Physical Geography of the US

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Chapter 2 - PHYSICAL GEOGRAPHY OF THE US

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As indicated in chapter 1, physical geography is divided into three basic areas of emphasis: geomorphology (landforms and physiography), climatology (climates and weather patterns), and biogeography (flora and fauna). Geomorphology is the branch of geography (and geology) that studies and explains how the geologic surface of the earth came to be the way that it is today. Much of this chapter focuses on the overall geomorphology of the US. Climate refers to the annual average pattern of weather in a region. This is discussed overall for the US in this chapter, though not as much as geomorphology. Biogeography, or the geographic distribution of plants and animals, is closely related to climatic patterns and is covered only minimally in this chapter.

When most people speak of topography, they are actually referring to physiography. Topography refers to all of the surface features of an area. This includes the surface elevation, the vegetation, roads and buildings, and other features commonly found on topographic maps. Physiography is only the geologic surface or terrain features of an area. These are also known as landforms. Among the more common landforms are mountains, hills, valleys, cliffs, and plains. In addition, there are a wide variety of more specialized landforms that you will be introduced to in this book. The study of landforms is known as geomorphology. Geomorphology is studied by both geographers and geologists. It is that portion of geology that is of most interest to geographers because it helps to explain the physical environment which provides both the settings and the opportunities for human settlement.

The surface physiography of a region is shaped by its underlying rock structure. The three main types of rock are sedimentary, igneous, and metamorphic. Sedimentary rock consists of successive layers of wind or waterborne materials. Under pressure, these deposits turn into rock such as sandstone, shale, limestone and coal. Sedimentary rocks are the most common type found in the US.

Igneous rock originates deep within the surface of the earth. High temperatures in the center of the earth cause rock to melt. Molten magma "plutons" push their way up through the earth's crust. If the pluton solidifies before it reaches the earth's surface, it is known as an "intrusive igneous formation." A batholith is a massive intrusive feature. Batholiths exposed through the erosion of upper rock layers, such as California's Sierra Nevada Range, are usually composed of granite. Plutons that reach the surface of the earth extrude as lava.

Metamorphic rocks were once either sedimentary or igneous but have been changed through intense heat and pressure. This often happens where there is folding and crumpling of the earth's crust. Marble is a metamorphosed form of sedimentary limestone, while diamonds are a metamorphosed form of sedimentary coal.

Although the planet earth seems to be a solid, it is actually much more of a liquid. The very core of the earth is believed to be comprised of solid iron, measuring some 780 miles from the center to its edge. The next 3,000 miles out is all liquid. The first 1,380 miles is liquid iron and nickel, while the remaining 1,550 to 1,750 miles (it varies in different locations) is a mixture of molten and solid rocks known as the mantel. The outermost 5 to 40 miles is the solid crust which we stand on and feel so secure about. In fact, the molten iron ore and rock beneath us is in constant motion, churning upwards in the same way the bubbles boil from the bottom of a boiling pot of water. This churning motion causes the hard crust to crack and shake (earthquakes). The mantel material pushes its way to the surface through volcanoes and undersea oceanic rift zones. The lightest material in the earth's crust has gradually come together to form the continents, which literally float on top of the heavier crustal material. The churning molten rock below pushes the continents around in a process known as the theory of continental drift. In addition to the continents, large blocks of heavier ocean floor are also being pushed about the surface of the earth. The study of this process is known as "plate tectonics." Some 20 ocean and continent plates have been identified on the surface of the earth today.

It is believed that the earth solidified as a planet approximately 4.7 billion years ago. For the first 1.5 billion years of its existence, there was no life on the planet. Rocks from the period, some 4 billion years old, have been found at the surface of the Canadian Shield and Greenland. Most of the rest of the surface of North America is much younger.

The first plant life, algae, probably came into existence about 3.2 billion years ago. Two billion years of plant development passed before the first waterborne animal life came into existence (1.2 billion years ago). Trilobites dominated the seas 600 million years ago, and the first fish came into existence 500 million years ago.

Five hundred million years ago, the North American continent was an island, largely underwater, and not connected to any of the other continents that drifted across the surface of the earth. About 440 million years ago the first surface life appeared in the form of plants. Fish began to crawl from the sea 400 million years ago, leading to the development of the first amphibians.

About 360 million years ago, all of the continental land masses came to form a single land mass, which we call Pangaea (meaning "all lands"). Most of North America was still underwater, although the compression of the continents colliding with one another caused the Appalachian Mountains to rise and a great swamp to form across the midsection of the continent. A few highland areas existed in areas of Texas, Nevada, and the Great Lakes. The first reptiles came into existence during this time, while great layers of organic matter in the swamp collected to later become coal. (This is known as the Carboniferous Period.) After some 75 million years of evolution, the reptiles became what we now call dinosaurs and dominated animal life over the entire planet. The trilobites finally became extinct at this same time (225 million years ago).

Geologic evidence indicates that starting about 200 million years ago, the continent of North America changed course and began drifting away from Pangaea. The Appalachian Mountains split, with a portion now on North America and the rest in what is today Scotland and the Scandinavian Peninsula in Europe. The mountains also stopped growing at this time and have been eroding ever since. The first birds came into existence some 160 million years ago. Most of the dinosaurs suddenly died out 70 million years ago. It was at this time that the Rocky Mountains began to be pushed up in the western portion of North America. The cause was probably the rapid speed at which the continent was moving westward.

Current evidence indicates that the first primates were also coming into existence about 65 million years ago, as mammals replaced the dinosaurs as the dominant species on all the continents, except Australia. Elephants appeared 45 million years ago, with the mammoth coming to populate North America. After over 60 million years of evolution, the primates eventually evolved into humans--a mere 2 million years ago (in the form of the Homo habilis species). Modern humans (the Homo sapiens species) evolved 40,000 years ago, at about the same time as what may have been the earliest crossing of humans into North America from Asia. (Most scholars accept that humans crossed into North America 12,000 years ago--evidence of crossing prior to that is somewhat controversial.) The earliest recorded history of any civilization dates back 5,000 years. Our modern calendar dates back 2,000 years. The modern period of European settlement in North America is only 500 years old. If the 110-story Sears Tower* in Chicago were to represent the entire history of the planet earth, the top 12 inches would be that in part which the various human species have been present here.

There are two types of glaciation: continental and alpine. Alpine glaciers are still seen in the high mountains of the western US. Continental glaciers no longer exist but were significant in shaping the landscape of the northern Interior Lowlands. The Pleistocene Epoch (a.k.a. the Great Ice Age), started about 2 million years ago and has been characterized by several periods in which glaciers advanced from the north to blanket large portions of North America and Eurasia. Up to a mile (5,280 feet) thick, these glaciers scoured the land and flattened the hills which they covered. The last of these glacial periods ended about 12,000 years ago. The glaciers have left behind numerous small lakes (kettles) and erratically meandering (or lost) rivers. The Ohio and Missouri Rivers roughly mark the southern boundary of the glacial advances.

The scoured material carried by the glaciers were deposited in a variety of ways. This material is known as "glacial till." The largest depositional features are moraines which define the edges of a glacier. Long Island (NY) and Nantucket and Martha's Vineyard islands (Mass.) are terminal or end moraines that marked the end of massive glaciers. They are both composed of material carried by glaciers from the interior of the continent.

Because large amounts of the earth's surface water was stored in ice during the glacial advances, sea levels were about 450 feet below their current levels. Much of the North American continental shelf was exposed and a land bridge existed to Asia. Because rivers seek to erode until they reach sea level, the lower sea levels allowed rivers to cut deeper valleys, especially in coastal areas. When sea levels rose, these valleys were filled in, creating Chesapeake Bay (Susquehanna River), Delaware Bay (Delaware River), and New York Harbor (Hudson River). Bays that are created by the filling in of river valleys are called "estuaries."

Weather refers to the day-to-day changes in the atmosphere of a place. Climate, on the other hand, refers to the annual pattern of atmospheric conditions in a place. Climate classifications are primarily based on (1) the seasonal variations in temperature and (2) the annual rainfall amount and seasonality. The major factors which influence climate are (1) latitude, (2) continentality, (3) air masses, and (4) surface physiography.

Because the earth moves around the sun on a tilted axis, the northern hemisphere is closest to the sun in June and farthest from the sun in December. These extremes become most pronounced as one gets closer to the poles. North of the Arctic Circle, Alaska's summer days have 24 hours of sun, while winter days have 24 hours of cold darkness. At the other extreme, the length of Florida's days stays about the same all year round, and average winter temperatures are only 10 degrees F less than average summer temperatures. Higher latitudes (closer to the poles), therefore, have greater annual temperature extremes and especially cold winters.

Land (rocks and dirt) heats and cools faster than does water. Climatically, this means that the large continents of the world are warmer than the oceans in their summer months and colder than the oceans in their winter months. This further contributes to the intense winter cold experienced by the interior of the North American continent. Conversely, summer temperatures are as much as 50 degrees F hotter in the interior of the continent than in coastal areas. Along the edges of the continent, the moderating influence of the oceans keeps summer temperatures down and winter temperatures relatively high. The onshore ocean winds hitting San Francisco, for example, give it almost the same temperature all year round. The maritime climate of San Francisco is the opposite of the continentality of the interior US.

Large movements of air over the surface of the earth generally have either rising or falling patterns. Air that is rising from the earth's surface forms low pressure air masses. When the rising air falls, it forms a high pressure air mass (because the air is pushing down on the earth's surface). Rings of predominantly high and low pressure encircle the earth. Low pressure on the equator brings year-round rainfall, creating tropical rain forests. Descending dry air creates high pressure north and south of the equator. These are where the large deserts of the world (including the Mojave and Sonoran Deserts of the US) are located. Humid low pressure systems ring the globe north of here, creating cool weather rain forests on the coasts of Washington and British Columbia. Finally, a dry high pressure system is situated over the North and South Poles, turning the north coast of Alaska into a frozen desert.

The rings of high and low pressure encircling the globe create air masses that shape the weather and climate of the US in different ways and seasons. The eastern half of the US is primarily influenced by interactions between cold and dry continental air coming down from the high pressure North Pole and warm, humid air rising from the equator. The warm tropical air bring the most of the region's rainfall in the summer months. It spins around the 'Bermuda High' pressure system, which circulates in a clockwise direction in the Atlantic Ocean. The cold polar air keeps the winter months frigid and comparatively drier. Winter precipitation occurs when humid, low pressure systems move in from the Pacific Coast to intermix with the polar air.

The Rockies and prevailing west-to-east winds keep most of the tropical and polar air masses in the eastern US. The western US is primarily influenced by the desert-creating high pressure system in the Southwest, and the upper latitude humid low pressure system in the Northwest. In the winter, the low pressure storms from the northwest occasionally migrate as far south as the Mexican border. Most of the rainfall in the Southwest, however, comes in the summer months from humid equatorial low pressure systems originating in either the Gulf of Mexico or the Gulf of California. Unlike in the nation's midsection, these seldom penetrate deeper than the border states of Arizona and New Mexico.

One of the two ways that surface physiography influences climate is through altitudinal zonation. "Altitudinal zonation" results from the fact that higher elevations are colder than lower elevations. As one moves up a high mountain slope, the climate and resulting vegetation change from warmer to colder weather types. The effect is similar to moving from lower to higher latitudes (i.e., from the equator to the poles). Altitudinal zonation is most pronounced in the southwestern US, where mountain peaks stand out as lush islands in a sea of desert.

The second way that physiography influences climate is through the creation of a "rainshadow." Water molecules slow down when the air gets colder, allowing condensation and clouds to form. When the temperature is warmer, the molecules warm up and move faster, and the clouds dissipate. As a mass of warm air rises into an area of colder air, it will eventually cool to a temperature at which condensation will occur and a cloud will form. This is known as the "dew point" temperature. If the air mass continues to rise and cool, condensation increases until precipitation (rain, snow, hail, etc.) occurs.

This process takes place regularly in the Cascade Range (in Oregon and Washington) and Sierra Nevada Range (in California). Air masses are pushed up the western side of the mountains in a process known as 'orographic uplift'. This cools the air and typically results in precipitation (rain or snow). By the time the air mass reaches the top of these ranges, it has lost much of its original moisture. The air mass then starts down the back side of the mountain. This causes the air to warm up and the molecules to speed up, with the result that the clouds disappear and the little moisture that is left is now spread out over a wider area. We say that the downwind side is in the "rainshadow" of the mountain. The deserts of the Southwest extend as far north as eastern Washington because of this rainshadow effect. In Hawaii, the wet side of an island is referred to as its "windward" side, while the dry, rainshadow side is known as the "leeward" or "lee" side.

Climate has a direct impact on the biogeographic distribution of the natural vegetation in a region. In fact, the two are often inseparable. The major factors of climate that affect vegetation patterns are humidity (precipitation) and temperature. High humidity and high temperature are associated with lush, green vegetation, such as that found in the Southeastern US. High temperature and low humidity are associated with arid, desert vegetation, such as is found in the southwestern US. Humidity becomes less important as temperatures decrease. The lowest temperature regions are associated with spruce coniferous trees (found in most of Canada) and tundra (a treeless type of grass and moss found in the northern extremes of the continent). Between these three extremes (1 high temperature and humidity, 2 high temperature and low humidity, and 3 low temperature), is a mixture of evergreen trees, deciduous trees (those that lose their leaves in winter), and grasslands.

The discussion and diagrams above focus on "natural vegetation" patterns. Human settlement in North America has significantly modified these natural vegetation patterns. Native Americans may have been responsible for changing the natural woodland areas of the western Midwest region into grasslands through the use of fire in the hunting of big game at the end of the last period of ice coverage about 10,000 years ago. Europeans cut down almost all of the virgin forests of the eastern US. All we see today are secondary growth that is very different from the original vegetation. In the West, the suppression of forest fires has resulted in forests that are far more dense (and dangerous) than the natural patterns. Humans have also turned biologically diverse habitats into monocultural agricultural and forestry farms. Exotic plants and animals, introduced through human migration and trade, have threatened indigenous varieties in many parts of the US -- and have caused some to become extinct. While long term climate and vegetation change is a normal process, such changes have been greatly accelerated by human activity since the beginning of the industrial revolution (mid-1800s). This situation has made climate change a major research area for climatologists and biogeographers.

Anglo North America is defined as that part of the North American continent comprised primarily by the US and Canada. This area comprises almost 7.5 million square miles and extends over 50 degrees of latitude from the Florida Keys to the Queen Elizabeth Islands in northernmost Canada. The distance from Newfoundland in the east to San Francisco in the west is more than 3,300 miles. North America can be divided into three physically distinct sections, each of which has important physical geography divisions within it. These are

1. The Eastern Lowlands

the Central or Interior Lowlands,
the Great Plains, and
the Canadian Shield.

2. The Eastern Mountain System
the Appalachian Mountains, and
the Ozark Plateau.

3. The Western Mountains and Basins
the Rocky Mountains, the Intermountain Basins and Plateaus, and
the Pacific Coast Mountains and Valleys.

Over half of the US, and even more of Canada, is flat land. The largest expanse of flat land is located between the Appalachian Mountains in the East and Rocky Mountains in the West. This vast Interior Lowland stretches from the Gulf of Mexico in the south to Hudson Bay in the north. The southern and eastern portions of the Eastern Lowlands consist of the Gulf and Atlantic Coastal Plains, which wrap around the eastern mountains in a gradual slope to the sea. The northernmost portion of the Interior Lowland is the 4-billion-year-old igneous rock of the Canadian Shield. Between the Great Lakes and the Gulf Coastal Plain is the Central Lowlands of the Midwest and Great Plains. This area is higher in elevation and consists of more consolidated sedimentary rock. All three of these areas are very flat.

The Gulf Coastal Plain faces the Gulf of Mexico, while the Atlantic Coastal Plain borders the Atlantic Ocean. Together, these two plains wrap around the southern end of the Appalachian Mountains, filling in an area that was under water up until only 75 million years ago. This is the flattest of the Interior Lowlands subregions. In Texas, these plains are 300 miles wide. Northeastward, from the Florida Peninsula, they gradually narrow as more of the plain is under the sea. Near New York, the plain comes to an end--reappearing north of the Hudson only in parts of Cape Cod and the islands off the coast of southern New England.

These plains are also among the youngest in the US. When the Rocky Mountains were first beginning to rise some 70 million years ago, most of the Gulf and Atlantic Coastal Plains were under water. The coastal plains were gradually built up by unconsolidated (and easily eroded) sediments washed out from the Appalachian Mountains, the Central Lowlands, and the Great Plains. Successive layers of sedimentation form belts with low scarp edges facing inland, but the general slope of the plains' surface is very gentle and no true coastline is formed. Instead, swamps (wet forests), marshes (wet grasslands), and lagoons are fronted by almost continuous sand bars and reefs, which characterize a transition zone between land and sea. Deposits of coastal marine life in these areas is the basis for vast reservoirs of oil and natural gas.

The climate throughout this region is humid subtropical, with summer rainy seasons. Annual rainfall is over 40 inches for the entire coastal plains area, with some areas reaching over 60 inches at the southern extremes. Temperatures are moderate in the winter and hot in the summer. The major air mass which influences the climate of this region comes from the south, bringing warm and moist tropical air. In the spring, this southerly flow may come in the form of hurricanes. This air mass is weaker in the winter, and occasional Arctic air masses reach deep into the South, often destroying the region's fruits and vegetables. The heavy precipitation washes out many of the nutrients in the soils, creating low nutrient ultisols. Vast areas of needleleaf evergreens make the lowland South one of the two major timber-producing regions in the US.

Florida lies entirely within the Gulf and Atlantic Coastal Plains. It is entirely flat (the highest elevation is 345 feet), is mostly sandy, and has large swamp areas. The southern tip is dominated by the 5,000-square-mile Everglades swamp. The coastal plains spread inland to include the Mississippi River Valley as far north as Cairo, IL., at the confluence of the Mississippi and Ohio Rivers. The Mississippi Valley is wide and flat, and the heavy silt load of the river continues to build its delta as the river empties into the Gulf.

A shallow shelf extends up to 250 miles from the coastline to the edge of the North American continent. This situation is known as a "trailing coastline," because it is often found on the side of the continent that is trailing behind a migrating continent. North America is moving westward, toward the Pacific Ocean and away from the Atlantic Ocean.

The Central Lowlands of the Midwest and Great Plains farther to the west are composed of sedimentary rock from material eroded from the Appalachian Mountains, the Rocky Mountains, and ancient upland areas to the north and west of the Great Lakes. As with the coastal plains, the Midwest and Great Plains were once covered by a vast inland extension of the Gulf of Mexico. The Central Lowlands and Great Plains are older, higher in elevation, and consist of harder sedimentary rock than along the coastal plains. Waterborne erosion has carved some gently rolling hill areas in the Midwest and Great Plains. In the north, continental glaciation from the ice ages has influenced the surface physiography.

Winter temperatures are cooler here than in the South, but summer temperatures can still be very warm. This difference is due to the greater degree of continentality experienced in the central part of the continent. The rainy season is in the summer, when moist tropical air moves into the region from the Gulf of Mexico. This moisture is very predictable for the Central Lowlands, which relies on it for intensive agriculture and animal husbandry. Summer rains are much less predictable for the Great Plains, where periodic drought is common. The Central Lowlands receive an average of over 30 inches of precipitation a year, while the Great Plains receives only 20 inches. Precipitation amounts under 20 inches (which is common west of 100 degrees W. Longitude) are considered drought conditions.

Soils on the Central Lowlands are primarily Alfisols, which are produced under the mixed deciduous and evergreen forests that used to cover Ohio and large portions of Indiana and Illinois. The soils of the Great Plains is Mollisols, which develop under grasslands. Mollisols are one of the richest soils and are well suited for nutrient demanding wheat. Alfisols are less nutrient rich but do well with corn and soybeans, the two crops which dominate the Central Lowlands.

The eastern transition belt between the Appalachian Mountains and the Central Lowlands is sometimes called the interior low plateaus. It is the result of slight folds which dip gently to the west. It is, for the most part, sandstone underlain by limestone creating a karst topography of limestone caves, such as Mammoth Cave in Kentucky. When the sandstone cover is removed by erosion, limestone basins are created, such as the Blue Grass Basin of Kentucky and the Nashville Basin of Tennessee. Both have rich soils and good farm lands.

The Great Lakes section of the Central Lowlands is a true structural depression consisting of a large, bow-like feature with numerous layers of different rock. Continental glaciers carved out the weaker layers of rock in a circular pattern (centered on central Michigan). Glacial action has also affected the river drainage pattern of the Great Lakes and upper Mississippi, Ohio, and Missouri Rivers.

The Great Lakes have a significant impact on the regional climate of the upper Midwest. Winds that cross the lakes pick up moisture, which increases precipitation (including snowfall) on the eastern shores of each lake. The lakes also moderate air temperatures (as the oceans do), creating longer growing seasons on the eastern lake shores. The Niagara Peninsula, north of Lake Erie and surrounded by three of the Great Lakes, is the major fruit-growing region in Canada.

During the Pennsylvanian (or Carboniferous) Period, 300 million years ago, the Interior Lowlands of the Midwest were covered with vast swamps, shallow seas, and thick, tropical-like vegetation. Dead plants in the swamps were covered by river-borne sediments. Under pressure and over time, this organic material was turned into coal, oil, and natural gas. This period in the earth's history is known as the "Carboniferous Period" and is the main source of the fossil fuel deposits throughout the world. It resulted in rich coal deposits under Missouri and in the western plateaus of the Appalachian Mountains, as well as oil and natural gas in Texas, Oklahoma, and Arkansas.

The entire Midwest and most of the Great Plains are drained by the Mississippi River and its tributaries, including the Missouri and Ohio Rivers. Good soils, flat land, and the navigable rivers made the Midwest an easy region to cross and settle in. Large grain shipments, coal, and iron ore (on the Canadian Shield) could also be transported easily on these waterways. These attributes contributed to making the Midwest both the agricultural and the industrial core of the US and Canada.

The Great Plains rise in elevation from under 100 feet at the lower Mississippi River to just over a mile high (1 mile = 5,280 feet) at the foot of the Rocky Mountains. It is composed of layer on layer of sedimentary rock washed out from the Rocky Mountains to the west. Rivers such as the Platte, Missouri, and Red parallel one another as they flow from west to east down this gentle grade. Much of the Great Plains was a swampy area during the Carboniferous Period. Large portions were still under water prior to the uplift of the Rocky Mountains some 70 million years ago. This history has resulted today in large deposits of coal, oil, and natural gas from Texas through Colorado, Wyoming, and Montana, and into Alberta in Canada.

While most of the Great Plains is flat and undistinguished, it does contain some distinct physical anomalies worthy of mention. In Nebraska, a belt of sand hills has been formed by wind and the outwash of the last continental glaciers. In South Dakota, however, water erosion has created a vast badlands landscape of deeply cut and barren soils. The one major mountain system that interrupts the evenness of the Great Plains is the Black Hills of South Dakota, which rise to over 7,000 feet. Here old crystalline igneous rock breaks throughout the surface in a dome-like swelling believed to be an outlier of the Rocky Mountain system. The oldest and largest gold mine in the US, as well as the famous Mount Rushmore, are both located in the Black Hills.

The Great Plains have a considerably drier and more unpredictable climate than do the Midwest and Coastal Plains. Prairie grasses are the natural vegetation of this area. For this reason, early settlers from northern Europe referred to it as the "Great American Desert." Soils that formed under the prairie grasses, however, were actually richer than those of the more forested Midwest, making the region the primary wheat-growing area in the US and Canada. The Great Plains become more narrow as they extend into Canada and gradually disappear in northern Alberta. North of Edmonton (Alberta), the prairie grasslands give way to a belt of mixed coniferous (pine) deciduous trees. Eventually, these become spruce and aspen forests which dominate the cold northern latitudes all the way to Alaska, both on the Canadian Shield to the east and the Rockies to the west.

Underlying more than 1 million square miles of the Interior Lowlands of Canada is the Canadian Shield. While the Canadian Shield is almost entirely in Canada, geologically important extensions of it lie in northern Michigan, Minnesota, and Wisconsin. In New York, the Adirondack Mountains are an extension of the Shield. The Canadian Shield contains some of the oldest exposed rock on the surface of the earth. It is primarily composed of hard, metamorphic rock created 4 billion years ago in the Precambrian Period. The metamorphic process has created zones rich in metallic mineral deposits. Successive periods of glacial cover have removed most of the sedimentary material that may have once covered the Shield, exposing rich deposits of metallic ores. Canadian Shield iron ores from Minnesota and Canada were one of the main resources allowing for the development of the US and Canadian manufacturing core area around the Great Lakes. Hudson Bay, the largest bay in North America, is situated at the center of the Canadian Shield. During the great ice ages, vast sheets of snow extended out of the Hudson Bay area, covering much of lowland Canada and the northern US.

The Canadian Shield dips from between 1,000 and 2,000 feet in elevation in the south and west to sea level at Hudson Bay. A string of lakes and other water bodies surrounds the edge of the Shield to the south and west. The Great Lakes, Lake Winnipeg, Great Slave Lake, and Great Bear Lake are the largest lakes in North America. They, along with the St. Lawrence River and Lake Champlain Lowland (New York and Vermont), and the Mackenzie River in the far north demarcate the boundary between the Canadian Shield and surrounding uplands. The St. Lawrence and Mackenzie Rivers are two of the largest in North America. Unlike the rivers that flow parallel across the Great Plains, those located on the Canadian Shield form a zigzag pattern interspersed with numerous lakes. This pattern reflects the underlying hard rock and is typical of landscapes created by continental glaciation. Most of the Shield is vegetated with skinny spruce and aspen forests, with the exception of the far north, which has treeless tundra vegetation. The tundra line also marks the southern limits of continuous permafrost soils.

The southern two-thirds of the Canadian Shield experiences mild summers (50 to 70 degrees F) and cold (below freezing) winters. The region north of Hudson Bay, however, remains at freezing and below throughout most of the year, with very short summers when high temperatures reach into the 40s. (Only interior Greenland averages below freezing all year round.) The major air mass influencing this region is the dry and cold continental polar system. This air mass is weaker in the summer, but comes on strong in the winter months. Annual precipitation is under 10 inches for the entire Canadian Shield, except for the areas to the east and south of the Great Lakes.

Created when North America first collided with Europe and Africa some 570 million years ago, the Appalachian Mountain Range was once the principal mountain system in North America, rising as high as the Rocky Mountains do today. Starting about 200 million years ago, North America began to break away from Europe and Africa, forming the Atlantic Ocean. Tectonic forces of swirling molten magma inside the earth cause continental and ocean plates to move about on the earth's surface.

The Appalachian Mountain Range is still the dominant mountain system in the eastern US. It was created by the intense folding of sedimentary rock. In some places, younger rocks were pushed completely over older rock as Africa and North America shifted and slid against one another. The Ozarks and Ouachita Mountain systems west of the Mississippi formed the southwest end of the Appalachian system. More intrusive igneous rock is located in the northeast portion, where the Appalachians continue into New Brunswick and Newfoundland in Canada and across to northern Ireland, Scotland, and Norway (this area was connected to North America over 200 million years ago).

The Appalachians generally stopped growing after the continents separated from one another 200 million years ago. Continuous erosion has now brought most of the peaks down to under 5,000 feet in elevation. Narrow valleys, steep hillsides, and thin soils have resulted in low population densities and has kept cities small. This scenario holds true for the Ozarks and Ouachitas. Metamorphic metallic minerals (especially copper and iron ore) have been largely mined out of the Appalachians, though coal still remains in abundant supply.

The Appalachian Mountains form a wide belt which runs from Newfoundland in the northeast to Alabama in the southwest. The Appalachian Mountains can be divided into three sections: (1) a northeastern section which covers northern Maine and the Maritime Provinces of Canada, (2) a New England Section, (3) and the Appalachian Mountains proper.

The maritime provinces of Canada and northern Maine were also subjected to mountain building processes which involved huge intrusions of igneous rock (known as batholiths). These formations constitute the Central Highlands of New Brunswick and Nova Scotia, as well as pockets of Newfoundland Island. The hard rock underlying this land has experienced considerable scraping and erosion from continental glaciers and resembles the landscapes of the Canadian Shield. The St. Lawrence River and the Gulf of St. Lawrence form a water boundary between the northern Appalachians and the Canadian Shield of Quebec and Labrador.

The New England section of the Appalachians extends north and east of the Hudson into central Maine. Two north-south trending mountains dominate this area, both of which are a mix of folded sedimentary and intrusive igneous rock. Granite (igneous) and marble (metamorphosed limestone) rock are both quarried here. The Green Mountains in the west are lower in elevation. Its peaks have been rounded due to continental glaciers that once covered them. The higher White Mountains include Mount Washington, the tallest peak in the northeastern US at 6,288 feet (in New Hampshire). The uplands of New England drop gently eastward to the sea. Here can be seen lone peaks called monadnocks which are remnants of mountains eroded by continental glaciers.

Even though this part of the Appalachian Mountains is close to the coast, the climate is continental because of the west-to-east flow of air. Thus, cold polar air masses from the Canadian Shield regularly blow across the northern Appalachians region, bringing winter snows all the way to Boston Harbor. The climate is generally one of mild summers and cold winters in the northern portion, with hot summers and cool winters farther to the south.

In the south, the Appalachians proper are found. The eastern side of the mountains here are made of ancient Precambrian igneous and metamorphic rocks. On the west, they are made of a new belt formed by the upthrust edge of the carboniferous swamp that covered the interior lowlands of North America 300 million years ago. These old swamp lands form the basis for the rich coal deposits found throughout the Appalachians proper.

Where the Appalachians meet the Atlantic Coastal Plain the old rocks have been severely eroded to form a gently sloping dissected plateau surface known as the Piedmont. The edge of the piedmont is known as the fall line, due to the presence of waterfalls along rivers flowing from the higher inland elevations. The piedmont rises gradually to 1,200-1,500 feet, where it merges with the first chain of wooded mountains, the Blue Ridge Mountains. These mountains reach an elevation of over 6,000 feet in the section known as the Great Smoky Mountains along the North Carolina and Tennessee border. (The Smokies are the highest mountains east of the Rocky Mountains, with the highest peak being Mt. Mitchell at 6,684 ft.) In northern Virginia and Pennsylvania, however, the Blue Ridge Mountains subside, becoming low and discontinuous.

In the west, the mountains are heavily folded in parallel lines known as ridge and valley topography. This corrugated area is 25-80 miles wide. The north-flowing Shenandoah River and the south-flowing Tennessee River run through it. West of the fold zone and above a high scarp face, known as the Allegheny Front in Pennsylvania, lies the plateau section. The two main plateaus are the Allegheny Plateau in Pennsylvania and the Cumberland Plateau of Tennessee, West Virginia, and Kentucky. They generally rise to about 2,000 feet, although the Cumberland Plateau reaches over 4,000 feet in West Virginia. The Allegheny Plateau stretches almost to the Great Lakes and has the richest coal deposits in the US.

Heavy precipitation falls in the high Great Smoky Mountains/Blue Ridge Mountains in the southern portion of the Appalachians proper. A rainshadow area of low precipitation exists in the central portion of the lateral mountain chains. Most of the Appalachians proper, however, experience a hot summer and cool to cold winter.

The Ozark Plateau and Ouachita Mountains consist of a group of low mountains sometimes called the interior highlands. The Ozark Plateau is composed of a domed plateau flanked by rocks of the Carboniferous Age. On the southern edge of the plateau are the Boston Mountains, the peaks of which are exposed Precambrian granites. To the south, beyond the Arkansas River, are the Ouachita Mountains, a folded belt that resembles the ridge and valley of the Appalachians. At the highest point, the area does not exceed 2,700 feet. The climate of the Ozarks and Ouachitas is similar to that of the surrounding lowland South. This is in part because they are an east-west trending system, which benefits little from altitudinal zonation and rainshadow impacts.

Although earthquakes are more commonly associated with the western US, in fact the strongest earthquakes to ever occur in the US were in the Memphis, Tennessee, and Madrid, Missouri, area in 1811 and 1812. Other devastating quakes have occurred in Charleston, South Carolina, Jamaica Bay (New York City) in the past 100 years. Geologists estimate an 85% probability of a destructive earthquake occurring in the eastern US before the year 2000 (which did not happen), and a near 100% chance of one by the year 2010. Because earthquakes are so infrequent in the East none of the buildings have been constructed to resist tremors, as they have in California.

The three major physiographic regions in the western US are: (1) the Rocky Mountains, (2) the Intermountain Basin and Plateaus region (the largest and oldest part of the West), and (3) the Pacific Coast Mountain and Valley region. The mountainous West was created by the rapid speed with which North America moved westward into and over the Pacific Ocean Plate. The Intermountain Basin and Plateaus region was the first portion to be uplifted, creating a large mountain range through present-day Nevada some 80 million years ago. The Rocky Mountains were formed 60 million years ago. Today, the Front Range of the Rockies still rises a mile above the Great Plains at Denver (Colorado).

The Rocky Mountains contain a mix of sedimentary, igneous, and metamorphic rock. Late in Cretaceous times (about 70 million years ago), tremendous mountain-building processes disturbed the western half of the continent where a long series of sedimentary beds lay evenly spread over the ancient continental floor. This disturbance was accompanied by volcanic activity and is known as the Laramide Revolution. It resulted in a great uplift of the sedimentary beds, folding and faulting, and metamorphic changes. The uplift was followed by the erosion of the sedimentary layers and exposure of the underlying igneous rock. In some places, the cover was preserved, usually in pockets formed by downfaulting. This diversity gives the West its varied and interesting physiography. The Rockies include the Brook Range in Alaska, the Canadian Rockies, the Northern Rockies (Idaho and Montana), the Central Rockies (Colorado), and the Southern Rockies (New Mexico). The highest peaks rise to over 14,000 feet.

The Rockies are breached in only a few places. In the far north, the Lizard River flows through a break in the Mackenzie Mountains. There is another break between the Northern and Central Rockies where the Wyoming Basin is formed. Here, the old Oregon Trail made use of the route, so that wagons could traverse the continental divide at about 7,000 feet.

Climate in the Rocky Mountains is generally cold in the winter and mild in the summer. Highland climates, however, are difficult to classify because of great variation from valley floors to mountain peaks. In addition, south-facing slopes which receive much sunshine can be very different from colder, north-facing slopes in climate and vegetation. Latitude plays a major role as the mountain chains existing farther to the north are much colder and have more permanent snow and active alpine glaciers than those to the south.

West of the Rockies lies an area whose astonishing physical features are the product of crustal faulting, volcanic activity, and intense recent erosion and downcutting by rivers. The basic form of this region is that of a series of high elevation plateaus. In the southwest, these plateaus are flat and descend in an almost step-like fashion from the Rocky Mountains into the Great Basin. The vast plateau in the northwest was shaped by volcanic activity, while in Canada the plateaus are narrow, steep, and mountainous until they open into the rolling hills of interior Alaska.

The development of these plateaus, like that of the Rockies, resulted from the Laramide Revolution. Their different levels are the result of faulting. The two main plateaus of the West are the Colorado Plateau and the Columbia Plateau. The Colorado plateau covers portions of Colorado, Utah, Arizona, and New Mexico. It is composed of relatively flat sedimentary rock that was uplifted. The Colorado River cuts the Grand Canyon through this uplifted sedimentary rock. The Columbia Plateau is located in eastern Washington and parts of Oregon and Idaho. It is composed of lava flows that combine to be several thousand feet thick. Hells Canyon, on the Snake River between Oregon and Washington, is the deepest gorge in the US and is cut into this plateau.

The climate of the plateaus varies with latitude (those farther north are colder) but they generally have a hot summer and mild to cold (depending on elevation) winter. Precipitation is typically under 10 inches a year (although it reaches 20 inches on the Fraser Plateau). Runoff from the surrounding mountain peaks provides a major source of water for irrigated farm fields, which often do well. The Columbia Plateau is one of the richest producing irrigated wheat regions on the continent.

The Fraser Plateau comprises much of central British Columbia, situated between a narrow band of the Rockies and the Canadian Coast Mountains. Like the Columbia Plateau to the south, it is comprised of large areas of extrusive igneous lava. The entire plateau is drained by the Fraser River, which enters the sea at Vancouver. North of here the Plateau region becomes narrow and is bisected by rivers cutting through mountains composed of a mixture of various rocks of different ages and composition. The older of these rocks is the source of the gold that was first discovered in the Yukon Territory of Canada in the late 1800s. The Klondike gold region sits astride the Yukon Plateau and the Yukon Interior (a northern basin and range-like region in Alaska). The Yukon Interior reaches over 500 miles in width in western Alaska.

At the western edge of the Colorado Plateau lies a great fault line at the base of the Wasatch Mountains. West of this lies the Great Basin, which stretches for 500 miles from north to south and east to west. Waters flow into, but not out, of the Great Basin. The Great Basin is centered on Nevada and is composed of the caved-in remnants of a pre-Rockies mountain system which was formed 80 million years ago. Numerous faulted mountain blocks run north and south through the Great Basin, with areas between these ranges consisting of unconsolidated alluvium. This basin and range landscape also extends into southern California, Arizona, and New Mexico. The Great Salt Lake occupies some 2,000 square miles of the eastern edge of the Great Basin. It is the remnant of the former Lake Bonneville, which was 10 times larger at the end of the last ice age.

Rainfall is generally under 5 inches throughout the basin and range deserts of the American West. Irrigated fields rely on water from the plateaus and higher peaks. Groundwater is another important source of irrigation, although it is becoming more and more difficult to reach as water tables drop. Most of the rain that does fall comes in the summer months, from moisture originating in the Gulf of California and the Gulf of Mexico. Altitudinal zonation on the higher mountains is well pronounced, as one can often travel from desert to tundra within a hundred miles.

The Pacific Coast province is very diverse. Within it are found the highest points and the last remaining active volcanoes on the continent. Like the rest of the mountainous West, the Pacific Coast portion was also shaped by plate tectonic activity. The granite peaks of the Sierra Nevada Mountains in California are an exposed intrusive igneous formation, while the Cascade Range to the north are of extrusive igneous (lava) origin. Farther to the north, Canada's Coast Mountains and the Alaska Range (in Alaska) are intrusive igneous formations, similar to the Sierra Nevada. The Coast Mountains are the world's highest coastal mountain system, while the Alaska Range contains the highest mountains on the continent, including McKinley (also known as Denali) at 20,320 feet.

Both intrusive and extrusive igneous rock come from Pacific Ocean sea floor plates that have been pushed under the North American plates, a process known a subduction. The ocean plates melted and the molten magma then rose to form the Alaska Range, Coast Mountains, Sierra Nevada, and Cascade Range. (A similar process pushed up the Rocky Mountains and Colorado Plateau.) As these mountains were uplifted, the area immediately to their west was sunken to form a depression. The depression originates as the Gulf of California in Mexico and continues as the Central Valley in California, the Willamette Valley in Oregon, the Puget Sound lowland in Washington, the coastal straits of British Columbia, and the Alexander Archipelago in the Alaska Panhandle.

Along the Pacific shoreline of California, Oregon, and Washington, the low-lying Coast Range is formed by the crumpling edge of the North American continent. The continued subduction of a small ocean plate under Oregon and Washington ensures volcanic activity in the Cascades. In California, however, the North American Plate is sliding sideways against the large Pacific Ocean Plate. This causes lateral or strike-slip faulting throughout the Coast Range of California. The Pacific Plate moves northward at about 2 inches a year, causing an ever present danger of serious earthquake activity. The Coast Range continues as Baja California in the south and Vancouver Island and Queen Charlotte Island in the north. The Alaska Peninsula, which contains more active volcanoes than anywhere else on the continent, could also be considered an extension of this coastal edge.

Unlike the eastern trailing coastline, the western coasts are all leading coastlines. They are steep and show signs of emergence--i.e., rising up out of the sea with older coastlines now hundreds of feet in the air.

California is famous for its Mediterranean climate. This climate exhibits a distinct summer dry and winter wet pattern, which is different from that found anywhere else in the US or Canada. This climate gradually changes to the Marine West Coast climate as one moves north through Oregon and into Washington. The Marine West Coast climate is wet all year round, although it, too, has a winter peak. The main air masses creating these climates are a north Pacific low pressure system, which bring cool and moisture storms, and a mid-Pacific high pressure system, which brings dry air to the coast of California. These two air masses move north in the summer and south in the winter. Because the moisture hitting the West Coast comes off the ocean, it has a moderating maritime influence, with coastal snow being uncommon, except in northern British Columbia and Alaska.

The two north-south mountain systems on the West Coast create two rainshadow patterns. The first, smaller rainshadow occurs in the inland valleys of California, Oregon, and Washington and the coastal channels of British Columbia. Thus, Sacramento, Portland, Seattle, and Vancouver (British Columbia) all lie in a warmer and less rainy rainshadow area. A more significant rainshadow exists east of the Sierra Nevada, Cascade Range, and Canada's Coast Mountains. This rainshadow extends the naturally occurring deserts of the southwestern US far north to the Canadian border. In Canada and Alaska, the dry polar desert is extended southward by the same rainshadow process behind the Coast Mountains. Dry rainshadow conditions also extend into Alaska, where the north-south mountains shift to an east-west moisture barrier in the Alaska Range.

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