Appalachians

The Appalachian Mountains form one of the most prominent features of the North American continent, dominating the topography of the Atlantic Coast. They mark the eastern continental divide, influence the climate of the region, and provide a record of the transformation of the Earth through a vast range of geological epochs.

Location and Characteristics

The Appalachian Mountains, in their present form, are a chain of relatively low but steeply inclined ridges that follow the East Coast of North America for 3,200 kilometers (2,000 miles). From Newfoundland in easternmost Canada to Alabama in the southeastern United States, they extend across nearly 20 degrees of latitude and more than 30 degrees of longitude. They were named for an Indigenous Nation known as the Apalachees (or Apalachis) by the Spaniards who followed Hernando de Soto in 1540 across the southern part of the chain, as de Soto had stayed with the tribe in southern Georgia before his expedition into the mountains. The term Appalachian Mountains was gradually applied to the entire range, superseding local designations such as "Allegheny," even though the mountains are not a single continuous range but rather a complex of separate mountain groups whose boundaries tend to overlap. They vary from 25 to 125 kilometers in width and are described by geologist and author John McPhee as a "long continuous welt… long ropy ridges of the eastern sinuous welt."

Running roughly parallel to the mountain ridges, valleys of varying width divide the highland ranges. Some are narrow and deeply wooded, some broad and gently contoured. They all are essentially a part of the Great Valley of the Appalachians, known locally as the Shenandoah or Tennessee Valley, or some even more specific local designation. In most places, the valley is several hundred meters above sea level, while the highest points of the range rise to 2,037 meters (Mount Mitchell) in the Black Mountains in northwest North Carolina, to 2,024 meters (Clingman's Dome) in the Great Smoky Mountains near the Tennessee border, and to 1,917 meters (Mount Washington) in the White Mountains of New Hampshire. There is no continuous crest to the chain; the highest peaks are found either at the center of the mountain range or on its northwest side. This geologically interconnected valleys and ridges has been subdivided into geologic provinces with common general features but distinct local characteristics.

In the southern segment, four belts (or provinces) have been described that run the length of the chain. The farthest northwest is the Appalachian Plateau Province, including the Poconos in Pennsylvania, the Cumberlands in Kentucky, and the Alleghenies in West Virginia. Despite the term "plateau," the land frequently is cut by deep valleys and rugged hills. The area supports some small-scale farming but is primarily valuable as a source of coal, oil, and gas, which has led to the growth of Rust Belt industry around its rim.

The second belt is designated the Valley and Ridge Province because it is most characteristic of the pattern that typifies the entire chain. The most prominent mountain groups in this province are the Clinch Mountains in Virginia, the Shawangunks in New York, and the Kittatinny Mountains in Pennsylvania. The mountains are arranged along stretches of fertile valleys among the most productive farming regions in the eastern United States, valleys fed by the rivers that have formed them in the rivers' courses from the higher peaks. The Delaware Water Gap is typical, with fields and woodlots sculpted by the rush of water over land and, above the flatland, with huge rock deposits such as the Martinsburg formation, a very large aggregate of slate used in blackboards and pool tables.

The third province is called the Blue Ridge, running from the low Highlands of New Jersey to a single, massive ridge through the central Appalachians. There, it rises abruptly from the west and drops off just as sharply in the east. From 10 to 20 kilometers across, it diverges into two ridges south of Roanoke, Virginia, where it runs in two roughly parallel chains through the Southern Highlands. Between these two ridges, additional connecting ridges (including the Black Mountains) occur before the two border ridges end fairly abruptly in northern Georgia. The well-known Blue Ridge Parkway, built as a Works Progress Administration project in the 1930s, follows the crest of the eastern ridge from Virginia to the Great Smoky Mountains National Park. This section is the least populated of the Appalachians; not especially suitable for agriculture or industry but scenically exceptional, the Blue Ridge Province has many summer resorts scattered amid the previously isolated communities of mountain pioneer families.

The fourth belt is the Piedmont Plateau, which slopes gradually from the Blue Ridge toward the Atlantic coastal plain. The availability of timber and water resources in this belt has led to the development of an extensive furniture industry as well as textile mills in many of its small cities, especially along the fringes toward the coastal flatlands.

The Northern Appalachians correspond primarily to the Blue Ridge and Piedmont Plateau Provinces but are not as clearly demarcated. The Blue Ridge, while not actually connected, is replicated by the Hudson River highlands through the Berkshire Hills in Massachusetts and the Green Mountains of Vermont. The remainder of New England and the Canadian Maritime Provinces resemble the Piedmont Plateau. Toward the ocean, the land is generally regular, with minor topographic features, while linear ridges and hilly belts predominate closer to the line of the mountains, where the effect of glaciation is more pronounced. During the early days of the United States, this was a thriving agricultural region, but the immense growth of urban centers transformed it into a manufacturing and communications complex, and extensive efforts are underway to preserve the unique features of the Appalachian wilderness.

The prominence of the Appalachian Mountains in the eastern United States has had a significant influence on the culture and economy of the region. Perhaps most notably, the area roughly from southwestern New York to northern Georgia and Alabama is often referred to as "Appalachia." Though definitions and actual characteristics of this cultural territory vary, public perception tends to stereotype the mountainous area as a backwoods, "redneck" community. This belief is somewhat rooted in the isolation provided for centuries by the rugged terrain, and the interrelated economic difficulties that persist into the twenty-first century. At the same time, the mountains account for some of the region's most important economic activity, from mining to tourism. People have long been drawn to the natural beauty of the Appalachians, and outdoor recreation was boosted by the opening of the Appalachian Trail, running between Maine and Georgia, in the 1920s and 1930s.

Formation

The present form of the Appalachians is very different from that maintained during earlier geological eras. The mountains have risen and then been worn down several times within the scope of historical observation—what McPhee called "the result of a series of pulses of mountain-building, the last three of which have been spaced across two hundred and fifty million years." Before that, the North American continent formed part of a larger, supercontinental entity. During the latter part of the Proterozoic eon (approximately 600 million years ago), the cratonic cores of North America, Europe, and an immense entity known as Gondwanaland separated and began to drift apart. During that time, the evidence of marine deposits in sedimentary rock indicates that the North American continent was a low-lying, relatively flat landmass. The effects of weathering and erosion over a long period produced this level surface, and glaciation contributed to a worn, scarred appearance. Because of what is known as epeirogenic movement (the rise and fall of landmasses), the craton was sometimes covered to a large extent by water. During the Cambrian period (from 544 to 505 million years ago), much of the North American landmass was submerged, and a tropic or subtropic climate existed.

At that time, the first stirrings of the Taconic orogeny began. This surge of energy, an upheaval in which sheets of oceanic lithosphere were thrust toward the continental interior, produced the first "modern" Appalachian chain. In the Ordovician period (from 505 to 440 million years ago), the sedimentary and volcanic rock that had been built up was fractured and crumpled. The deformation (or deconstruction) of the original mountains was so great that slices of the range were transported considerable distances, folding the mountains upon themselves. The general direction of plate motion was westward. As the high-density material of the upward-thrusting oceanic plate depressed the low-density material of the craton, forcing it downward into what is called a subduction zone, the low-density material eventually rebounded to produce orogenic waves, which further crumpled and folded the mountain chain.

In the Early Paleozoic era, the continents converged, producing the Acadian orogeny (about 350 million years ago). As the North American and African landmasses moved toward a collision, the violence of the Acadian orogeny folded and faulted the sedimentary rock that remained from the erosion of the Taconian orogeny. The intense temperatures and pressures generated by the mountain-forming process also metamorphosed some of the rock, changing the shales into slates, the sandstones into quartzites, and the limestones and dolomites into marble. The final episode of mountain building took place between 280 and 240 million years ago and is known as the Alleghenian orogeny. Occurring quite some time after the initial contact between North America and Africa, it may have been the consequence of an additional impact, possibly with a microplate (wandering island formation) such as New Guinea, Madagascar, Fiji, or the Solomon Island system. These smaller masses, known also as exotic terrains because of their separation from the main continental blocks, may have collided with the North American continent to trigger the most recent orogenic episode. The proto-Atlantic Ocean was probably in the process of closing when the first collision between North America and Africa occurred, but by the Alleghenian orogeny, the continents were closer to relative fixture (or stability), which suggests that another island arc may have been responsible for the orogeny.

This last orogenic episode was a time of tremendous upheaval, and one of its products was the creation of the extensive anthracite deposits of eastern Pennsylvania. These coal fields were formed when severe pressure folded carbonaceous rock at least one kilometer beneath the surface. Erosion and an eventual balancing of forces that ended the upheavals brought coal seams closer to the surface, which led to the mining industry of the Appalachian region. Similarly, petroleum deposits—the transmuted fossils of ocean algae—were formed when rocks were heated to about 50 degrees Celsius and remained between 50 and 150 degrees Celsius for at least one million years. The rocks that were once part of the western Atlantic Ocean were driven by orogenic waves onto the North American continent, and, in the process of mountain building, in some areas and under the proper conditions of temperature and pressure, they formed oil deposits. The richest deposits in the Appalachians were in Pennsylvania and Ohio, where oil seeped out of the ground in such purity that it could be used prior to refining and was once sold as a health tonic. Much of this oil has been extracted, but geologists speculate that there may still be deposits of oil and natural gas off the South Atlantic Coast—the product of the process that formed the Appalachian Mountains.

Although the Appalachians in their present form are not as spectacular as they once must have appeared when their highest peaks stood from eight to ten kilometers above the Earth's surface, the terrain is continuously varied and visually engaging. In the deformed, sedimentary Appalachians—the present topographical form of the chain—the rock "not only has been compressed like a carpet shoved across a floor," as John McPhee observed, "but in places had been squeezed and shoved until the folds tumbled forward into recumbent positions." An overview reveals a consistent sinuosity: The mountains bend right into Georgia, left into Tennessee, right into North Carolina, and so on toward Newfoundland. Some geologists speculate that this pattern represents the coastline of North America in Precambrian times.

In the 2020s, the range was noted to store more than 50 percent of the eastern region's above-ground carbon. Preserving these mountains is critical in the fight against climate change. They also serve as a critical migration corridor, with sixty-four high-priority bird species rotating annually in the early 2020s. Additionally, 80,000 rare species are found in the Appalachians, including many bobcat, bear, wolf, bison, elk, and moose species.

Principal Terms

craton: a part of a continent that has been free of significant structural rupture for a long time; often a region with a thin covering of newer rocks

erosion: the process by which the surface of the Earth's crust is gradually broken down and worn away

fault: a fracture in the Earth's crust along which there has been some displacement or deformation

fold: the deformation of rocks caused by external pressure

glaciation: the effect of a glacier on the terrain it transverses as it advances and recedes

lithosphere: the outer, rigid shell of the Earth's crust; it includes the continental plates and the oceans

metamorphic rock: rock altered by means of intense heat

orogeny: a major event in which tectonic processes combine to radically alter the Earth's crust; mountain building

tectonics: the process by which the broken shell of the Earth's crust (the lithospheric plates) moves in varying directions to create geological features

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