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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 (see 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 northeastern 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). During these slow but relentless collisions of crustal plates, the rigid surface rocks buckled and cracked. Large pieces of crust were forced downward into the hot interior of the earth, where they warmed and eventually melted. This lighter molten material then moved upward through the crustal cracks, occasionally erupting as a volcano, but more often pushing surface rock upward in a broad bulge, or dome. In time, the molten material cooled and crystallized. The characteristic rock of the shield is granite, a slowly cooled and therefore coarse-grained rock. The margins of the ancient continent are more complex in structure, with zones of granite, darker ocean-bottom rocks, fine-grained volcanic rocks, hardened ocean sediments, and rocks of all types that were altered by heat and pressure during later crustal activity. The iron deposits of Minnesota, Upper Michigan, Wisconsin, and northern New York all occur in contorted rocks near the edges of the ancient shield.

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 northwest through the Great Plains states and Canadian prairies to Arctic Alaska.

The period of crustal calm ended when the North American and European landmasses collided early in the fossil-forming period; southern Massachusetts and Rhode Island are actually parts of the European landmass that became attached to the American plate at this time. Later, after the coal-forming age, the African and American landmasses converged. The modern Appalachians are the worn-down remnants of lofty mountains that were built during this majestic collision. This was an age of earthquakes and volcanoes, the same kind of intermittent but relentless geologic activity that characterizes California, Washington, and Alaska today. 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 farther west and north, less intense folding created broad domes and basins. Present-day Michigan and Iowa occupy geologic basins. Structural domes are centered 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 geologic calm. The lofty Appalachians began to erode, and the resulting sediment accumulated on the midcontinental 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 onto 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, Utah's Wasatch Range, Nevada's aligned mountain ranges, and the Teton Range in Wyoming. The land of Arizona and southern Utah was lifted, and rivers cut canyons in the level sedimentary rocks.

The motion of crustal plates thus accounts for the broad panorama of the landforms of the United States. Interpreting the finer details of the present-day landscape requires consideration of the effects of past and present climates. The major past climatic event is the Pleistocene epoch, more commonly known as the Ice Age (see Ice Ages). 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, like a powerful bulldozer, scraped soil and rock from Canada and the northern United States and deposited material farther south. The aligned lakes and exposed rocks of New England and northern Minnesota are results 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 mark the beds and low beach ridges mark the shores of Ice Age lakes on both the eastern and western edges of Vermont, around the Maumee River of northwestern 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, the Central Valley of California, and hundreds of smaller areas in many states. Ice Age dust storms left thick deposits of loess (fine-grained silt or clay) on the undulating plains around the Mississippi River and Missouri River, 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; on the other hand, Chesapeake Bay and many similar drowned river valleys along the Atlantic coast from Georgia to Connecticut are results of periods of lower sea level.

The climate of a place is the seasonal pattern of its inputs of solar energy, wind, and precipitation. With the exception of the principal islands of Hawaii, no place in the United States ever sees the sun directly overhead. 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 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; this is especially true of the western coast, where the ocean is cool and the wind is usually onshore. Finally, mountains are somewhat cooler by day and much colder at night than surrounding lowlands. Effects of each of these factors can be seen in the accompanying table of temperatures.

The other significant part of atmospheric circulation is the jet stream, a shifting zone of fast winds blowing generally from west to east high above the ground. 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, although on any given day it may loop as far north as Alaska or as far south as Louisiana. The jet 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.

Rivers respond to the quantity and seasonal pattern of precipitation in a region. The streams of the eastern United States, principal among which are the Hudson, Delaware, Susquehanna, Potomac, and Savannah, receive rainfall in every month and therefore are reliable avenues for waterborne 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 snow months of winter. Some flow regulation and flood control have 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. Along the Pacific seaboard are a few coastal streams that have their seasonal peaks during the winter rainy season. In Alaska the drainage system is dominated by the Yukon, a river as long as the Rio Grande but considerably greater in volume.

The surface water sources of the nation also include the five Great Lakes, which occupy an interconnected set of glacially scoured basins and together serve as a major transportation artery. Glaciers also left tens of thousands of smaller lakes throughout the northeastern United States, the upper Midwest, and much of Alaska. Among the larger of these are Champlain, Winnipesaukee, and Cayuga in the northeast and Winnebago, Red, and Mille Lacs in 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.

During the Ice Age, glaciers scoured material from the already thin and poor soils of the north and moved a substantial amount of earth southward. Illinois, Indiana, Iowa, and Ohio were the principal beneficiaries; many parts of these states had more than 30 m (100 ft) of new earth laid down as gently undulating till plain. Parts of New England, New York, Michigan, Wisconsin, and Minnesota also gained material, but often of a more coarse and rocky nature. Meltwater floods filled many valleys in the Midwest and West with rich sediment, and upland soils were often enriched by fine windblown dust.

The United States can thus be divided into five major soil regions that are a result of these continuing formative forces: (1) the deep, black mollisols of the midcontinent (the country's most fertile), in an area stretching from Illinois west to Nebraska and the Dakotas; (2) the leached red ultisols of the southeastern and western coast mountains, which are considerably less fertile; (3) the coarse and acid spodosols in the northeastern United States and in the high western mountains; (4) the moderately fertile gray 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 geologic 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. Younger soils, such as entisols and inceptisols, occur on shifting sand dunes, recent volcanoes, river floodplains, and steep slopes, where disturbances such as floods or landslides prevent the development of mature soil profiles. These immature soils range from barren rock outcrops in Arizona and grassy sandhills in Nebraska to fertile volcanic ash soils in Hawaii and the extraordinarily productive floodplain of the Mississippi River.

Still farther south, the frost-free season exceeds six months, rainfall becomes more reliable, and the forest reaches its maximum diversity. The Great Smoky Mountains National Park in western North Carolina and eastern Tennessee contains more tree species than the entire continent of 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 favor 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 armor the shore against the eroding forces of wind and water.

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, outwardly similar to Arctic vegetation, although the two kinds of tundra have different patterns of solar radiation, day length, and diurnal (day-to-night) temperature changes. 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, piУon, 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 found ponderosa and piУon pines.

The hot, dry summers and mild, moist winters of coastal southern California produce a distinctive shrub vegetation known as chaparral. The plants here have adapted to rainy winters, dry summers, autumn fires, and thin soils. Farther north on the western slopes of the coastal hills and Sierra Nevada, the winter rainy season is longer; forests of giant sequoia and redwood occupy this favored location, where there is enough rain to permit rapid growth but a long enough dry season to discourage competition from numerous species. Still farther north, in western Oregon and Washington, a true rain forest appears as the dry periods shrink to less than a few weeks in midsummer. This luxuriant forest consists primarily of a great variety of needle-leaf trees: Douglas firs, true firs, hemlocks, cedars, spruces, and pines, each occupying its own preferred elevation zone here, and together constituting the second richest forest resource for the nation. The coastal forests of Alaska have fewer species than the rich rain forest to the south but a faster growth than the taiga to the north.

The natural vegetation of Hawaii is conditioned by its isolation, which has served to limit the number of species, and by the interplay of its mountains and the moist trade winds. Forests dominated by guava trees on the windward (northeast) coasts of the islands grade upward to a rich but swampy rain forest at moderate elevations, where the annual rainfall may exceed about 10,000 mm (about 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 (southwest) coasts are virtual deserts, with spiny koa and kiawe shrubs growing on the slightly wetter slopes.

Animals depend, either directly or indirectly, on plants for their survival. Each major vegetation region thus has its own characteristic kinds of animal life. 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 elk spend summers on the tundra but migrate into the conifer forest for winter. Many birds migrate even farther, going from the polar regions to the Tropics each winter. The hardwood forests of the eastern United States contain moose, black bears, deer, foxes, raccoons, skunks, squirrels, and a diversity of small birds. Along the Gulf of Mexico coast live larger and more colorful birds such as pelicans, flamingos, and green kingfishers, as well as alligators and warmwater fish such as catfish. Several varieties of venomous snakes are also found here.