The territory now part of the United States has been inhabited for from 15,000 to 40,000 years, as attested by local evidence. The aboriginal peoples, ancestral to today's American Indians, left no firm monuments on the scale of contemporaneous cultures elsewhere, but both the pueblos of the Southwest and the great mounds of the Mississippi River valley antedate the arrival of the European colonial powers. The original 13 British colonies that became the United States of America in 1776 were just one of several attempts by European powers to build empires in North America. All seized land from the native Indians, who then were usually either assimilated or driven off by superior European weapons. The Spaniards reached Florida as early as 1513 and New Mexico in 1540. The French began their exploration of the Mississippi River valley in 1673. The Russians reached Alaska in 1741. x1o8oc
Of all the colonizers, the British were the most successful. In 1607 Jamestown became the first permanent British settlement in North America and the foundation of the Virginia colony. It was followed 13 years later by the Pilgrim settlement at Plymouth, which was soon dwarfed by the Puritan colony of Massachusetts Bay. Most of New England was settled by Puritans fleeing either the harassment of Charles I or the orthodoxy of Massachusetts Bay. Pennsylvania was given to the Quaker William Penn as payment for a debt, and Maryland, a grant to the Roman Catholic George Calvert, was the first colony to establish religious freedom. New York, New Jersey, and Delaware were taken from the Dutch by the British in 1664, a year after the Carolinas had been granted to eight British noblemen. The 13th colony was Georgia, founded by James Oglethorpe in 1732 as a refuge for debtors and convicts.
When the British successfully evicted the French from North America in 1763, they embarked on a number of policies that the colonials found increasingly onerous. Settlement was prohibited west of the Appalachians and measures were passed to raise revenue in the colonies. These revenue-raising measures and Britain's generally exploitive mercantilist economic policy irked the colonials, who began to band together to oppose and subvert the measures. Britain increased its military presenceto enforce compliance (a presence part of whose cost was exacted from the colonials), and fighting broke out in 1775. The Second Continental Congress, acting for the 13 colonies, declared independence on July 4, 1776, and created. Articles of Confederation to govern the new nation. Victory over the British came in 1783, and the resulting Treaty of Paris established U.S. boundaries, except for Spanish Florida, west to the Mississippi River.

The Articles of Confederation provided a weak central government and proved inadequate to govern the growing nation. A new constitution was created in 1787, ratified in 1788, and took effect in 1789. George Washington was the first president, and his sober and reasoned judgments were instrumental in establishing both the tenor of the country and the precedents of the executive office. Under the new Constitution, the country began to grow almost immediately. By the Louisiana Purchase of 1803, the United States acquired from France the entire western half of the Mississippi River basin, thereby nearly doubling the size of the national territory. The movement into the lands west of the Appalachians thenceforth became a flood. The United States' victory in the Mexican War (1846-48) brought all or part of the future territory of seven more states (including California and Texas) into American hands.
As the United States moved west, the issue of slavery was intensifying strains between the rapidly industrializing North and the slave-based agricultural South. The South was determined to maintain the institution of black slavery against the federal government's efforts to curtail the latter's spread. Several compromises over the slavery issue held the Union together for more than a half-century, but the election as president in 1860 of Abraham Lincoln, whose Republican Party clearly advocated the prohibition of slavery in the Western territories, led South Carolina to secede, joined by 10 other Southern states by the next year.
Lincoln denied the Southern states' right to secede. The North's defeat of the South in the ensuing Civil War (1861-65) resulted in the preservation of the Union, the abolition of slavery, the establishment of citizenship for former slaves, and the institution of universal adult male suffrage. Lincoln's plans for magnanimity to the defeated South were cut short by his assassination, and Congress, completely dominated by northern Radical Republicans, embarked on its own, more punitive scheme of reconstruction. This system, which protected black civil rights in the South, came to an end with the withdrawal of federal (Northern) troops by 1877. Thereafter, Southern blacks were gradually disenfranchised and forcibly segregated within the larger society.
The post-Civil War United States was characterized by rapid industrialization, a continuing westward movement across the Great Plains, a massive influx of foreign immigrants, and the slow emergence of the United States into a position of world power. The westward movement fueled by the desire for land, led to a long series of evictions of Plains Indians from their lands onto less desirable reservations. Immigration from Europe exceeded 13,000,000 between 1900 and 1914 alone and provided labour for the North's burgeoning factories. When Cuba revolted against Spain in 1895, American sympathies and interests ultimately led to war with Spain (1898). Victory brought the United States its first overseas territories (the Philippines, Guam, Puerto Rico) and marked it as an emergent international power. The United States' rise to great-power status had its price. Though President Woodrow Wilson pledged neutrality in World War I, the United States was unable to remain outside the struggle. Its entry into the war in 1917 was decisive in bringing about an Allied victory and commenced American involvement in the European balance of power.

The prosperity of the decade that followed World War I came to a sudden end in 1929 when the stock market crashed and the Great Depression began. It ushered in an era of increased federal involvement in economic and social policy under President Franklin D. Roosevelt. His New Deal legislation revolutionized the country, but full economic recovery was still not achieved until war production became massive on the eve of World War II. The Japanese attack on Pearl Harbor brought the United States into World War II on the side of Britain and the Soviet Union against the fascist nations of Germany, Japan, and Italy. The war effort galvanized the American economy's productive capacity, and after victory was achieved in 1945 the United States experienced three decades of unprecedented economic growth and prosperity.
The Allied victory in 1945 left the United States the leader of the Western world, deeply involved in the reconstruction of Europe and Japan, but embroiled in 40-year-long rivalry with the Soviet Union that became known as the Cold War. In 1949 the United States formed the North Atlantic Treaty Organization in an effort to counter the Soviet military presence in eastern Europe, and a Soviet-inspired attack on South Korea involved the United States in the Korean War (1950-53), which ended in stalemate. The United States subsequently became involved in the Vietnam War (1955-75) in an effort to prevent communist North Vietnam from taking over South Vietnam. The prolonged and unsuccessful American war effort ended in a withdrawal of the United States from the conflict in 1973 and the fall of South Vietnam to the communists two years later.
At home the 1960s witnessed a successful protest movement by American blacks to outlaw racial segregation and discrimination and to obtain full voting rights in the South and other parts of the country. The expense of the Vietnam War drained resources away from liberal programs of social reform in the 1960s and early '70s, however, and the end of American involvement in the Vietnam War was accompanied by the Watergate scandal, which forced the resignation of President Richard M. Nixon in 1974.

The Cold War ended with the breakup of the Soviet Union in 1991, leaving the United States the undisputed superpower in the world. The most serious challenges late in the 20th century were economic ones, however. Beginning in the 1970s, rates of economic growth slowed and living standards stagnated or even fell as the American economy was forced to cope with increased foreign competition, its own steadily declining vigour, and the effects of massive budget deficits and a huge national debt.

Grand Canyon

Immense gorge cut by the Colorado River into the high plateaus of northwestern Arizona, U.S., noted for its fantastic shapes and coloration.
The broad, intricately sculptured chasm of the Grand Canyon contains between its outer walls a multitude of imposing peaks, buttes, canyons, and ravines. It ranges in width from about 0.1 to 18 miles (0.2 to 29 km) and extends in a winding course from the mouth of the Paria River, near the northern boundary of Arizona, to Grand Wash Cliffs, near the Nevada line, a distance of about 277 miles (446 km). The canyon includes many tributary side canyons and surrounding plateaus. The deepest and most impressively beautiful section, 56 miles (90 km) long, is within Grand Canyon National Park, which encompasses the river's length from Lake Powell to Lake Mead. In its general colour, the canyon is red, but each stratum or group of strata has a distinctive hue--buff and gray, delicate green and pink, and, in its depths, brown, slate-gray, and violet. At 8,200 feet (2,500 m) above sea level, the North Rim is 1,200 feet (350 m) higher than the South Rim.
The first sighting of the Grand Canyon by a European is credited to the Francisco Coronado expedition of 1540 and subsequent discovery to two Spanish priests, Francisco Garcés and Silvestre Vélez de Escalante, in 1776. In the early 1800s trappers examined it, and sundry government expeditions exploring and mapping the West began to record information about the canyon. By the 1870s, following the exploration of John Wesley Powell and others, extensive reports on the geography, geology, botany, and ethnology of the area were being published.
Grand Canyon National Park, now containing 1,904 square miles (4,931 square km), was created in 1919. Its area was greatly enlarged in 1975 by the addition of the former Grand Canyon National Monument and Marble Canyon National Monument and by portions of Glen Canyon National Recreation Area, as well as other adjoining lands. The North and South rims are connected by a 215-mile- (346-kilometre-) long paved road and by a transcanyon trail. Scenic drives and trails lead to all important features. Mule-pack trips down the canyon and rides down the river in rafts and power-driven craft are intensively sought-after ways of viewing and experiencing the vast beauty of the canyon. Many pueblo and cliff-dweller ruins, with accompanying artifacts, indicate prehistoric occupation. There are five Indian tribes living on nearby reservations.

Geologic history

Although its awesome grandeur and beauty are the major attractions of the Grand Canyon, perhaps its most vital and valuable aspect lies in the time scale of Earth history that is revealed in the exposed rocks of the canyon walls. No other place on Earth compares with the Grand Canyon for its extensive and profound record of geologic events. The canyon's record, however, is far from continuous and complete. There are immense time gaps; many millions of years are unaccounted for by gaps in the strata in which either vast quantities of Earth materials were removed by erosion or there was little or no deposition of Earth materials. Thus rock formations of vastly different ages are separated only by a thin, distinct surface that reveals the vast unconformity in time.
Briefly summarized, the geologic history of the canyon strata is as follows. The crystallized, twisted, and contorted unstratified rocks of the inner gorge at the bottom of the canyon are granite and schist about two billion years old. Overlying these very ancient rocks is a layer of limestones, sandstones, and shales that are more than 500 million years old. On top of these are rock strata composed of more limestones, freshwater shales, and cemented sandstones that form much of the canyon's walls and represent a depositional period stretching over 300 million years. Overlying these canyon rocks is a thick sequence of Mesozoic Era rocks (245 to 66.4 million years old) that form precipitous butte remnants and the vermilion, white, and pink cliff terraces of southern Utah but which have been entirely eroded away in the area of the Grand Canyon proper. Of relatively recent origin are overlying sheets of black lava and volcanic cones that occur a few miles southeast of the canyon and in the western Grand Canyon proper, some estimated to have been active within the past 1,000 years.

The cutting of the mile-deep Grand Canyon by the Colorado River is an event of relatively recent geologic history that began not more than six million years ago, when the river began following its present course. The Colorado River's rapid velocity and large volume and the great amounts of mud, sand, and gravel it carries swiftly downstream account for the incredible cutting capacity of the river. Prior to the building of the Glen Canyon Dam, the sediments carried by the Colorado River weremeasured at an average of 500,000 tons per day. Conditions favourable to vigorous erosion were brought about by the uplift of the region, which steepened the river's path and allowed deep entrenchment. The depth of the Grand Canyon is due to the cutting action of the river, but its great width is explained by rain, wind, temperature, and chemical erosion, helped by the rapid wear of soft rocks, all of which steadily widened it. Amazingly, the canyon was cut by a reverse process, for the river remained in place and cut through the rocks as the land moved slowly upward against it. Only thus can be explained the canyon's east-to-west course across a south-facing slope and the presence of plateaus that stand across the river's course without having deflected it.
The most significant aspect of the environment that is responsible for the canyon is frequently overlooked or not recognized. Were it not for the arid climate in the surrounding area, there would be no Grand Canyon. Slope wash from rainfall would have removed the canyon walls, the stairstep topography would long ago have been excavated, the distinctive sculpturing and the multicoloured rock structures could not exist, the Painted Desert would be gone, and the picturesque Monument Valley would have only a few rounded hillocks.

Biological past and present

Plant and animal fossils are not abundant in the Grand Canyon's sedimentary rocks and are confined mostly to primitive algae and mollusks, corals, trilobites, and other invertebrates. Animal life in the Grand Canyon area today is varied and abundant, however. The common animals are the many varieties of squirrels, coyotes, foxes, deer, badgers, bobcats, rabbits, chipmunks, and kangaroo rats. Plant life is also varied. In the bottom of the canyons are willows and cottonwoods, which require abundant water during the growing season. At the other end of the moisture scale are drought-resistant plants such as the yucca, agave, and numerous species of cactus.
On the canyon rims, north and south, there is a wide assortment of plant life. Typical of the South Rim is a well-developed ponderosa pine forest, with scattered stands of piñon pine and juniper. Bush vegetation consists mainly of scrub oak, mountain mahogany, and large sagebrush. On the North Rim are magnificent forest communities of ponderosa pine, white and Douglas fir, blue spruce, and aspen. Under less optimum conditions the plant life reverts to the desert varieties.

Grand Canyon Series

Major division of rocks in northern Arizona dating from Precambrian time (about 3.8 billion to 540 million years ago). The rocks of the Grand Canyon Series consist of about 3,400 m (about 10,600 feet) of quartz sandstones, shales, and thick sequences of carbonate rocks. Spectacular exposures of these rocks occur in the Grand Canyon of the Colorado River in northwestern Arizona, where they overlie the strongly deformed and contorted Vishnu Schist, the angularity of which stands in bold contrast to the almost horizontal bedding of the Grand Canyon Series. The Grand Canyon Series actually dips slightly eastward and is separated from the overlying Cambrian sandstones by a major erosion surface unconformity. A conglomerate was deposited on the eroded surface of the Vishnu Schist. Limestones, shales, and sandstones occur over the conglomerate and are thought to represent shallow water deposits. The area of deposition was probably a large deltaic region that was slowly subsiding, allowing great thicknesses of sediment to accumulate near sea level . The presence of Precambrian organisms is indicated by calcareous algaelike structures in the carbonate rocks, as well as by tracks and trails of wormlike creatures in other rocks. Initially, in a generalized outline of the Precambrian history of the region, the Vishnu Schist was upraised, folded, and metamorphosed and then slowly eroded and worn down to a flat surface. The Grand Canyon Series was deposited perhaps as part of a slowly subsiding geosynclinal trough. The region was then subjected to uplift and tilting, and a Precambrian period of erosion for the Grand Canyon Series began. This action was later followed by a long period of deposition during the Paleozoic Era (540 to 245 million years ago) and then further erosion during the Cenozoic Era (beginning 66.4 million years ago) until the region assumed its modern configuration.