig. 1.1). The source of the cold,continental polar air mass is the frozen Arctic and, in winter, snow-covered northwestern Canada and Alaska. The moist, mild, maritime Pacific air mass originates in the North Pacific and is ushered eastward by the midlatitude westerlies. The Gulf of Mexico, Caribbean Sea, and subtropical western Atlantic are the source areas for the moist, warm, tropical air mass. Climate in northeastern North America is also affected, especially in winter, by a fourth air mass: the cold, moist, oceanic North Atlantic air mass that finds its source over the cold waters off Newfoundland, Labrador, and Greenland. The influence is sporadic along the coast, not penetrating beyond the Appalachians or Cape Hatteras.

Air masses converge along fronts. The Polar Front is the fundamental climatic phenomenon at midlatitudes,where temperature contrasts are sharpest. It consists of the zone of convergence between the polar and tropical air masses,and coincides with a pressure trough with abundant cyclonic storms. The Polar Front intersects the earth slightly south of the high-altitude, high-velocity jet stream, and it migrates seasonally in relation with the latter. The jet stream thus represents an important element in the control of weather. This wind is circumpolar and has a meandering course around 30°--35° N, farther north in winter (35°--45° N), and south in summer (20°--25° N). Poleward of the jet stream, air is colder, and equatorward, warmer.

Precipitations are concentrated in areas south of the stream, along the zones of convergence. In the subtropical zone (20°--30° N), an area dominated by anticyclones, a minimum of precipitation is encountered; the great deserts are located in this zone. Between 45° and 55° N, a maximum of precipitation occurs in the region of convergence associated with the Polar Front and the belt of maximal cyclonic activity. From 50° to 55° N poleward, precipitation declines sharply to an absolute minimum in the cold regions at very high latitudes (greater than 75° N). Seasonal variations occur in this pattern: the subtropics are mostly dry all year, and the Mediterranean climate at the western margin of the continent has rainy winters and dry summers. In temperate latitudes, precipitation is present in all seasons, but it is most abundant in summer over the continents. Polar regions have slight precipitations in all seasons.

Annual cycling in the position of the meteorological equator (the latitude at which the noon sun is vertical to the earth surface at a given time of the year) causes pressure and wind belts to migrate latitudinally. This shifting of wind belts is significant in regions intermediate between different air mass systems; these transition zones experience contrasting weather conditions at opposite seasons. In North America, transition zones exist: (1) in the area between latitudes 30° and 40° N, (a) over the eastern parts of the Pacific and adjacent coastal California (part of the western facade of the continent), a region located between the subtropical anticyclone and the westerlies that experiences summer drought and winter rain, and (b) on the subtropical east side of the continent, along the Gulf of Mexico, where onshore monsoon winds favor maximum summer rainfall; and (2) in the area of subpolar lows between 60° and 70° N, intermediate between stormy westerlies and polar easterlies, a region of alternating colder and warmer air, as is encountered in Hudson Bay, northwestern Canada, and Alaska.

Oceanic currents affect the heat balance of the continental margins along which they flow (fig. 1.1); winds must be onshore, however, for currents to affect land climates. The major currents flow clockwise both in the Atlantic and in the Pacific. Because of the direction of the prevailing westerlies, the east coast of North America is less affected by the temperature of the sea currents than the west coast. The currents flowing along the southeastern sides of North America are warm. Thus the warm Equatorial Current enters the Caribbean Sea and the Gulf of Mexico from the tropical Atlantic, before returning to the North Atlantic through the Florida Strait as the Gulf Stream (fig.1.1). The Gulf Stream then passes up along the east side of North America, transferring heat from subtropical to cold latitudes, but it leaves the shore at Cape Hatteras. Cold currents either rise from oceanic depths at subtropical latitudes (e.g., California Current) or are polar in origin (e.g., Labrador Current from the Arctic Sea on both sides of Greenland). On eastern seaboards, warm and cold currents converge at about 40° latitude. Thus the Labrador Current and Gulf Stream meet off Cape Hatteras (fig. 1.1). At such sites, temperature gradients are sharp, and fog is frequent. Therefore at subtropical latitudes the east coast has a warm and rainy climate; at lower middle latitudes it has a modified continental climate with relatively cold winters and warm to hot summers; and at higher midlatitudes, where cold currents flow offshore, cool summers are characteristic.

On the west coast of North America, cool currents flow equatorward from about 40° latitude, warm ones poleward, and they diverge from each other. When the prevailing westerlies blow over cool offshore water onto land that is warmed in summer, as in coastal California, the summers are dry; the moisture-holding capacity of the air increases when it passes onto the land. Such climates, known as Mediterranean climates, are found at similar latitudes along the western side of the other continents as well. Warm currents at midlatitudes result in maritime climates with cool summers and mild winters, characterized by small annual ranges in temperature and adequate precipitation. Such climates, which are highly equable, are encountered along the temperate Pacific Coast of North America.

R.A. Bryson and F.K. Hare (1974b) described the climate of North America north of Mexico through a topographic model, that was correlated to the global atmospheric circulation between subtropical latitudes at 26° N, which experience an excess of radiation, and the polar latitudes at 85° N, where net radiation is in deficit. They stated that two geographical features have major effects on North American climates: the western Cordillera, trending north-south, and the Interior Plains to the east. The former constitutes a major obstacle to westerlies and trade winds, while the latter provides an uninterrupted path for the flow of the Arctic and tropical air masses (fig. 1.1).

The major effect of the high Cordillera on climate is in intercepting large-scale patterns of air circulation. The prevailing, low-altitude winds at midlatitudes (30°--65°N) are from the west (westerlies, fig. 1.1), due to the rotation of the earth, i.e., Coriolis force. Equatorward (0°--30° N), easterlies or trade winds predominate; poleward, winds are also easterlies. Where air masses are forced above extensive mountain ranges, they discharge much of their water vapor windward as orographic precipitations, while leeward, a rain shadow is formed. Consequently westerlies that blow onshore from the Pacific must pass over the Cordilleran high plateaus (1500--2000 m) and mountains (to more than 4000 m) before reaching the interior. Near the region where westerlies are strongest (45°--50° N), surface winds are deflected both north and south, while north of the main flow, they are deflected northward.

Few low passes in the Cordillera provide direct routes for surface winds through the Cordillera (fig. 1.1). The major passes are: (1) near 45°--50° N (maximum westerlies); (2) the Columbia River--Snake River--Wyoming Basin gap; and (3) the region of lower relief near the Mexican border. Most of the Pacific air mass that penetrates to the interior is from the upper part of the air mass that enters north of the Cordillera. Along the eastern slopes, the air descends and is warmed in the process, i.e., adiabatically, with a concomitant decrease in relative humidity. As a result the Pacific air mass has a mild, dry character east of the Cordillera. By this process, the moist Pacific air mass provides precipitations on the windward side of the ranges, most notably the Coastal Ranges, and leaves a rain shadow on the leeward side. Over the plains, the Pacific air forms a wedge that invades eastern North America at midlatitudes, dragging along air from both the Arctic and tropical masses (fig. 1.1).

East of the Cordillera, no significant obstacle impedes the southward flow of the Arctic air mass or the northward flow of the tropical air mass. Arctic air may rapidly cross the Interior Plains and reach Mexico, with little temperature change. Tropical air rarely extends beyond the Canadian border n the continent, although it may reach farther north over the Atlantic or the Pacific. On average, Arctic air reaches south into mid-Canada in summer, and into the northern United States in winter. Tropical air stretches to the northern United States in summer, but in winter it is restricted to the extreme southern United States. In winter, an anticyclonic eddy often develops in the southeastern United States as a relict of Arctic or Pacific air masses, at a latitude where subtropical anticyclones normally develop.

The behavior of air masses produces discrete patterns recognized as climatic regions. The position of fronts at various seasons coincides with the limits of major biomes, as was shown for eastern North America by R.A. Bryson et al. (1970; fig. 1.2). Historical shifts in the boundaries between air masses help explain the biogeographic history of species and biomes (P.A. Delcourt and H.R. Delcourt, chap.4). R.Hengeveld (1990, p. 213) stated this concept:

Global climate can thus be considered as a process of regional atmospheric warming and cooling and, related to this, of regional humidity variation. The resulting, highly dynamic spatial patterning of climatic conditions accounts for relatively sharply, though temporarily defined regions. Biogeographical processes, both present and past, should be viewed against this background of regional and global climatic patterns, shifting on various time scales.

In the following paragraphs, the climatic classification of North America developed by C. Troll and K.H. Paffen (M.J. Müller 1982; fig. 1.3) is presented. The descriptions are from M.J. Müller (1982) unless otherwise ndicated. The system by Troll and Paffen is used here because, among other reasons, the level of resolution is comparable to that of the soil map by D.Steila (chap. 2) and of the vegetation map by M.G. Barbour and N.L. Christensen (chap. 5). Other systems of classification and a discussion of their relative value may be found in climatology or biogeography textbooks (e.g., F.W. Cole 1980; J.R. Eagleman 1985; F.K. Lutkens and E.J. Tarbuck 1989; P.Pagney 1973; G.T. Trewartha and L.H. Horn 1980).

Much of the northern and eastern North American climatic pattern is latitudinal; it integrates both the average seasonal positions of air masses and the temperature and moisture gradients. In the boreal region, air masses and gradients tend to slope eastward to lower latitudes, so that a particular climatic regime is found farther south in the east than in the west. Patterns in the west are more complex due to the impact of the Cordillera on air movement and on temperature.

Polar and subpolar climates occur north of a line running from northern Alaska to northern Quebec-Labrador. They are dominated by the Arctic air mass. The areas of icecaps and glaciers of Greenland and the Arctic Archipelago have a high polar icecap climate. Adjacent areas of the Arctic islands have a polar climate, with the warmest month averaging below 6° C; this is the High Arctic climate or belt (G.Halliday 1989; T.W. Böcher et al. 1968).

Along the coasts of the Arctic mainland and of Hudson Bay, in the southern Arctic Archipelago and in southern Greenland, a subarctic tundra climate occurs, characterized by cool summers (warmest month 6°--10° C) and intensely cold winters (coldest month below -8° C); this is the Low Arctic climate or belt (G.Halliday 1989; T.W. Böcher et al. 1968). The Aleutians and the Alaska Peninsula have a highly oceanic subpolar climate with cool summers (warmest month 5°--12°C), moderately cold winters (coldest month -8° to 2°C) with little snow cover, and annual fluctuations in temperature of less than 13° C. Such a climate type may also occur at Cape Farewell, Greenland, an area of high precipitation (H.Walter and H.Lieth 1967; the maps published by M.J. Müller [1982] do not cover Greenland).

The cold temperate boreal climates stretch across the continent south of the Polar Zone. This belt barely reaches south of the Canadian border; its southern limit corresponds to that of the average winter position of the Arctic air mass. The west coast from Kodiak Island (Alaska) to Prince Rupert (British Columbia) has an oceanic boreal climate with moderately warm summers (warmest month 10°--15° C; growing season of 120--180 days), moderately cold winters (coldest month -3° to 2° C) with abundant snow (winter maximum in precipitation), and annual fluctuations in temperatures between 13° and 19° C. A broad, extensive band running from the Bering Sea to the North Atlantic is subject to a continental boreal climate with short, warm summers (warmest month 10°--20° C; growing season of 100--150 days) and long, cold winters with abundant snow, and annual fluctuations in temperature of 20°--40° C.

A zone of highly continental boreal climate occupies interior Alaska and northwestern Canada eastward to Hudson Bay in northern Manitoba. This region is part of the source area for the Arctic air mass in winter. The summers are short but relatively warm (warmest month 10°--20° C), permafrost is prevalent, the winters are long, extremely cold (coldest month below -25° C) and dry, and annual temperature fluctuations exceed 40° C.

Cool temperate climates span about one-third of the continent. Two groups of climates are recognized: woodland (forest) and steppe.

Woodland climates are classified as highly oceanic, oceanic, subcontinental, continental, humid and warm summer, humid and warm summer with humid winters, and permanently humid with warm summer. Climates of the west coast benefit from the abundant precipitation brought by the warm, moist Pacific air mass. A narrow band along the west coast from Washington to northern California has a highly oceanic climate characterized by cool to moderately warm summers (warmest month below 15° C), very mild winters (coldest month 2°--10°C) with high precipitation, and annual temperature fluctuation of less than 10° C. From the Queen Charlotte Islands to Vancouver Island, and a short distance inland from Washington to northern California, an oceanic climate dominates. Summers are moderately warm (warmest month below 20° C), winters are mild (coldest month 2°--10° C), maximum precipitation occurs in fall and winter, and annual fluctuation in temperature is less than 16° C.

Subcontinental climates are found on both sides of the continent. A sizable area extending from the Great Lakes--St. Lawrence region to the St. Lawrence estuary and the coast of Maine has this climate. Water vapor from the Great Lakes humidifies the Pacific air mass traversing the region (fig. 1). This vapor and the cyclonic storms of the Polar Front contribute to the precipitation regime of this zone. A second area of subcontinental climate occurs near the west coast, inland from the oceanic climate zone, in the form of a narrow north-south band with an eastward extension to the mountains of Montana and Wyoming and a small outlier in the middle Rocky Mountains. This extension reflects a greater penetration of moist westerlies at that latitude through the Columbia River--Snake River--Wyoming Basin gap. Summers are moderately warm and receive the maximum precipitation (growing season 160--210 days), winters are cold (coldest month -3° to -13° C), and annual fluctuation in temperature is 20°--30° C. A narrow band north of the prairies, from Minnesota northwestward toward Edmonton (Alberta), has a continental climate characterized by moderately warm and humid summers (warmest month 15°--20°C; growing season 150--180 days), cold, slightly dry winters (coldest month -10° to -20° C), and annual temperature fluctuations of 30° to 40° C.

A broad triangle, with its base between Minneapolis (Minnesota) and Oklahoma City (Oklahoma) and its apex at the west end of Lake Erie (Michigan and Ohio), corresponds with the eastern limit of penetration of the dry Pacific air mass, and it is strongly influenced by the tropical air mass in summer and the Arctic air mass in winter. Summers are warm and humid (warmest month 20°--26° C), winters are moderately cold and dry (coldest month -8° to 0° C), and annual temperatures fluctuate by 25° to 35°C. In the southern Rocky Mountains and at the margins of the Colorado Plateau, a similar climate prevails, but with mild to moderately cold, slightly humid winters (coldest month -6° to 2°C). A band stretching from southern New England to northeastern Arkansas has a permanently humid, warm summer climate. It straddles the southern part of the Polar Frontal Zone, although the influence of the tropical air mass is more prevalent than in the subcontinental climate to the north. The climate exhibits warm and humid summers (warmest month 20°--26°C), mild to moderately cold winters (coldest month -6° to 2°C), and annual temperature fluctuations of 20° to 30°C.

Steppe climates occupy the western interior half of the continent at cool temperate latitudes. The prairies in a narrow north-south band along the eastern edge of the region are designated humid steppe with cold winters. Temperature averages below 0°C in the coldest month. More than 6 months are humid (months without water deficit), and the period of vegetative growth lasts from spring to early summer. At the southern end of this zone, winters are milder (coldest month above 0°C). West of this band is a broad area extending westward to the Sierra Nevada, dominated by a dry steppe climate. Summers are arid, and winters are cold (coldest month less than 0°C). Less than 6 months of humidity occur. A small area to the southeast, in Colorado-New Mexico, has milder winters (coldest month 0°--6°C). Some basins of the Basin and Range physiographic province in Utah, Nevada, and Arizona experience a semidesert and desert climate with cold winters (coldest month less than 0°C).

Except for south Florida, the southern United States has warm temperate and subtropical climates. Much of California has a dry summer Mediterranean climate with humid winters (more than 5 humid months). The Central Valley and the southwestern coast of California experience a dry summer steppe climate with humid winters (fewer than 5 humid months). An irregular band along the Rio Grande in southern Texas, extending into New Mexico and parts of mountainous Arizona, has a steppe climate with summer humidity of short duration (fewer than 5 humid months) and dry winters. East of this zone in Texas, a narrow region has a dry winter climate with long-lasting summer humidity (8--9 humid months).

Semidesert and desert climates are encountered in two areas of the United States: in the Mojave and Sonoran deserts of Arizona and California, and in the Chihuahuan Desert reaching extreme southern New Mexico and western Texas. They are characterized by fewer than 2 humid months; winters are not hard, but transient or night frosts occur. Southeastern North America east of the Edwards Plateau, Texas (fig. 1.4), is characterized by a permanently humid climate. Summers are hot and are the time of maximum precipitation.

North America has only a small tropical zone, which is in southern Florida. It has a tropical humid summer climate, characterized by 7--9 1/2 humid and 3--5 1/2 arid months.

Physiography and Geology

The diversity and complexity of North American geology is well illustrated by the volumes produced for the Geology of North America Decade program (see the list in A.W. Bally and R.A. Palmer 1989). The classical map of landforms by Erwin Raisz (fig. 1.4) expresses this diversity. The map was an attempt to reproduce realistically the continental landscape from information gathered on the ground. Today, in contrast, satellites with remote-sensing devices help us to explore the globe, and from the vast amounts of data, computers produce stunning images and maps (fig. 1.5). In both Raisz's map and the mosaic image, diversity and complexity are revealed. Before reviewing the physiography and geology of North America, we shall examine the conditions that prevailed in the past and the processes responsible for the patterns observed today.

Tectonic Geology of North America

The movement of continents as a result of plate tectonics has led to their present configurations and has resulted in most mountain-building (orogeny) around the world. A chronology of the geologic periods mentioned below is provided in figure 1.6.

Most of North America is on a single major plate, the North American Plate, while Baja California and California west of the San Andreas Fault are part of the Pacific Plate. These two plates are colliding along the deep Aleutian Trench, whereas strike-slip motions (i.e., lateral motions along a fault line) dominate along the west coast of the United States (San Andreas Fault and associated faults, fig. 1.7). North America is drifting away from Europe with the widening of the Atlantic Ocean.

The major tectonic and geomorphic elements of North America are given in figure 1.7 (A.W. Bally et al. 1989). The craton (the stable core of a continent) of North America includes a Precambrian basement, of which the Canadian and Greenland shields are outcrops, and the sedimentary platforms that overlie it. The shields have been coherent for almost 2 billion years, and the oldest part (Greenland) is more than 3.5 billion years old. It comprises some of the oldest rocks on Earth. The basement extends underneath the folded belts of much of North America. In the southwest, the craton was deformed and uplifted during the Paleozoic, either in the formation of the Wichita Mountains and of the ancestral Rocky Mountains or in the Laramide and southern Rocky Mountains orogenies.

Three major folded belts are found around the periphery of the craton: (1) the Innuitian folded belt in the Canadian Arctic and northern Greenland, of Paleozoic age; (2) the Appalachian folded belt (including the Ouachita Mountains)of the east coast and the Caledonian folded belt of northeastern Greenland, also of Paleozoic age; and (3) the younger Cordilleran folded belt along the whole western edge of the continent, of Mesozoic-Cenozoic age. Superimposed on the Cordilleran folded belt are the Tertiary extensional systems of the Basin and Range Province and the widespread Tertiary volcanics encountered from the Basin and Range Province into southern British Columbia. Quaternary volcanoes of the circum-Pacific "ring of fire" are found from Central America to the Aleutians. The Atlantic and Gulf coastal plains and the adjacent continental shelf are a passive continental margin. The west coast of North America is an active continental margin that coincides with the western zone of the Cordillera (A.W. Bally et al. 1989).

A. W. Bally et al. (1989) provided a description of events and reconstructions of the relative position of North America in the context of the global development of oceans and continents since the Precambrian. The flowering plants---the plant group dominant today---had arisen near the beginning of the Cretaceous (e.g., E.M. Friis et al. 1987b), but probably not much earlier. Contemporary families of angiosperms mostly originated between the mid-Cretaceous (100 Ma, or million years ago) and the Paleocene (65--58 Ma), so that it is the last 100 million years of Earth history that is of prime importance in explaining their contemporary distributions, and those of their constituent genera and species. The contemporary representatives---families and genera---of modern vascular plants have also developed largely during this period.

To help understand the present-day distribution of vascular plants, therefore, we describe and illustrate the tectonic events that have occurred since the Cretaceous, using maps published by A.G. Smith et al. (1981; fig. 1.8). Epicontinental seas are not represented (see A.Graham, chap. 3, fig. 3.1), although they represented significant barriers to dispersal at various times. The paleogeography and paleoclimates of the late Cretaceous and early Tertiary in relation to plant geography and evolution, and the land bridges that linked North America to Europe and Asia, have been thoroughly discussed and summarized (P.H. Raven and D.I. Axelrod 1974; D.I. Axelrod and P.H. Raven 1985; J.T. Parrish 1987; M.E. Collinson 1990; B.H. Tiffney 1985b; D.W. Taylor 1990; D.I. Axelrod et al. 1992; A.Graham, chap. 3).

In the Jurassic, the continents were close together and formed the super-continent Pangaea. In the Cretaceous, they became increasingly separate due to the expansion of new oceanic basins. In the early Cretaceous, a narrow oceanic basin formed in the position of the central Atlantic Ocean between South America and Africa, and possibly also in the Gulf of Mexico. South of the Gulf, the Proto-Caribbean Sea formed in the widening gap between North and South America (fig. 1.8a). Expansion of the central Atlantic and Pacific oceans occurred. In the late Cretaceous, the spreading of the seafloor continued between North America and Africa, and a new rift system began between North America and Europe (fig. 1.8b). The Proto-Caribbean stopped widening, and the Greater Antilles arc (including Cuba) began its collision with the Bahama Platform, causing the initiation of the Panamanian arc system.

The south Atlantic and the west Pacific oceans expanded during that time. Intensive volcanism occurred, associated with the subduction (phenomenon whereby the lighter oceanic plate is driven underneath the denser continental plate along their collision front) of the Pacific Plate. A large meteoric impact may have occurred at the end of the Cretaceous. This cataclysm may have been responsible for mass extinction of living groups (volcanism is given as the alternative explanation of this phenomenon; M.E. Collinson 1990). Although Europe and North America formed a single geological unit, sea level was high during the Cretaceous, and shallow continental seas between Europe and eastern North America and between eastern and western North America probably prevented direct overland migration until the beginning of the Tertiary (A.Graham, chap. 3, fig. 3.1). The relative position of North America and Siberia had been stable since the end of the Mesozoic (fig. 1.8c--f).

In the early Oligocene (the period immediately following that illustrated in fig. 1.8d), Baffin Bay and the Labrador Sea opened, and Greenland separated from Norway. Strike-slip motions along the Nares Strait occurred between Greenland and Ellesmere Island. Formation of the Eurekan folded belt in eastern Ellesmere Island resulted in the opening of northern seaways. Seafloor spreading continued in the central and southern Atlantic. In the Caribbean, the eastern island arc system of the Caribbean Plate began to approach its modern location. The leading edge of the Pacific Plate impinged on the margin of North America during the Oligocene. Consequently, the San Andreas strike-slip fault system developed, and the Gulf of California opened as present-day Baja California commenced moving northward from approximately the location of the Mexican state of Jalisco.

The San Andreas Fault runs through the Gulf of California to the vicinity of Cajon Pass north of San Bernardino, California, turns west and runs along the San Bernardino and San Gabriel mountains to the vicinity of Mount Pinos, and then turns again, running north-northwest, leaving the continent at the north end of the Point Reyes peninsula just north of San Francisco. All of the lands west of the fault are on the Pacific Plate and have moved to their present positions during the past 35 million years. The Transverse Ranges of southern California, which have formed along the east-west trending section of the fault, are a zone of violent tectonic activity and uplift, and movements along the fault may well also be related to the development of the Basin and Range physiographic province. Changes in plate motion and stress systems may have been responsible for the development of the Basin and Range physiographic province.

North America and Europe remained in contact via Greenland until the Miocene (fig. 1.8e), when the mid-Atlantic Ridge extended to the Arctic Ocean. Before the Miocene, migration of terrestrial organisms could have occurred intermittently, hindered at times by shallow seaways. A narrow seaway existed between Europe and North America from the Cretaceous rise in sea level onward; during the Eocene, high sea levels contributed to submersion of continental regions. Bridges may have emerged for short times during drops in sea level in early Paleocene and early Eocene. Direct overland migration between North America and Europe could not have occurred through eastern Asia during that period.

In Paleocene and early Eocene, eastern Asia was isolated from Europe by the Turgai Strait, a shallow seaway. This strait was closed by the early Oligocene, but the European landmass was then separated from the Russian platform by the Polish Seaway. The land between Alaska and northeastern Siberia was continuous throughout late Cretaceous and early Tertiary. Submergence of this region occurred in the late Tertiary. The Isthmus of Panama was closed in the late Pliocene, allowing the direct overland interchange of terrestrial biotas between North and South America for the first time during the history of the flowering plants.

Continental Glaciation in North America

The beginning of the Quaternary is set by convention at 2 Ma (H.E. Wright Jr. 1989). During the Quaternary major glaciations that strongly affected North American biotas repeatedly occurred . Glaciations resulted from both climatic and geological phenomena. Glaciers affected regional weather (P.A. Delcourt and H.R. Delcourt, chap.4), and they modified the landscape, scouring the surface or producing surficial deposits of considerable extent. These deposits can result directly from glacial erosion, or less directly from proglacial lacustrine deposition or from transport of silt and sand by wind along modified wind circulation patterns.

Several episodes of glaciation occurred during the Quaternary, and much debate surrounds the interpretation of regional and local events. Geoclimatic events prior to the late Wisconsinan glaciation (also called the Wisconsin glaciation) doubtless affected the distribution of plants and animals, but their reconstruction is uncertain. Also, the last glacial maximum, the Wisconsinan, is the episode that contributed most to the definition of the distribution of modern North American biotas, and to the distributions of the taxa treated in this flora. Therefore, we have limited our summary of physical glacial events (i.e., position and contour of glaciers, proglacial lakes, and marine invasions) to the late Wisconsinan, from maximum glaciation to total deglaciation, i.e., 18,000--ca. 5000 yr B.P. (fig. 1.9). These physical factors directly determined the areas where living organisms could survive (refugia; see R.W. Mathewes 1989) or migrate (e.g., C.E. Schweger 1989) during the Pleistocene. (For discussions of various reconstructions and models proposed by Quaternary geologists, see R.F. Flint 1971; A.Weidick 1976; H.E. Wright Jr. 1989; and authors in W.F. Ruddiman and H.E. Wright Jr. 1987 and in R.J. Fulton 1989. Vegetation and climatic changes during the late Wisconsinan are discussed in P.A. Delcourt and H.R. Delcourt, chap. 4.)

Glacial maximum occurred at about 18,000 yr B.P. (fig.1.9a), but it was not reached simultaneously all along the ice limits. At that time, ice covered nearly the whole of Canada (except for much of Yukon) and lowland areas in the Arctic. The extent to which the Gulf of St. Lawrence was ice-free is controversial. Glaciers in Alaska were restricted to the Alaska Range and adjacent areas, and to parts of the Brooks Range. The Bering Strait area was dry land, allowing migration between eastern Siberia and Alaska. Greenland was completely ice-covered, and pack ice reached to the southern end of the island. Two major ice masses, confluent at that time, constituted the ice cover: the Cordilleran Ice Sheet in the Canadian Cordillera, and the Laurentide Ice Sheet that covered half of the continent east of the Rocky Mountains. Separate, smaller ice sheets, also confluent, occupied the High Arctic and Newfoundland. Discontinuous mountain glaciers were present in several high ranges of the western United States, and pluvial lakes (P.A. Delcourt and H.R. Delcourt, chap. 4, fig. 4.9) developed at various times during glacial retreat. Sea levels lowered as much as 200 m, and broad expanses of the continental shelf were exposed and available for plant colonization and migration. Meltwater discharge was mainly to the south of the ice, notably via the Mississippi River system.

At 14,000 yr B.P. (fig. 1.9b), the ice sheets had receded perceptibly, particularly in the east. An ice-free corridor was gradually developing between the Cordilleran and Laurentide ice masses. The Greenland sheet had receded, but the island was still fully covered. Mountain glaciers were disappearing. Proglacial lakes were forming at the margins of the retreating ice sheets between morainic deposits and the ice sheet; some of them were forerunners of the Great Lakes. The Des Moines lobe reached its maximum in Iowa, contributing to the delimitation of the Driftless Area of Wisconsin. Between 18,000 and 12,000 yr B.P., strong winds from the northwest deposited sand and loess over the Great Plains. Drainage to the ocean was largely to the south.

At 12,000 yr B.P. (fig. 1.9c), ice was globally receding. The Cordilleran and Laurentide ice sheets were separated in Alberta by a corridor through which migration could occur between Beringia and the southern, unglaciated part of the continent. Mountain glaciers in the United States became more restricted. Major proglacial lakes were found all along the southern and western margins of the Laurentide Ice Sheet, notably Lake Agassiz in the northeastern prairies, and the precursors of the Great Lakes. In the east, the ice was retreating from the St. Lawrence Valley. A local dome existed over Newfoundland. Sea levels were slowly rising, inundating the continental shelf and reducing the area of the coastal plain.

Around 10,000 yr B.P. (fig. 1.9d), in response to the warming of the climate, recession of the ice accelerated. The Cordilleran Ice Sheet was broken into several domes; mountain summits became ice-free before lowlands. Ice had disappeared from most mountain areas of Alaska and the western United States, except in the St. Elias and Alaska ranges; isolated glaciers still persisted locally. Much of the Arctic was also ice-free, and the coastal areas of Greenland were uncovered. Retreat of the Laurentide ice mass north of the St. Lawrence Valley allowed marine invasion by the Champlain Sea to occur. Large proglacial lakes existed along the western margin, notably Lake Agassiz. The Great Lakes Basin was occupied by smaller water bodies. Drainage could now occur to the east, via the St. Lawrence estuary, as well as to the south. The Laurentide ice mass started receding over the Torngat Mountains of Labrador, and little ice was left on Newfoundland.

By 8400 yr B.P. (fig. 1.9e), ice remained in the Canadian Cordillera and Arctic only in areas where glaciers persist today. The Laurentide ice mass was centered on Hudson Bay, having receded from west of the Torngat Mountains in the east, the Keewatin District in the west, and the south of James Bay. A separation was forming between the Keewatin and Laurentide domes. Lake Agassiz occupied nearly all of Manitoba and extended in the east to join Lake Ojibway-Barlow, responsible for the deposition of the Clay Belt of northern Ontario and western Quebec. The Champlain Sea was receding from the St. Lawrence Valley and the Laflamme Gulf occupied the Saguenay--Lake St. John area. The Atlantic Coastal Plain was nearly as it is now.

Soon after this time, a sudden marine invasion occurred around the periphery of Hudson Bay, provoking a fast recession of the ice and its separation into the Keewatin and Labrador domes by 8000 yr B.P. (fig. 1.9f). By then, the Tyrrel Sea was transgressing over much of the Hudson Bay Lowlands, Hudson Strait was open, and the Champlain Sea was nearly gone. The Great Lakes slowly achieved their present extent. By 7000 yr B.P. (fig. 1.9g), the ice had disappeared from Keewatin, but a dome was left on Baffin Island. The Labrador dome slowly melted in central New Quebec and adjacent Labrador. Through that whole period, ice receded from the periphery of Greenland, and the Greenland Ice Sheet reached a minimum at about 5000 yr B.P. (fig. 1.9h), followed by a readvancement to the position it occupies today. No ice was left on the continent at that time, except in the glacier areas of the Cordillera. The domes on Baffin Island receded to the areas where glaciers are found at present.

Geographical Features

River systems, surficial geology, edrock types, and permafrost are features of North American geography that transgress the boundaries of the physiographic units described below. Thus a general description of these features is provided here.

River Systems

North America has several major river basins (fig. 1.4). The St. Lawrence River drains the Great Lakes Basin and opens into the Gulf of St. Lawrence. The Mississippi River system drains the center of the continent between the Appalachian and Rocky mountains from near the Canadian border southward, and terminates at the Gulf of Mexico. Flowing at the border of Texas and Mexico, the Rio Grande River system drains the southern Rocky Mountains and reaches the Gulf of Mexico. The Colorado River Basin drains a part of the Basins and Ranges and the Colorado Plateau, and finishes at the Gulf of California. The Columbia River and its tributaries, which drain the Columbia-Snake plateaus, and the Fraser River, which drains the interior of British Columbia, both empty into the Pacific at midlatitudes.

The mighty Yukon River drainage system covers the interior of Yukon and Alaska, and terminates at the Bering Sea. The Mackenzie River Basin takes its source in the Canadian Rocky Mountains and in the great northern lakes (Great Bear, Great Slave, Athabaska), and empties into the Beaufort Sea. The Canadian Prairies and adjacent areas of Minnesota and North Dakota are drained by several rivers (Saskatchewan North, Saskatchewan South, Souris, Red) that flow from the Rocky Mountains or the border area, near or through Lake Winnipeg, north of which they merge into the Nelson River, which reaches Hudson Bay. Other rivers on the continent are smaller, drain relatively small areas, and empty directly into the adjacent oceans (fig. 1.4).

Surficial Geology

The following description is based on the map by R.C. Heath (1989b). The diversity of surface deposits encountered on the continent reflects the erosional and depositional processes that have predominated in various parts of North America at various times (fig. 1.10). The northern half of the continent is covered with deposits (e.g., tills, moraines) that reflect the action of the ice sheets, especially during the Wisconsinan glaciation (fig.1.9). Some areas were laid bare by glacial action, notably mountainous British Columbia, ice-free Greenland, parts of the Arctic Archipelago and of the Ungava Peninsula, the north shore of the Gulf of St. Lawrence, and much of Newfoundland (fig. 1.10). Where proglacial lakes or marine inundations occurred (fig. 1.9), extensive silts and clays were left; these are prominent in central Canada and adjacent United States, south and west of Hudson Bay, in the Arctic Coastal Plain, in the Arctic Lowlands, south of the Bay of Ungava, and in the St. Lawrence Valley. Lake deposits are also encountered in areas where pluvial lakes (P.A. Delcourt and H.R. Delcourt, chap. 4) existed within the Great Basin physiographic province during the time of glaciation (fig. 1.10).

The Central Valley of California was also inundated by sea at that time. Alluvial plain and loess deposits in the Great Plains and in the Central Lowlands are associated with fluvial and aeolian action during the Wisconsinan glaciation, particularly in the central United States, east of the Rocky Mountains (fig. 1.10). Surficial material in much of the area outside of the limits of the Laurentide and Cordilleran ice sheets, in Alaska-Yukon and in the conterminous United States, results mostly from in situ weathering. In the mountainous, arid western United States, water and wind erosion played a major role, leaving bare rock and alluvial fans and plains (fig. 1.10). The Atlantic and Gulf coastal plains are covered by coarse, stratified marine deposits of Cretaceous to Quaternary age, with marine limestone prominent in southern Florida. Finally, the alluvial plains and deltas of the Yukon, Mackenzie, and Mississippi rivers have fine-grained deposits (fig. 1.10).

Bedrock Geology

The following description is based on the map of R.C. Heath (1989). The rock formations that underlie surficial deposits and that are also sometimes exposed (fig. 1.11) are diverse and reflect tectonic processes. Unconsolidated deposits (see also fig. 1.10) predominate in the Atlantic and Arctic coastal plains, in the western Great Plains, in intermontane basins and valleys, in the Central Valley of California, and in parts of Alaska (fig. 1.11). Intrusive igneous and metamorphic rocks (e.g., granites, gneiss) dominate the Canadian and Greenland shields, Newfoundland, and parts of the Appalachian Mountains and adjacent Piedmont; and several batholiths are found in the western Cordillera, including a large part of British Columbia and Yukon. Sedimentary rocks predominate on the stable platform surrounding the craton (fig. 1.7) and cover the Great Plains and Central Lowlands, the St. Lawrence Lowlands, the Hudson Bay Basin, the Colorado Plateau, and the Wyoming Basin, and parts of Alaska, the Arctic Archipelago, and northern Greenland. Metamorphic formations have been folded notably in the Queen Elizabeth Archipelago, along the Appalachian Mountains and in the Ouachita Mountains, in the Rocky Mountains and Brooks Range, and in the Pacific Coast Ranges. Volcanic rocks are encountered mainly in the Aleutian Archipelago, in the interior of British Columbia, in the Columbia-Snake plateaus, and in various areas of the Basins and Ranges physiographic province (fig. 1.11).

Glaciers and Permafrost

Because of its high latitude, the Arctic Ocean is almost completely covered by a permanent sea pack of ice. In winter, the continuous sea pack extends southward to cover the Bering Sea as far south as the Alaska Peninsula in the west, and reaches south of Greenland and along the Labrador Coast in the east. Looser pack ice also covers the northeastern coast of Newfoundland and the Gulf of St. Lawrence. The Great Lakes, the large northern lakes (fig.1.4), and the St. Lawrence River also freeze during the winter months. Ellesmere Island and northern Greenland are the landmasses closest to the North Pole (figs. 1.4, 1.12), and the whole Arctic Archipelago and parts of the continental Northwest Territories and of Alaska are located north of the Arctic Circle (fig. 1.4). An icecap covers about 90% of Greenland (figs. 1.4, 1.10). The highlands of the Queen Elizabeth Islands and of Baffin Island (Arctic Archipelago), and the St. Elias Range (Alaska-Yukon) have glaciers (fig.1.10). In the western Cordillera, smaller glaciers are found farther south along the Coast Ranges and in the Rocky Mountains.

Permafrost is ground that remains permanently frozen to a certain depth; in summer only the surface thaws. Figure 1.12 illustrates the distribution of permafrost in North America (T.L. Péwé 1983b). Continuous permafrost is found in the northern two-thirds of Greenland, the Canadian Arctic Archipelago, the Ungava Peninsula and the Torngat Mountains, and the south coast of Hudson Bay, northwestwardly to Great Bear Lake, northern Yukon, and the Brooks Range area. Discontinuous permafrost occurs in the southern third of Greenland, Alaska, and the Northwest Territories, and the northern third of British Columbia, then southeastward across the northern Prairie Provinces to James Bay and southern Labrador. South of this area, only alpine permafrost is encountered. In the west, it is found in the Coast Ranges of British Columbia, the northern Cascade Mountains, the Sierra Nevada, and along the Rocky Mountains (fig. 1.12). Small areas of alpine permafrost also occur in the east in the Shickshock Mountains of the Gaspé Peninsula and on the highest mountains of New England.

Physiographic Regions

The division of North America north of Mexico into physiographic units (fig. 1.13) has been addressed in detail by several authors (notably N.M. Fenneman 1931, 1938; C.B. Hunt 1974; J.B. Bird 1980; M.J. Bovis 1987; W.D. Tidwell 1972; R.F. Madole et al. 1987; and E.C. Pirkle and W.H. Yoho 1982). This section is a synthesis of their information. Absolute consistency does not exist concerning the circumscriptions or nomenclature of physiographic units among the sources consulted. The following classification reflects these uncertainties, and it should not be construed as definitive. The formal physiographic units (usually provinces) of the various authors were grouped into informal "systems." These systems are aggregations of physiographic units that have similar geologic histories and maintain geographic continuity and distinctness. The systems are described in a clockwise sequence, from the northeast to the northwest.

Arctic System

The Arctic system (fig. 1.13) comprises the extreme north of the continent and is fronted on the north by the Arctic Ocean. The system encompasses about 1.3 million km², including intervening waters. Included are the Caledonian Uplands, High Arctic Uplands, Arctic Lowlands, and Arctic Coastal Plain provinces.

Quaternary glaciers covered most of this system (fig.1.9); they created straight, deep channels and numerous fjords, and removed most of the weathered mantle from the islands, often leaving bare rock (fig. 1.10). Marine transgressions over the Arctic Coastal Lowlands and Arctic Coastal Plain were extensive after glaciation (fig.1.9). Glaciers remain on promontories of Axel Heiberg, Ellesmere, and Devon islands, and on Greenland (fig. 1.13).

The rugged North Greenland Mountains and the High Arctic Uplands are geologically related (figs. 1.7, 1.13). Both were uplifted during the Innuitian (Paleozoic) and Eurekan (Cretaceous) orogenies (H.P. Trettin 1989). Subsequently, they were separated by the movements of the Greenland Plate. The rocks are mostly folded sedimentary formations (fig. 1.11). Elevations on Ellesmere Island reach 2604 m (Barbeau Peak). The Paleozoic Caledonian orogeny uplifted the mountain ranges of northeastern Greenland that constitute the Caledonian Province (A.Escher and W.S. Watt 1976). The topography of the western part of the system includes plateaus, lowlands, and coastal plains dissected by channels and fjords. The Mackenzie Delta comprises fine-grained deposits of sand, silt, and clay (fig. 1.10). The Arctic Coastal Plain (figs. 1.4, 1.11), also referred to as the Arctic Slope, is low and dotted with ponds and lakes. It stretches 1200 km from a point 160 km east of Point Hope (Alaska) to the Mackenzie Delta. At its widest, it is about 160 km across.

Canadian Shield System

The Canadian and Greenland shields form the core of the continent (figs. 1.7, 1.13). The Greenland Shield rifted from the Canadian Shield during the Tertiary and now is separated from the mainland by Davis Strait (fig. 1.4); it constitutes most of Greenland. The north-south axis of the Greenland Shield is about 2700 km, and the east-west axis, 1200 km. Most of it lies below sea level because of the weight of the 3-km-thick icecap of the Greenland Icecap Province. This ice occupies almost 90% of the surface of Greenland (G.Halliday 1989). A narrow, discontinuous, coastal band, mostly along the west side, is ice-free. This zone comprises deep fjords and a rocky coast (Western Mountain Fjords Province).

The Canadian Shield is a gently rolling peneplain, about 3400 km long and 3000 km wide, covering over 5 million km². The elevation is mostly below 600 m, with half lower than 300 m; some peaks exceed 1500 m. It comprises mostly acidic, granitic uplands separated by igneous and sedimentary metamorphic formations (fig. 1.7). At one time, sedimentary rocks covered it, but subsequent erosion exposed Precambrian formations and only left remnants of the sedimentary formations. A series of major lakes is found along its western margin with the Interior Plains: Winnipeg, Athabasca, Great Slave, and Great Bear lakes. The entire region was glaciated, and the cores of the continental glaciers were found in this system, one in Franklin District and the other on the Quebec-Labrador peninsula. Eastern and western portions of this shield are covered by coarse-grained tills. Areas peripheral to Hudson Bay were inundated during glaciation by extensive proglacial lakes and seas that left behind silts and clays (fig. 1.6).

The Canadian Shield has been divided into a series of provinces based on geological history, but they are not particularly relevant to modern-day plant distributions. Mountainous highlands with eroded summits are found in several belts on the eastern half of the shield. These highlands include the Davis Mountains (1500 m; maximum 2591m) on Baffin Island, which are partially glaciered, the Torngat Mountains (1200 m; maximum 1595 m; fig. 1.4) in northern Labrador, and the Otish Mountains (1135 m) in the Laurentian Highlands of Quebec. Thelon Plains (Northwest Territories) and Athabasca Plains (Alberta-Saskatchewan), two extensive low-lying plains of sandstone, limestone, and dolomite, are found on the western half of the shield (fig.1.11). Sand dunes occur on the south shore of Lake Athabasca. In the middle of the shield, Hudson Bay north to Southampton Island, James Bay, and the Hudson Bay Lowlands form a shallow basin that is over 1600 km across from north to south (figs. 1.4, 1.11, 1.13). These lowlands are limestone overlain by marine sediments of Quaternary age (figs. 1.10, 1.11).

Appalachian System

The Appalachian system consists of a series of roughly parallel plateaus, valleys, and ridges that extend for more than 3000 km, from Newfoundland southwestward to central Alabama (fig. 1.13). The system resulted from Paleozoic orogenic events followed by erosion (fig. 1.7). The central ranges are thrust-fault mountains flanked by plateaus. This system includes several physiographic provinces: Maritime and New England, Adirondack Mountains, Piedmont Plateau, Blue Ridge, Valley and Ridge, and Appalachian Plateau.

The Maritime and New England Province is made of hilly uplands ranging from sea level to about 450 m, with a few peaks exceeding 1800 m (Mt. Washington, 1907 m). In New England, rocks are mostly gneiss and granite, whereas in the Gaspé Peninsula and in the Maritimes, rocks are mostly sedimentary (sandstones) and igneous intrusions (fig.1.11). Within this province, substrates are mainly acidic. Pleistocene glaciers covered the entire area, leaving till as they receded; much of Newfoundland is bare rock (fig.1.10).

The Adirondack Mountains Province is a nearly circular dome of Precambrian acidic rocks surrounded by Paleozoic sedimentary formations (figs. 1.11, 1.13). The highest peak, Mt. Marcy, is 1629 m. The Adirondacks were completely glaciated and are now covered by coarse till (fig.1.10).

The Piedmont Plateau Province extends 1600 km from southern New York to central east Alabama (fig.1.13). It is a rolling plateau tilted eastward from 450 m to about 150m at the Fall Line, which represents the boundary with the coastal plain. Rocks are metamorphic and diverse, but most are acidic gneiss and schist (fig. 1.11). Along the eastern edges of the plateau, expanses of granites are exposed as flatrocks.

The Blue Ridge Province consists of a narrow range extending from southeastern Pennsylvania southward to northern Georgia (fig. 1.13). The ridge system is 900km long and 8 to 80 km wide. Elevation is from 750 to 1950m, and Mt. Mitchell (2326 m) is the highest point of eastern North America. Substrates are mostly Precambrian granitic rocks (fig. 1.11).

The Valley and Ridge Province s 2000 km long and 40 to 120 km wide, a complex of ridges and valleys extending from southern Quebec to central Alabama (fig. 1.13). Sandstones, shales, and limestones of varying degrees of metamorphosis constitute major rock formations (fig. 1.11). The area is noted for its fold mountains and broad flat valleys.

The westward sloping Appalachian Plateau Province stretches from the Adirondacks south to central Alabama and covers a distance of 1500 km (fig. 1.13). The plateau surface is mostly at 600 to 900 m. The Catskill Mountains, at the northern end, reach 900 m. The surface comprises mostly resistant, acidic sandstones deeply incised by streams. Underlying the sandstones are shales and limestones, which lie exposed along the edges of the plateau (fig. 1.11). The glaciated, northern portion has lakes and till (fig. 1.10).

Coastal Plains System

The Atlantic and Gulf Coastal Plains Province forms a belt along the Atlantic Ocean and the Gulf of Mexico, extending from Cape Cod in Massachusetts south to the Mexican border (fig. 1.13). The coastal plain is an elevated sea bottom of low relief that rises inland from below sea level to an altitude of about 150 m. The submerged portion forms the Continental Shelf of eastern North America, which slopes gently to a depth of about 200m. The coastal plain dips to the north and east; this slope results in extensive tidal inlets, such as Chesapeake Bay (fig. 1.4), and submerged banks, such as George Banks and the Grand Banks. The 1800-km shore consists of sandy beaches, notably barrier islands (outer banks, such as those at Cape Hatteras; fig.1.4). Behind these beaches, low-lying areas are occupied by sounds, estuaries, swamps, or wetlands.

The province comprises mostly unconsolidated marine sediments (figs. 1.10, 1.11), eroded from the Appalachian Mountains, of Cretaceous, Tertiary, and Quaternary ages, and occurring in this sequence from inland to the coast. These formations form belts of low ridges and valleys roughly parallel to the coastline. Peninsular Florida is underlain by limestones, and the area is characterized by sinkholes with numerous wet prairies and lakes. Stratified marine limestones, including coral reefs, are exposed extensively in the Everglades and Florida Keys (fig. 1.10).

The Mississippi alluvial plain (fig. 1.4) divides the province into three areas: eastern (Atlantic and eastern Gulf coasts), central (Mississippi flood plain), and western (coastal band of Texas). This alluvial plain extends from the confluence of the Ohio and Mississippi rivers, where the coastal plain penetrates far inland, to the extensive Mississippi Delta (fig. 1.13). The Mississippi River meanders through the low-lying plain for about 300 km, and drains mostly through its delta. It comprises fine-grained floodplain and delta deposits of silts, sands, and clays (fig.1.10).

Interior Highlands System

This system comprises two nearly disjunct areas (figs. 1.4, 1.11), the Interior Low Plateau Province east of the Mississippi River, and the Ozark Plateau Province and the Ouachita Mountains Province west of it. The Ozarks and Ouachitas are very old (Paleozoic). The Interior Highlands lie south of the glacial limits.

Elevations of the Interior Low Plateau Province are generally less than 300 m and are characterized by moderate, rolling relief, sloping from the Appalachians toward the Mississippi River. The province is 470 km long and 400 km wide. The Lexington Plain and the Nashville Basin (fig.1.4) are mostly limestone exposures with sinkholes and underground drainages.

The Ozark Plateau Province (figs. 1.4, 1.13) is primarily a rolling upland of moderate elevation, mostly above 300 m. The province is about 470 km north-south and 270 km east-west. Limestone and dolomite are the most abundant substrates (fig. 1.11). Some of the higher summits have sandstone caps, such as the Boston Mountains in the south that reach more than 600 m (maximum elevation 781m).

The Ouachita Mountains Province is approximately 360km long and 100 km wide and is mostly separated from the Ozarks by the Arkansas River Valley. The Ouachita Mountains arose from the same uplift as the Appalachians. They comprise folded linear ridges and valleys ranging east to west, with a maximum elevation of 839 m (Magazine Mountain). Predominant rocks are sandstone and shale (fig. 1.11).

Interior Plains System

The Interior Plains system stretches from the Appalachian Mountains, Interior Highlands, and Canadian Shield on the east to the Rocky Mountains on the west, and from southern Texas northward to the Arctic Lowlands. It comprises three provinces: St. Lawrence Lowlands, Central Lowlands, and Great Plains (figs. 1.4, 1.13). The Interior Plains constitute the stable platform bordering the Precambrian Shield (fig.1.7). They cover one-third of the continent and are the most extensive system. An epicontinental sea (A.Graham, chap.3, fig. 3.1) linked the Gulf of Mexico and the Beaufort Sea across this area during the Cretaceous and early Tertiary. It deposited extensive sediments on the Great Plains and the western part of the Central Lowlands (fig.1.11).

The St. Lawrence Lowland Province follows the course of the St. Lawrence River from the Frontenac Axis (a narrow outcrop of Precambrian rock linking the Canadian Shield to the Adirondacks, just east of Lake Ontario) to western Newfoundland on the Gulf of St. Lawrence (fig. 1.13). This narrow lowland is constricted between the Canadian Shield and the Appalachians. It is oriented southwest to northeast and stretches 1500 km. Elevations vary from sea level to 150 m near the Frontenac Axis. The province consists of three disjunct land areas: the St. Lawrence Valley, Anticosti Island and the Mingan Archipelago, and northwestern Newfoundland. The valley is 500 km long, about 150 km at its widest bulge, and it terminates near Quebec City. In the Gulf of St. Lawrence area, the lowlands are submerged except at Anticosti Island and the Mingan Archipelago, and on the coastal plain of northwestern Newfoundland. The entire area was glaciated. During ice retreat, much of the St. Lawrence River Valley was inundated by the Champlain Sea (fig. 1.9), which left behind marine sediments, mainly clay (fig. 1.10). The province is characterized by flat limestone and sandstone outcrops (fig.1.11). The Monteregian Hills are igneous intrusions and represent a conspicuous feature on the flat valley. In many respects, the St. Lawrence Province can be considered an extension of the Central Lowland Province.

The Central Lowland Province (figs. 1.4, 1.13) is a vast plain oriented north-south just west of the Mississippi River Valley; it projects into the south central Canadian Prairies, and a broad wedge extends eastward to the Great Lakes. It is 2400 km long (north-south) in the west and 2200 km at its greatest width (east-west). Elevations range from 150 m along the Mississippi River to 600 m in the west. The northern part was glaciated, except for the Driftless Area of southwestern Wisconsin and adjacent Minnesota and Iowa. The shallow proglacial Lake Agassiz covered much of the northern extension of the province, mostly in Manitoba, and left a flat plain of clay and silt sediments (fig.1.10).

Similar deposits in southern Ontario resulted from various developmental phases of the proglacial Great Lakes. Gravel and sand were deposited on glacial outwash plains in the Lower Peninsula of Michigan and southwest of Lake Superior. Other glaciated areas were covered by fine-grained tills (figs. 1.9, 1.10). The southern portion, the Osage Plain, was not glaciated; it is located south of the Missouri River. Loess deposits (fig. 1.10) cover much of the surface, and they are deeply dissected by steep-sided river valleys; there is little relief otherwise. The Red Hills of south Kansas are noteworthy features caused by stream erosion.

The Great Plains Province is a western continuation of the Central Lowlands but is distinguished by having Tertiary deposits washed eastward from the Rocky Mountains. It stretches from southern Texas to the Arctic Lowlands, east of the Rocky Mountains (figs. 1.4, 1.13). Elevations range from 600m in the east to 1500 m at the foot of the Rocky Mountains. Most rivers run transversely across the province and drain to the Mississippi River in the United States, and to Hudson Bay and the Beaufort Sea in Canada (fig. 1.4). Since aridity increases toward the west, soils are more alkaline than in the Central Lowlands.

The portion located in Canada and the adjacent United States was glaciated and is covered by fine-grained till and proglacial lake deposits (figs. 1.9, 1.10). The area is flat with rolling hills, the most conspicuous being the gravel-topped, rocky Cypress Hills (Alberta-Saskatchewan; fig. 1.4), which reach 300 to 600 m above the surrounding plains (maximum elevation about 1400 m). The unglaciated southern half lies within the United States.

Most of the western side is an elevated plain with varying degrees of dissection due to stream cutting. Extensive erosion sculpted impressive canyons, hills, and bluffs, referred to as the Badlands, that are located mostly in South Dakota on the Missouri Plateau. Several dome mountains rise 500 to 600 m above the plain; the most prominent are the Black Hills of South Dakota (1000 m above the adjacent plain; highest elevation is Harney Peak, 2207m; figs. 1.4, 1.11). The Black Hills are an uplift of Precambrian rock topped with limestone eroded in the eastern half, exposing granite and schist. Extensive areas of sand sheets formed by wind from alluvial deposits are distributed over large portions of this section (fig. 1.10). Striking results of this process are stabilized dunes such as the Nebraska Sandhills (fig. 1.4), which occupy 62,000 km². At the southern end is an extensive limestone tableland, the Edwards Plateau (fig. 1.4), demarcated from the coastal plain by an escarpment.

Intermountain System

The Intermountain system derives its name from being located between the Rocky Mountains and the Sierra Nevada--Cascades axis. It extends south of the Mexican border (fig. 1.13). The system comprises four distinct and geologically diverse provinces: Basin and Range, Colorado Plateau, Wyoming Basin, and Columbia River plateaus. It was uplifted during the Laramide orogeny (Cretaceous and Tertiary) along with the flanking mountain ranges (fig. 1.7). The intermountain region consists mainly of a series of plateaus that are the highest on the continent.

In arid and semiarid areas, winds cause sheet erosion of cliff faces, valley floors, and plateau surfaces, as opposed to gully erosion by water. At the bottoms of basins that lack external drainage are playas, i.e., mud flats that often have accumulation of salts. These playas derive from lakes that evaporate in times of limited precipitation (P.A. Delcourt and H.R. Delcourt, chap. 4, fig. 4.9). Other characteristic features are alluvial fans that are built from sediments eroded from the adjacent mountains, rock pediments that are sloped and eroded by stream floods, desert pavements, and wind-accumulated sand dunes (fig.1.10).

The Basin and Range Province is bounded to the north by the lava flows of the Columbia Plateau and extends south into Mexico. The province occupies about 780,000 km². Elevations vary from below sea level (Death Valley, -86 m; Salton Sea, -71 m) to nearly 4000 m above sea level at the highest peaks; most of the plateau is at about 1500 m. Relief from basin floor to adjacent mountain tops is from 900 to 1500 m. The province is characterized by broad, level desert basins and narrower, elongate, isolated, parallel mountain ranges trending north to south (fig.1.4). In the south, this pattern becomes irregular. In the north, most basins lack external drainage. This topography originated by block faulting in the Oligocene, accompanied by extensional phenomena (fig. 1.7). Paleozoic formations predominate; they consist of limestone, siltstone, shale, and sandstone (fig. 1.11). The province is geologically very diverse, and several sections may be recognized (described below).

The Sacramento Mountains Section adjoins the Great Plains. It is mostly plateau with diverse topography and geology. The western and eastern rims are montane, with a basin between, which is occupied by salt basins, clay hills, and dunes rising to 30 m. Erosion of gypsum deposits by water and wind led to the formation of alkaline flats and dunes, such as those at White Sands (New Mexico). The Mexican Highland Section, from southern Arizona to trans-Pecos Texas, is topographically varied. Basins decline westwardly from 1500 m to 600 m, and ranges are 900 to 1500m higher. The New Mexico and Texas portion drains into the Rio Grande and to the Gulf of Mexico, while the Arizona portion drains to the Gila River and to the Gulf of California (fig. 1.4). Lava flows are found in this section (fig. 1.11).

The Sonoran Desert Section includes the Mojave Desert of southeastern California and the Arizona deserts. Elevations there are mostly below 600 m and rarely exceed 900m. Mountains are mostly granite and volcanics. Basins are drained, except in the Mojave; aridity, however, often prevents drainage to the sea. The Salton Trough Section is a continuation of the Gulf of California Trough. Imperial Valley is south of the Salton Sea, which occupies the lowest part of the trough.

The northern Great Basin Section is north of the Mojave Desert and is delimited by the Garlock Fault. Centered on Nevada, it has topography typical of the Basin and Range Province: isolated mountain chains oriented north to south, with broad, intermontane basins. John C. Frémont gave the area the name "Great Basin." Only small portions have external drainage. The lowest and highest elevations of the province occur in this section.

In the central area of elevated basins and ranges, valley floors are at 1500 m and have no perennial lakes. Pluvial Lake Bonneville formerly occupied the entire Bonneville Basin and left behind alluvial fans, salt playas, and salt lakes such as Great Salt and Sevier lakes in Utah (figs. 1.4, 1.10; P.A. Delcourt and H.R. Delcourt, chap.4, fig.4.9). Similar to the Bonneville Basin is the Lahontan Basin to the west, except that volcanism is more prevalent in the latter. The southern area is similar to the central area. A noteworthy feature of the section is Death Valley, which includes a dry, salt-encrusted playa, the lowest point on the continent.

The Colorado Plateau Province is delimited by the southern and middle Rocky Mountains to the north and east, and by the Basin and Range Province to the south and west (figs. 1.7, 1.13). It covers 340,000 km² and is roughly circular, with a diameter of about 470 km. Uplifted to its present level during the Pliocene, it is the highest plateau on the continent: most of it is above 1500 m, some tablelands and several peaks reaching to 3300 m. The surface is deeply dissected by hundreds of steep-walled canyons that expose an impressive display of geological history. Among them, the Grand Canyon is the most famous. The province consists of high plateaus of nearly horizontal sedimentary formations of sandstone, shale, and limestone, volcanic mountains, lava plateaus, and intrusive dome mountains, sand deserts, and shale deserts with badlands (figs. 1.10, 1.11). Rainfall charges streams that drain externally, such as the Colorado River. Glaciation was slight in the area and occurred mainly in south central Utah. Several sections are recognized.

The Grand Canyon Section constitutes the high southwestern part of the province. Rock formations there are Carboniferous, and the northern third of the section is covered by volcanics. The Grand Canyon (fig. 1.4) exposes these formations. Several discrete plateaus are found within this section, such as the Kaibab Plateau, the highest of them (Pt. Sublime, 2273 m). The Datil Section, at the southern end of the Province, is covered by thick lavas (fig. 1.11). North of these two sections is the Navajo Section, a depression with broad flats on shaley formations separated by sandstone cuestas. The Painted Desert is located in this region. Further northward, the Canyon Lands Section is characterized by gorges carved in sandstone and by badlands in areas of thick shale capped by sandstone. The Uinta Basin Section is a deep bowl in Tertiary formations that rises steeply northward onto the flank of the Uinta Mountains (fig. 1.4). Along the western side, the High Plateau Section comprises tablelands higher than 2700m, with some exceeding 3300 m. Mesozoic and Tertiary formations are often capped by lava flows. Where exposed, the Tertiary formations are severely eroded into badlands, such as Bryce Canyon.

Disjunct from the bulk of the Intermountain system, the Wyoming Basin Province (fig. 1.13) is surrounded by the three Rocky Mountain ranges and opens to the Great Plains along a northeast corridor. Elevations are between 1500 and 2400 m. The province comprises semiarid basins and isolated low mountains.

The Columbia--Snake River Plateau Province occupies the northern end of the Intermountain system (figs. 1.4, 1.13). It is a series of semiarid plateaus of rolling, mostly laminar, basaltic lava flows (figs. 1.7, 1.11). Much of the surface is covered by loess (fig. 1.10). Along with the extensive basalts, huge amounts of sand, gravel, and clay occur in alluvial fans and washes. Average elevation is about 900 m. The plateau is deeply dissected by the Columbia and Snake rivers. Substrates exposed in gorges are mostly igneous with lowest walls of granite and schist. Several sections are recognized.

The Snake River Plain Section (fig. 1.4) of southern Idaho is covered mostly with Quaternary basalts (figs. 1.7, 1.11). Along the northern border is a series of volcanic craters, such as Craters of the Moon, and numerous sinks and ponds. The Snake River flows along the southern side of the section. The Payette Section to the west is generally lower in elevation. Former lake beds of basalt overlain by lake sediments constitute much of its surface, now dissected by stream valleys. Two batholiths frame this basin traversed by the Snake River, which, after flowing through a gorge, meanders through extensive lava plains. The Owyhee Mountains rise sharply above the lake beds to 2400 m.

The Harney Lake Section is a volcanic plain at the southwestern corner of the province with little local relief except at centers of volcanism, where cones rarely exceed 60m above adjacent floors. Much moisture percolates through the porous surface: Harney (alkaline) and Malheur (fresh) lakes, included within a marshy tract, fluctuate greatly throughout the year. The Blue Mountain Section (fig. 1.4) is an uplift of Paleozoic and Mesozoic sediments surrounded by elevated plateaus of basalt. Here the Snake River carves Hell's Canyon, a gorge deeper than Grand Canyon. The Blue Mountains rise 900 m above the dissected surface to an elevation of 2700 m.

The Walla Walla Section is underlain by basalts and is covered by lake sediments and loess (fig.1.10). The eastern side has gently rolling relief and canyons to 600m deep. The Columbia and Yakima rivers cut gorges in the east, which become shallower to the west. The Spokane River flows at the northern edge of the basalt. After confluence with the Spokane River, the Columbia River is joined by the Snake River in the Pasco Basin. The northern part of the plateau is the Scabland, an area stripped of loess by a catastrophic flood as the ice dam holding a proglacial lake broke during the late Wisconsinan. This flood also cut the Grand Coulee and other large valleys.

Rocky Mountains System

The Rocky Mountains system occurs along the western edge of the stable craton of the continent and is part of the folded belt uplifted during the Laramide orogeny of the early Tertiary (fig. 1.7). The system runs northward some 5000km from the vicinity of Santa Fe (New Mexico) to Cape Hope (Alaska), with a bend westward near the Beaufort Sea (figs.1.4, 1.13).

The Southern Rocky Mountains Province extends from central Wyoming south to north central New Mexico. Mountain ranges in this province are mostly on a north to south axis, sometimes exceeding elevations of 4200 m, such as Pike's Peak (4301 m) in Colorado. The main ranges are arrayed in eastern and western parallel series, separated in the north by the North Platte River Basin and the Arkansas River Valley, and in the south by the San Luis Valley and the Rio Grande River (fig. 1.4). Two-thirds of the drainage is eastward of the Continental Divide. Underlying rocks are granitic formations flanked by sedimentary rocks (fig.1.11). The San Juan Mountains of Colorado are mainly formed by volcanics.

The Middle Rocky Mountains Province extends nearly 600km from near Payson (Utah) almost to Livingston (Montana). The ranges have diverse orientations, and they are separated by semiarid, intermontane basins (fig. 1.4). Elevations vary from 1200 to 3000 m. All the major ranges were glaciated during the Wisconsinan (figs. 1.9, 1.10), and snow and ice still cover most of the mountains above 2400m, particularly in the northern part. The Yellowstone Plateau is a high (2300--2600 m) volcanic tableland contiguous with the Intermountain system, demarcated by the Snowy and Absaroka ranges. The plateau was also glaciated. Numerous alkaline and calcareous hot springs and geysers occur in the area, among which is Old Faithful. Another noteworthy feature is Bighorn Basin, nearly surrounded by the Bighorn Mountains and connecting with the Great Plains (fig. 1.4). This whole area was extensively glaciated, and the glaciers left impressive cirques and U-shaped valleys. In western Wyoming, the Teton Range rises majestically to 4197 m (Grand Teton). In the south, the Uinta Mountains, the largest east-west trending chain in North America (fig. 1.4), form the southern border where the province abuts the Colorado Plateau.

The Northern Rocky Mountains Province is a narrow, north-trending chain of mountains running 1700 km from Boise (Idaho) to the Liard River in British Columbia (figs. 1.4, 1.13). The area is characterized by linear block-faulted mountains of mostly igneous and metamorphic rocks separated by long straight valleys of Tertiary sediments (fig. 1.11). This province includes the Rocky Mountain Trench, the longest terrestrial trench in the world, which begins near Flathead Lake (Montana) and terminates 1200 km to the north at the Liard River. The range is 300 km wide at its southern end, narrows to less than 120 km near the Canadian border, and flares north of the Liard River into the Liard River Plateau and Plain. Elevations range from 1200 m to 3000 m.

In Montana, most peaks above 2400 m were centers of Pleistocene glaciers (figs. 1.9, 1.10). The Canadian Rockies were covered by the Cordilleran Ice Sheet that extended eastward on the Great Plains to meet the Laurentide Ice Sheet. In Montana, numerous Tertiary lake basins and mountains occur. Much of central Idaho comprises separate mountain groups that were formed by dissection of the Idaho batholith during the Cretaceous (fig. 1.4). This area is characterized by block faulting. Most valleys are deep gorges; few are broad basins. Some peaks harbor mountain glaciers, such as in the Lewis Ranges (Glacier National Park). The Canadian Rocky Mountains are a series of long, sharp-crested ranges. They consist mostly of Paleozoic limestones, with some older rocks thrust into them. To the east, the foothills are underlain by Cretaceous rocks. The highest peak of the Canadian Rockies is Mt. Robson (3954 m).

The Brooks Range Province is delimited from the northern Rocky Mountains by the Liard Plateau and Plain. It is bordered to the north by the Arctic Coastal Plain. The province forms an arch 2200 km long from the northern extremity of the Northern Rocky Mountain Province to Cape Hope. The area comprises a series of ranges and plateaus along the northern side of the Yukon Basin. This province was strongly folded, thrust-faulted, intruded by molten masses, and metamorphosed. It was last uplifted during the Tertiary and Quaternary. Two major ranges compose the Brooks Range Province: the Mackenzie Mountains and adjacent mountain chains in Northwest Territory and Yukon, and the Brooks Range of Yukon and Alaska (fig. 1.4). The Brooks Range is rugged with large areas above 1500 m and some reaching above 2400 m near the Mackenzie Delta (Mt. Chamberlin, 2749 m; Mt. Isto, 2761 m). The Mackenzie Mountains and associated ranges rarely exceed 2100 m (although Keele Peak is 2972 m). The province was glaciated during the Pleistocene, and some glaciers persist today. The ranges are interrupted by lowland plateaus and plains.

Pacific Interior System

The Pacific Interior system extends 4900 km from the southernmost Sierra Nevada in California to the mouth of the Yukon River in Alaska. Narrow in the south, it broadens near the Canadian border, and it arches and further widens in Yukon. It is characterized by belts of rugged mountains and dissected uplands. Some geographers subdivide this complex system into numerous regions, corresponding to terranes, i.e., distinctive geological formations, associated with the Cordilleran orogenies. Several episodes of uplift occurred in late Cretaceous and early Tertiary, with renewed uplift and erosion during the Pliocene-Pleistocene (fig. 1.11).

The Sierra Nevada--Cascades Province is located between the Intermountain system and the Pacific Border system (fig.1.4). It consists of narrow (80--100 km wide) mountains trending north-south for a distance of more than 1600 km. The province is underlain by a great granitic batholith (fig. 1.11). Elevations are generally highest along the east side of the Sierra Nevada, where many peaks rise to more than 3800 m (Mt. Whitney, 4418 m, highest peak in the conterminous United States). These summits are flanked to the west by a large upland above 3000 m that slopes gently westward to the Central Valley of California. Mountains of the Sierra Nevada are blocks of granite that range for more than 600 km north and south. Rivers cut deep valleys, some to 1500 m, that drain mostly to the west. Most peaks and higher valleys were glaciated, but glaciers affected little terrain south of Owens Lake (California).

To the north, the Cascade Mountains extend over a distance of 1100 km from Lassen Peak (northern California) to Meager Mountain (southwestern British Columbia). The Cascade Mountains include 12 major stratovolcanoes, some of which approach the elevation of peaks in the Sierra Nevada, such as Mount Shasta (4317 m), Mount Adams (3751 m), and Mount Rainier (4392 m). The only rivers to breach the Cascades are the Columbia, Klamath, and Pit, and all flow westward.

Three sections are recognized within the Cascade Mountains. The Southern Section is a lava-covered sag in the granite, mostly less than 1500 m high. Numerous volcanic cones are encountered, including Mount Shasta and Mount Lassen (3187 m; an eruption in 1914--1915, and some activity in the 1970s). The Middle Section is an uplift of middle Tertiary lavas dominated by Quaternary volcanic cones. The east side is higher, overlooking the Columbia Plateau; the crest is marked by the High Cascades. The Western Cascades are so severely eroded that no trace of the original landscape persists. Crater Lake (2486 m) was formed after the eruption of the high volcanic cone of Mount Mazama, which occurred during the late Pleistocene. The cone subsequently collapsed and filled with water. Mount St. Helens (now 2550 m) violently erupted in 1980, losing its top 400 m. Glaciers formed on peaks above 2700 m during the Pleistocene, and some still persist.

The Northern Section of the Cascades is composed of two kinds of mountains, volcanic and granitic (e.g., Mount Baker, volcanic, and Glacier Peak, granitic). These mountains comprise mostly folded, metamorphosed sediments and intruded granites. The peaks and ridges are approximately uniform in elevation, although they are rugged and steep-sloped. None of these mountains rises above 2400m. In British Columbia, the range reaches its northern limit south of the lower Fraser Valley (fig. 1.4). The northern Cascades were glaciated extensively, and the effect of the ice stops near their southern end, except for higher elevations. Several small glaciers persist today.

The northern part of the system is wider than the Sierra Nevada--Cascades Province (fig. 1.13) and consists of a series of plateaus bordered to the east and west by the northern Rocky Mountains and the Coastal Mountains, respectively. In British Columbia two mountain chains invade the plateaus. Interior British Columbia was fully glaciated during the Pleistocene by the Cordilleran Ice Sheet, but the Yukon-Alaska segment was not.

The Columbia Mountains Province is comparable in elevation to the Sierra-Cascades. Two peaks exceed 3500 m elevation. The province consists of four parallel ranges, the Purcell, Selkirk, Monashee, and Cariboo mountains (fig. 1.4). It forms a triangle with the narrow side abutting the Columbia Plateaus on the south, the east side delimited by the Rocky Mountain Trench, and the western edge delimited by the northern Cascades and the Fraser Plateau. Geologically, the granitic Nelson batholith is combined with metamorphic and sedimentary formations of older ages (fig.1.11). Orogenic activity ended here long before it did in the Rocky Mountains.

The Fraser Plateau Province is delimited to the east by the Columbia and northern Rocky mountains, to the west by the northern Cascades and Coastal Mountains, and to the north by the Cassiar--Skeena Mountains Province. The Fraser Plateau is a rolling upland of 1000 m elevation tilted toward the north, mostly higher than the Columbia Plateaus to the south. Isolated ranges reaching to 800 m cross the semiarid plateau, which comprises Tertiary lava flows and intrusions (fig. 1.11). The Fraser River and tributaries traverse the plateau (fig. 1.4); in the south, they have cut gorges 500 to 1000 m deep; in the north the plateau is less deeply incised.

The Cassiar--Skeena Mountains Province lies north of the Fraser Plateau. It is bordered on the west by the Coastal Mountains, on the north by the Stikine Plateau, and on the east by the Liard Plateau and the northern Rocky Mountains. Two rugged mountain belts comprise the province: the Cassiar-Omineca mountains in the east and the Skeena-Hazelton mountains in the west. The eastward-flowing Peace and Liard rivers cut the Cassiar-Omineca mountains, while the westward-flowing Skeena (fig. 1.4), Nass, and Iskut rivers dissect the Skeena-Hazelton mountains. Most of the area is below 2000 m, but some peaks reach 2400 m. The province developed on a granitic batholith (fig. 1.11).

The Stikine Plateau Province (fig. 1.4) is bordered by the Cassiar Mountains to the east, by the Skeena Mountains to the south, by the Coastal Mountains to the west, and it extends to the Yukon Plateau on the north. Elevations are mostly from 600 to 1200 m. On the western edge, volcanic Mount Edzina towers over the plateau, reaching a height of 2700m. The Stikine River drains the plateau to the west.

The Yukon Plateau Province is delimited on the east by the Mackenzie Mountains (fig. 1.4), on the south by the Cassiar Mountains and the Stikine Plateau, on the west by the Shakwak Trench that borders the St. Elias Mountains, and on the north by the Yukon-Tanana Upland. The Yukon Plateau averages 1000 m, but it has high points at almost 2000 m on the plateau proper and in the Ogilvie Mountains to the northeast. Part of the eastern side is occupied by the Selwyn Mountains. The plateau is deeply dissected into hills and lowlands by the Yukon River and other rivers that have cut the surface by 600 m. Rock types are diverse, including granitic intrusions and metamorphic formations (fig. 1.11). The plateau is bisected east of the Pelly Mountains by the Tintina Trench, which runs northwestward parallel with the Shakwak Trench; Kluane Lake lies in the latter.

The Porcupine Plateau Province is a lowland wedged between the Brooks Range (fig. 1.4) and Richardson Mountains on the north and east. It is bounded on the south by the Yukon Plateau, and by the Yukon-Tanana Upland on the west.

The Alaskan portion of the Pacific Interior system consists of a series of uplands separated by large basins trending east to west. The area lies between the Brooks Range to the north and the Coastal Mountains to the south (figs. 1.4, 1.13); it is bounded to the east by the Porcupine and Yukon plateaus. It includes 11 physiographic provinces (M.J. Bovis 1987), which are, from east to west: the Yukon-Tanana Upland, the Yukon Flats, the Tanana-Kuskokwim Lowland, the Ray Mountains (fig. 1.4), the Kuskokwim Mountains, the Nushagak Lowland, the Yukon-Kuskokwim Lowland, the Kobuk-Selawik Lowland, the Nulato Hills, the Ahklun Mountains, and the Seward Peninsula. Most of the area was unglaciated during the Pleistocene.

Most of the uplands of central Alaska are drained by the Yukon River and its tributaries, and generally they are lower than 1500 m. Highlands include the hills of the Seward Peninsula and the Nulato Hills (450--600 m) between the Yukon River and Norton Sound. The uplands fronting the Bering Sea consist of a rugged, dissected plateau of 300 to 750 m with some low mountains. The Kuskokwim Mountains (Kiokluk Mountain, 1248 m) east of the Yukon delta extend into central Alaska; they are part of a Quaternary uplift that extends into Siberia. The vast Yukon-Tanana Upland is located in eastern Alaska. The arched Tanana-Kuskokwim Lowland lies just north of the Alaska Range, and runs from Canada to the Kuskokwim Mountains (fig. 1.4). The Yukon River flows through a complex of lowlands, the most extensive being the Yukon Flats with an area of 23,300 km², located near Porcupine Plateau where the Yukon River changes its course to the west. Flowing 3200 km, the Yukon River is one of the longest in the world. It has several major tributaries: the Koyukuk, Tanana, Porcupine, and Klondike rivers. The Kuskokwim River, which originates within the Alaska Range, converges with the Yukon River to form a low deltaic flat more than twice the size of the Mississippi Delta.

Pacific Border System

The Pacific Border system fronts the Pacific Ocean. It is the longest North American system, extending for more than 7000 km from western Alaska to Baja California (fig.1.13). In the conterminous United States, a series of low ranges (mostly less than 600 m) with an inland trough constitutes the Pacific Border Province. The system continues in British Columbia and Alaska as the Coastal Mountains Province, a series of high, rugged, coastal and insular mountains with glaciers and fjords. Where the coast curves to the west, the mountains are heavily glaciated and form the Glaciered Coast Province. Farther west, an inland arcuate belt of mountains constitutes the South Central Alaska Province, which is succeeded westward by the Alaska Peninsula and Aleutian Islands Province.

The Lower California, or Peninsular Range, Province represents the north end of the Baja California peninsula. It is a plateau tilted westward, divided into ranges oriented northwest by faults, and with an east-facing escarpment. San Jacinto Peak (3293 m) at the north end towers above the Salton Trough. The province comprises a granitic batholith of Cretaceous age that is intrusive into Cretaceous sedimentary formations.

The Pacific Border Province comprises a series of coastal mountains separated from the Sierra-Cascades mountain chains (Pacific Interior system) by a trough that is partly submerged in the north (Puget Sound) and elsewhere is less than 150 m in elevation. At the southern end of the province, the Transverse Ranges Section consists of ranges and basins trending east to west, perpendicular to the orientation of adjacent mountain ranges (Sierra Nevada, Coastal Ranges) (fig. 1.4). The northern Channel Islands (Santa Barbara, Santa Cruz, and Santa Rosa) represent peaks of the submerged part of the Santa Monica Mountains (maximum elevation 861 m); the southern islands are not part of this chain, however. The basins have thick Tertiary deposits overlain by mostly marine sediments; the Ventura Basin is submerged in the Santa Barbara Channel; and the Los Angeles Basin is the only coastal plain of the province.

The western mountains of the Transverse Ranges are mostly marine, Tertiary formations; the higher eastern ranges comprise older rocks, including much granite (fig.1.11). The San Bernardino (maximum elevation 3506 m) and the San Gabriel mountains (Mt. San Antonio, 3068 m), the highest ranges, are separated by the San Andreas Fault (fig.1.7). They consist of granitic rocks and volcanics. The Central Valley of California Section (figs. 1.4, 1.10, 1.11, 1.13) is a trough between the coastal ranges and the Sierra Nevada. Most of the valley is below 150 m, and one-third is lower than 30 m. It is mostly filled with recent sedimentary deposits from the Sierra Nevada that bury the western edge of the underlying granitic batholith (figs.1.10, 1.11). The Sacramento River drains the northern part, and the San Joaquin River the southern part, both of them reaching the Pacific through San Francisco Bay. The southernmost part of the valley is a closed basin containing two playas, Tulare and Buena Vista lakes. Two intrusive, domed mountains, the Marysville or Sutter Buttes, rise 600 m above the valley floor.

The California Coast Ranges Section (fig. 1.4) consists of a series of four ridges and three valleys that parallel the coast. Ridges rarely exceed 1500 m, with many below 900m. Rocks are mostly sedimentary. Numerous faults, including the San Andreas Fault (fig. 1.7), among others, are involved in the geological history of the Coast Ranges and continue to affect them. The Pacific Plate to the west of the faults is moving north relative to the American Plate and has provoked the displacement northward of the Coast Ranges area by more than 160 km since the Cretaceous, relative to the area just east of the faults.

Near the Pacific Coast of northern California and southern Oregon, the Klamath Mountains Section is an upland deeply incised (450--750 m) by the Rogue and Klamath rivers. Isolated peaks rise to elevations 1500 to 2000 m above the upland. The rock formations are deformed, metamorphosed, and intruded by granite. They were uplifted during the Cretaceous and again during the past 2 million years. The Oregon Coast Range Section is a gently rolling plateau of Tertiary rocks along the coasts of northern Oregon and southwestern Washington. The highest summits are mostly less than 900 m, the hills are rounded, and the valleys are open. The Willamette Valley forms the eastern side of the section and is the southern end of Puget Trough.

The Puget Trough Section is a lowland partially submerged in the north, and it does not exceed 150 m in elevation at its southern end. This lowland was submerged during the Pleistocene. To the north, the trough extends 2500 km along the coast of British Columbia and Alaska, creating the Inside Passage. The Olympic Mountains Section (fig. 1.4) of northwestern Washington is a domed uplift that reaches an elevation of 2408 m (Mt. Olympus). The core of resistant rocks is surrounded by Tertiary formations. The Olympic Mountains were glaciated during the Pleistocene, and some cirques still contain glaciers.

The Coastal Mountains Province extends from just north of the Fraser River (British Columbia) to the coastal glaciers of the St. Elias Mountains (southeast Alaska). The province is about 1600 km long and sometimes reaches a width of 160 km (figs. 1.4, 1.13). The rugged coastal and insular mountains reach 3900 m. The greatest batholith of the Pacific Coast underlies the province (fig. 1.11). The batholith was uplifted during the Columbian orogeny (Tertiary). The province is divided longitudinally into two ranges separated by the coastal trough and associated lowlands: the Coastal Mountains on the mainland, and the Queen Charlotte Island and Vancouver Islands seaward (fig.1.4). Portions of the islands escaped glaciation during the Wisconsinan glaciation. The interior ranges are associated with lava plateaus and three large shield volcanoes, including Mount Garibaldi (southeastern British Columbia). The Fraser River (fig. 1.4) cuts through this plateau and reaches the Strait of Georgia in a valley that curves around the south end of the Coastal Mountains. Most peaks in the Coast Range are about 2400 m above sea level, but a few reach 2700 m. Several rivers cross the range, notably the Skeena (fig. 1.4) and the Stikine rivers, and some of the valleys are very deep. Glaciers are present along inlets, where they calve icebergs into the sea.

The Glaciered Coast Province lies between the Coastal Mountains and the South Central Alaska provinces, where the coast bends westward. The St. Elias Mountains reach elevations near 6000 m, including Mount Logan (Yukon, 5951m), the highest peak in Canada. Glaciers cover about 13,000 km² in the province; perpetual snow exists above 750m. Along the coast, formations are sedimentary.

The South Central Alaska Province includes the Aleutian Range, the inland Alaska Range, and other belts of mountains and lowlands (troughs) that curve around the Gulf of Alaska. The mountains were uplifted during the Tertiary, and in the late Tertiary the Aleutian Trench developed (fig. 1.7). The Chugach and Kenai mountains are composed of metamorphics and volcanics (fig. 1.11), and they are separated from the Alaska and Aleutian ranges by lowlands (fig. 1.4). The Alaska Range is nearly 1000 km long. Elevations reach 6000m, including Mt. McKinley (Denali) (6194 m), the highest peak in North America. The range is mostly sedimentary with extensive intrusions of granite (fig. 1.7). Pleistocene glaciations were extensive, and many glaciers persist today.

The Aleutian arc represents the northern segment of the "ring of fire" (fig.1.7), the volcanically active margin of the Pacific Plate. The arc is highest at its eastern end, where it begins at Mt. Spurr (3330 m), and sweeps westward as the Aleutian Range. As the range dips beneath oceanic waters, the tops of the volcanoes (maximum elevation 2857 m, Shishaldin Volcano, Unimak Island) form a 2500-km, narrow, rugged archipelago that ends at Attu Island, the westernmost part of North America (ca. 173° E). Glaciation was heavy in the eastern portion; icecaps and glaciers persist on high peaks. Geological evidence indicates that the Aleutian Islands formed a corridor of land south of the Bering Land Bridge during the Wisconsinan, when sea level was about 50 m lower than today.