Cordilleras of North America
Cordilleras of North America
part of the Cordilleran mountain system found in the western part of North America and extending through the coterminous United States and Alaska, Canada, and Mexico. Total length exceeds 7,000 km (from 19°N to 69°N). In Alaska the width of the mountain belt reaches 1,100–1,200 km; in Canada, up to 800 km; in the coterminous United States, about 1,600 km; and in Mexico, up to 1,000 km. The tectonic depression of the Balsas Valley, dividing North and Central America, serves as the southern boundary of the Cordilleras of North America.
Orography. Three longitudinal belts—an eastern, inner, and western—may be clearly discerned in the North American Cordilleras. The eastern, or Rocky Mountain, belt is represented by a chain of high massive ranges, which, for the most part, function as a drainage divide between the Pacific Ocean basin and the basins of the Gulf of Mexico and the Arctic Ocean. In the east the belt slopes sharply toward piedmont plateaus (Arctic plateau, Great Plains). In the west the belt is bounded in places by deep tectonic depressions, such as the Rocky Mountain Trench, or by the valleys of large rivers (Rio Grande), and in certain areas it gradually gives way to massifs and uplands. The Rocky Mountain belt includes the Brooks Range in Alaska and, in northwest Canada, the Richardson and Mackenzie Mountains, bounded on the north and south by the water gaps of the Peel and Liard rivers.
The Rocky Mountains proper extend further to the south, in Canada and the USA as far as 32°N. Between 45°N and 32°N the eastern belt achieves its greatest width and is represented by isolated, high (over 4,000 m) but not long ranges and massifs, divided by extensive plateaus (”parks”): the Sawatch Massif, the San Juan Mountains, the Front Range, and the Uinta Mountains. In the area between 32°N and 26°N, cut by the Rio Grande Valley, the belt is not clearly expressed. The mountain ranges are divided by plateaus and basins, which in the west merge with the bolsons of the Mexican Highlands and in the east give way to Edwards Plateau. The southernmost stretch of the eastern belt forms the Sierra Madre Oriental (elevations up to 4,054 m).
The inner belt of the North American Cordilleras, or belt of inner plateaus and highlands, lies between the eastern belt and the belt of Pacific ranges in the west. In the interior of Alaska this belt includes broad tectonic depressions, dissected by river valleys and alternating with flat-topped mountain massifs up to 1,500–1,700 m in elevation (the Kilbuck, Kuskokwim, and Ray mountains). In Canada the inner belt consists of many high plateaus (Yukon, Stikine, Fraser), massifs, and ranges as high as those of the Rockies (Cassiar and Omineca mountains, 2,590 m; Columbia Mountains, up to 3,581 m). Within the coterminous United States and Mexico the inner belt comprises high-mountain massifs in the region of batholiths in Idaho (up to 3,857 m), the Snake and Columbia volcanic plateaus (average elevation about 1,000 m), the Great Basin, and the northeast region of Mexico, as well as the scalariform Colorado Plateau and Mexican Highlands.
The western belt consists of a belt of Pacific ranges, a belt of intermontane depressions, and a belt of coastal chains. The belt of Pacific ranges, rimming the inner region of the North American Cordilleras from the west, contains the highest ranges in the mountain system, including the Alaska Range (with the highest peak in North America, Mount McKinley, 6,193 m), the chain of volcanic Aleutian Islands, the Aleutian Range (volcano Iliamna, 3,075 m), the high mountain junction of the St. Elias Massif (Mount Logan, 6,050 m), and the strongly dissected Coast Mountains (Mount Waddington, 4,042 m) forming a characteristic fiord coastline along their entire length. In Mexico and the coterminous United States this belt includes the Cascade Range with a series of volcanic peaks (Mount Rainier, 4,392 m), the Sierra Nevada (Mount Whitney, 4,418 m), the ranges of Lower California (elevations up to 3,078 m), separated from the inner belt by the Gulf of California, and the Transverse Volcanic Sierra, with the volcanoes Orizaba (5,700 m), Popocatepetl (5,-452 m), and Nevado de Colima (4,265 m).
Intermontane longitudinal depressions are represented both by inlets and straits (Cook Inlet, the Shelikof and Georgia straits, Sebastian Vizcaino Bay) and by a series of lowlands and plateaus (the Susitna Lowland, the Copper River plateau, the Willamette Valley, and the Central Valley in California). The belt of coastal ranges, rimming the western edge of the continent, is the most fragmented part of the North American Cordilleras. It is represented by low- and middle-altitude ranges (Coast Ranges in the USA, the Sierra Vizcaino in Lower California) and by a series of rugged coastal islands (Kodiak, Queen Charlotte, and Vancouver islands and the Alexander Archipelago). The highest peak of this belt is found in the Chugach Mountains (Mount Marcus Baker, 4,016 m) in southern Alaska.
A. V. ANTIPOVA and G. M. IGNAT’EV
Geological structure and minerals.. The Cordilleras of North America have been formed by various tectonic elements. To the south, in the United States, they include the western part of the Precambrian North American Platform (Colorado Plateau and the ranges of the Rockies, to the east), elevated by recent movements. Here a folded basement (absolute age about 2.4 billion years) has been covered by a horizontal Paleozoic and Mesozoic mantle. To the west extend miogeosynclinal and eugeosynclinal troughs of Mesozoics of the Sierra Nevada and Rockies. In Canada the Mesozoics are separated from the platform by the Pre-Cordilleran Foredeep, filled with carbonaceous and saline formations of the Middle Paleozoic and molasses from the Jurassic and Lower Cretaceous. In Alaska the Mesozoics are sepa-rated from the ancient Yukon Massif by the deep Tintin fracture. Similar fractures separate the Mesozoics of Mexico from the Precambrian Central American Massif. The initial stage of the geosynclinal troughs of the region developed in the Late Precam-brian, and the accumulation of sediments continued until the end of the Jurassic. Carbonaceous (Paleozoic) and terrigenous (Mesozoic) miogeosynclinal series up to 10 km thick formed in the eastern part of the orogenic belt. The eugeosyncline is com-posed of volcanic and volcanic-sedimentary series about 15 km thick. During the Late Jurassic the Mesozoics of Canada and the United States experienced folding, and in the Early Cretaceous they were intruded by granitoids. In the Sierra Madre Occidental and Lower California, folding and orogenic processes occurred during Late Cretaceous and Paleocene times (Laramide), and the intrusion of granites dates to the Late Cretaceous and Oligocene.
A Cenozoic geosynclinal system extends to the west of the Mesozoics on the Alaskan Peninsula, in the Coast Ranges of California and Oregon, and in the southern part of Central America. It is composed of thick layers (up to 25 km) of volcanic and sedimentary rock from the Upper Jurassic, Cretaceous, and Cenozoic. These regions are characterized by volcanism, great seismic activity, and intensive contemporary tectonic movements. Geosynclinal structures in the North Pacific include the Aleutian Trench, and in the South Pacific, the Central American Trench. The formation of the deep trough of the Gulf of California is associated with geosynclinal development.
Petroleum deposits occur in the Pre-Cordilleran Foredeep (Canada) and in the young depressions of Alaska and California. In the mesoides of the Rocky Mountains, Sierra Nevada, and Sierra Madre there are deposits of gold, tungsten, copper, molybdenum, and polymetals, and the Cenozoic structures of the Coast Ranges contain mercury and coal.
N. A. BOGDANOV
Relief.. The eastern belt is characterized by large domed massifs dissected by river valleys (Brooks Range, Mackenzie Mountains, Canadian Rockies, and Sierra Madre Oriental) and by short anticlinal ranges, formed in regions of marginal platform structures (American Rockies).
Prominent relief features of the inner belt include high plateaus (Yukon, Stikine) joining large flat-topped massifs and broad basins dissected by river valleys, lava plateaus (Fraser, Columbia, Mexican) deeply cut by canyons, partially buried highlands (Great Basin) with a folded foundation emerging at the surface in the form of numerous short ridges surrounded by extensive subsidences, and deeply dissected plateaus (Colorado Plateau) constituting areas of platform structures that have been included in the Cordilleran mountain belt.
Large anticlinal ranges with outcrops of intrusive rock in axial areas (Alaska Range) are characteristic of the belt of Pacific ranges. The massive, long batholith ranges (Sierra Nevada, Coast Mountains) are similar to the anticlinal ranges. Also typical are volcanic ranges with folded foundations on which a series of volcanoes, including active volcanoes, have been superim-posed. Aggradation lowlands, such as the Central Valley in California, are widespread in the belt of longitudinal depressions. Most commonly found in the belt of coastal chains are low, weakly dissected ranges, forming straight coastlines.
Widely found in the northern part of the North American Cordilleras (north of 40°–49°N) are both ancient glacial relief forms (troughs, corries, moraine ridges, and loess, outwash, and lake plains) and contemporary nival forms (rock streams, mountain terraces), confined to the highest mountain levels (Alaska Range, Rockies). Thermokarst and polygonal relief forms, associated with the distribution of rocks and soils, are widely represented in areas that did not experience glaciation (inner Alaska). Water-erosion forms predominate in the remaining areas of the North American Cordilleras: valley dissection in the regions with the greatest rainfall (Canadian Cordilleras) and table forms and canyons in arid regions (Colorado and Columbia plateaus). Denudation and aeolian forms are characteristic of desert regions (Great Basin, Mexican Highlands).
Climate.. The northern part of the Cordilleras of North America lies within the arctic (Brooks Range) and subarctic (the greater part of Alaska) zones. The area as far as 40°N lies in the temperate zone, and further south the Cordilleras lie in the subtropical zone; Lower California and the Mexican Highlands are in the tropical zone. On slopes facing the Pacific Ocean the climate is generally mild and oceanic (the climate at the latitude of San Francisco is Mediterranean). In interior regions the cli-mate is continental. On the Yukon Plateau the average January temperature is −30°C, and the average July temperature 15°C. Winter frosts in the Great Basin reach −17°C, and summer temperatures frequently exceed 40°C (absolute maximum 57°C). In July the highest temperatures occur in the southern intermontane valleys (32°C along the lower reaches of the Colorado River) and the lowest temperatures in the high mountain areas of southern Alaska (8°C in the Chugach Mountains and St. Elias Massif). Precipitation is extremely uneven. In the temperate zone, the extreme west receives the greatest amount of precipitation, and in the tropical zone, the extreme east. The smallest amount of precipitation occurs in the inner highlands. Annual precipitation totals 3,000–4,000 mm in the southern ranges of Alaska and up to 2,500 mm along the coast of British Columbia, decreasing to 400–200 mm in the inner plateaus of the United States. Precipitation in the Mojave Desert totals 50 mm per year, but in the southeastern part of the Mexican Highlands it rises to 2,000 mm annually. The thickest snow cover (150 cm and over) is found in southern Alaska (Chugach, St. Elias, and Wrangell mountains) and in the Coast and Columbia mountains in Canada.
Glaciation.. The uneven development of contemporary glaciation has resulted from the great differences in latitude and elevation of the North American Cordilleras, as well as from sharp differences in precipitation. The lowest snow line (300–450 m) is found on the Pacific slope of the mountains in southern Alaska, descending, in places to sea level. On the northern slopes of the Chugach and St. Elias mountains the snow line occurs at elevations of 1,800–1,900 m, and in the Alaska Range it occurs at altitudes ranging from 1,350–1,500 m on the southern slope and to 2,250–2,400 m on the northern slope. Here, the area of con-temporary glaciation totals 52,000 sq km. Glaciation is developed only on the highest peaks of the Brooks Range and the Mackenzie Mountains. Further to the south the snow line is found at 1,500–1,800 m in the Coast Ranges and 2,250 m in the Canadian Columbia Mountains. The resulting area of glaciation in the interior of Alaska and in the Canadian Cordilleras totals 15,000 sq km. In the coterminous United States the snow line rises to 2,500–3,000 m in the Cascades and Rockies and to over 4,000 m in the Sierra Nevada; in Mexico the snow line occurs at 4,500 m and higher. The area of contemporary glaciation is estimated at 500–600 sq km in the United States and 11 sq km in Mexico.
All the basic types of glaciers are represented in the North American Cordilleras: vast ice sheets and ice caps, washed glaciers (Lepont Glacier in the Coast Ranges), piedmont, or foot-hill, glaciers (Malaspina), valley glaciers (Hubbard, 145 km long, Coast Ranges), and corrie and short hanging glaciers, which, for the most part, are disappearing (Sierra Nevada). Stellate glaciers, sending out numerous glacial streams, form on volcanic summits (there are more than 40 streams on the Rainier volcano).
Rivers and lakes.. The sources of many continental river systems lie within the North American Cordilleras, including the Yukon, Peace-Mackenzie, Saskatchewan-Nelson, Missouri-Mississippi, Colorado, Columbia, and Fraser rivers. Since the eastern belt is the main drainage divide, most of the precipitation in the North American Cordilleras flows westward, into the Pacific. Along the Pacific coast, north of 45°–50°N, rivers are fed primarily by snow, with clearly expressed spring high water. In the south, rivers are fed chiefly by rain, with a winter maximum along the Pacific coast and a spring-summer maximum in the interior regions. In the southern part of the North American Cordilleras large areas do not have ocean runoff and are watered primarily by temporary streams ending in drainless salt lakes, the largest of which is the Great Salt Lake. In the north are numerous freshwater lakes of glacial-tectonic and dam origin, such as Lakes Atlin, Kootenay, and Okanagan.
The deepest mountain rivers have considerable drops, and their flow is regulated by lakes. They have enormous hydroelectric potential and are used for electric energy and irrigation. The Columbia River has more than ten locations suitable for the construction of hydroelectric power plants, some of which have already been used (Grand Coulee, The Dalles).
Natural regions.. Because of the considerable elevations throughout the entire length of the North American Cordilleras, there is a pronounced altitude zonation of natural landscapes. Moreover, the extension of mountain ranges in a direction perpendicular to the prevailing flow of water causes substantial differences between the landscapes of the coastal (Pacific Ocean) and interior regions. The most important landscape changes are associated with latitude, with transition from subarctic to temperate, subtropical, and tropical zones. Four major natural regions may be identified: the Northwestern Region, the Canadian Cordilleras, the United States Cordilleras, and the Mexican Cordilleras.
The Northwestern Region, or Alaskan Cordilleras, embraces the greater part of the state of Alaska and the Yukon Plateau in northwestern Canada. High mountain ranges with thick glaciation predominate in the south, and uplands cover the remaining area. The climate is subarctic, but along the southern coast it is temperate. Permafrost occurs everywhere except the coast of the Gulf of Alaska. The range of altitude zones is represented by piedmont sparse forests (forest tundra) in the river valleys and by mountain tundra on the high plateaus. On the western coast subarctic meadows are well developed. On the southern Pacific slopes a belt of high coniferous forests of hemlock and arborvitae (called coastal forests) is replaced by a belt of subalpine sparse woods, which at the summits give way to alpine meadows and glaciers. Reindeer, arctic foxes, polar hares, and lemmings live in the tundra. Elk, grizzly bears, wolves, foxes, and other predators inhabit the forests. There are many birds. The bulk of the population and most of the cities are on the southern coast.
The Canadian Cordilleras constitute the narrowest part of the mountain belt, including the southeastern coast of Alaska and partially extending into the United States, as far as 44°N. Dominant relief forms consist of high mountain ranges with extensively developed old glaciation forms and contemporary glaciation. The climate is temperate, ranging from humid to arid. The spectrum of vertical zones includes steppes on the floors of intermontane valleys, pine forest-steppes on the high plateaus, mountain coniferous forests of firs, spruce, red cedar, and balsam pine on slopes where podzolic brown forest and mountain-forest soils are well developed, and subalpine coniferous sparse woods and alpine meadows, growing at the summit on mountain-meadow and skeletal soils. The Pacific slopes are covered with high forests of Douglas fir, Sitka spruce, hemlock, and arbor-vitae, extending from the southern regions of Alaska. Many different animals are found in the mountain forests, including wapiti, American elk, caribou, and grizzly bears. Also encountered are wolves, foxes, wolverines, lynx, cougars, and mountain sheep. Fur-bearing animals include marten, ermine, mink, coypu, and muskrat. The population is concentrated in the south and in coastal cities (Vancouver). The steppe lands of the valleys are cultivated, and the forest-steppe plateaus are used for pasture.
The United States, or Southern, Cordilleras are the widest part of the mountain belt and vary greatly in natural conditions. High, forested ranges covered with neve and glaciers, directly border on vast drainless desert plateaus. The climate is subtropical—Mediterranean on the coast and arid inland. On the slopes of high ranges such as the Front Range and the Sierra Nevada are zones of mountain pine forests (American spruce, larch), coniferous subalpine sparse woods, and alpine meadows. The low Coast Ranges are covered with mountain pine forests, relict groves of sequoias, and dense thickets of hard-leaved evergreen shrubs (chaparral). The western slopes of this part of the Cordilleras are rich in forest resources. During the 19th century and particularly in the 20th century, however, the forests were heavily cut and suffered from frequent fires, so that the forest area has been significantly reduced. Sitka spruce and Douglas fir have been preserved in small stands along the Pacific coast. On the vast inner plateaus are found sagebrush and shrub semideserts and deserts, and the low ranges are covered with sparse pine and pine-juniper forests. In areas developed by man, larger animals have been entirely or almost entirely destroyed. Bison are almost extinct, and the pronghorn antelope is rarely encountered. The rich animal world has been preserved only in national parks, such as Yellowstone and Yosemite. Rodents, snakes, lizards, and scorpions are numerous in the semidesert regions. Population is concentrated near the Pacific coast, where the major cities are found (Los Angeles, San Francisco). In the river valleys, large tracts of irrigated land are used for subtropical fruit cultivation. Subtropical sparsely wooded areas and shrub deserts are used for pasture.
The Mexican Cordilleras include the Mexican Highlands and Lower California. The relief is dominated by high plateaus and highlands, strongly dissected in places (Sierra Madre Occidental). There is great seismic activity, and the climate is tropical, mainly arid. Low thorn forests (at the foot of mountains) and deciduous tropical forests (at summits) are well developed on the windward slopes. Creosote bush deserts, alpine succulent deserts, cactus-acacia savannas, and coniferous and hard-leaved mountain forests are found in the interior regions. Animals en-countered in the deserts and semideserts include the cougar, pronghorn antelope, coyote, and many rabbits, field mice, and other rodents. Black bears, lynx, and other predators inhabit the forests. Monkeys, tapirs, and jaguars are found in the tropical forests. The majority of the population is concentrated on the Meseta Central, where the major cities are located (Mexico City, Guadalajara, San Luis Potosi), and along the coast of the Gulf of Mexico (the ports of Tampico and Veracruz). In the south, large tracts of land are devoted to growing tropical and grain crops.
REFERENCESIgnat’ev, G. M. Severnaia Amerika. Moscow, 1965.
Rel’f Zemli. Moscow, 1967.
Vitvitskii, G. N. Klimaty Severnoi Ameriki. Moscow, 1953.
King, P. B. Geologicheskoe razvitie Severnoi Ameriki. Moscow, 1961. (Translated from English.)
Bostock, H. S. Physiography of the Canadian Cordillera. Ottawa, 1948.
Landscapes of Alaska. Los Angeles, 1958.
Tamayo, J. L. Geografia general de México, 2nd ed., vols. 1–4. Mexico City, 1962.
Thornbury, W. D. Regional Geomorphology of the United States. New York, 1965.
A. V. ANTIPOVA and G. M. IGNAT’EV