THE HISTORY OF AMERICA
United States, officially United States of America, abbreviated U.S. or U.S.A., byname America, country in North America, a federal republic of 50 states. Besides the 48 conterminous states that occupy the middle latitudes of the continent, the United States includes the state of Alaska, at the northwestern extreme of North America, and the island state of Hawaii, in the mid-Pacific Ocean. The conterminous states are bounded on the north by Canada, on the east by the Atlantic Ocean, on the south by the Gulf of Mexico and Mexico, and on the west by the Pacific Ocean. The United States is the fourth largest country in the world in area (after Russia, Canada, and China). The national capital is Washington, which is coextensive with the District of Columbia, the federal capital region created in 1790.
The major characteristic of the United States is probably it's great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rainforest to the arid desert, from the rugged mountain peak to the flat prairie. Although the total population of the United States is large by world standards, its overall population density is relatively low. The country embraces some of the world’s largest urban concentrations as well as some of the most extensive areas that are almost devoid of habitation. The United States contains a highly diverse population. Unlike a country such as China that largely incorporated indigenous peoples, the United States has a diversity that to a great degree has come from an immense and sustained global immigration. Probably no other country has a wider range of racial, ethnic, and cultural types than does the United States. In addition to the presence of surviving Native Americans (including American Indians, Aleuts, and Eskimos) and the descendants of Africans taken as enslaved persons to the New World, the national character has been enriched, tested, and constantly redefined by the tens of millions of immigrants who by and large have come to America hoping for greater social, political, and economic opportunities than they had in the places they left. (It should be noted that although the terms “America” and “Americans” are often used as synonyms for the United States and its citizens, respectively, they are also used in a broader sense for North, South, and Central America collectively and their citizens.)
The United States is the world’s greatest economic power, measured in terms of gross domestic product (GDP). The nation’s wealth is partly a reflection of its rich natural resources and its enormous agricultural output, but it owes more to the country’s highly developed industry. Despite its relative economic self-sufficiency in many areas, the United States is the most important single factor in world trade by virtue of the sheer size of its economy. Its exports and imports represent major proportions of the world total. The United States also impinges on the global economy as a source of and as a destination for investment capital. The country continues to sustain an economic life that is more diversified than any other on Earth, providing the majority of its people with one of the world’s highest standards of living.
The United States is relatively young by world standards, being less than 250 years old; it achieved its current size only in the mid-20th century. America was the first of the European colonies to separate successfully from its motherland, and it was the first nation to be established on the premise that sovereignty rests with its citizens and not with the government. In its first century and a half, the country was mainly preoccupied with its own territorial expansion and economic growth and with social debates that ultimately led to civil war and a healing period that is still not complete. In the 20th century the United States emerged as a world power, and since World War II it has been one of the preeminent powers. It has not accepted this mantle easily nor always carried it willingly; the principles and ideals of its founders have been tested by the pressures and exigencies of its dominant status. The United States still offers its residents opportunities for unparalleled personal advancement and wealth. However, the depletion of its resources, the contamination of its environment, and the continuing social and economic inequality that perpetuates areas of poverty and blight all threaten the fabric of the country. The District of Columbia is discussed in the article Washington. For discussion of other major U.S. cities, see the articles Boston, Chicago, Los Angeles, New Orleans, New York City, Philadelphia, and San Francisco. Political units in association with the United States include Puerto Rico, discussed in the article Puerto Rico, and several Pacific islands, discussed in Guam, Northern Mariana Islands, and American Samoa.
The two great sets of elements that mould the physical environment of the United States are, first, the geologic, which determines the main patterns of landforms, drainage, and mineral resources and influences soils to a lesser degree, and, second, the atmospheric, which dictates not only climate and weather but also in large part the distribution of soils, plants, and animals. Although these elements are not entirely independent of one another, each produces on a map patterns that are so profoundly different that essentially they remain two separate geographies. (Since this article covers only the conterminous United States, see also the articles Alaska and Hawaii.) Relief
The centre of the conterminous United States is a great sprawling interior lowland, reaching from the ancient shield of central Canada on the north to the Gulf of Mexico on the south. To east and west this lowland rises, first gradually and then abruptly, to mountain ranges that divide it from the sea on both sides. The two mountain systems differ drastically. The Appalachian Mountains on the east are low, almost unbroken, and in the main set well back from the Atlantic. From New York to the Mexican border stretches the low Coastal Plain, which faces the ocean along a swampy, convoluted coast. The gently sloping surface of the plain extends out beneath the sea, where it forms the continental shelf, which, although submerged beneath shallow ocean water, is geologically identical to the Coastal Plain. Southward the plain grows wider, swinging westward in Georgia and Alabama to truncate the Appalachians along their southern extremity and separate the interior lowland from the Gulf.
West of the Central Lowland is the mighty Cordillera, part of a global mountain system that rings the Pacific basin. The Cordillera encompasses fully one-third of the United States, with an internal variety commensurate with its size. At its eastern margin lie the Rocky Mountains, a high, diverse, and discontinuous chain that stretches all the way from New Mexico to the Canadian border. The Cordillera’s western edge is a Pacific coastal chain of rugged mountains and inland valleys, the whole rising spectacularly from the sea without benefit of a coastal plain. Pent between the Rockies and the Pacific chain is a vast intermontane complex of basins, plateaus, and isolated ranges so large and remarkable that they merit recognition as a region separate from the Cordillera itself.
These regions—the Interior Lowlands and their upland fringes, the Appalachian Mountain system, the Atlantic Plain, the Western Cordillera, and the Western Intermontane Region—are so various that they require further division into 24 major subregions, or provinces. Andrew Jackson is supposed to have remarked that the United States begins at the Alleghenies, implying that only west of the mountains, in the isolation and freedom of the great Interior Lowlands, could people finally escape Old World influences. Whether or not the lowlands constitute the country’s cultural core is debatable, but there can be no doubt that they comprise its geologic core and in many ways its geographic core as well. This enormous region rests upon an ancient, much-eroded platform of complex crystalline rocks that have for the most part lain undisturbed by major orogenic (mountain-building) activity for more than 600,000,000 years. Over much of central Canada, these Precambrian rocks are exposed at the surface and form the continent’s single largest topographical region, the formidable and ice-scoured Canadian Shield. In the United States most of the crystalline platform is concealed under a deep blanket of sedimentary rocks. In the far north, however, the naked Canadian Shield extends into the United States far enough to form two small but distinctive landform regions: the rugged and occasionally spectacular Adirondack Mountains of northern New York and the more-subdued and austere Superior Upland of northern Minnesota, Wisconsin, and Michigan. As in the rest of the shield, glaciers have stripped soils away, strewn the surface with boulders and other debris, and obliterated preglacial drainage systems. Most attempts at farming in these areas have been abandoned, but the combination of a comparative wilderness in a northern climate, clear lakes, and white-water streams has fostered the development of both regions as year-round outdoor recreation areas. Mineral wealth in the Superior Upland is legendary. Iron lies near the surface and close to the deepwater ports of the upper Great Lakes. Iron is mined both north and south of Lake Superior, but best known are the colossal deposits of Minnesota’s Mesabi Range, for more than a century one of the world’s richest and a vital element in America’s rise to industrial power. In spite of depletion, the Minnesota and Michigan mines still yield a major proportion of the country’s iron and a significant percentage of the world’s supply. South of the Adirondack Mountains and the Superior Upland lies the boundary between crystalline and sedimentary rocks; abruptly, everything is different. The core of this sedimentary region—the heartland of the United States—is the great Central Lowland, which stretches for 1,500 miles (2,400 kilometres) from New York to central Texas and north another 1,000 miles to the Canadian province of Saskatchewan. To some, the landscape may seem dull, for heights of more than 2,000 feet (600 metres) are unusual, and truly rough terrain is almost lacking. Landscapes are varied, however, largely as the result of glaciation that directly or indirectly affected most of the subregion. North of the Missouri–Ohio river line, the advance and readvance of continental ice left an intricate mosaic of boulders, sand, gravel, silt, and clay and a complex pattern of lakes and drainage channels, some abandoned, some still in use. The southern part of the Central Lowland is quite different, covered mostly with loess (wind-deposited silt) that further subdued the already low relief surface. Elsewhere, especially near major rivers, postglacial streams carved the loess into rounded hills, and visitors have aptly compared their billowing shapes to the waves of the sea. Above all, the loess produces soil of extraordinary fertility. As the Mesabi iron was a major source of America’s industrial wealth, its agricultural prosperity has been rooted in Midwestern loess.
The Central Lowland resembles a vast saucer, rising gradually to higher lands on all sides. Southward and eastward, the land rises gradually to three major plateaus. Beyond the reach of glaciation to the south, the sedimentary rocks have been raised into two broad upwards, separated from one another by the great valley of the Mississippi River. The Ozark Plateau lies west of the river and occupies most of southern Missouri and northern Arkansas; on the east the Interior Low Plateaus dominate central Kentucky and Tennessee. Except for two nearly circular patches of rich limestone country—the Nashville Basin of Tennessee and the Kentucky Bluegrass region—most of both plateau regions consist of sandstone uplands, intricately dissected by streams. Local relief runs to several hundreds of feet in most places, and visitors to the region must travel winding roads along narrow stream valleys. The soils there are poor, and mineral resources are scanty.
Eastward from the Central Lowland the Appalachian Plateau—a narrow band of dissected uplands that strongly resembles the Ozark Plateau and Interior Low Plateaus in steep slopes, wretched soils, and endemic poverty—forms a transition between the interior plains and the Appalachian Mountains. Usually, however, the Appalachian Plateau is considered a subregion of the Appalachian Mountains, partly on grounds of location, partly because of geologic structure. Unlike the other plateaus, where rocks are warped upward, the rocks there form an elongated basin, wherein bituminous coal has been preserved from erosion. This Appalachian coal, like the Mesabi iron that it complements in U.S. industry, is extraordinary. Extensive, thick, and close to the surface, it has stoked the furnaces of northeastern steel mills for decades and helps explain the huge concentration of heavy industry along the lower Great Lakes.
The western flanks of the Interior Lowlands are the Great Plains, a territory of awesome bulk that spans the full distance between Canada and Mexico in a swath nearly 500 miles (800 km) wide. The Great Plains were built by successive layers of poorly cemented sand, silt, and gravel—debris laid down by parallel east-flowing streams from the Rocky Mountains. Seen from the east, the surface of the Great Plains rises inexorably from about 2,000 feet (600 metres) near Omaha, Nebraska, to more than 6,000 feet (1,825 metres) at Cheyenne, Wyoming, but the climb is so gradual that popular legend holds the Great Plains to be flat. True flatness is rare, although the High Plains of western Texas, Oklahoma, Kansas, and eastern Colorado come close. More commonly, the land is broadly rolling, and parts of the northern plains are sharply dissected into badlands.
The main mineral wealth of the Interior Lowlands derives from fossil fuels. Coal occurs in structural basins protected from erosion—high-quality bituminous in the Appalachian, Illinois, and western Kentucky basins; and sub bituminous and lignite in the eastern and northwestern Great Plains. Petroleum and natural gas have been found in nearly every state between the Appalachians and the Rockies, but the Midcontinent Fields of western Texas and the Texas Panhandle, Oklahoma, and Kansas surpass all others. Aside from small deposits of lead and zinc, metallic minerals are of little importance.
The Appalachian Mountain system
The Appalachians dominate the eastern United States and separate the Eastern Seaboard from the interior with a belt of subdued uplands that extends nearly 1,500 miles (2,400 km) from northeastern Alabama to the Canadian border. They are old, complex mountains, the eroded stumps of much greater ranges. Present topography results from erosion that has carved weak rocks away, leaving a skeleton of resistant rocks behind as highlands. Geologic differences are thus faithfully reflected in topography. In the Appalachians these differences are sharply demarcated and neatly arranged, so that all the major subdivisions except New England lie in strips parallel to the Atlantic and to one another. The core of the Appalachians is a belt of complex metamorphic and igneous rocks that stretches all the way from Alabama to New Hampshire. The western side of this belt forms the long slender rampart of the Blue Ridge Mountains, containing the highest elevations in the Appalachians (Mount Mitchell, North Carolina, 6,684 feet [2,037 metres]) and some of its most handsome mountain scenery. On its eastern, or seaward, side the Blue Ridge descends in an abrupt and sometimes spectacular escarpment to the Piedmont, a well-drained, rolling land—never quite hills, but never quite a plain. Before the settlement of the Midwest the Piedmont was the most productive agricultural region in the United States, and several Pennsylvania counties still consistently report some of the highest farm yields per acre in the entire country.trips parallel to the Atlantic and to one another.
West of the crystalline zone, away from the axis of primary geologic deformation, sedimentary rocks have escaped metamorphism but are compressed into tight folds. Erosion has carved the upturned edges of these folded rocks into the remarkable Ridge and Valley country of the western Appalachians. Long linear ridges characteristically stand about 1,000 feet (300 metres) from base to crest and run for tens of miles, paralleled by broad open valleys of comparable length. In Pennsylvania, ridges run unbroken for great distances, occasionally turning abruptly in a zigzag pattern; by contrast, the southern ridges are broken by faults and form short, parallel segments that are lined up like magnetised iron filings. By far the largest valley—and one of the most important routes in North America—is the Great Valley, an extraordinary trench of shale and limestone that runs nearly the entire length of the Appalachians. It provides a lowland passage from the middle Hudson valley to Harrisburg, Pennsylvania, and on southward, where it forms the Shenandoah and Cumberland valleys, and has been one of the main paths through the Appalachians since pioneer times. In New England it is floored with slates and marbles and forms the Valley of Vermont, one of the few fertile areas in an otherwise mountainous region.
Topography much like that of the Ridge and Valley is found in the Ouachita Mountains of western Arkansas and eastern Oklahoma, an area generally thought to be a detached continuation of Appalachian geologic structure, the intervening section buried beneath the sediments of the lower Mississippi valley.
The once-glaciated New England section of the Appalachians is divided from the rest of the chain by an indentation of the Atlantic. Although almost completely underlain by crystalline rocks, New England is laid out in north–south bands, reminiscent of the southern Appalachians. The rolling, rocky hills of southeastern New England are not dissimilar to the Piedmont, while, farther northwest, the rugged and lofty White Mountains are a New England analogue to the Blue Ridge. (Mount Washington, New Hampshire, at 6,288 feet [1,917 metres], is the highest peak in the northeastern United States.) The westernmost ranges—the Taconics, Berkshires, and Green Mountains—show a strong north–south lineation like the Ridge and Valley. Unlike the rest of the Appalachians, however, glaciation has scoured the crystalline rocks much like those of the Canadian Shield, so that New England is best known for its picturesque landscape, not for its fertile soil.
Typical of diverse geologic regions, the Appalachians contain a great variety of minerals. Only a few occur in quantities large enough for sustained exploitation, notably iron in Pennsylvania’s Blue Ridge and Piedmont and the famous granites, marbles, and slates of northern New England. In Pennsylvania the Ridge and Valley region contains one of the world’s largest deposits of anthracite coal, once the basis of a thriving mining economy; many of the mines are now shut, oil and gas having replaced coal as the major fuel used to heat homes.
The Atlantic Plain
The eastern and southeastern fringes of the United States are part of the outermost margins of the continental platform, repeatedly invaded by the sea and veneered with layer after layer of young, poorly consolidated sediments. Part of this platform now lies slightly above sea level and forms a nearly flat and often swampy coastal plain, which stretches from Cape Cod, Massachusetts, to beyond the Mexican border. Most of the platform, however, is still submerged, so that a band of shallow water, the continental shelf, parallels the Atlantic and Gulf coasts, in some places reaching 250 miles (400 km) out to sea.
The Atlantic Plain slopes so gently that even slight crustal upwarping can shift the coastline far out to sea at the expense of the continental shelf. The peninsula of Florida is just such an upwarp: nowhere in its 400-mile (640-km) length does the land rise more than 350 feet (100 metres) above sea level; much of the southern and coastal areas rise less than 10 feet (3 metres) and are poorly drained and dangerously exposed to Atlantic storms. Downwards can result in extensive flooding. North of New York City, for example, the weight of glacial ice depressed most of the Coastal Plain beneath the sea, and the Atlantic now beats directly against New England’s rock-ribbed coasts. Cape Cod, Long Island (New York), and a few offshore islands are all that remain of New England’s drowned Coastal Plain. Another downwarp lies perpendicular to the Gulf coast and guides the course of the lower Mississippi. The river, however, has filled with alluvium what otherwise would be an arm of the Gulf, forming a great inland salient of the Coastal Plain called the Mississippi Embayment.
South of New York the Coastal Plain gradually widens, but ocean water has invaded the lower valleys of most of the coastal rivers and has turned them into estuaries. The greatest of these is Chesapeake Bay, merely the flooded lower valley of the Susquehanna River and its tributaries, but there are hundreds of others. Offshore a line of sandbars and barrier beaches stretches intermittently the length of the Coastal Plain, hampering entry of shipping into the estuaries but providing the eastern United States with a playground that is more than 1,000 miles (1,600 km) long.
Poor soils are the rule on the Coastal Plain, though rare exceptions have formed some of America’s most famous agricultural regions—for example, the citrus country of central Florida’s limestone uplands and the Cotton Belt of the Old South, once centred on the alluvial plain of the Mississippi and belts of chalky black soils of eastern Texas, Alabama, and Mississippi. The Atlantic Plain’s greatest natural wealth derives from petroleum and natural gas trapped in domal structures that dot the Gulf Coast of eastern Texas and Louisiana. Onshore and offshore drilling have revealed colossal reserves of oil and natural gas.
West of the Great Plains the United States seems to become a craggy land whose skyline is rarely without mountains—totally different from the open plains and rounded hills of the East. On a map the alignment of the two main chains—the Rocky Mountains on the east, the Pacific ranges on the west—tempts one to assume a geologic and hence topographic homogeneity. Nothing could be farther from the truth, for each chain is divided into widely disparate sections.
The Rockies are typically diverse. The Southern Rockies are composed of a disconnected series of lofty elongated upwarps, their cores made of granitic basement rocks, stripped of sediments, and heavily glaciated at high elevations. In New Mexico and along the western flanks of the Colorado ranges, widespread volcanism and deformation of colourful sedimentary rocks have produced rugged and picturesque country, but the characteristic central Colorado or southern Wyoming range is impressively austere rather than spectacular. The Front Range west of Denver is prototypical, rising abruptly from its base at about 6,000 feet (1,825 metres) to rolling alpine meadows between 11,000 and 12,000 feet (3,350 and 3,650 metres). Peaks appear as low hills perched on this high-level surface, so that Colorado, for example, boasts 53 mountains over 14,000 feet (4,270 metres) but not one over 14,500 feet (4,420 metres).
The Middle Rockies cover most of west-central Wyoming. Most of the ranges resemble the granitic upwards of Colorado, but thrust faulting and volcanism have produced varied and spectacular country to the west, some of which is included in Grand Teton and Yellowstone national parks. Much of the subregion, however, is not mountainous at all but consists of extensive intermontane basins and plains—largely floored with enormous volumes of sedimentary waste eroded from the mountains themselves. Whole ranges have been buried, producing the greatest gap in the Cordilleran system, the Wyoming Basin—resembling in geologic structure and topography an intermontane peninsula of the Great Plains. As a result, the Rockies have never posed an important barrier to east–west transportation in the United States; all major routes, from the Oregon Trail to interstate highways, funnel through the basin, essentially circumventing the main ranges of the Rockies.
The Northern Rockies contain the most varied mountain landscapes of the Cordillera, reflecting a corresponding geologic complexity. The region’s backbone is a mighty series of batholiths—huge masses of molten rock that slowly cooled below the surface and were later uplifted. The batholiths are eroded into rugged granitic ranges, which, in central Idaho, compose the most extensive wilderness country in the conterminous United States. East of the batholiths and opposite the Great Plains, sediments have been folded and thrust-faulted into a series of linear north–south ranges, a southern extension of the spectacular Canadian Rockies. Although elevations run 2,000 to 3,000 feet (600 to 900 metres) lower than the Colorado Rockies (most of the Idaho Rockies lie well below 10,000 feet [3,050 metres]), increased rainfall and northern latitude have encouraged glaciation—as elsewhere a sculptor of a handsome alpine landscape.
The western branch of the Cordillera directly abuts the Pacific Ocean. This coastal chain, like its Rocky Mountain cousins on the eastern flank of the Cordillera, conceals bewildering complexity behind a facade of apparent simplicity. At first glance the chain consists merely of two lines of mountains with a discontinuous trough between them. Immediately behind the coast is a line of hills and low mountains—the Pacific Coast Ranges. Farther inland, averaging 150 miles (240 km) from the coast, the line of the Sierra Nevada and the Cascade Range includes the highest elevations in the conterminous United States. Between these two unequal mountain lines is a discontinuous trench, the Troughs of the Coastal Margin.
The apparent simplicity disappears under the most cursory examination. The Pacific Coast Ranges actually contain five distinct sections, each of different geologic origin and each with its own distinctive topography. The Transverse Ranges of southern California are a crowded assemblage of islandlike faulted ranges, with peak elevations of more than 10,000 feet but sufficiently separated by plains and low passes so that travel through them is easy. From Point Conception to the Oregon border, however, the main California Coast Ranges are entirely different, resembling the Appalachian Ridge and Valley region, with low linear ranges that result from erosion of faulted and folded rocks. Major faults run parallel to the low ridges, and the greatest—the notorious San Andreas Fault—was responsible for the earthquake that all but destroyed San Francisco in 1906. Along the California–Oregon border, everything changes again. In this region, the wildly rugged Klamath Mountains represent a western salient of interior structure reminiscent of the Idaho Rockies and the northern Sierra Nevada. In western Oregon and southwestern Washington the Coast Ranges are also different—a gentle, hilly land carved by streams from a broad arch of marine deposits interbedded with tabular lavas. In the northernmost part of the Coast Ranges and the remote northwest, a domal upwarp has produced the Olympic Mountains; its serrated peaks tower nearly 8,000 feet (2,440 metres) above Puget Sound and the Pacific, and the heavy precipitation on its upper slopes supports the largest active glaciers in the United States outside of Alaska.
East of these Pacific Coast Ranges the Troughs of the Coastal Margin contain the only extensive lowland plains of the Pacific margin—California’s Central Valley, Oregon’s Willamette River valley, and the half-drowned basin of Puget Sound in Washington. Parts of an inland trench that extends for great distances along the east coast of the Pacific, similar valleys occur in such diverse areas as Chile and the Alaska panhandle. These valleys are blessed with superior soils, easily irrigated, and very accessible from the Pacific. They have enticed settlers for more than a century and have become the main centres of population and economic activity for much of the U.S. Still farther east rise the two highest mountain chains in the conterminous United States—the Cascades and the Sierra Nevada. Aside from elevation, geographic continuity, and spectacular scenery, however, the two ranges differ in almost every important respect. Except for its northern section, where sedimentary and metamorphic rocks occur, the Sierra Nevada is largely made of granite, part of the same batholithic chain that creates the Idaho Rockies. The range is grossly asymmetrical, the result of massive faulting that has gently tilted the western slopes toward the Central Valley but has uplifted the eastern side to confront the interior with an escarpment nearly two miles high. At high elevation glaciers have scoured the granites to a gleaming white, while on the west the ice has carved spectacular valleys such as the Yosemite. The loftiest peak in the Sierras is Mount Whitney, which at 14,494 feet (4,418 metres) is the highest mountain in the conterminous states. The upfaulting that produced Mount Whitney is accompanied by downfaulting that formed near Death Valley, at 282 feet (86 metres) below sea level, the lowest point in North America.
The Cascades are made largely of volcanic rock; those in northern Washington contain granite like the Sierras, but the rest are formed from relatively recent lava outpourings of dun-coloured basalt and andesite. The Cascades are in effect two ranges. The lower, older range is a long belt of upwarped lava, rising unspectacularly to elevations between 6,000 and 8,000 feet (1,825 and 2,440 metres). Perched above the “low Cascades” is a chain of lofty volcanoes that punctuate the horizon with magnificent glacier-clad peaks. The highest is Mount Rainier, which at 14,410 feet (4,392 metres) is all the more dramatic for rising from near sea level. Most of these volcanoes are quiescent, but they are far from extinct. Mount Lassen in northern California erupted violently in 1914, as did Mount St. Helens in the state of Washington in 1980. Most of the other high Cascade volcanoes exhibit some sign of seismic activity.
The Western Intermontane Region
The Cordillera’s two main chains enclose a vast intermontane region of arid basins, plateaus, and isolated mountain ranges that stretches from the Mexican border nearly to Canada and extends 600 miles from east to west. This enormous territory contains three huge subregions, each with a distinctive geologic history and its own striking topography.
The Colorado Plateau, nestled against the western flanks of the Southern Rockies, is an extraordinary island of geologic stability set in the turbulent sea of Cordilleran tectonic activity. Stability was not absolute, of course, so that parts of the plateau are warped and injected with volcanics, but in general the landscape results from the erosion by streams of nearly flat-lying sedimentary rocks. The result is a mosaic of angular mesas, buttes, and steplike canyons intricately cut from rocks that often are vividly coloured. Large areas of the plateau are so improbable picturesque that they have been set aside as national preserves. The Grand Canyon of the Colorado River is the most famous of several dozen such areas.
West of the plateau and abutting the Sierra Nevada’s eastern escarpment lies the arid Basin and Range subregion, among the most remarkable topographic provinces of the United States. The Basin and Range extends from southern Oregon and Idaho into northern Mexico. Rocks of great complexity have been broken by faulting, and the resulting blocks have tumbled, eroded, and been partly buried by lava and alluvial debris accumulating in the desert basins. The eroded blocks form mountain ranges that are characteristically dozens of miles long, several thousand feet from base to crest, with peak elevations that rarely rise to more than 10,000 feet, and almost always aligned roughly north–south. The basin floors are typically alluvium and sometimes salt marshes or alkali flats.
The third intermontane region, the Columbia Basin, is literally the last, for in some parts its rocks are still being formed. Its entire area is underlain by innumerable tabular lava flows that have flooded the basin between the Cascades and Northern Rockies to undetermined depths. The volume of lava must be measured in thousands of cubic miles, for the flows blanket large parts of Washington, Oregon, and Idaho and in southern Idaho have drowned the flanks of the Northern Rocky Mountains in a basaltic sea. Where the lavas are fresh, as in southern Idaho, the surface is often nearly flat, but more often the floors have been trenched by rivers—conspicuously the Columbia and the Snake—or by glacial flood waters that have carved an intricate system of braided canyons in the remarkable Channelled Scablands of eastern Washington. In surface form the eroded lava often resembles the topography of the Colorado Plateau, but the gaudy colours of the Colorado are replaced here by the sombre black and rusty brown of weathered basalt.
Most large mountain systems are sources of varied mineral wealth, and the American Cordillera is no exception. Metallic minerals have been taken from most crystalline regions and have furnished the United States with both romance and wealth—the Sierra Nevada gold that provoked the 1849 gold rush, the fabulous silver lodes of western Nevada’s Basin and Range, and gold strikes all along the Rocky Mountain chain. Industrial metals, however, are now far more important; copper and lead are among the base metals, and the more exotic molybdenum, vanadium, and cadmium are mainly useful in alloys.
In the Cordillera, as elsewhere, the greatest wealth stems from fuels. Most major basins contain oil and natural gas, conspicuously the Wyoming Basin, the Central Valley of California, and the Los Angeles Basin. The Colorado Plateau, however, has yielded some of the most interesting discoveries—considerable deposits of uranium and colossal occurrences of oil shale. Oil from the shale, however, probably cannot be economically removed without widespread strip-mining and correspondingly large-scale damage to the environment. Wide exploitation of low-sulphur bituminous coal has been initiated in the Four Corners area of the Colorado Plateau, and open-pit mining has already devastated parts of this once-pristine country as completely as West Virginia.
As befits a nation of continental proportions, the United States has an extraordinary network of rivers and lakes, including some of the largest and most useful in the world. In the humid East they provide an enormous mileage of cheap inland transportation; westward, most rivers and streams are unnavigable but are heavily used for irrigation and power generation. Both East and West, however, traditionally have used lakes and streams as public sewers, and despite efforts to clean them up, most large waterways are laden with vast, poisonous volumes of industrial, agricultural, and human wastes.
The Eastern systems
Chief among U.S. rivers is the Mississippi, which, with its great tributaries, the Ohio and the Missouri, drains most of the midcontinent. The Mississippi is navigable to Minneapolis, nearly 1,200 miles (1,900 km) by air from the Gulf of Mexico, and along with the Great Lakes–St. Lawrence system it forms the world’s greatest network of inland waterways. The Mississippi’s eastern branches, chiefly the Ohio and the Tennessee, are also navigable for great distances. From the west, however, many of its numerous Great Plains tributaries are too seasonal and choked with sandbars to be used for shipping. The Missouri, for example, though longer than the Mississippi itself, was essentially without navigation until the mid-20th century, when a combination of dams, locks, and dredging opened the river to barge traffic.
The Great Lakes–St. Lawrence system, the other half of the midcontinental inland waterway, is connected to the Mississippi–Ohio via Chicago by canals and the Illinois River. The five Great Lakes (four of which are shared with Canada) constitute by far the largest freshwater lake group in the world and carry a larger tonnage of shipping than any other. The three main barriers to navigation—the St. Marys Rapids, at Sault Sainte Marie; Niagara Falls; and the rapids of the St. Lawrence—are all bypassed by locks, whose 27-foot (8-metre) draft lets ocean vessels penetrate 1,300 miles (2,100 km) into the continent, as far as Duluth, Minnesota, and Chicago.
The third group of Eastern rivers drains the coastal strip along the Atlantic Ocean and the Gulf of Mexico. Except for the Rio Grande, which rises west of the Rockies and flows about 1,900 circuitous miles (3,050 km) to the Gulf, few of these coastal rivers measure more than 300 miles (480 km), and most flow in an almost straight line to the sea. Except in glaciated New England and in arid southwestern Texas, most of the larger coastal streams are navigable for some distance.
The Pacific systems
West of the Rockies, nearly all of the rivers are strongly influenced by aridity. In the deserts and steppes of the intermontane basins, most of the scanty runoff disappears into interior basins, only one of which, the Great Salt Lake, holds any substantial volume of water. Aside from a few minor coastal streams, only three large river systems manage to reach the sea—the Columbia, the Colorado, and the San Joaquin–Sacramento system of California’s Central Valley. All three of these river systems are exotic: that is, they flow for considerable distances across dry lands from which they receive little water. Both the Columbia and the Colorado have carved awesome gorges, the former through the sombre lavas of the Cascades and the Columbia Basin, the latter through the brilliantly coloured rocks of the Colorado Plateau. These gorges lend themselves to easy damming, and the once-wild Columbia has been turned into a stairway of placid lakes whose waters irrigate the arid plateaus of eastern Washington and power one of the world’s largest hydroelectric networks. The Colorado is less extensively developed, and proposals for new dam construction have met fierce opposition from those who want to preserve the spectacular natural beauty of the river’s canyonlands.
Climate affects human habitats both directly and indirectly through its influence on vegetation, soils, and wildlife. In the United States, however, the natural environment has been altered drastically by nearly four centuries of European settlement, as well as thousands of years of Indian occupation.
Wherever land is abandoned, however, “wild” conditions return rapidly, achieving over the long run a dynamic equilibrium among soils, vegetation, and the inexorable strictures of climate. Thus, though Americans have created an artificial environment of continental proportions, the United States still can be divided into a mosaic of bioclimatic regions, each of them distinguished by peculiar climatic conditions and each with a potential vegetation and soil that eventually would return in the absence of humans. The main exception to this generalisation applies to fauna, so drastically altered that it is almost impossible to know what sort of animal geography would redevelop in the areas of the United States if humans were removed from the scene.
The pattern of U.S. climates is largely set by the location of the conterminous United States almost entirely in the middle latitudes, by its position with respect to the continental landmass and its fringing oceans, and by the nation’s gross pattern of mountains and lowlands. Each of these geographic controls operates to determine the character of air masses and their changing behaviour from season to season.
The conterminous United States lies entirely between the tropic of Cancer and 50° N latitude, a position that confines Arctic climates to the high mountaintops and genuine tropics to a small part of southern Florida. By no means, however, is the climate literally temperate, for the middle latitudes are notorious for extreme variations of temperature and precipitation.
The great size of the North American landmass tends to reinforce these extremes. Since land heats and cools more rapidly than bodies of water, places distant from an ocean tend to have continental climates; that is, they alternate between extremes of hot summers and cold winters, in contrast to the marine climates, which are more equable. Most U.S. climates are markedly continental, the more so because the Cordillera effectively confines the moderating Pacific influence to a narrow strip along the West Coast. Extremes of continentality occur near the centre of the country, and in North Dakota temperatures have ranged between a summer high record of 121 °F (49 °C) and a winter low of −60 °F (−51 °C). Moreover, the general eastward drift of air over the United States carries continental temperatures all the way to the Atlantic coast. Bismarck, North Dakota, for example, has a great annual temperature range. Boston, on the Atlantic but largely exempt from its influence, has a lesser but still-continental range, while San Francisco, which is under strong Pacific influence, has only a small summer–winter differential.
In addition to confining Pacific temperatures to the coastal margin, the Pacific Coast Ranges are high enough to make a local rain shadow in their lee, although the main barrier is the great rampart formed by the Sierra Nevada and Cascade ranges. Rainy on their western slopes and barren on the east, this mountain crest forms one of the sharpest climatic divides in the United States.
The rain shadow continues east to the Rockies, leaving the entire Intermontane Region either arid or semiarid, except where isolated ranges manage to capture leftover moisture at high altitudes. East of the Rockies the westerly drift brings mainly dry air, and as a result, the Great Plains are semiarid. Still farther east, humidity increases owing to the frequent incursion from the south of warm, moist, and unstable air from the Gulf of Mexico, which produces more precipitation in the United States than the Pacific and Atlantic oceans combined.
Although the landforms of the Interior Lowlands have been termed dull, there is nothing dull about their weather conditions. Air from the Gulf of Mexico can flow northward across the Great Plains, uninterrupted by topographical barriers, but continental Canadian air flows south by the same route, and, since these two air masses differ in every important respect, the collisions often produce disturbances of monumental violence. Plainsmen and Midwesterners are accustomed to sudden displays of furious weather—tornadoes, blizzards, hailstorms, precipitous drops and rises in temperature, and a host of other spectacular meteorological displays, sometimes dangerous but seldom boring.
The change of seasons
Most of the United States is marked by sharp differences between winter and summer. In winter, when temperature contrasts between land and water are greatest, huge masses of frigid, dry Canadian air periodically spread far south over the midcontinent, bringing cold, sparkling weather to the interior and generating great cyclonic storms where their leading edges confront the shrunken mass of warm Gulf air to the south. Although such cyclonic activity occurs throughout the year, it is most frequent and intense during the winter, parading eastward out of the Great Plains to bring the Eastern states practically all their winter precipitation. Winter temperatures differ widely, depending largely on latitude. Thus, New Orleans, Louisiana, at 30° N latitude, and International Falls, Minnesota, at 49° N, have respective January temperature averages of 55 °F (13 °C) and 3 °F (−16 °C). In the north, therefore, precipitation often comes as snow, often driven by furious winds; farther south, cold rain alternates with sleet and occasional snow. Southern Florida is the only dependably warm part of the East, though “polar outbursts” have been known to bring temperatures below 0 °F (−18 °C) as far south as Tallahassee. The main uniformity of Eastern weather in wintertime is the expectation of frequent change.
Winter climate on the West Coast is very different. A great spiralling mass of relatively warm, moist air spreads south from the Aleutian Islands of Alaska, its semipermanent front producing gloomy overcast and drizzles that hang over the Pacific Northwest all winter long, occasionally reaching southern California, which receives nearly all of its rain at this time of year. This Pacific air brings mild temperatures along the length of the coast; the average January day in Seattle, Washington, ranges between 33 and 44 °F (1 and 7 °C) and in Los Angeles between 45 and 64 °F (7 and 18 °C). In southern California, however, rains are separated by long spells of fair weather, and the whole region is a winter haven for those seeking refuge from less agreeable weather in other parts of the country. The Intermontane Region is similar to the Pacific Coast, but with much less rainfall and a considerably wider range of temperatures.
During the summer there is a reversal of the air masses, and east of the Rockies the change resembles the summer monsoon of Southeast Asia. As the midcontinent heats up, the cold Canadian air mass weakens and retreats, pushed north by an aggressive mass of warm, moist air from the Gulf. The great winter temperature differential between North and South disappears as the hot, soggy blanket spreads from the Gulf coast to the Canadian border. Heat and humidity are naturally most oppressive in the South, but there is little comfort in the more northern latitudes. In Houston, Texas, the temperature on a typical July day reaches 93 °F (34 °C), with relative humidity averaging near 75 percent, but Minneapolis, Minnesota, more than 1,000 miles (1,600 km) north, is only slightly cooler and less humid.
Since the Gulf air is unstable as well as wet, convectional and frontal summer thunderstorms are endemic east of the Rockies, accounting for a majority of total summer rain. These storms usually drench small areas with short-lived, sometimes violent downpours, so that crops in one Midwestern county may prosper, those in another shrivel in drought, and those in yet another be flattened by hailstones. Relief from the humid heat comes in the northern Midwest from occasional outbursts of cool Canadian air; small but more consistent relief is found downwind from the Great Lakes and at high elevations in the Appalachians. East of the Rockies, however, U.S. summers are distinctly uncomfortable, and air conditioning is viewed as a desirable amenity in most areas.
Again, the Pacific regime is different. The moist Aleutian air retreats northward, to be replaced by mild, stable air from over the subtropical but cool waters of the Pacific, and except in the mountains the Pacific Coast is nearly rainless though often foggy. In the meanwhile, a small but potent mass of dry hot air raises temperatures to blistering levels over much of the intermontane Southwest. In Yuma, Arizona, for example, the normal temperature in July reaches 107 °F (42 °C), while near Death Valley, California, holds the national record, 134 °F (57 °C). During its summer peak this scorching air mass spreads from the Pacific margin as far as Texas on the east and Idaho to the north, turning the whole interior basin into a summer desert.
Over most of the United States, as in most continental climates, spring and autumn are agreeable but disappointingly brief. Autumn is particularly idyllic in the East, with a romantic Indian summer of ripening corn and brilliantly coloured foliage and of mild days and frosty nights. The shift in dominance between marine and continental air masses, however, spawns furious weather in some regions. Along the Atlantic and Gulf coasts, for example, autumn is the season for hurricanes—the American equivalent of typhoons of the Asian Pacific—which rage northward from the warm tropics to create havoc along the Gulf and Atlantic coasts as far north as New England. The Mississippi valley holds the dubious distinction of recording more tornadoes than any other area on Earth. These violent and often deadly storms usually occur over relatively small areas and are confined largely to spring and early summer.
The bioclimatic regions
Three first-order bioclimatic zones encompass most of the conterminous United States—regions in which climatic conditions are similar enough to dictate similar conditions of mature (zonal) soil and potential climax vegetation (i.e., the assemblage of plants that would grow and reproduce indefinitely given stable climate and average conditions of soil and drainage). These are the Humid East, the Humid Pacific Coast, and the Dry West. In addition, the boundary zone between the Humid East and the Dry West is so large and important that it constitutes a separate region, the Humid–Arid Transition. Finally, because the Western Cordillera contains an intricate mosaic of climatic types, largely determined by local elevation and exposure, it is useful to distinguish the Western Mountain Climate. The first three zones, however, are very diverse and require further breakdown, producing a total of 10 main bioclimatic regions. For two reasons, the boundaries of these bioclimatic regions are much less distinct than boundaries of landform regions. First, climate varies from year to year, especially in boundary zones, whereas landforms obviously do not. Second, regions of climate, vegetation, and soils coincide generally but sometimes not precisely. Boundaries, therefore, should be interpreted as zonal and transitional, and rarely should be considered as sharp lines in the For all of their indistinct boundaries, however, these bioclimatic regions have strong and easily recognized identities. Such regional identity is strongly reinforced when a particular area falls entirely within a single bioclimatic region and at the same time a single landform region. The result—as in the Piedmont South, the central Midwest, or the western Great Plains—is a landscape with an unmistakable regional personality.
The Humid East
The largest and in some ways the most important of the bioclimatic zones, the Humid East was where the Europeans first settled, tamed the land, and adapted to American conditions. In early times almost all of this territory was forested, a fact of central importance in American history that profoundly influenced both soils and wildlife. As in most of the world’s humid lands, soluble minerals have been leached from the earth, leaving a great family of soils called pedalfers, rich in relatively insoluble iron and aluminum compounds.
Both forests and soils, however, differ considerably within this vast region. Since rainfall is ample and summers are warm everywhere, the main differences result from the length and severity of winters, which determine the length of the growing season. Winter, obviously, differs according to latitude, so that the Humid East is sliced into four great east–west bands of soils and vegetation, with progressively more amenable winters as one travels southward. These changes occur very gradually, however, and the boundaries therefore are extremely subtle.
The Sub-Boreal Forest Region is the northernmost of these bands. It is only a small and discontinuous part of the United States, representing the tattered southern fringe of the vast Canadian taiga—a scrubby forest dominated by evergreen needle-leaf species that can endure the ferocious winters and reproduce during the short, erratic summers. Average growing seasons are less than 120 days, though localities in Michigan’s Upper Peninsula have recorded frost-free periods lasting as long as 161 days and as short as 76 days. Soils of this region that survived the scour of glaciation are miserably thin podzols—heavily leached, highly acidic, and often interrupted by extensive stretches of bog. Most attempts at farming in the region long since have been abandoned.
Farther south lies the Humid Microthermal Zone of milder winters and longer summers. Large broadleaf trees begin to predominate over the evergreens, producing a mixed forest of greater floristic variety and economic value that is famous for its brilliant autumn colours. As the forest grows richer in species, sterile podzols give way to more productive grey-brown podzolic soils, stained and fertilized with humus. Although winters are warmer than in the Sub-Boreal zone, and although the Great Lakes help temper the bitterest cold, January temperatures ordinarily average below freezing, and a winter without a few days of subzero temperatures is uncommon. Everywhere, the ground is solidly frozen and snow covered for several months of the year.
Still farther south are the Humid Subtropics. The region’s northern boundary is one of the country’s most significant climatic lines: the approximate northern limit of a growing season of 180–200 days, the outer margin of cotton growing, and, hence, of the Old South. Most of the South lies in the Piedmont and Coastal Plain, for higher elevations in the Appalachians cause a peninsula of Northern mixed forest to extend as far south as northern Georgia. The red-brown podzolic soil, once moderately fertile, has been severely damaged by over cropping and burning. Thus much of the region that once sustained a rich, broadleaf-forest flora now supports poor piney woods. Throughout the South, summers are hot, muggy, long, and disagreeable; Dixie’s “frosty mornings” bring a welcome respite in winter.
The southern margins of Florida contain the only real tropics in the conterminous United States; it is an area in which frost is almost unknown. Hot, rainy summers alternate with warm and somewhat drier winters, with a secondary rainfall peak during the autumn hurricane season—altogether a typical monsoonal regime. Soils and vegetation are mostly immature, however, since southern Florida rises so slightly above sea level that substantial areas, such as the Everglades, are swampy and often brackish. Peat and sand frequently masquerade as soil, and much of the vegetation is either salt-loving mangrove or sawgrass prairie.
The Humid Pacific Coast
The western humid region differs from its eastern counterpart in so many ways as to be a world apart. Much smaller, it is crammed into a narrow littoral belt to the windward of the Sierra–Cascade summit, dominated by mild Pacific air, and chopped by irregular topography into an intricate mosaic of climatic and biotic habitats. Throughout the region rainfall is extremely seasonal, falling mostly in the winter half of the year. Summers are droughty everywhere, but the main regional differences come from the length of drought—from about two months in humid Seattle, Washington, to nearly five months in semiarid San Diego, California.
Western Washington, Oregon, and northern California lie within a zone that climatologists call Marine West Coast. Winters are raw, overcast, and drizzly—not unlike northwestern Europe—with subfreezing temperatures restricted mainly to the mountains, upon which enormous snow accumulations produce local alpine glaciers. Summers, by contrast, are brilliantly cloudless, cool, and frequently foggy along the West Coast and somewhat warmer in the inland valleys. This mild marine climate produces some of the world’s greatest forests of enormous straight-boled evergreen trees that furnish the United States with much of its commercial timber. Mature soils are typical of humid midlatitude forest lands, a moderately leached grey-brown podzol.
Toward the south, with diminishing coastal rain the moist marine climate gradually gives way to California’s tiny but much-publicized Mediterranean regime. Although mountainous topography introduces a bewildering variety of local environments, scanty winter rains are quite inadequate to compensate for the long summer drought, and much of the region has a distinctly arid character. For much of the year, cool, stable Pacific air dominates the West Coast, bringing San Francisco its famous fogs and Los Angeles its infamous smoggy temperature inversions. Inland, however, summer temperatures reach blistering levels, so that in July, while Los Angeles expects a normal daily maximum of 83 °F (28 °C), Fresno expects 100 °F (38 °C) and is climatically a desert. As might be expected, Mediterranean California contains a huge variety of vegetal habitats, but the commonest perhaps is the chaparral, a drought-resistant, scrubby woodland of twisted hard-leafed trees, picturesque but of little economic value. Chaparral is a pyrophytic (fire-loving) vegetation—i.e., under natural conditions its growth and form depend on regular burning. These fires constitute a major environmental hazard in the suburban hills above Los Angeles and San Francisco Bay, especially in autumn, when hot dry Santa Ana winds from the interior regularly convert brush fires into infernos. Soils are similarly varied, but most of them are light in colour and rich in soluble minerals, qualities typical of subarid soils.
The Dry West
In the United States, to speak of dry areas is to speak of the West. It covers an enormous region beyond the dependable reach of moist oceanic air, occupying the entire Intermontane area and sprawling from Canada to Mexico across the western part of the Great Plains. To Americans nurtured in the Humid East, this vast territory across the path of all transcontinental travellers has been harder to tame than any other—and no region has so gripped the national imagination as this fierce and dangerous land.
In the Dry West nothing matters more than water. Thus, though temperatures may differ radically from place to place, the really important regional differences depend overwhelmingly on the degree of aridity, whether an area is extremely dry and hence desert or semiarid and therefore steppe.
Americans of the 19th century were preoccupied by the myth of a Great American Desert, which supposedly occupied more than one-third of the entire country. True desert, however, is confined to the Southwest, with patchy outliers elsewhere, all without exception located in the lowland rain shadows of the Cordillera. Vegetation in these desert areas varies between nothing at all (a rare circumstance confined mainly to salt flats and sand dunes) to a low cover of scattered woody scrub and short-lived annuals that burst into flamboyant bloom after rains. Soils are usually thin, light-coloured, and very rich with mineral salts. In some areas wind erosion has removed fine-grained material, leaving behind desert pavement, a barren veneer of broken rock.
Most of the West, however, lies in the semiarid region, in which rainfall is scanty but adequate to support a thin cover of short bunchgrass, commonly alternating with scrubby brush. Here, as in the desert, soils fall into the large family of the pedocals, rich in calcium and other soluble minerals, but in the slightly wetter environments of the West, they are enriched with humus from decomposed grass roots. Under the proper type of management, these chestnut-coloured steppe soils have the potential to be very fertile.
Weather in the West resembles that of other dry regions of the world, often extreme, violent, and reliably unreliable. Rainfall, for example, obeys a cruel natural law: as total precipitation decreases, it becomes more undependable. John Steinbeck’s novel The Grapes of Wrath describes the problems of a family enticed to the arid frontier of Oklahoma during a wet period only to be driven out by the savage drought of the 1930s that turned the western Great Plains into the great American Dust Bowl. Temperatures in the West also fluctuate convulsively within short periods, and high winds are infamous throughout the region.
The Humid–Arid Transition
East of the Rockies all climatic boundaries are gradational. None, however, is so important or so imperceptibly subtle as the boundary zone that separates the Humid East from the Dry West and that alternates unpredictably between arid and humid conditions from year to year. Stretching approximately from Texas to North Dakota in an ill-defined band between the 95th and 100th meridians, this transitional region deserves separate recognition, partly because of its great size, and partly because of the fine balance between surplus and deficit rainfall, which produces a unique and valuable combination of soils, flora, and fauna. The native vegetation, insofar as it can be reconstructed, was prairie, the legendary sea of tall, deep-rooted grass now almost entirely tilled and planted to grains. Soils, often of loessial derivation, include the enormously productive chernozem (black earth) in the north, with reddish prairie soils of nearly equal fertility in the south. Throughout the region temperatures are severely continental, with bitterly cold winters in the north and scorching summers everywhere.
The western edge of the prairie fades gradually into the shortgrass steppe of the High Plains, the change a function of diminishing rainfall. The eastern edge, however, represents one of the few major discordances between a climatic and biotic boundary in the United States, for the grassland penetrates the eastern forest in a great salient across humid Illinois and Indiana. Many scholars believe this part of the prairie was artificially induced by repeated burning and consequent destruction of the forest margins by Indians.
The Western mountains
Throughout the Cordillera and Intermontane regions, irregular topography shatters the grand bioclimatic pattern into an intricate mosaic of tiny regions that differ drastically according to elevation and exposure. No small- or medium-scale map can accurately record such complexity, and mountainous parts of the West are said, noncommittally, to have a “mountain climate.” Lowlands are usually dry, but increasing elevation brings lower temperature, decreased evaporation, and—if a slope faces prevailing winds—greater precipitation. Soils vary wildly from place to place, but vegetation is fairly predictable. From the desert or steppe of intermontane valleys, a climber typically ascends into parklike savanna, then through an orderly sequence of increasingly humid and boreal forests until, if the range is high enough, one reaches the timberline and Arctic tundra. The very highest peaks are snow-capped, although permanent glaciers rarely occur outside the cool humid highlands of the Pacific Northwest.
The dominant features of the vegetation are indicated by the terms forest, grassland, desert, and alpine tundra.
A coniferous forest of white and red pine, hemlock, spruce, jack pine, and balsam fir extends uninterruptedly in a narrow strip near the Canadian border from Maine to Minnesota and southward along the Appalachian Mountains. There may be found smaller stands of tamarack, spruce, paper birch, willow, alder, and aspen or poplar. Southward, a transition zone of mixed conifers and deciduous trees gives way to a hardwood forest of broad-leaved trees. This forest, with varying mixtures of maple, oak, ash, locust, linden, sweet gum, walnut, hickory, sycamore, beech, and the more southerly tulip tree, once extended uninterruptedly from New England to Missouri and eastern Texas. Pines are prominent on the Atlantic and Gulf coastal plain and adjacent uplands, often occurring in nearly pure stands called pine barrens. Pitch, longleaf, slash, shortleaf, Virginia, and loblolly pines are commonest. Hickory and various oaks combine to form a significant part of this forest, with magnolia, white cedar, and ash often seen. In the frequent swamps, bald cypress, tupelo, and white cedar predominate. Pines, palmettos, and live oaks are replaced at the southern tip of Florida by the more tropical royal and thatch palms, figs, satinwood, aThe grasslands occur principally in the Great Plains area and extend westward into the intermontane basins and benchlands of the Rocky Mountains. Numerous grasses such as buffalo, grama, side oat, bunch, needle, and wheatgrass, together with many kinds of herbs, make up the plant cover. Coniferous forests cover the lesser mountains and high plateaus of the Rockies, Cascades, and Sierra Nevada. Ponderosa (yellow) pine, Douglas fir, western red cedar, western larch, white pine, lodgepole pine, several spruces, western hemlock, grand fir, red fir, and the lofty redwood are the principal trees of these forests. The densest growth occurs west of the Cascade and Coast ranges in Washington, Oregon, and northern California, where the trees are often 100 feet (30 metres) or more in height. There the forest floor is so dark that only ferns, mosses, and a few shade-loving shrubs and herbs may be found.
The alpine tundra, located in the conterminous United States only in the mountains above the limit of trees, consists principally of small plants that bloom brilliantly for a short season. Sagebrush is the most common plant of the arid basins and semi deserts west of the Rocky Mountains, but juniper, nut pine, and mountain mahogany are often found on the slopes and low ridges. The desert, extending from southeastern California to Texas, is noted for the many species of cactus, some of which grow to the height of trees, and for the Joshua tree and other yuccas, creosote bush, mesquite, and acacias. The United States is rich in the variety of its native forest trees, some of which, as the species of sequoia, are the most massive known. More than 1,000 species and varieties have been described, of which almost 200 are of economic value, either because of the timber and other useful products that they yield or by reason of their importance in forestry.
Besides the native flowering plants, estimated at between 20,000 to 25,000 species, many hundreds of species introduced from other regions—chiefly Europe, Asia, and tropical America—have become naturalised. A large proportion of these are common annual weeds of fields, pastures, and roadsides. In some districts these naturalised “aliens” constitute 50 percent or more of the total plant population.
With most of North America, the United States lies in the Nearctic faunistic realm, a region containing an assemblage of species similar to Eurasia and North Africa but sharply different from the tropical and subtropical zones to the south. Main regional differences correspond roughly with primary climatic and vegetal patterns. Thus, for example, the animal communities of the Dry West differ sharply from those of the Humid East and from those of the Pacific Coast. Because animals tend to range over wider areas than plants, faunal regions are generally coarser than vegetal regions and harder to delineate sharply.
The animal geography of the United States, however, is far from a natural pattern, for European settlement produced a series of environmental changes that grossly altered the distribution of animal communities. First, many species were hunted to extinction or near extinction, most conspicuously, perhaps, the American bison, which ranged by the millions nearly from coast to coast but now rarely lives outside of zoos and wildlife preserves. Second, habitats were upset or destroyed throughout most of the country—forests cut, grasslands ploughed and overgrazed, and migration paths interrupted by fences, railroads, and highways. Third, certain introduced species found hospitable niches and, like the English sparrow, spread over huge areas, often preempting the habitats of native animals. Fourth, though their effects are not fully understood, chemical biocides such as DDT were used for so long and in such volume that they are believed at least partly responsible for catastrophic mortality rates among large mammals and birds, especially predators high on the food chain. Fifth, there has been a gradual northward migration of certain tropical and subtropical insects, birds, and mammals, perhaps encouraged by gradual climatic warming. In consequence, many native animals have been reduced to tiny fractions of their former ranges or exterminated completely, while other animals, both native and introduced, have found the new anthropocentric environment well suited to their needs, with explosive effects on their populations. The coyote, opossum, armadillo, and several species of deer are among the animals that now occupy much larger ranges than they once did.
Arrangement of the account of the distribution of the fauna according to the climatic and vegetal regions has the merit that it can be compared further with the distribution of insects and of other invertebrates, some of which may be expected to fall into the same patterns as the vertebrates, while others, with different modes or different ages of dispersal, have geographic patterns of their own.
The transcontinental zone of coniferous forest at the north, the taiga, and the tundra zone into which it merges at the northern limit of tree growth are strikingly paralleled by similar vertical zones in the Rockies, and on Mount Washington in the east, where the area above the timberline and below the snow line is often inhabited with tundra animals like the ptarmigan and the white Parnassius butterflies, while the spruce and other conifers below the timberline form a belt sharply set off from the grassland or hardwood forest or desert at still lower elevations.
A whole series of important types of animals spread beyond the limits of such regions or zones, sometimes over most of the continent. Aquatic animals, in particular, may live equally in forest and plains, in the Gulf states, and at the Canadian border. Such widespread animals include the white-tailed (Virginia) deer and black bear, the puma (though only in the remotest parts of its former range) and bobcat, the river otter (though now rare in inland areas south of the Great Lakes) and mink, and the beaver and muskrat. The distinctive coyote ranges over all of western North America and eastward as far as Maine. The snapping turtle ranges from the Atlantic coast to the Rocky Mountains.
In the northern coniferous forest zone, or taiga, the relations of animals with European or Eurasian representatives are numerous, and this zone is also essentially circumpolar. The relations are less close than in the Arctic forms, but the moose, beaver, hare, red fox, otter, wolverine, and wolf are recognizably related to Eurasian animals. Even some fishes, like the whitefishes (Coregonidae), the yellow perch, and the pike, exhibit this kind of Old World–New World relation. A distinctively North American animal in this taiga assemblage is the Canadian porcupine.
The hardwood forest area of the eastern and the southeastern pinelands compose the most important of the faunal regions within the United States. A great variety of fishes, amphibians, and reptiles of this region have related forms in East Asia, and this pattern of representation is likewise found in the flora. This area is rich in catfishes, minnows, and suckers. The curious ganoid fishes, the bowfin and the gar, are ancient types. The spoonbill cat, a remarkable type of sturgeon in the lower Mississippi, is represented elsewhere in the world only in the Yangtze in China. The Appalachian region is headquarters for the salamanders of the world, with no less than seven of the eight families of this large group of amphibians represented; no other continent has more than three of the eight families together. The eel-like sirens and amphibians (congo snakes) are confined to the southeastern states. The lungless salamanders of the family Plethodontidae exhibit a remarkable variety of genera and a number of species centred in the Appalachians. There is a great variety of frogs, and these include tree frogs whose main development is South American and Australian. The emydid freshwater turtles of the southeast parallel those of East Asia to a remarkable degree, though the genus Clemmys is the only one represented in both regions. Much the same is true of the water snakes, pit vipers, rat snakes, and green snakes, though still others are peculiarly American. The familiar alligator is a form with an Asiatic relative, the only other living true alligator being a species in central China.
In its mammals and birds the southeastern fauna is less sharply distinguished from the life to the north and west and is less directly related to that of East Asia. The forest is the home of the white-tailed deer, the black bear, the grey fox, the raccoon, and the common opossum. The wild turkey and the extinct hosts of the passenger pigeon were characteristic. There is a remarkable variety of woodpeckers. The birdlife in general tends to differ from that of Eurasia in the presence of birds, like the tanagers, American orioles, and hummingbirds, that belong to South American families. Small mammals abound with types of the worldwide rodent family Cricetidae, and with distinctive moles and shrews.
Most distinctive of the grassland animals proper is the American bison, whose nearly extinct European relative, the wisent, is a forest dweller. The most distinctive of the American hoofed animals is the pronghorn, or prongbuck, which represents a family intermediate between the deer and the true antelopes in that it sheds its horns like a deer but retains the bony horn cores. The pronghorn is perhaps primarily a desert mammal, but it formerly ranged widely into the shortgrass plains. Everywhere in open country in the West there are conspicuous and distinctive rodents. The burrowing pocket gopher is peculiarly American, rarely seen making its presence known by pushed-out mounds of earth. The ground squirrels of the genus Citellus are related to those of Central Asia, and resemble them in habit; in North America the gregarious prairie dog is a closely related form. The American badger, not especially related to the badger of Europe, has its headquarters in the grasslands. The prairie chicken is a bird distinctive of the plains region, which is invaded everywhere by birds from both the east and the west.
The Southwestern deserts are a paradise for reptiles. Distinctive lizards such as the poisonous Gila monster abound, and the rattlesnakes, of which only a few species are found elsewhere in the United States, are common there. Desert reptile species often range to the Pacific Coast and northward into the Great Basin. Noteworthy mammals are the graceful bipedal kangaroo rat (almost exclusively nocturnal), the ring-tailed cat, a relative of the raccoon, and the piglike peccary.
The Rocky Mountains and other western ranges afford distinctive habitats for rock- and cliff-dwelling hoofed animals and rodents. The small pikas, related to the rabbit, inhabit talus areas at high elevations as they do in the mountain ranges of East Asia. Marmots live in the Rockies as in the Alps. Every western range formerly had its own race of mountain sheep. In the north the Rocky Mountain goat lives at high altitudes—it is more properly a goat antelope, related to the taking of the mountains of western China. The dipper, remarkable for its habit of feeding in swift-flowing streams, though otherwise a bird without special aquatic adaptations, is a Rocky Mountain form with relatives in Asia and Europe.
In the Pacific region the extremely distinctive primitive tailed frog Ascaphus, which inhabits icy mountain brooks, represents a family by itself, perhaps more nearly related to the frogs of New Zealand than to more familiar types. The Cascades and Sierras form centres for salamanders of the families Ambystomatidae and Plethodontidae second only to the Appalachians, and there are also distinctive newts. The burrowing lizards, of the well-defined family Anniellidae, are found only in a limited area in coastal California. The only family of birds distinctive of North America, that of the wren-tits, Chamaeidae, is found in the chaparral of California. The mountain beaver, or sewellel (which is not at all beaverlike), is likewise a type peculiar to North America, confined to the Cascades and Sierras, and there are distinct kinds of moles in the Pacific area.
The mammals of the two coasts are strikingly different, though true seals (the harbour seal and the harp seal) are found on both. The sea lions, with longer necks and with projecting ears, are found only in the Pacific—the California sea lion, the more northern Steller’s sea lion, and the fur seal. On the East Coast the larger rivers of Florida are inhabited by the Florida manatee, or sea cow, a close relative of the more widespread and more distinctively marine West Indian species.