Here is a look at Tennessee physical geography beginning with the eastmost region and moving westward to the Mississippi River.
Unaka Mountains: The bedrock here consists of a variety of igneous and metamorphic rocks, and is quite resistant to erosion. Due to the resistance of these rocks to erosion, and uplift associated with the mountain-making processes of the past, and the isostasy (equilibrium in the earth's crust such that the forces tending to elevate landmasses balance the forces tending to depress landmasses) of this area, the elevation throughout this area is generally 1000's of feet above sea level.
Valley and Ridge: This area consists of a large number of thrust-faulted layers or thrust sheets of rock dipping to the east at low angles. Imagine a deck of cards lying in a neat pile on a table. Now imagine a dealer spreading those cards out so that the stack is now splayed out over a much larger surface area of the table top. That should give you some idea of the nature of these thrust sheets -- except that each is several hundred to thousands of feet thick. Because the sheets dip shallowly to the east, their edges are exposed at the surface as a series of linear outcrops that are roughly oriented north to south. These thrust sheets were created as a result of continental convergence (mountain building).
Outcrops that contain mostly resistant rocks (such as sandstone or siltstone) form ridges. Outcrops that consist primarily of less resistant rocks (such as limestone or soft shale) form valleys. The result of this arrangement on a large scale is a series of north-south oriented ridges and valleys. Superimposed on these thrust sheets are smaller scale anticlines (folds with strata sloping downward on both sides from a common crest) and synclines (folds in rock in which the rock layers dip inward from both sides toward the axis) that complicate the geology somewhat. Elevations are highly variable, but generally 100's to a couple thousand feet lower than those in the Unaka's.
Cumberland Plateau: Structural geology played an important role in the development of this area. Continental convergence triggered mountain making in the Unakas and thrust faulting in the Valley and Ridge during the development of Pangaea. Much of the bedrock of this area is weather resistant, flat-lying, hardened sandstone. The resulting landscape is a tableland, or plateau, with typical elevations of 1200 to 2000 feet above sea level. These elevations are equal to, or higher than, those of the Valley and Ridge. This plateau is capped by a thick, nearly continuous sheet of resistant sandstone.
Eastern Highland Rim, Central Basin, and Western Highland Rim: Uplift of the Nashville Dome accompanied each mountain building episode in Tennessee. As a result, the regions of the Eastern and Western Highland Rims and the Central Basin all experienced periodic increases in surface elevation during the Paleozoic and early Mesozoic. At one time, the sandstones of the Cumberland Plateau probably extended westward over these areas as well. Fractures, resulting from uplift along the crest of the Nashville Dome, however, made the sandstones and the underlying limestones more susceptible to erosion. Consequently, the only remnants of these sandstones in Middle Tennessee are preserved in features such as Short Mountain. Isolated, resistant bedrock features like Short Mountain are termed erosional remnants.
Elsewhere in the Eastern Highland Rim, erosion has exposed carbonate bedrock of Late Paleozoic age. These carbonate rocks contain variable amounts of chert, and are often interbedded with fine grained, fragmented (clastic) rocks. As a result, these rocks are more resistant to erosion than the underlying, purer limestones of the Lower (Early) Paleozoic. Therefore, the Eastern Highland Rim stands above the Central Basin where Lower Paleozoic limestones crop out and erode rapidly. Structural fracturing would have been most intense over the top of the dome; therefore, the Central Basin is more deeply eroded than the adjacent Highland Rims. The geologic characteristics of the Western Highland Rim closely parallel those of the Eastern Highland Rim, resulting in very similar physical geography as well. Elevations in the Highlands Rims typically range from 600 to 1200 feet. Within the Central Basin, the elevation rarely exceeds 800 feet, with 500 to 600 foot elevations more typical.
Mississippi Embayment/Coastal Plain: The Coastal Plain is the western-most physical geographic area in Tennessee. This geographic region roughly corresponds with that of the Mississippi Embayment. In other words, the Coastal Plain was once covered by a shallow sea; when that sea regressed southward, this area became a low relief coastal plain for a while. This sea deposited numerous, flat-lying sequences of sand, silt, and mud, lying between beds of strata which together form a thick blanket of sediment. This blanket is draped over a much older carbonate bedrock surface consisting of Lower Paleozoic carbonate rock.
These relatively young marine fragmented sediments have never been deeply buried, and so are not very hard. As a result, they do not resist erosion very effectively. Instead they form a subdued, low elevation, low relief landscape, consisting of rolling hills, poorly drained lowlands, and shallow, wide stream valleys. Elevations are usually less than 500 feet and decrease rather steadily toward the Mississippi River. Recent (i.e. geologically very young) terrestrial deposits, which are simply reworked marine sediments, are slowly accumulating in many lakes and streams.
If you would like to contribute information on Tennessee geology, or if you have a question about Tennessee geology, please leave a comment to this post.