The dominant topographic features of the United States tend to extend north-south across the country (Map 2). The interior of the country is a vast, sprawling lowland that stretches from the Gulf of Mexico to the Canadian border and then on to Alaska. Geographers with an interest in landform development place this expanse of flat land and gently rolling hills in three different physiographic regions - the Atlantic and Gulf coastal plains, the interior lowland (which some split into the Great Plains and the interior plains), and the Canadian Shield.

The Atlantic and Gulf coastal plains reach north along the east coast of the United States as far as the southern margins of New England. Underlying this area are beds of young, soft, easily eroded rock deposited in recent geologic time as shallow seas lapped back and forth across the land. These low plains extend well out under the ocean surface to form a continental shelf, which in places extends as much as 400 kilometers beyond the shore.

Northward, the interior lowland, although noticeably hillier than the coastal plains, has almost no rough terrain. This region is like a saucer, turned up at the edges and covered with a deep series of sedimentary rocks. These sedimentary beds are generally quite flat; most topographic variation is the result of local erosion or, in the North, of glacial debris deposited during the Ice Age.

The geologic structure of the Great Plains differs little from that of the interior plains. The sedimentary beds dominate, although in the north they are broken by some eroded domes, most notably the Black Hills of western South Dakota. While nearly horizontal, the sedimentary beds do dip gently toward the west to a trough at the foot of the Rocky Mountains, where the Colorado cities of Denver and Colorado Springs are located.

The boundary between the Great Plains and the interior plains is marked by a series of low escarpments that indicate the eastern edge of the mantle of loose sediments, eroded from the Rocky Mountains, that covers the plains.

The character of this massive interior lowland area has had a number of important influences on the economic and settlement history of the United States. In addition to the vast agricultural potential it provides, fully half the country can be crossed without encountering significant topographic barriers to movement. This facilitated the integration of both this region and the distant West into the economic fabric of the country. Nearly all of the interior lowland is drained by the Mississippi River or its tributaries. This drainage pattern assisted regional integration by providing a transport and economic focus for the land west of the Appalachian Mountains.

North and northeast of the central lowland is the Canadian Shield, where old, hard crystalline rocks lie at the surface. Farther south in the lowlands, similar rocks are covered by the sedimentary beds deposited under the sea that once filled the midsection of the country. Erosion has worn down the surface of the Shield into a lowland of small local relief.

The Shield, more than any other North American physiographic region, has had its landforms remolded and shaped by massive continental glaciers during the last million years. These glaciers covered most of Canada east of the Rocky Mountains and the Coast Ranges, and they reached southward to approximately the present valleys of the Missouri and Ohio Rivers.

The ice could pluck rocks weighing many tons off the surface and carry them great distances: Massive boulders are strewn across the landscape of the Shield, resting where they were dropped by the glaciers. Ice melt along the peripheries of the glaciers created major rivers and cut broad new pathways to the sea.

Glaciation scoured much of the Shield's surface. Today, the soil cover of the region remains thin or nonexistent. The heavily disrupted drainage pattern dammed many streams with debris and led others into the area's labyrinth of lakes and swamps rather than to the sea. Central and northern Minnesota, for example, called the "Land of 10,000 Lakes," is part of the southern lobe of the glaciated shield that extends into the states of Minnesota, Michigan, and Wisconsin.

Southward, where the ice was not as thick and its force correspondingly less, the glaciers were diverted or channeled by higher elevations. For example, the ice was blocked in central New York by the highlands south of the Mohawk River. However, narrow probes did push up the valleys of streams tributary to the Mohawk, gradually broadening and deepening them. Today, the deep, narrow Finger Lakes of New York State fill these glacially enlarged valleys and form one of America's truly beautiful landscapes.

All along and beyond the southern edges of the glaciers, deposition replaced erosion as the prime result of glaciation. Large areas of the interior lowland are covered by a mantle of glacial till (rocks and soil dropped by the glaciers), which covers the land to depths varying from a meter or less to more than 100 meters. Where the glaciers remained stationary for long periods of time, higher hills, called moraines, were created. In the east, Staten Island, Long Island, Martha's Vineyard, Nantucket, and Cape Cod are end moraines that mark the farthest major extension of glaciers toward the southeast. The landscape south of the Great Lakes is laced with long, low, semicircular moraine ridges and other glacial deposits.

One section of the interior lowland escaped glaciation. The southwestern quarter of Wisconsin and the adjoining 400-kilometer stretch of the Mississippi River valley were apparently spared by the barrier effect on the flowing ice of the Superior upland to the north and by the channeling of the ice by the deep valleys of Lakes Michigan and Superior. The result is the "driftless area" (drift is another name for till), a local landscape that is more angular, with fragile rock formations like natural bridges and arches.

As the ice retreated, massive lakes were created along the glacial margins. On the northern Great Plains, two huge lakes, Agassiz and Regina, together covered an area larger than today's Great Lakes. With continued glacial retreat, these lakes mostly disappeared. Their existence is now marked by the former lake bed, a flat area covering parts of North Dakota and Minnesota.

Sea level was significantly lower during periods of widespread glaciation. This lowered the base level of many rivers and thus fostered increased erosion by those streams. Furthermore, many of these stream valleys extended well into what is today the ocean. Along with many others, the Susquehanna and Hudson Rivers cut much deeper valleys during this period. As the ice retreated and sea level rose, the ocean filled these deepened valleys. Two of the world's finest harbor areas were formed in this way: New York Bay, with the deep Hudson River and the protective barriers formed by Staten Island and Long Island; and Chesapeake Bay, the drowned valley of the Susquehanna River and some of its major former tributaries, such as the Potomac and James Rivers.

In the East, the coastal plains are gradually squeezed against the coast northward along the ocean by the Appalachian Highlands until the lowland disappears entirely at Cape Cod. From there northeastward, the coastal landscape is a part of the northern extension of the Appalachian Mountain system. The Appalachians - eroded remnants of what were once much higher mountain ranges - separate the seaboard from the interior lowlands along much of the eastern United States.

Soils in most parts of this region are shallow, and the steep slopes, difficult to farm under any circumstances, are totally unsuited to modern agricultural practices that emphasize mechanization. Large-scale urban or industrial growth is cramped by the small, local lowlands. Early settlers found the Appalachians from the Mohawk River in New York southward to northern Alabama to be a surprisingly effective barrier to western movement; there are few breaks in the mountains' continuity.

The western United States is a land of mountains and of sudden, great changes in elevation. The physiography, again, is arranged in a series of three large north-south trending bands, with the Rocky Mountains on the east separated from the mountains and valleys of the Pacific coastlands by a series of high, heavily dissected plateaus.

Starting in the east, the Rocky Mountains generally present a massive face to the Great Plains, with peaks occasionally rising 2 kilometers or more. Elsewhere, as in south-central Wyoming, the Rockies almost seem not to exist at all. In the northern Rockies in Idaho, the north-south linearity of most of the region's mountains is replaced by massive igneous domes irregularly eroded into a rugged, extensive series of mountain ranges that contain the largest remaining area of wilderness in the United States outside Alaska.

The high plateaus of the interior West are also varied in their origin and appearance. The southernmost subsection, the Colorado Plateau, is a series of thick beds of sedimentary rocks rising more than 1,000 meters above the lowlands' elevation and tilted upward toward the northeast. The plateau is a land of spectacular canyonlands, volcanic peaks, and sandy deserts.

Farther north, the Columbia-Snake Basin has been filled by repeated lava flows to a depth of more than 1,000 meters. Rivers, both past and present, have eroded into the rock. The resultant landscape is similar to that of the Colorado Plateau, although the stepped appearance resulting from the variable resistance to weathering of the eroded sedimentary rocks of the Colorado Plateau is missing. Volcanic cones also dot portions of the region, especially across south-central Oregon and in the Snake River Valley in Idaho.

The plateaus gradually widen northward, encompassing the valley of the Yukon River in Alaska. In comparison, much of central Alaska is a broad, flat lowland that is poorly drained.

In the conterminous United States (excluding Alaska and Hawaii), the Pacific Coast seems to consist largely of two north-south trending mountain chains separated by a discontinuous lowland. In southern California, the Coast Range is fairly massive, with peaks reaching 3,000 meters. From there almost to the Oregon border, the mountains are low and linear, seldom rising above 1,000 meters. This also is the major fault zone of the state and a region of frequent earthquake activity. Along the California-Oregon border, the Klamath Mountains are higher, more extensive, and much more rugged and irregular. Except for the Olympic Mountains in northwestern Washington, the Coast Ranges in the rest of Oregon and Washington State are low and hilly rather than mountainous.

The interior lowlands along the coast - the Central Valley of California, the Willamette Valley in Oregon, and the Puget Sound lowland in Washington - are the only extensive lowlands near the West Coast. Filled with relatively good soils, these lowlands have supported much of the Pacific Coast's agriculture.

East of the lowlands are the Sierra Nevada and the Cascade mountain ranges. The Sierra Nevada appears as though a massive section of earth was tilted upward relative to the areas to the east and west in what is called a fault block, with the highest, sharpest exposed face toward the east. Although the western approaches into the Sierra Nevada are reasonably gentle, on the eastern side the mountains rise in some places more than 3,000 meters. Volcanic activity was important in the formulation of the Cascades. Some of America's best known volcanic peaks, such as Mt. Rainier and Mt. St. Helens in Washington, are found there.

Climate is the aggregate of day-to-day weather conditions over a period of many years. It is the result of the interaction of many different elements, the most important of which are temperature and precipitation.

Climatic patterns are a result of the interaction of three geographic controls. The first is latitude. The earth is tilted on its axis with reference to the plane of its orbit around the sun. As it makes its annual revolution around the sun, first the Northern Hemisphere and then the Southern are exposed to the more direct rays of the sun. During the Northern Hemisphere's summer, higher latitude locations have longer days, with far northern points experiencing a period of continuous daylight. Daylight periods during the winter months are shorter at higher latitudes, whereas more southerly locations have both longer days and exposure to more direct rays of the sun.

The second control is based on the relationship between land and water. Land tends to heat and cool more rapidly than water. In a tendency called continentality, places far from large bodies of water experience greater seasonal extremes of temperature than do coastal communities. Parts of the northern Great Plains experience annual temperature ranges close to 65C; annual differences of as much as 100C (from 50C to -50C) have been recorded in some locations.

The converse effect occurs at maritime locations, especially on the western coast of continents in the mid-latitudes. These locations have smaller temperature ranges as a result of what is called a maritime influence. Summer and winter extremes are moderated by the movement onshore of prevailing westerly wind systems from the ocean. Horizontal and vertical ocean currents minimize seasonal variations in the surface temperature of the water. The moderated water temperature serves to curb temperature extremes in the air mass above the surface.

Proximity to large water bodies also tends to have a positive influence on precipitation levels, with coastal locations receiving generally higher amounts. The reason for this should be obvious; large water bodies provide greater levels of evaporation and thus increase the amount of moisture in the atmosphere. That, in turn, increases the possibility of precipitation. There are, however, notable exceptions to this rule, including the dry coast of southern California and the Arctic coastline of Alaska.

The third prime geographic influence on climate is topography. Most obvious is the relationship between elevation and temperature, with higher elevations cooler than lower elevations. The influence of topography can be broader, however, because of its effect on wind flow. If a major mountain chain lies astride a normal wind direction, the mountains force the air to rise and cool. As the air mass cools, the amount of moisture that it can hold is reduced. Precipitation results if the cooling causes the relative humidity to reach 100 percent. Moisture falls on the windward side, and the lee is dry. The wettest area in North America is along the Pacific coast from Oregon to southern Alaska, where moisture-laden winds strike mountains along the shore. Average annual precipitation is more than 200 centimeters throughout the area, and in some places exceeds 300 centimeters.

Mountains also can reduce the moderating effects of maritime conditions on temperature, as happens in the interior of the Pacific Northwest. The Western Cordillera (mountain mass) confines West Coast maritime climatic conditions to that coast. Some of the greatest variations in both precipitation and temperature to be found across a small distance anywhere in America exist between the west and east sides of parts of the Coast Ranges. The aridity of the central and northern interior West is due in large part to the barrier effect of the north-south-trending mountain ranges of the West.

East of the Rockies, the topographic effect on precipitation eventually disappears, partly because the eastern mountains are lower and thus pose less of a barrier to moving air, and partly because much of the weather of the interior is a result of conflict between two huge air masses that are unimpeded, one flowing northward from the Gulf of Mexico, and the other flowing southward out of Canada. The contact of these air masses creates what are often violent displays of weather in the region.

This illustrates a fourth major and complex influence on climate, the impact of air mass characteristics and wind systems. America's weather is affected markedly by the confrontation between polar continental air masses (usually cold, dry, and stable) and tropical maritime air masses (warm, moist, and unstable). The former push farthest south in winter, whereas the latter extend farthest north in summer. Most parts of America are subject to a generally westerly wind flow that tends to move weather systems eastward. The continental climate of the interior is thus pushed onto the East Coast.

The interaction of these climatic controls creates a pattern of climatic regionalization. In the East, the principal element in climatic variation is temperature; in the West, it is precipitation. In the East, the divisions between the climate regions are based largely on the length of the growing season - the period from the average date of the last frost in spring to the first frost in fall - and on the average summer maximum temperature or winter minimum temperature. In the West, average annual precipitation is the key, although moderated temperatures are an important aspect of the marine West Coast climate. In the East, the more northerly areas are generally drier; in the West, they are colder. In the East, the major influence on climatic variation is latitude; in the West, it is topography.

Botanists speak of something called climax vegetation, which is defined as the assemblage that would grow and reproduce indefinitely at a place given a stable climate and average conditions of soil and drainage. For most of the inhabited portions of America today, that concept has little meaning. The "natural" vegetation, if it ever existed, has been so substantially removed, rearranged, and replaced that it seldom is found now. In the Southeast, for example, the original mixed broadleaf and needleleaf forests were cut and replaced by the economically more important needleleaf forests. The grasses of the plains and prairies are mostly European imports. Their native American predecessors are gone either because they offered an inferior browse for farm animals or because they could not withstand the onslaught of modern humanity and its imported weeds. Most of what climax vegetation remains is in the West and North.

There are several ways of creating vegetation regions. Perhaps the simplest is to divide the United States into three broad categories - forest, grasslands, and scrublands. Forests once covered most of the East, the central and northern Pacific Coast, the higher elevations of the West, and a broad band across the interior North. Forests of the Pacific coast, the interior West, the North, and a narrow belt in the Deep South were all needleleaf and composed of many different trees. Much of the Ohio and lower Mississippi River Valleys and the middle Great Lakes region was covered by a deciduous broadleaf forest.

Grasslands covered much of the interior lowlands, including nearly all of the Great Plains from Texas and New Mexico to the Canadian border. This is an area of generally subhumid climate where precipitation amounts are not adequate to support tree growth. An eastward extension of the grasslands, the Prairie Wedge, reached across Illinois to the western edge of Indiana, where precipitation is clearly adequate to support tree growth.

Scrublands usually develop under dry conditions. They are concentrated in the lowlands of the interior West. Actual vegetation varies from the cacti of the Southwest to the dense, brushy chaparral of southern California and the mesquite of Texas.

The tundra of the far North is the result of a climate that is too cold and too dry for the growth of vegetation other than grasses, lichens, and mosses. Tundra exists in small areas far southward into the United States, where climatic conditions at high elevations are inhospitable to tree growth. Northward, the altitudinal tree line is found at lower elevations until, eventually, the latitudinal tree line is reached.

The soil of a place owes its characteristics to such things as the parent rock material, climate, topography, and decaying plants and animals. Hundreds of different types of soil result from the interaction of these elements. Any particular soil is unique because of its mix of properties (such as color and texture) and composition (including organic content and the action of soil colloids).

"Colloids" are small soil particles. Their properties and influences on soil are complex and often important. Soil acidity (or alkalinity), for example, is a result of the alteration and integration of soil colloids. Acid soils are characteristic of cold, moist climates; alkaline soils typically are found in dry areas. Most soils of the major agricultural zones of the eastern United States are moderately to strongly acidic. Lime must be added periodically to neutralize that acidity before these soils can be used to produce most row crops.

Color is perhaps the most obvious soil property. A dark color usually indicates an abundance of organic materials, and red, the presence of iron compounds. Generally, however, color is a result of the soil-forming processes. For example, the pale-gray soil of the northern needleleaf forest results from the leaching of organic matter and minerals from the soil's surface layer.

Soil texture, which determines a soil's ability to retain and transmit water, refers to the proportion of particles of different size in the soil. Sand is the coarsest measure of soil texture, silt is intermediate, and clay is the finest. Soils called "loams" contain substantial proportions of each of the three particle grades and are considered best. They are fine enough to hold moisture yet are not so fine that they cannot easily take up water.

The U.S. Department of Agriculture has developed a soil classification system that indicates the most important soil types for an area of the country. "Aridisols," found mostly in the Southwest, gain their name from arid. These soils of dry climates are low in organic content and have little agricultural value. "Spodosols" generally develop in cool, moist climates, although they are found in northern Florida. They are quite acidic and low in nutrients, and are of agricultural value only for acid-loving crops. "Tundra soils," which also have little agricultural value, are associated with a cold, moist climate such as Alaska. The soil is shallow, frequently water saturated, and with a subsurface of perennially frozen ground. "Highland soils," found in West Virginia, Utah, and Alaska, are little developed and agriculturally worthless.

"Mollisols" are grassland soils of the semiarid and subhumid climates of the Central, North Central, and Pacific Northwest sections of the country. They are thick dark brown to black and have a loose texture and high-nutrient content. They are among the most naturally fertile soils in the world and produce most of America's cereals.

"Alfisols" are second only to mollisols in agricultural value. They are soils of the mid-latitude forest and the forest-grassland boundaries. They are very much "middle" soils in a climatic sense. They are located in areas moist enough to allow for the accumulation of clay particles but not so moist as to create a heavily leached or weathered soil.

Alfisols are divided into three categories, each with its own characteristic climatic association. "Udalfs" are soils of the deciduous forests of the Middle West. Somewhat acidic, they are nevertheless highly productive when lime is used to reduce the acidity. "Ustalfs," found in warmer areas with a strong seasonal variation in precipitation, are most common in Texas and Oklahoma. They are highly productive if irrigated. "Xeralfs" are soils of cool, moist winters and hot, dry summers. Found in central and southern California, they too are highly productive.

"Ultisols" represent the ultimate stage of weathering and soil formation in the United States. They develop in areas with abundant precipitation and a long frost-free period, such as the South. Particle size is small, and much of the soluble material and clay has been carried downward. These soils can be productive, but high acidity, leaching, and erosion are often problems.

"Entisols" are recent soils, too young to show the modifying effects of their surroundings. They are widely scattered and of many types, from the Sand Hills of Nebraska to the alluvial floodplains of the Mississippi River Valley. The agricultural potential of entisols varies, but the alluvial floodplain soils, drawn from the rich upper layers of upstream soils, are among America's most productive.

There is a distinct association between the location of minerals that meet the needs of heavy manufacturing and the land's subsurface rock structure. Each of the three major forms of rock - sedimentary, metamorphic, and igneous - is capable of containing a type of mineral economically useful to humans. Sedimentary and metamorphic rocks are the most prevalent rock substructure and are more likely to contain minerals of broad utility than are igneous rocks.

Sedimentary rock is the result of the gradual settling of small solid particles in stationary water. For example, if a shallow sea were located adjacent to an arid landscape subject to occasional rainstorms, sand particles would be washed into the sea and spread across its bottom by water currents and the force of gravity. As this process continued, each layer of sand would press down on the layers beneath it, squeezing and solidifying the sandy mass that had been deposited only a few thousand years before. When this seabed was raised and folded into mountains by shifts in the earth's crust, the method by which at least some of the rocks were formed was betrayed by the presence of layers of sandstone.

About 300 million years ago during what earth historians call the Carboniferous period of the Paleozoic Era, conditions present in most existing land areas were such that unusual sedimentary sequences were created. Heavily vegetated and thick, swampy regions were drowned and covered with another layer of sediment. In some cases, the organic matter came to be represented in liquid form, trapped between folds of impermeable rock and eventually drawn off as petroleum. Most of these petroleum deposits are found in conjunction with another by-product of the period - natural gas. In other cases, the organic matter became solid layers of coal that were sometimes only centimeters thick but occasionally found dozens of meters thick.

In North America, vast regions are underlaid by sediments formed during the Carboniferous period. These areas where coal, oil, or natural gas might be found are located in the interior and Great Plains, sections of the Gulf coastal plain, portions of the Pacific mountains and valleys, the Arctic rimland, and in folded and broken form along the western margins of the Appalachian Highlands and into the eastern Rockies.

Large deposits of mineral fuels have been identified across extensive portions of these sedimentary lowlands. The most important coal deposits in America have been mined in the more rugged Appalachian field. Mines throughout this nearly continuous field in eastern Kentucky, West Virginia, and western Pennsylvania were the earliest to be brought into production, and they continue to supply over half of America's coal needs.

Until recently, much of the remaining coal mined in the United States has been obtained from the Eastern Interior Field, which underlies most of Illinois and extends into western Indiana and western Kentucky. Although some of the Eastern Interior Field's coal has been used in iron and steel production, its higher sulfur content has restricted most use to heating and electric-power generation.

The Western Interior Field is also large, located under Iowa and Missouri with a narrowing extension southward into eastern Oklahoma. The coal found in this field is of slightly poorer quality than that found in the eastern fields and has only recently begun to be mined.

There are many small and a few large bituminous deposits scattered through and along the eastern margins of the Rocky Mountains. Extensive deposits in Wyoming and Montana have come into production in the last two decades. There are also several extensive fields of lignite (brown coal) in the northern Great Plains.

Scattered deposits of petroleum and natural gas are found throughout the Appalachian coal field. Southern Illinois and south-central Michigan produce some petroleum, as do scattered sites across the northern Great Plains and the northern Rockies.

Easily the most important petroleum fields, however, have been those in the southern plains, along the Gulf coast, and in southern California. One great arc of producing wells is located along the full length of the Texas and Louisiana coasts. Another slightly broken arc extends from central Kansas south through Oklahoma and westward across central Texas to New Mexico. Between and beyond these two large areas lie two more fields of great importance, the East Texas field and the Panhandle field in northwest Texas. Separate from these fields but also of major importance are those located in southern California. In the mid-1960s, exploitation of deposits of petroleum and natural gas was begun along the north Alaska slope.

Metamorphic rock is formed in a quite different manner than sedimentary rock. Under the tremendous pressure exerted through the gradual deformation of the earth's crust, the internal structure of previously formed rocks can be metamorphosed, or changed. So great is the pressure exerted over thousands of years and so great is the heat generated that the very molecular structure of the rock is altered. This transformation indicates why metallic minerals in economically extractable quantities are located most often in areas of metamorphic rock.

Many of the mining sites for early exploitation of the metallic minerals were located near the margins of the Canadian Shield. The pattern of mineral production follows a long arc extending from the North Atlantic and St. Lawrence River estuary across the Great Lakes and northward through Canada to the Arctic Ocean. The arc continues on both sides of Lake Superior: in northern Michigan, Wisconsin, and Minnesota with copper and iron.

A second zone of metamorphic rock is located along the eastern Appalachian Mountains. Copper and iron were important minerals found locally by New England colonists.

A third and extensive region of metallic minerals is formed by the western mountains. Scatter deposits of gold and silver, a few of them rich, drew prospectors and mining companies to isolated locations from south of the Mexican border to central Alaska. Of great industrial importance are the large deposits of copper, zinc, lead, molybdenum, and uranium found in this western region, as well as smaller deposits of tungsten, chromite, manganese, and other minerals.

It should not be assumed that America's industrial requirements are met fully by the tremendous variety of minerals found in these three zones of metamorphic rock. A few minerals needed by modern industry (for example, tin, manganese, and high-grade bauxite for aluminum) have not been located in America in sufficient quantities to satisfy domestic needs. In addition, the growth of industrial capacity has been matched by a growth in demand for many minerals. Nevertheless, few countries have equaled or even approached the original quantity and diversity of metallic minerals and mineral fuels located in the United States.

This abundance of minerals has been critical in assisting the development of the immense American manufacturing-industrial complex.