United States

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. Downwarps 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.

The Western Cordillera

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 upwarps 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.

Grand Teton National Park: Teton RangeThe Teton Range at sunset, with Jenny Lake in the foreground, Grand Teton National Park, northwestern Wyoming, U.S.Robert Glusic/Getty Images