Geology

The complexity of the Appalachian’s geology and physical geography is as impressive as their immensity and biological diversity.

Compared to the Earth's tallest mountains, the Appalachians are hardly more than forest-covered hummocks that were rounded, weathered, and diminished by erosion long before the summits of the Alps, Andes, Himalayas, or Rockies were created. While the still-growing Himalayas exceed 25,000 feet at more than thirty places and Alaska’s Mount McKinley rises to 20,300 feet, the loftiest point in the Appalachians—Mount Mitchell in North Carolina—stands at a modest 6,684 feet. Only in the southern Blue Ridge, the highest and easternmost subdivision of the ancient highlands, are there numerous peaks above 6,000 feet. The Alleghenies, extending from central Pennsylvania, across western Maryland, and down through eastern West Virginia, average only 2,000 feet in the north and 4,000 feet in the south. Nevertheless, the contrasts between the steep flanks of the mountains and the sprawling Great Valley extending from Pennsylvania to Alabama endow the entire region with marvelously scenic topography.

The complexity of the Appalachians’ geology and physical geography is as impressive as their immensity and biological diversity. Rather than a chain, the Appalachians constitute a system encompassing no fewer than four distinct geological provinces: the Piedmont, the Blue Ridge, the Appalachian Plateaus, and the Ridge and Valley, each with a distinctive topography and geological history. South to north, they span several climate zones. Summers in Birmingham, Alabama, rival the tropics while winters on New Hampshire’s 6,288-foot Mount Washington record temperatures that approach fifty degrees below zero and wind gusts far in excess of one hundred miles per hour.

Though muted beside snow-capped mountain chains of the world, the rugged, forested Appalachian landscape and its underlying geologic structures and resources have shaped life and culture in the region from the arrival of humankind. Fluted projectiles made from local stone have been found throughout the southern mountains, indicating that Paleo-Indians were living in the region about 10,000 B.C. Caves and rock shelters, common from north Alabama to eastern Kentucky, have yielded evidence of animal butchering. In the Great Smoky Mountains, the discovery of tools and projectiles made of stone from plateaus far to the west suggests that hunting parties traveled eastward into the highlands in search of game. Centuries before Christopher Columbus arrived on the shores of the New World, permanent villages existed in valleys where rich soil had accumulated from the inexorable erosion of the mountains and near rocky shoals where fish could be readily taken from major streams.

Just as the mountains framed the lives of native peoples for millennia before the arrival of the first Europeans, so did they influence the course of European exploration, migration, and displacement of Indian tribes. Early European settlers made use of Indian hunting trails and followed their leads in selecting settlement sites and places to farm. Until the rise of the salt, coal, and timber industries, mountain topography primarily determined where new arrivals put down roots on the frontier.

But after European colonists took up life on the coastal plain, a century and a half passed before settlers made a concerted push from the Tidewater and the Piedmont across the daunting Blue Ridge. (The Spanish explorer Hernando de Soto had led an expedition up from the southeast to the North Carolina Piedmont, crossing the Blue Ridge into present-day Tennessee in 1540. Though he failed to find the gold he sought, his men unleashed infectious diseases among the Cherokee people, and the expedition apparently led to the later appearance of “Appalachee” on sixteenth-century maps.) Crossing the Blue Ridge was particularly difficult, with the southern portion presenting the greatest challenge. In North Carolina, the nearvertical Blue Ridge Escarpment rose to more than six thousand feet, including the highest elevations in the eastern United States. Natural openings were few. Indeed, along the Blue Ridge from southern Pennsylvania to north Georgia, only three natural water gaps provided passageways through the mountains into the Great Valley. One was in northwestern North Carolina along the New River, another near Roanoke, Virginia, where the James River passed through the ridge on its way to the Chesapeake Bay, and a third in southern Pennsylvania. The last, in the area of the Susquehanna River, was a seventy-mile stretch where the “ridge” was nothing more than rolling, low-lying hills. Through the area, beginning around 1730, a stream of northern Europeans—Germans, Scots, Irish, and Quakers—poured into the Appalachian Valley from the east, establishing productive farms in the limestone soils and pressing ever southward in a wave of migration that reached all the way to the Tennessee Valley.

From the Great Valley, westward-bound settlers moved into the folded highlands of the parallel Ridge and Valley Province by following valleys that streams had carved from sandstone ridges and shale-bottomed lowlands. But travelers bound for the West found before them the imposing Allegheny Structural Front, a stone escarpment rising as high as 1,000 feet, separating the mountain ridges from the sprawling Appalachian Plateaus. There, even the passages opened by major streams were steep and narrow gorges. Historians would later consider the Allegheny Front second only to the Rocky Mountains as a natural obstacle to western migration in the United States. Except for broad interstate highways blasted through the rock, even modern-day roads take steep and winding courses across the escarpment. Beyond it, the topography again changes dramatically, though the contrast is far less striking than the demarcation between the Great Valley and the ridges on either side of it. In the Appalachian Plateaus Province, consisting basically of the Allegheny Plateau in the north and the Cumberland Plateau from Kentucky southward, the underlying rock has been less deformed and lies generally flat, its topographic features more random and its drainage patterns similar to the branches of a tree.

Far more than in the rolling Piedmont, the Great Valley, or the broad, rounded ridges of the Alleghenies in the northern Appalachians, topography has constrained and shaped human circumstance across both the Ridge and Valley and Appalachian Plateaus Provinces of the central and southern highlands.

There, early settlers, often taking their cue from the native peoples they displaced, gravitated to coves, hollows, and valleys that provided shelter from storms and offered arable soil, good water, and abundant timber. Mountain ridges and streams served as property lines, isolated developing communities, and sometimes marked political jurisdictions. The Virginia-Kentucky border follows the crest of Pine Mountain, for example, and the crest of the Blue Ridge separates North and South Carolina along a forty-mile stretch. More commonly, mountain ridges served as lines separating counties.

While early settlers settled in natural bowls with the best soil, those who came later established themselves in the heads of hollows, on mountainsides, and on remote ridges where survival was more problematic, thus beginning what would become enduring social hierarchies. As historian Ronald D. Eller observed in his widely read study Miners, Millhands, and Mountaineers, the land shaped the culture and social patterns of Appalachia perhaps more profoundly than any other rural area of the country.

The rise of twentieth-century technology, construction of modern roads, and social intervention by state, local, and federal governments notwithstanding, the mountains remain an elemental force in the identity and self-perception of people who live in the highlands, in the organization of society, and in the evolution of the region’s economic life. People who left the region in waves of migration, particularly from the coalfields of the Appalachian Plateaus, often expressed feelings of deep, persistent, and visceral longing, not only for friends and family but for the mountains themselves.

For geologists, the saga of the Appalachians’ creation and the evolution of the region’s topography are sources of endless fascination and continuing research. Some have contended for generations that the entire system is much older than the tallest mountains now on Earth and that far back in geologic time antecedents of the modern Appalachians towered as high as the present-day Rockies. But only in the 1960s did geologists settle on a satisfactory hypothesis for the origin and evolution of the major mountains chains of the world. Now generally accepted, the theory of plate tectonics was discussed for decades before research produced decisive new data. The last crucial piece of the explanation fell into place with confirmation that the floor of the Atlantic Ocean is spreading—America and Europe are drifting apart by a couple of inches a year. Moreover, since magnetite particles in molten rock line up with Earth’s magnetic field as they cool, scientists were able to deduce that rocks, indeed continents, were formed far from their present location.

The world’s great mountain chains, it is now accepted, formed when continental masses of Earth’s outer crust, drifting on a molten or semi-molten mantle, collided and their margins buckled upward under millions of years of indescribable force.

The last of several mountain-building epochs in Appalachian prehistory, an event known as the Alleghenian Orogeny, came about 248 million years ago when the present North American continent came together with the landmass that is now western Africa. The eastern margin of North America was shoved about 160 miles to the west, and the grinding pressure raised the southern Appalachians to heights comparable to the tallest mountains on Earth today. The highlands of that event were leveled by erosion over the more than 150 million years that followed. About 65 million years ago, in an episode still not clearly understood, a general continental uplifting raised the entire eastern United States. The modern Appalachians appeared in that mighty uplift as rejuvenated streams sculpted the rising land. They are, therefore, remnants of earlier mountains and the product of episodic uplift, destruction, and rebirth. Estimates are that 50 million years from now, the Appalachians will have again worn down to an essentially featureless plain.

Confirming the mountains’ most distant origins, geologists have found and dated rocks much older than the mountains themselves—billion-year-old “basement rocks” raised from the depths during Earth’s first mountain-building episode back in deep time, long before life appeared on the planet. The saga of the Appalachian system itself began about 700 million years ago with the breakup of a supercontinent called Rodinia. Over a span of some 500 million years, collisions between five continental masses left from Rodinia’s destruction produced three distinct periods of upheaval, the last the Alleghenian Orogeny. The “Tectonics” entry in the following section traces these events in more detail.

Besides creating a topographical template for human settlement and cultural evolution, the global wandering of the tectonic plates led to the formation of bituminous coal beds found beneath 63,000 square miles of the Appalachians from Pennsylvania to Alabama, as well as the famed anthracite beds in northeastern Pennsylvania. During the millions of years when the present-day Appalachian region lay near the equator, tree-sized ferns, horsetails, and mosses grew and died in steamy swamps. As the plants fell into oxygen-depleted water, they decayed into deepening beds of peat, which mounting temperature and pressure transformed into coal. Three hundred million years later, the Sun’s energy thus stored by the plants’ process of photosynthesis was released in burning coal, powering the steam engines and industries of the American Industrial Revolution.

While no match for the Rockies or other major mountain systems, the Appalachians have produced significant metallic deposits in times past and continue to be a source of nonmetallic minerals vital to a number of industries. Gold was first discovered in North America in North Carolina and later mined in the Piedmont Province in Virginia and Georgia as well as the Carolinas. Reminders of the gold-mining days remain in the names of communities such as Goldvein, Virginia. Iron, tungsten, copper, lead, and zinc have been profitably extracted from the region as well. Important from colonial times forward, Silurian Clinton iron ore combined with locally available limestone and coal to provide the basis for the steel industry of Birmingham, making the area a southern counterpart of Pittsburgh, the de facto capital of the Industrial Revolution in the United States. The fact that the Appalachians were once more than double their present height, however, has led to speculation that their chief mineral lodes were carried away during 200 million years of relentless erosion.

In spite of erosion and pulses of uplift, the strata of the modern Appalachians and the debris of the previous highlands record not only the tectonic history of the region but the evolution of life from shelled animals, vertebrates, amphibians, and reptiles as well. Evidence shows that Appalachia’s first true forests, consisting of treesized ferns, appeared in the late Devonian period, a harbinger of the coal formation to come during the Pennsylvanian period.

Striking fossilized tracks of dinosaurs that walked the Appalachian Basin during the Triassic and Jurassic periods some 248 to 144 million years ago have been found in basins of the Piedmont Province. Outside the town of Stevensburg in Culpeper County, Virginia, for example, a six-acre area contains no fewer than two thousand dinosaur tracks, most of them left by a three-toed carnivore called Dilophosaurus. In the Ridge and Valley Province are two significant vertebrate sites: a sinkhole at Gray, Tennessee, that formed about five million years ago and contains fossils of many plants and animals, including tapirs, rhinoceroses, turtles, and mastodons; and Saltville, Virginia, notable for fossils of Pleistocene mastodons.

The Appalachians are undoubtedly the most thoroughly studied mountains on Earth. Early investigations of the structures from the Blue Ridge to the Ridge and Valley gave rise to modern concepts concerning the formation of folded mountains. Recognition of the extraordinary thickness of sedimentary rocks involved in the formation of the Appalachians led to the understanding of sediment accumulation in the geoclines that border opening oceans such as the Atlantic. But besides playing an important role in the evolving science of geology, the mountains have exerted profound influence on the spirit and the character of the people of the region, from the Native Americans who lived in harmony with the land, water, and forests to modern Americans whose technologies have brought accelerating and permanent change to the landscape.

 

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