Geologic Zones
Tennessee is a narrow state over 500 miles long, with its long axis running east-west across the grain of the geology. Most of the geological provinces of the east-central United States are represented somewhere in Tennessee. Since geology exercises a controlling influence on topography, hydrology, mineral resources, and soil type and thickness, which in turn affect the activities of humankind, it has strongly influenced the history and development of the state.
Tennessee consists of three grand divisions which have been recognized from the earliest days of settlement and simply called East, Middle, and West Tennessee. Each of the grand divisions is divisible into three geologic zones, all with differences in basic geology that distinguish them from each other and cause some areas to resemble parts of neighboring states more than adjacent parts of Tennessee.
East Tennessee includes the Unaka, or Great Smoky, Mountains along the border with North Carolina, the Valley and Ridge Province (also known as the Great Valley of East Tennessee), and all or part of the Cumberland Plateau/Mountains region. In this part of the state, boundaries between regions and geologic zones run northeast-southwest rather than north-south, and geologists and geographers disagree on definitions for them. Some consider the Cumberland Plateau to be in East Tennessee, others say Middle Tennessee; most compromise on an irregular, north-south line running up the middle. No matter how it is defined, parts of Middle Tennessee near Kentucky actually lie farther east than parts of East Tennessee near Chattanooga.
Middle Tennessee includes a low-lying central area called the Central (or Nashville) Basin completely surrounded by the higher ground of the Highland Rim. The valleys of the Cumberland River, flowing into the state from the north, and the Elk River, flowing southward from the basin into Alabama, bisect the Highland Rim, dividing the area into eastern and western provinces.
West Tennessee begins at the northward-flowing reach of the Tennessee River, where it crosses Tennessee from the Mississippi-Alabama corner to Kentucky. West Tennessee includes the Western Valley of the Tennessee, the Plateau Slope of West Tennessee, and the Tennessee portion of the Mississippi River Flood Plain at the western edge of the state.
Travelers moving from west to east across Tennessee alternately cross higher and lower topographical belts representing each of the nine geologic provinces. Parts of the Mississippi River Flood Plain, the lowest area, are less than 200 feet above sea level. East of the Chickasaw Bluffs is the Plateau Slope, some 200 to 300 feet higher, which forms a comparatively level stretch across most of West Tennessee. Near the eastern edge of the Plateau Slope, a region of eroded, sandy hills borders the 300-foot drop-off into the Western Valley, where the Tennessee River (now impounded and called Kentucky Lake) stands at a level of about 360 feet. Across the river, the elevation climbs more than 400 feet to the Western Highland Rim before descending about 300 feet into the Central Basin. Across the basin the rise onto the Eastern Highland Rim is about 500 feet, and across the rim another rise of approximately 1,000 feet reaches the Cumberland Plateau. The eastern edge of the plateau is about 750 feet higher than the lowlands of the adjacent Valley and Ridge Province. Very long linear valleys paralleled by ridges, all running northeast to southwest, with local relief of several hundred feet or more, characterize the Valley and Ridge. The Unaka Range at the eastern end of the state has the highest elevations and the greatest local relief in the state, ranging from a high of more than 6,600 feet atop Clingman's Dome to about 1,000 feet near Pigeon Forge, more than a mile of vertical difference across half the width of Sevier County. A visual profile across the length of Tennessee forms a series of terraces stepping up and down, with the height of the terraces generally increasing to the east.
West Tennessee, except for the Western Valley, is geologically part of the Gulf Coastal Plain, with the valley of the Mississippi River cut into it and its flood plain sediments deposited on top. This part of the state is geologically young, consisting of Cretaceous, Tertiary, and Holocene Age deposits. Unique in Tennessee, at a depth below the northwestern part of the region, is the New Madrid seismic zone, capable of generating devastating earthquakes.
The Western Valley, Highland Rim, and Central Basin are all part of the Interior Low Plateaus physiographic region of the east-central United States. Topography and surface geology are controlled by differential rates of erosion of rocks of the Ordovician, Silurian, Devonian, and Mississippian Ages, modified by the tectonic uplift of broad swells and arches such as the Nashville Dome (essentially coextensive with the Central Basin).
The Cumberland Plateau and Cumberland Mountains are part of the great Appalachian Plateau extending from New York to Alabama. The strong relief of this coal-bearing region is due to its cap of hard, erosion-resistant sandstone layers of Pennsylvanian Age.
The Valley and Ridge Province is also part of a much larger region extending beyond the borders of Tennessee to New England and Alabama. The rocks, ranging in age from Cambrian to Pennsylvanian, have been folded and faulted repeatedly by pressure from the southeast that built the Appalachian Mountains and a number of predecessor ranges.
The Unaka Mountains are part of an extensive system of mountain ranges extending from New England to Georgia and Alabama. The system is very ancient, more than a billion years old, and has been reactivated several times. The present mountains owe their relief to uplift and erosion during and since the Permian Period.
