The major characteristic of the United States is probably its great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rain forest 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 slaves 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.

Land

The two great sets of elements that mold 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.

The Interior Lowlands and their upland fringes

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 upwarps, 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 consists 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 subbituminous 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 major characteristic of the United States is probably its great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rain forest 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 slaves 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.

Land

The two great sets of elements that mold 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.

The Interior Lowlands and their upland fringes

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 upwarps, 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 consists 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 subbituminous 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 major characteristic of the United States is probably its great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rain forest 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 slaves 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.

Land

The two great sets of elements that mold 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.

The Interior Lowlands and their upland fringes

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 upwarps, 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 consists 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 subbituminous 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 major characteristic of the United States is probably its great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rain forest 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 slaves 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.

Land

The two great sets of elements that mold 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.

The Interior Lowlands and their upland fringes

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 upwarps, 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 consists 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 subbituminous 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 major characteristic of the United States is probably its great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rain forest 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 slaves 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.

Land

The two great sets of elements that mold 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.

The Interior Lowlands and their upland fringes

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 upwarps, 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 consists 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 subbituminous 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 major characteristic of the United States is probably its great variety. Its physical environment ranges from the Arctic to the subtropical, from the moist rain forest 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 slaves 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.

Land

The two great sets of elements that mold 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.

The Interior Lowlands and their upland fringes

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 upwarps, 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 consists 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 subbituminous 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.