Map of Baja Peninsula - Mexico Maps, Baja Peninsula Facts Landforms - World Atlas The Baja Peninsula (Baja California Peninsula) extends from Tijuana, near the U.S. border, to Cabo San Lucas - approximately 747 miles, (1202 km). It's divided into two Mexican states; Baja California Norte (north), and Baja California Sur (south). It's bordered by the Pacific Ocean and the Gulf of California, which is alternately known as the"Sea of Cortes," named for Herman Cortes, a 16th century Spanish explorer. Archaeologists have concluded that the peninsula was separated from mainland Mexico by the San Andreas Fault, and over eons of time, the eastern edges were uplifted by tectonic plate movements and the lower areas then filled with seawater, thus forming the Gulf of California. The peninsula is mountainous, especially in the north and far south, and those mountains ring an inhospitable, rocky (and hot) desert landscape. Most tourists travel to the tropical south, visiting the city of La Paz and the resort communities of Cabo San Lucas and San Jose del Cabo.

A daily, reliable ferry service runs from La Paz to many cities on mainland Mexico's Gulf of California coastline. The Baja Peninsula landscape.... The Gulf of California Geology of California's Imperial Valley A Monograph by Eugene Singer CHAPTER 9 THE GULF OF CALIFORNIA SEVEN MILLION YEARS AGO, in Pliocene time, the Baja California peninsula did not exist, nor did the Gulf of California, known in Mexico as the Sea of Cortez. Their mutual evolution and geologic history not only make a fascinating story, but one that has had a profound effect upon the Salton Valley. The Gulf of California and the Salton Trough are components of a single geologic structure that averages less than 100 miles in width yet is more than 1,000 miles long. In this context, the Salton Trough is considered the landward extension of the Gulf. This point becomes clear when one considers that the Salton Valley exists only because the Colorado River delta is a natural earthen dam that completely excludes the waters of the Gulf. If it was not for that dam, the Valley would be submerged as far north as Indio.

So, any study of the valley's physical history and environment must include the Gulf. More so than usual, in fact, since this area is one of earth's most dynamic and changing regions, and the long term future of the valley has many speculations of wonder. Geology of the Gulf Briefly stated, the Gulf of California is an elongated depressed block along a zone of major, roughly parallel faults. It is a complex fault zone in which total relief, i.e., the elevation difference between the sea floor of the Gulf relative to the mountains of Baja California to the west and the Sierra Madre of mainland Mexico to the east is greater than 10,000 feet. The San Andreas fault is considered a major element of this fault zone. The fault presently disappears beneath the alluvium of the Imperial Valley, but is thought to continue southeastward into the Gulf, merging into the East Pacific Rise along the Gulf's axis. The general structure of the Gulf is very well observed in this satellite composite photo. It is linear in direction, with the landward portion, the Salton Valley comprising a triangular area in the north.

Clearly visible is the Colorado River delta, the Mexicali Valley and the Imperial Valley represented by dark tan color, the Salton Sea's conspicuous gourd-shaped area and the Coachella Valley at the apex. One may also interpolate the East Pacific Rise as the axis of the Gulf translating into the San Andreas Fault Zone beneath the delta. The northern one fourth of the Gulf is shallow; at no place greater then 600 feet deep. Here, the Colorado River delta has contributed greatly toward filling the north end with fine sand and silt, making the sea floor much shallower than it is in the south. The sea bottom of the Gulf grades upward to the north, gradually and eventually mergeing into the marshlands of the delta. This character of the Gulf is observed at the beaches of San Felipe, 120 miles south of Mexicali. The basement rocks under the north end of the Gulf are covered by as much as 25,000 to 30,000 feet of these deltaic sediments. This enormous volume of silt suggests the amount of erosional debris taken from the Colorado Plateau to the Gulf by the Colorado River over several million years.

Further south, the east side of the Gulf floor slopes gently from the mainland Mexico coast to an irregular escarpment near the center of the Gulf. Between the 25th and 26th parallels of latitude, one section of this escarpment is a great submarine cliff nearly 6,000 feet high. Much of the south half is occupied by a remarkable depression in the sea floor, extending 250 miles southeastward from a point east of tiny Isla Tortuga. It widens into enormous proportions in places. The enclosing contour of this depression is 5,400 feet below sea level. This great submerged basin is occupied by three separate basins. The deepest basin, 10,740 feet, lies in the center of the Gulf between the 25th and 26th parallels. Between 30 and 40 islands varying in size are found in the Gulf. Some islands, while very small in area, tower to surprising elevations with great ocean deeps close offshore. Many islands are elongated, with shorelines generally parallel to the axis of the Gulf, suggesting their origin as uplifted blocks along faults.

Isla Angel de la Guarda, the second largest island, is unique. It is at the 29th parallel, in the upper half of the Gulf. This 45-mile long island lies about ten miles offshore from Baja California, yet is separated from it by a long, narrow and deep submarine trough which averages 4,000 feet deep. The deepest portion is more than 5,000 feet below sea level. (This island is conspicuous in the satellite photograph at the top of this page.) Considering that the island lies only a few miles offshore from the peninsula proper, the extreme depth of the intervening basin suggests significant vertical displacement along faults on the sea bottom. The several broad, deep basins in the Gulf are probably wedge-shaped blocks, or grabens, bounded by faults. The most convincing evidence of this is that the deep basins are true closed depressions, with elongated steep sides, rather than open-ended submarine valleys. Volcanic activity in the Gulf area was intense in Miocene time, about 25 million years ago. Extensive basalt flows are found in the southern part of Baja California. Volcanoes were active through the Pleistocene, and have continued active to the present.

Isla Tortuga is a very young volcano in the Gulf. Las Tres Virgenes and Isla Coronada have been active in historic times, as well. A Hypothesis as to the Origin of the Gulf of California The Baja peninsula, including most of southwestern California is an elongated slab of the North American continent that has been sheared from the mainland and moved to its present position. The postulated movement of the peninsula may be retraced backward in time and in direction. Looking back to the Cretaceous, the peninsula is brought into a likely former position with the mainland. In this arrangement, the Sierra Madre del Sur appears to continue without break from mainland Mexico into Baja California. In Pliocene time, about seven million years ago, a zone of separation developed on the East Pacific Rise. The future Baja California peninsula and a piece of future California were sheared from mainland Mexico along a lateral fault, possibly the ancestral San Andreas fault which was then, as now, oriented northwest-southeast. During this early period of development, movement was right lateral, with the sheared-off slab moving northwest, but always in close contact with the mainland.

[1] The northwest movement seems to have been repeated slippage along the principal members of the San Andreas fault system -- the Elsinore fault, the San Jacinto fault, and the main San Andreas strand itself. About four million years ago, the San Andreas fault proceeded to play a key role in the next phase, the opening of the mouth of the Gulf of California. While the Baja California peninsula continued to move to the northwest as a whole, its southern end began to rotate westward, opening a seaway between the new peninsula and the mainland. This movement is thought to have been caused by torsional stress. That is, while the primary movement continued to be northwest, the northern end of the Baja California peninsula became locked against the mainland, causing the southern end to rotate westward, creating the seaway that was to become the Gulf of California. As the Gulf continued to open, Baja California moved out to sea while the Peninsular Ranges of California remained attached to the mainland block. Movement along the San Andreas fault system was vigorous, and the San Jacinto fault became the most active member. At this time, the San Jacinto Mountains and the Santa Rosa Mountains were sharply compressed and uplifted.

Like a massive lever, the rotation caused intense compression forces at the north end, near the fulcrum. The squeezing effect helped to raise the Transverse Ranges of the Los Angeles basin (the Santa Monica, San Gabriel and San Bernardino Mountains), orienting them east-west, as opposed to the northwest-southeast direction of other major ranges of central and southern California. Coincident with these forces, there were stresses that formed the offshore basins, such as the Santa Barbara Channel and the underwater grabens off the southern California coastline. It thus appears that the East Pacific Rise extends well into the Gulf of California and sea floor spreading is widening the Gulf. Tensional thinning at the spreading center between the North American plate and the Pacific plate is further supported by the existence of a large intrusion of magma beneath the sediments of the Salton Sea. The result is regional metamorphism of the sedimentary fill beneath the Salton Sea, resulting in high heat flow and the geothermal activity and volcanism in the Imperial Valley.

United States | history - geography | Britannica.com United States Written By: Alternative Titles: America, U.S., U.S.A., United States of America United States federal republic with two legislative houses (Senate [100]; House of Representatives [4351]) Head of state and government (2010) 308,745,538; (2016 est.) 323,349,0002 Total area (sq mi) Literacy: percentage of population age 15 and over literate Male: (2000–2004) 95.7% GNI per capita (U.S.$) (2014) 55,200 1Excludes 5 nonvoting delegates from the District of Columbia, the U.S. Virgin Islands , American Samoa , the Northern Mariana Islands , and Guam and a nonvoting resident commissioner from Puerto Rico . 2Includes military personnel overseas.

3Total area (excluding 42,334 sq mi [109,645 sq km] of coastal water and 76,804 sq mi [198,921 sq km] of territorial water) equals 3,677,649 sq mi (9,525,067 sq km), of which land area equals 3,531,925 sq mi (9,147,643 sq km), inland water area equals 85,631 sq mi (221,783 sq km), and Great Lakes water area equals 60,093 sq mi (155,641 sq km). list of cities and towns in the United States United States, officially United States of America, abbreviated U.S. or U.S.A., byname America, country in North America , a federal republic of 50 states. Besides the 48 conterminous states that occupy the middle latitudes of the continent, the United States includes the state of Alaska , at the northwestern extreme of North America, and the island state of Hawaii , in the mid-Pacific Ocean .

The conterminous states are bounded on the north by Canada , on the east by the Atlantic Ocean , on the south by the Gulf of Mexico and Mexico , and on the west by the Pacific Ocean. The United States is the fourth largest country in the world in area (after Russia , Canada, and China ). The national capital is Washington , which is coextensive with the District of Columbia, the federal capital region created in 1790. United States of America Fishing boat at the harbour at Portsmouth, New Hampshire. Craig Blouin/New England Stock Photo 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. Roger B. Taney 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.) USA Facts 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. Related Places Kansas City 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 .) United States of America The Colorado River in Marble Canyon at the northeastern end of Grand Canyon National Park, … © Gary Ladd Facebook Twitter YouTube Instagram Pinterest 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. Related Topics 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. Learn about the different physcial landscapes that make up the American Midwest. Encyclopædia Britannica, Inc. 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 High Plains, near Fort Morgan, Colorado. Epimethius 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 Appalachian Mountain system The Appalachians dominate the eastern United States and separate the Eastern Seaboard from the interior with a belt of subdued uplands that extends nearly 1,500 miles (2,400 km) from northeastern Alabama to the Canadian border. They are old, complex mountains, the eroded stumps of much greater ranges. Present topography results from erosion that has carved weak rocks away, leaving a skeleton of resistant rocks behind as highlands. Geologic differences are thus faithfully reflected in topography. In the Appalachians these differences are sharply demarcated and neatly arranged, so that all the major subdivisions except New England lie in strips parallel to the Atlantic and to one another. Screw Auger Falls in the Mahoosuc Range, northern Appalachian Mountains, Maine. © George Wuerthner The core of the Appalachians is a belt of complex metamorphic and igneous rocks that stretches all the way from Alabama to New Hampshire .

The western side of this belt forms the long slender rampart of the Blue Ridge Mountains, containing the highest elevations in the Appalachians ( Mount Mitchell , North Carolina, 6,684 feet [2,037 metres]) and some of its most handsome mountain scenery. On its eastern, or seaward, side the Blue Ridge descends in an abrupt and sometimes spectacular escarpment to the Piedmont , a well-drained, rolling land—never quite hills, but never quite a plain. Before the settlement of the Midwest the Piedmont was the most productive agricultural region in the United States, and several Pennsylvania counties still consistently report some of the highest farm yields per acre in the entire country. West of the crystalline zone, away from the axis of primary geologic deformation, sedimentary rocks have escaped metamorphism but are compressed into tight folds. Erosion has carved the upturned edges of these folded rocks into the remarkable Ridge and Valley country of the western Appalachians. Long linear ridges characteristically stand about 1,000 feet (300 metres) from base to crest and run for tens of miles, paralleled by broad open valleys of comparable length.

In Pennsylvania, ridges run unbroken for great distances, occasionally turning abruptly in a zigzag pattern; by contrast, the southern ridges are broken by faults and form short, parallel segments that are lined up like magnetized iron filings. By far the largest valley—and one of the most important routes in North America—is the Great Valley , an extraordinary trench of shale and limestone that runs nearly the entire length of the Appalachians. It provides a lowland passage from the middle Hudson valley to Harrisburg , Pennsylvania, and on southward, where it forms the Shenandoah and Cumberland valleys, and has been one of the main paths through the Appalachians since pioneer times. In New England it is floored with slates and marbles and forms the Valley of Vermont, one of the few fertile areas in an otherwise mountainous region. Ridge-and-valley topography in the Appalachian Highlands, near Bristol, Tennessee, U.S. Bantosh Topography much like that of the Ridge and Valley is found in the Ouachita Mountains of western Arkansas and eastern Oklahoma, an area generally thought to be a detached continuation of Appalachian geologic structure, the intervening section buried beneath the sediments of the lower Mississippi valley.

The once-glaciated New England section of the Appalachians is divided from the rest of the chain by an indentation of the Atlantic. Although almost completely underlain by crystalline rocks, New England is laid out in north–south bands, reminiscent of the southern Appalachians. The rolling, rocky hills of southeastern New England are not dissimilar to the Piedmont, while, farther northwest, the rugged and lofty White Mountains are a New England analogue to the Blue Ridge. ( Mount Washington , New Hampshire, at 6,288 feet [1,917 metres], is the highest peak in the northeastern United States.) The westernmost ranges—the Taconics , Berkshires , and Green Mountains —show a strong north–south lineation like the Ridge and Valley. Unlike the rest of the Appalachians, however, glaciation has scoured the crystalline rocks much like those of the Canadian Shield, so that New England is best known for its picturesque landscape, not for its fertile soil. Mount Washington, in the White Mountains, New Hampshire. William Hemmel/© New Hampshire Division of Travel and Tourism Development Typical of diverse geologic regions, the Appalachians contain a great variety of minerals.

Only a few occur in quantities large enough for sustained exploitation, notably iron in Pennsylvania’s Blue Ridge and Piedmont and the famous granites, marbles, and slates of northern New England. In Pennsylvania the Ridge and Valley region contains one of the world’s largest deposits of anthracite coal, once the basis of a thriving mining economy; many of the mines are now shut, oil and gas having replaced coal as the major fuel used to heat homes. The Atlantic Plain The eastern and southeastern fringes of the United States are part of the outermost margins of the continental platform, repeatedly invaded by the sea and veneered with layer after layer of young, poorly consolidated sediments. Part of this platform now lies slightly above sea level and forms a nearly flat and often swampy coastal plain, which stretches from Cape Cod , Massachusetts , to beyond the Mexican border. Most of the platform, however, is still submerged, so that a band of shallow water, the continental shelf, parallels the Atlantic and Gulf coasts, in some places reaching 250 miles (400 km) out to sea. Discover the climate, landscape, and waterways of the northeastern United States. Encyclopædia Britannica, Inc.

The Atlantic Plain slopes so gently that even slight crustal upwarping can shift the coastline far out to sea at the expense of the continental shelf. The peninsula of Florida is just such an upwarp: nowhere in its 400-mile (640-km) length does the land rise more than 350 feet (100 metres) above sea level; much of the southern and coastal areas rise less than 10 feet (3 metres) and are poorly drained and dangerously exposed to Atlantic storms. Downwarps can result in extensive flooding. North of New York City, for example, the weight of glacial ice depressed most of the Coastal Plain beneath the sea, and the Atlantic now beats directly against New England’s rock-ribbed coasts. Cape Cod , Long Island (New York), and a few offshore islands are all that remain of New England’s drowned Coastal Plain. Another downwarp lies perpendicular to the Gulf coast and guides the course of the lower Mississippi. The river, however, has filled with alluvium what otherwise would be an arm of the Gulf, forming a great inland salient of the Coastal Plain called the Mississippi Embayment.

South of New York the Coastal Plain gradually widens, but ocean water has invaded the lower valleys of most of the coastal rivers and has turned them into estuaries. The greatest of these is Chesapeake Bay , merely the flooded lower valley of the Susquehanna River and its tributaries, but there are hundreds of others. Offshore a line of sandbars and barrier beaches stretches intermittently the length of the Coastal Plain, hampering entry of shipping into the estuaries but providing the eastern United States with a playground that is more than 1,000 miles (1,600 km) long. Learn about the landforms and climate of the southeastern United States. Encyclopædia Britannica, Inc. Poor soils are the rule on the Coastal Plain, though rare exceptions have formed some of America’s most famous agricultural regions—for example, the citrus country of central Florida’s limestone uplands and the Cotton Belt of the Old South, once centred on the alluvial plain of the Mississippi and belts of chalky black soils of eastern Texas, Alabama, and Mississippi . The Atlantic Plain’s greatest natural wealth derives from petroleum and natural gas trapped in domal structures that dot the Gulf Coast of eastern Texas and Louisiana .

Onshore and offshore drilling have revealed colossal reserves of oil and natural gas. Page 1 of 41 The San Andreas Fault System Geology of California's Imperial Valley A Monograph by Eugene Singer CHAPTER 18 THE SAN ANDREAS FAULT SYSTEM The San Andreas fault is a major geologic feature of North America. Actually, the term San Andreas fault is a misnomer, suggesting that the fault is a single break. It is more accurately called the San Andreas fault system, or zone, with several major and numerous minor branches, particularly in southern California. The San Andreas Fault Despite its benign name (Saint Andrew), the San Andreas fault is a violent and destructive killer. Over its long history, spasms of its enormous energy have been released in countless earthquakes along its great length. These have ranged in magnitude from slight tremors to cataclysmic upheavals and rupturing of Earth's surface. It is a well known and familiar geologic feature in western America. The remarkably straight fault trace is prominent on any California map. It looks like a knife slash from hell in a northwest-southeast direction that attempts to separate the Coast Ranges from central and southern California. The fault ranks among the longest such structures in the world.

Overland, it is a visible break that can be traced for 625 miles from Point Arena, north of San Francisco, to the Salton Sea, near the border with Mexico. The inferred extensions of the fault are even more dramatic. Northward, the break is thought to extend under the sea another 400 miles, possibly merging into the Murray Fracture Zone on the deep sea floor. At the southern end, the fault is believed to follow the axis of the Gulf of California for almost 1,000 miles. Because of the fault's great length, as much as 2,000 miles, movement is intermittent as to time, and random as to place. At any place along the fault, movement may be a one-time event, or movement may be recurrent over thousands of years. In any one episode, displacement may be a fraction of an inch or it may be many feet. The surface trace of the fault first appears at Point Arena, a peninsula north of San Francisco. It takes a straight path southeastward through a series of elongated valleys to San Francisco Bay, it crosses the bay with San Francisco on the western block and the bay cities on the eastern or continental block.

In the San Francisco Bay area, the San Andreas has two major companion branches. Generally defining the east side of the Bay is the Hayward fault, and a few miles inland is the Calaveras fault. The Hayward fault presently carries a high probability forecast for an earthquake. In an undeviating course, the trace passes San Jose and enters the Coast Ranges. It runs parallel to the Gabilan Range, passes the towns of Coalinga and Parkfield (both with vivid earthquake histories), thence it defines the elongated Temblor Range to Tejon Pass. This segment of the fault in central California is very old. Between the Bay area and the Tehachapi Mountains, rock formations appear to have been displaced as much as 160 miles. This suggests that the northern section of the fault may have originated about 30 to 40 million years ago, during Oligocene time. The fault planes of the faults in the system are usually near vertical. In some sections, however, the fault plane is locally tilted as much as 60 degrees to the northeast. A local example of this effect is the Banning fault near Cabazon in San Gorgonio Pass.

Here, the fault tilted to the point where the moving western block slid under the eastern block, a thrust fault. The San Andreas fault is a shallow fault, generally extending into the earth's crust four to ten miles. Earthquakes associated with shallow faults are more destructive at the surface than those originating from deep-seated faults. The San Andreas is an uninterrupted zone of many braided fault traces. These fractures branch and interlace in a swath two to eight miles wide along the fault's great length. With these multiple fractures creating great slices or slabs, and over countless episodes of earth movement, the rocks within the fault zone have been severely deformed, smashed and ground up. Thus weakened and broken, they become subject to rapid weathering and erosion. So, the fault course in central California has been eroded into a series of elongated valleys for more than one hundred miles through the Coast Ranges as a shallow trench only a few miles wide. This distinctive landform was made both from erosion of the shattered and weathered rocks and from the effect of recent faulting. The valleys contain many typical fault-related features such as offset streams, ponds, and scarps. This string of continuous valleys is recognizable on any large scale road map.

In southern California, the fault is much younger. Major right-lateral movement began about 12 million years ago, or during late Miocene time. This considerably predates the opening of the present Gulf of California which began about 5.5 million years ago. The progressively younger age of segments of the fault from north to south is consistent with ideas concerning plate movement along the west coast of North America. The southern California segment of the San Andreas fault is described from the vicinity of Gorman, about 60 miles northwest of Los Angeles. There, it bends abruptly to the east for six miles, then resumes its original southeast heading. This "Big Bend" area is possibly the most significant tectonic area in California today. Here, the San Andreas intersects the left-lateral Garlock fault, the only major east-west trending fault in southern California. Earthquakes are common in this area. The Fort Tejon earthquake in 1857 (magnitude 8-plus) is thought to have been at least as violent as the San Francisco earthquake 49 years later. The San Fernando earthquake of 1971 (magnitude 6.6) was also associated with this zone of intersection.

From Tejon Pass, the main trace of the San Andreas fault passes through the high desert north of Los Angeles, defining the north face of the San Gabriel Mountains. It then separates the San Gabriel Mountains from the San Bernardino Mountains, creating Cajon Pass that takes Interstate 15 out of southern California. From the vicinity of Cajon Pass, the southern California segment of the San Andreas fault becomes very complex with no distinctive single trace as in the north. Instead, it is divided into several right-lateral elements, all somewhat parallel to each other. Principal among these are the San Jacinto and Elsinore faults. Several branches go through the San Bernardino Mountains and along the north margin of San Gorgonio Pass into the Coachella Valley. One important associated fault is the San Gabriel fault, defining the southern edge of the San Gabriel Mountains in the Los Angeles basin. This fault was probably the most active strand for thousands of years, but there is no evidence of recent activity. Another important branch in the Los Angeles area is the Newport-Inglewood fault, the source of the destructive Long Beach earthquake of 1923. The Banning Fault The Banning fault, a subordinate branch of the San Andreas, first appears east of Riverside, trending almost due east.

It defines the north side of San Gorgonio Pass where a section is unique in being a thrust fault. This thrust is responsible for the low, brown-colored hills along the north side of Interstate 10 between Cabazon and Whitewater Canyon. The fault has created a distinctive gash behind these hills. In crossing Whitewater Canyon, it forms a barrier to the passage of groundwater down-canyon, supporting lush riparian growth in the floor of the canyon. The Banning fault enters the Coachella Valley near Whitewater, crossing Route 62 about 1.5 miles north of the interchange with Interstate 10. It intersects Indian Avenue one mile north of the freeway, and crosses Palm Drive at 20th Avenue, then continues to the southwest past Palm Springs. The surface trace of the San Andreas fault is conspicuous on the valley floor by the lineup of vegetation along its north side, a result of fault-dammed groundwater flow. This prominent feature is conpicuous as viewed from the observation post at the top of the aerial tramway. The Banning fault trace defines the southern margin of the Indio Hills, and supports several beautiful palm oases in the canyons where ground water reaches the surface as seepage along the fault trace.

Similarly, the northern margin of the Indio Hills is defined by the Mission Creek fault, and here, too, may be found palm oases, principally Thousand Palms Oasis. While the Banning fault is very well defined by surface features, its relationship to the main San Andreas fault has not been firmly established. The area in question is the complex fault geology of San Gorgonio Pass. The Mission Creek Fault About six miles north of the Banning fault, another branch, the Mission Creek fault, enters the Coachella Valley near the mouth of Morongo Valley. The Mission Creek fault passes directly through the town of Desert Hot Springs, and is responsible for the many warm springs in that area. It intersects Dillon Road near Wide Canyon Road, and marks the northern margin of the Indio Hills. Thousand Palms Oasis is on this fault. Locally, the Mission Creek fault is the most active branch in the system. Seismological data relating to recent earthquakes and measures from precise triangulation surveys show current persistent movement. Disrupted alluvial deposits, truncated older alluvial fans, and vegetation lines are evidence of its trace.

In particular, low scarps may be seen where Indian Avenue meets Route 62, and again at Miracle Hill, east of Desert Hot Springs. The Banning fault and the Mission Creek fault join at Biskra Palms in the Indio Hills near the north end of Madison Street). The San Andreas fault continues along the northwest shore of Salton Sea. At Salt Creek Wash, the surface trace of the great fault ends, more than 600 miles from its northern end. Current thinking is that the San Andreas fault, buried under alluvium, merges with the Imperial fault. It then continues southeastward to the Gulf of California, defining the western margin of the Gulf, finally merging into the East Pacific Rise. The South Pass Fault The inferred South Pass fault defines the southern margin of San Gorgonio Pass along the edge of the San Jacinto mountains. The fault has no surface trace, being deeply buried in alluvium. The principal rationale for placing a fault here is (1) the uniform straight mountain scarp, and (2) without a fault, it would be difficult to account for the pass structure. Where the fault enters the Coachella Valley, it curves to the southwest, past Windy Point to about Chino Canyon.

The San Jacinto Fault The most seismically active fault in southern California today is the San Jacinto fault and its branches. It is also the fault that is potentially troublesome to the Salton Trough. Many earthquakes within historic time have been associated with it. The San Jacinto fault is a major element of the San Andreas system. Separating from the San Andreas west of Cajon Pass, it takes a more southerly but generally parallel track. The trace bisects the city of San Bernardino, and passes directly under the four-level freeway interchange at Colton. Its path takes it behind the San Jacinto Mountains, passing the communities of San Jacinto and Hemet. Along the mountain front in this area, the fault has dammed groundwater channels, forcing water to the surface as hot springs. It lies west and south of the San Jacinto Mountains, and for several miles forms the northeast edge of Borrego Valley. The fault cuts the Ocotillo Badlands near Ocotillo Wells, then enters the Imperial Valley where its trace is buried under recent lake bed sediments. The known length of the fault is approximately 180 miles. Like the San Andreas, it is a complex system of many faults with local names.

At its northwest end, following its separation from the San Andreas fault near San Bernardino, it generally has one or more well-defined breaks, especially where its trace cuts across rocks of somewhat recent age. Southeastward, it includes several zones of subparallel breaks, separating masses of bed rock into slabs ranging in width from a few feet to thousands of feet. Although they are parts of the San Jacinto fault zone, many of these subsidiary faults are themselves major features and commonly are given individual names. One important subordinate fault is the Thomas Mountain fault that forms the southwest margin of Garner Valley. This pattern of multiple faults continues to broaden to the southeast where the San Jacinto zone enters the Salton Trough. There, at least six subordinate faults are spaced half a mile to three miles apart. This pattern continues through the Imperial Valley and into Baja California. The major breaks in the San Jacinto fault zone dip very steeply to nearly vertical, and the entire zone appears to be a very deep-rooted feature. There is abundant evidence of recency of movement along most of the length of the fault.

In its central section, along the San Jacinto Mountains and the Santa Rosa Mountains, there are many fault scarps, elongate trenches, sag ponds, aligned canyons, and offset drainage. Many earthquakes, moderate to severe in scale, have been recorded from the City of San Jacinto south into Mexico. The most active branch at this time is the Imperial fault, the source of many tremors and quakes in recent years, some of them very destructive. Another active branch is the Superstition Hills fault along the west side of the Imperial Valley. The very active Cerro Prieto fault on the Mexican side of the border is a member of the San Jacinto system, and is probably an extension of the Imperial fault. The San Jacinto fault is a young, right lateral zone of seismic strain that has dominated fault movement in southern California for a least a century. Notwithstanding the notoriety of the San Andreas fault, since 1857 there have been 36 major earthquakes identified to faults in the San Jacinto system. Of these, 15 have originated along the Imperial fault in the Salton Trough.

Between 1915 and 1954, five historic large quakes, all with magnitudes between 6.0 to 6.8 on the Richter scale, occurred along this fault between the City of San Jacinto and the Salton Sea. While presently more active than the San Andreas fault, the San Jacinto fault is much younger in age, having slipped laterally only about 15 miles, compared to nearly 200 miles of displacement along the San Andreas fault. The age of the San Jacinto fault is uncertain, but it seems probable that the fault has been active since at least early Tertiary time. The Elsinore Fault A brief mention of the Elsinore Fault is made here only because it is one arm of a trilateral split of the San Andreas Fault. As a geographic feature it is not associated with the Salton Valley. The third major fault in the southland is the Elsinore fault, also a member of the San Andreas system. Its trace is parallel to the San Jacinto, and about 20 miles west. The fault originates beneath the alluvium of the Los Angeles Basin. Running southeast, it sharply defines Elsinore Valley and Lake Elsinore).

Passing south of the Vallecito Mountains bordering Borrego Valley, the fault enters the Salton Trough near the Coyote Mountains in the extreme southwest corner of the Imperial Valley. Of the three principal branches including the San Andreas and the San Jacinto faults the Elsinore fault has been considerably less active in historic time. Movement along the San Andreas fault The San Andreas is a right lateral fault, with the relative movement of adjacent land masses being sideways. Along the San Andreas system, movement of the western block has been consistently to the northwest. Over millions of years, the cumulative displacement amounts to several hundred miles. Or, as Armand Eardley put it, "The San Andreas marks such an important contact that rarely can it be crossed, except in recent alluvium, without passing into significantly different rocks. Displacement, or the amount of movement along lateral faults is determined by identifying distinctive rock units found on opposite sides of a fault, but are some distance apart. One such technique is identification of rock debris in a sedimentary basin that could not possibly have come from the local mountains. The geologist then attempts to match up the sedimentary material with the appropriate source on the other side of the fault.

A local example of this occurs in Whitewater Canyon. A rock formation known as the Coachella Fanglomerate a coarse, bouldery ancient alluvial fan deposit is exposed where the Mission Creek fault crosses the canyon. Geologists have studied the boulder fragments in the Coachella Fanglomerate in detail, finding several distinctive types south of the Mission Creek fault that do not appear north of the fault. These rock fragments appear to have been derived from a source area close to the Cargo Muchacho Mountains, near the Mexican border. Such a correlation requires 130 miles of right-lateral separation within the San Andreas fault system. Studies at many locations between Soledad Pass and the Salton Sea have established that the San Andreas fault has a total offset of at least 140 miles. As the southern section of the San Andreas fault can be dated back to the Miocene Epoch, about 12 million years ago, this suggests an average displacement of more than one inch per year. Offset of the rocks has also been observed at many points along the length of the San Andreas fault. Older rocks appear to have been displaced more than 350 miles, while progressively younger rocks are displaced progressively fewer miles.

It is difficult to absorb the scale on which nature works! A displacement of hundreds of miles along the San Andreas fault can only be accepted when we understand that it is the result of an immense number of small steps occurring over a vast span of time.

Question: How can the land to the west of the Gulf of California be described?
Answer: Peninsula