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A
The East and the Gulf Coast
The Atlantic Gulf Coastal Plain extends along the east and southeast coasts of the United States from Long Island to the Rio Grande; Cape Cod and the islands off Massachusetts are also part of this region. Although narrow in the north, the Atlantic Coastal Plain widens in the south, merging with the Guff Coastal Plain in Florida. The Atlantic and Gulf coasts are essentially coastlines of submergence, with numerous estuaries, islands and barrier beaches backed by lagoons. The northeast coast has many fine natural harbors, such as those of New York Bay and Chesapeake Bay. A principal feature of the lagoon-lined Gulf Coast is the great delta of the Mississippi River.
The Atlantic Coastal Plain rises in the west to the rolling Piedmont ( the falls along which were an early source of waterpower ), a hilly transitional zone leading to the Appalachian Mountains. These ancient mountains, a once towering system now worn low by erosion, extend southwest from Canada to the Gulf Coastal Plain in Alabama. In New England, the Appalachians extend in a few places to the Atlantic Ocean, contributing to a rocky, irregular coastline. The Appalachians and the Adirondack Mountains of New York ( which are geologically related to the Canadian Shield) include all the chief highlands of E United States; Mt. Mitchell (6,684ft/ 2,037m high), in the Black Mts. of North Carolina, is the highest point of North America.
B
The Plains and Highlands of the Interior
Extending more than 1,000 mi (1,610 km) from the Appalachians to the Rocky Mts. and lying between Canada (into which they extend) in the north and the Gulf Coastal Plain in the south are the undulating Interior Plains. Once covered by a great inland sea, the Interior Plains are underlain by sedimentary rock. Almost all of the region is drained by one of the world’s greatest river systems-the Mississippi-Missouri. The Interior Plains may be divided into two sections: the fertile central lowlands, the agricultural heartland of the United States; and the Great Plains, a treeless plateau that gently rises from the central lowlands to the foothills of the Rocky Mts. The Black Hills of South Dakota form the region’s only upland area.
C
The Western Mountains and Great Basin
West of the Great Plains are the lofty Rocky Mountains. This geologically young and complex system extends into United States from Canada and runs into New Mexico. There are numerous high peaks in the Rockies; the highest is Mt. Elbert (14,433ft/4,399m). The Rocky Mts. are divided into four sections-the Northern Rockies, the Middle Rockies, the Wyoming (Great Divide) Basin, and the Southern Rockies. Along the crest of the Rockies is the Continental Divide, separating Atlantic-bound drainage from that heading for the Pacific Ocean.
Between the Rocky Mrs. and the ranges to the west is the Intermontane Region, an add expanse of plateaus, basins, and ranges. The Columbia Plateau, in the north of the region, was formed by volcanic lava and is drained by the Columbia River and its tributary the Snake River, both of which have cut deep canyons into the plateau. The enormous Colorado Plateau, an area of sedimentary rock, is drained by the Colorado River and its tributaries; there the Colorado River has entrenched itself to form the Grand Canyon, one of the world’s most impressive scenic wonders. West of the plateaus is the Basin and Range province, an area of extensive semidesert.
The lowest point in North America, in Death Valley (282ft/86m below sea level), is there. The largest basin in the region is the Great Basin, an area of interior drainage (the Humboldt River is the largest stream) and of numerous salt lakes, including the Great Salt Lake. Between the Intermontane Region and the Pacific Ocean is the Pacific Mountain System, a series of ranges generally paralleling the coast, formed by faulting and volcanism. The Cascade Range, with its numerous volcanic peaks extends from Canada into California, and from there is continued south by the Sierra Nevada, a great fault block. Mt. Whitney (14,495ft/4,418m), in the Sierra Nevada, is the highest peak in the conterminous United States.
D
The Pacific Coast, Alaska, and Hawaii
West of the Cascades and the Sierra Nevada and separated from them by a structural trough are the Coast Ranges, which extend along the length of the U.S. Pacific coast. The Central Valley in California, the Willamette Valley in Oregon, and the Puget Sound lowlands in Washington are part of the trough. The San Andreas Fault, a fracture in the earth’s crust, parallels the trend of the Coast Ranges from San Francisco Bay to Mexico; earthquakes are common along its entire length. The Pacific Coastal Plain is narrow, and in many cases the mountains plunge directly into the sea. A coastline of emergence, it has few islands, except for the Channel Islands (see Santa Barbara Islands) and those in Puget Sound; there are few good harbors besides Puget Sound, San Francisco Bay, and San Diego Bay.
Alaska may be divided into four physiographic regions; they are, from north to south, the Arctic Lowlands, the coastal plain of the Arctic Ocean; the Rocky Mountain System, of which the Brooks Range is the northernmost section; the Central Basins and Highlands Region, which is dominated by the Yukon River basin; and the Pacific Mountain System, which parallels Alaska’s southern coast and which rises to Mt. McKinley (Denali; 20,320ft/6,194m), be highest peak of North America. The islands of Alaska and those of the Aleutian Islands chain are partially submerged portions of the Pacific Mountain System and are frequently subjected to volcanic activity and earthquakes. These islands, like those of Hawaii, are the tops of volcanoes that rise from the floor of the Pacific Ocean. Manna Kea and Mauna Loa on Hawaii are active volcanoes; the other Hawaiian islands are extinct volcanoes.
·is narrow in the north and wide in the south (74)
·has one of the world’s most impressive scenic wonders (75)
·has large basins with interior drainage and salt lakes (76)
·was once a towering system but now worn by erosion (77)
·was once covered by a great inland sea (78)
·is characterized by a structural trough (79)
·was formed by volcanic lava (80)
·is lined by lagoons with beautiful harbors (81)
·has the frequent occurrence of earthquake (82)
·is drained by one of the world’s greatest river systems (83)

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