II.

Land and Resources

The United States has an enormous variety of physical features and a wide diversity of animal and plant life, which are discussed more fully in the individual state entries.

A.

Geological History

The present-day pattern of the landforms of the United States is the result of a long sequence of collisions and separations of large blocks of the Earth’s surface crust, a process known as plate tectonics. The oldest part of the continent is the Canadian Shield, or Laurentian Plateau, a mass of granite and related rock that underlies eastern Canada and the north-eastern United States. The shield was formed during several long periods of crustal convergence in Precambrian time (a period that stretches from the formation of the Earth to about 570 million years ago). The characteristic rock of the shield is granite. The margins of the ancient continent are more complex in structure, and include zones of granite, darker ocean-bottom rocks, fine-grained volcanic rocks, and hardened ocean sediments.

A long period of inactivity in the crust followed the formation of the shield. Erosion reduced the mountainous continent to a low plain, and the adjoining seas were filled with thick beds of sediment. Near the end of this period, great forests covered the land, and the addition of organic material to the sediment formed the vast coal and petroleum layers that stretch in a broad curve from northern Pennsylvania through West Virginia to Alabama, then west to Texas and north-west through the Great Plains states and Canadian prairies to Arctic Alaska.

The period of crustal calm ended when the North American and European land masses collided early in the fossil-forming period; southern Massachusetts and Rhode Island are actually parts of the European land mass that became attached to the American plate at this time. Later, after the coal-forming age, the African and American land masses converged. The modern Appalachians are the worn-down remnants of the mountains that were built during this collision. Crustal uplift and subsequent erosion exposed ancient granite rocks all the way from New England to Alabama, as evidenced in the low Piedmont hills of Georgia and the Carolinas and the somewhat higher Blue Ridge Mountains of North Carolina, Virginia, and West Virginia. To the west, the layers of younger sedimentary rocks still remain at the surface, crumpled and eroded, notably in the long, even ridges and valleys of central Pennsylvania, eastern Tennessee, and the Ouachita region of Arkansas and Oklahoma. Still further west and north, less intense folding created broad domes and basins. Present-day Michigan and Iowa occupy geological basins. Structural domes are centred near the Wisconsin Dells, the Bluegrass area of Kentucky, and the Nashville area of Tennessee.

After the Appalachian collision, the continent reversed direction and drifted west. The Atlantic Ocean began to widen, and the eastern United States again became a region of geological calm. The Appalachians began to erode, and the resulting sediment accumulated on the mid-continental Great Plains and on the Atlantic and Gulf coastal plains. Meanwhile, new ranges of mountains were rising as the western United States collided with the Pacific plate. Lava erupted on to the surface in many places at different times: in northern New Mexico, central Arizona, eastern California, and southern Idaho, and especially in the region of the Cascade Range of Oregon and Washington. The sandy sediments of the Great Plains were thrust sharply upward along the Front Range of the Rocky Mountains in Colorado and Montana and around smaller mountain ranges such as the Black Hills of South Dakota. Rock movement along massive faults formed California’s Sierra Nevada, the Wasatch Range in Utah, the aligned mountain ranges of Nevada, and the Teton Range in Wyoming. The land of Arizona and southern Utah was lifted, and rivers cut canyons into the level sedimentary rocks.

The major past climatic event is the Pleistocene epoch, more commonly known as the Ice Age. At least four times in the past 1 million years, great ice sheets formed in eastern Canada and the mountains of the west and spread outward. The moving ice scraped up soil and rock from Canada and the northern United States and deposited the material further south. The aligned lakes and exposed rocks of New England and northern Minnesota are the result of glacial scouring. Long Island and Cape Cod are huge glacial deposits, characterized by hills composed of rock and soil, with associated swamps and sand outwash plains; similar features are abundant throughout the former glaciated areas, from New England to the Dakotas and in the western mountain valleys. Rivers such as the Hudson, Illinois, Minnesota, Missouri, and Columbia carried huge floods of glacial meltwater and carved valleys much larger than the present-day streams require. Glacial meltwater also formed many large lakes. Today, level plains and low beach ridges mark the beds and shores respectively of Ice Age lakes on both the eastern and western edges of Vermont, around the Maumee River of north-western Ohio, in the sand counties of central Wisconsin, around the Red River of Minnesota and the Dakotas, around the Great Salt Lake of Utah, and in the Missoula Basin of Montana, and the Central Valley of California. Ice Age dust storms left thick deposits of loess (fine-grained silt or clay) on the undulating plains around the Mississippi and Missouri rivers, on the steeper bluffs of western Wisconsin and western Tennessee, and in the Palouse Hills region of eastern Washington. Times of higher sea level built beaches far up on the Gulf Coastal Plain and on slopes overlooking the Pacific Ocean, while Chesapeake Bay and many similar drowned river valleys along the Atlantic coast from Georgia to Connecticut are the result of periods of lower sea level.

B.

Rivers and Lakes

The rivers of the eastern United States, principal among which are the Hudson, Delaware, Susquehanna, Potomac, and Savannah, receive rainfall in every month and are therefore reliable routes for water-borne commerce. Rivers of the interior, such as the Ohio, Tennessee, Illinois, and Mississippi, often flood in spring and decrease in size during the hot weeks of late summer and the snowy winter months. Some degree of flow regulation and flood control has been achieved on these rivers through a costly and controversial system of dams and levees. Argument over water projects is even more heated in the western United States, where mountain snowmelt is the principal source of water for the eastward-flowing Missouri, Platte, Arkansas, and Rio Grande rivers and the westward-flowing Colorado, Sacramento, Snake, and Columbia rivers. Most of these rivers shrink in volume as they flow away from their mountain sources; some, like the Colorado, are dammed and diverted for so many urban or agricultural uses that they no longer carry water to the sea. In Alaska the drainage system is dominated by the Yukon, a river as long as the Rio Grande but considerably greater in volume.

The five Great Lakes—Ontario, Erie, Huron, Michigan, and Superior—occupy an interconnected set of glacially scoured basins and together serve as a major artery of transport. Glaciers also left tens of thousands of smaller lakes throughout the north-eastern United States, the upper Midwest, and much of Alaska. Among the larger of these are Champlain, Winnipesaukee, and Cayuga in the north-east and Winnebago, Red, and Mille Lacsin the Midwest. The Great Salt Lake of Utah and many smaller salt basins of the Mountain states are remnants of much larger Ice Age lakes. Many groundwater aquifers, especially those of the Great Plains, are also relics of wetter climates of the past.

C.

Climate

In general, sun intensity and, consequently, temperatures decrease from south to north; in summer, however, the decrease in intensity is partly offset by longer days in the north. Montana, North Dakota, and Minnesota actually have higher record temperatures than New Mexico and Alabama. In winter, on the other hand, the short days in the north exaggerate the effect of low sun angles, creating wide temperature differences from south to north. Forests use up much solar energy to evaporate water, and therefore the humid states of the eastern United States do not get as warm as the dry western deserts. Oceans and lakes moderate temperatures, and mountains are somewhat cooler by day and much colder at night than surrounding lowlands.

The pattern of precipitation is largely a consequence of the interaction of wind and topography. The wind system of the Earth balances temperatures by taking heat from the equator and carrying it towards the poles. Two features of this global atmospheric circulation are particularly significant for the United States. One is a current of sinking air, a gentle but persistent downward movement of air from the upper atmosphere. This subsidence is part of the global convection cycle and starts with updrafts of warm and humid air near the equator; the air loses moisture as it rises to the upper atmosphere and begins to move polewards. At about latitude 30° north the air begins to sink, bringing hot and dry conditions to the south-western United States, especially in summer.

The other significant part of atmospheric circulation is the jet stream, a shifting zone of fast winds blowing high above the ground, generally from west to east. The path of the jet stream on any given day is a key to surface weather. In summer, the jet stream is usually near the Canadian border, though it may loop as far north as Alaska or as far south as Louisiana. It brings wet Pacific air onshore in Washington and Alaska, but in other western states dry air masses from Mexico and Canada dominate. In the east, by contrast, the jet can pull moist air masses northward from the Gulf of Mexico all the way to Canada. In winter, the entire wind system follows the sun southward. Pacific air masses now bring clouds and rain to the coastal mountains from California to southern Alaska. The jet usually crosses the country at the latitude of Oklahoma, and cold, dry Canadian air covers the northern half of the country; however, day-to-day shifts of the jet may pull warm, moist Gulf air as far north as Illinois or bring Canadian air to Florida.

Regional weather hazards are intimately associated with the seasonal position of the jet stream and associated fronts. Torrential rains are most common near the Gulf of Mexico, which is the major source of moisture for the country. Tornadoes occur in the centre of the United States, where Canadian and Gulf air masses often collide violently; hurricanes arise out of the late-summer warmth of the Atlantic Ocean and drift towards the south-eastern states in the autumn. Southern California experiences smog and forest fires in late summer.

Heavy winter snows in the eastern United States are caused by the rapid cooling of Gulf air, amplified in the Great Lakes region by local lake breezes. December and March are the major months for snow in Minnesota and the Dakotas; in January there is a time of intense cold and little snowfall, because Gulf air cannot penetrate that far north. Finally, the occasional kona (west coast) storms of Hawaii are wintertime incursions of North Pacific air that occur when the jet stream curves far to the south. Normal weather consists of trade winds that cause rain only on the north-eastern slopes of each island.

D.

Natural Resources

The United States is exceedingly rich in natural resources, and the climate is favourable for a diversity of crops and forest products. There are significant deposits of many important minerals, including more than a fifth of the world’s coal, and the Corn Belt, a region stretching from western Ohio to central Nebraska, is the largest expanse of prime farmland in the world. Despite this, the United States imports more than 80 per cent of its bauxite, magnesium, platinum, tin, and tungsten; and although it produced about 57 per cent of its petroleum needs in the mid-1990s, new domestic discoveries tend to be small and costly to recover.

Agricultural exports have helped compensate for mineral imports, but the price is high: more than a third of the nation’s topsoil has been lost to erosion. There is also concern about the rapid conversion of prime cropland to urban and other non-farm uses.

Nevertheless, the environmental picture is not all bleak. Because of government regulations, water pollution diminished considerably during the 1970s; forests are growing more rapidly than they are being cut in most regions; and substantial areas have been set aside as wilderness preserves and national parks. In the future the questions of acid rain, toxic-waste disposal, water supply, and climate change will continue to be major national environmental issues.

E.

Soils

The United States can be divided into five major soil regions: (1) the deep, black mollisols of the mid-continent (the country’s most fertile), in an area stretching from Illinois west to Nebraska and the Dakotas; (2) the leached red ultisols of the south-eastern and western coast mountains, which are considerably less fertile; (3) the coarse and acid spodosols in the north-eastern United States and in the high western mountains; (4) the moderately fertile grey or brown alfisols found around the Great Lakes, on the southern Great Plains, and in the Central Valley of California; and (5) the dry, salty aridsols of the desert Southwest. Superimposed on these broad patterns are local geological exceptions, such as the black clay vertisols of Texas and the organic histosols, which are peat and muck soils found in swampy places such as the Florida Everglades or the Minnesota bogs.

F.

Vegetation

At the time European settlement began, about 50 per cent of the United States was covered by forests; today, the figure is about 30 per cent. Similarly, grasslands and other natural vegetative cover decreased in extent as the continent was settled.

Northern Alaska, the northernmost part of the United States, is characterized by a windswept tundra, a region of lichens, mosses, hardy low shrubs, and flowering plants. Inland and to the south, the growing season lengthens and certain trees can survive. A few species of conifers, notably spruces and firs, dominate a vast evergreen forest, interspersed with lichen-covered rocky areas, grassy swamps, and aspen-choked fire scars. This forest, known as the taiga, stretches south-east from interior Alaska and reaches into northern New England and the Great Lakes region. South of the taiga the growing season is longer and more tree species can survive; the forest contains both conifer and deciduous trees, including pines, maples, elms, birches, oaks, hickory, beech, and sycamore. This type of mixed forest covered the region around the Great Lakes and most of the New England and Middle Atlantic states when European settlers arrived.

Still further south, the forest reaches its maximum diversity. The Great Smoky Mountains National Park in western North Carolina and eastern Tennessee contains more tree species than Europe. The Gulf of Mexico coast is even warmer than this mountainous area, but its plains and low hills do not support as complex a forest. Moreover, the sandy soils and hot summers encourage fires, which suppress oaks and other hardwoods and favour the fast-growing pines that now represent the major forest resource of the nation. Other species found here include southern magnolia, pecan, red gum, and black gum (tupelo). A number of subtropical and tropical trees flourish in southern Florida. Along the coast of the Gulf of Mexico, salt marshes and groves of cypress and mangrove help to buttress the shore against the eroding forces of wind and water.

The diversity of the forest also decreases west of the Appalachian Mountains. First, the mountaintop spruces, firs, and mountain ashes disappear. Then rainfall decreases in quantity and reliability, and fires become more frequent. The lush hardwood forests of the Mississippi Valley slowly dwindle in size and complexity; oak-hickory forests give way to isolated stands of oak and then to tall grass prairies, which, before cultivation, occupied the present Corn Belt from Indiana to the eastern Great Plains.

Further west, the climate becomes still drier, and the tall bluestem grasses yield to shorter grama and wheatgrass ranges. The grasses of the northern Great Plains grow only during the short summer and flower in late summer or early autumn. By contrast, the grasses of the southern Great Plains grow rapidly in spring, flower early, and then lie dormant during the hot, dry summers. Both kinds of grass become less productive as rainfall continues to diminish towards the west. Shrubby sagebrush (in the north) and mesquite and juniper (in Texas) are typical invaders on poorer grasslands that have been overgrazed or protected from fires.

A gradual transition to true desert vegetation is interrupted by the Rocky Mountains and other ranges, the elevation of which both increases rainfall and decreases temperature and evaporation. Trees become prominent on the lower and middle slopes. Hardy pines and junipers dominate at lower elevations, giving way to aspens, firs, and spruces at higher elevations. Still higher, the spruces and firs become stunted and widely spaced. Above this zone is treeless tundra. Shrubby low-lying deserts alternate with forested (and occasionally tundra- or ice-capped) mountains across all of the Mountain states and into the Pacific states. This region is agriculturally productive only when massive investments are made in irrigation. Death Valley, which lies below sea level, is but one of the many nearly barren lowlands. Vegetation in these regions includes species such as sagebrush, juniper, piñon, rabbitbrush, mesquite, creosote bush, and yucca; the cactus “forests” that form a popular image of deserts are actually found on the slopes of mountain ranges in the Mojave Desert of southern Arizona and California. On the higher but still relatively dry Colorado Plateau are ponderosa and piñon pines.

The hot, dry summers and mild, moist winters of coastal southern California produce a distinctive shrub vegetation known as chaparral. Further north on the western slopes of the coastal hills and Sierra Nevada, where there is enough rain to permit rapid growth but a long enough dry season to discourage competition from numerous species, forests of giant sequoia and redwood grow. Still further north, in western Oregon and Washington, where the dry periods extend only to a few weeks in midsummer, a true rainforest appears, consisting primarily of a great variety of conifers. Douglas firs, true firs, hemlocks, cedars, spruces, and pines each occupy their own preferred elevation zone, and together constitute the second-richest forest resource for the nation. The coastal forests of Alaska have fewer species than the rich rainforest to the south but a faster growth than the taiga to the north.

The natural vegetation of Hawaii is conditioned by its isolation and by the interplay of its mountains and the moist trade winds. Forests dominated by guava trees on the windward (north-east) coasts of the islands graduate to a rich but swampy rainforest at moderate elevations, where the annual rainfall may exceed 10,000 mm (400 in). The high mountains support shrub forest, and patches of tundra are found on the summits of the highest peaks, Mauna Loa and Mauna Kea. The dry leeward (south-west) coasts are virtual deserts, with spiny koa and kiawe shrubs growing on the slightly wetter slopes.

G.

Animals

In the Arctic areas and regions of mountain tundra are found burrowing marmots, ground squirrels, cold-water fish such as grayling and trout, and an occasional bear. Alaskan coastal waters are the habitat of a number of large mammals, including walrus and fur seals. Caribou and wapiti spend summers on the tundra but migrate into the conifer forest for winter. The hardwood forests of the eastern United States contain elk, black bears, deer, foxes, raccoons, skunks, squirrels, and a diversity of small birds. Along the Gulf of Mexico coast live larger and more colourful birds such as pelicans, flamingos, and green kingfishers; as well as alligators and warm-water fish such as catfish. Several varieties of venomous snakes are also found there.

Bison (buffalo) are popularly associated with the grasslands, although in fact they once ranged over most of eastern North America before becoming nearly extinct through hunting; they now exist only in captivity or in protected areas. Gophers, rabbits, prairie dogs, ferrets, and other burrowing species are the creatures best suited to the grasslands, which were once swept by fires. The mountainous western states, especially Alaska, are the last refuges in the United States of most big-game animals: elk, pronghorn, moose, deer, bighorn sheep, mountain goats, timber wolves, and, in a few remote areas, brown bears. The Kodiak bear, the largest carnivore in North America, is found in Alaska. The deserts have fewer small animals, and almost no large ones; kangaroo rats, lizards, and wide-ranging birds are typical animals in such harsh regions. The animals of Hawaii include many endemic species, but many of these have been driven nearly to extinction by human alteration of the habitat. Hawaii’s only indigenous mammal is the bat.