---

The lower figure is a cross-section of TN, and it shows the distribution of rock types with respect to the physiographic provinces. This is an important concept because it is the geology of these areas that is largely responsible for their characteristic terrain. You must also know these associations (rock type(s) vs. physiographic province) for the exam.

For this lesson, we will review the characteristics of the rock types that form the various provinces, and discuss why these characteristics lead to the development of the associated terrains.

Rock type and resistance to erosion - When weighing the various influences on the development of a landscape, bedrock composition is one of the most important factors to consider. For most areas in Tennessee, a fairly limited range of rock types is present. In fact, of the seven major provinces, only the Unaka Mtns. contain non-sedimentary rocks.

One of the most important influences on a sedimentary rock's resistance to erosion is its quartz content. The more quartz a rock contains, the more resistant it will be to erosion. In Tennessee, sandstones generally consist mostly of quartz. Siltstones and shales (i.e. "fine grained [f.g.] clastics" ) are usually admixtures of quartz and other less resistant minerals. Limestones (i.e. "carbonates"), of course, are composed of calcite, which is rather susceptible to chemical weathering. However, some carbonate rocks do contain a fair amount of quartz -- especially in the form of chert -- and can be a bit more resistant to erosion than their more pure relatives. In addition, the carbonate rock dolomite is generally somewhat more resistant to erosion than limestone.

Another equally important characteristic is how well the grains of a rock are consolidated -- known as its induration. Well indurated rocks have either intergrown crystals, or the grains are joined together with a lot of intergranular cement. Most limestones are initially well indurated. The induration of sandstones, siltstones, and shales (i.e. "clastics") varies from outcrop to outcrop. In addition, over time, weathering may dissolve the cement and weaken the rock. Quartz cements are very resistant to dissolution. Calcite cements dissolve rather readily. Iron oxides and clay minerals are fairly susceptible to chemical alteration.

As a result of these two factors -- composition and induration -- sedimentary rocks display a wide range of susceptibility to erosion. Generally, well-indurated quartz sandstones are the most resistant to erosion. Likewise, limestones and poorly indurated shales are the most susceptible to erosion. Other sedimentary rock types are a bit more difficult to place within this spectrum. For example, all else being equal, a moderately indurated siltstone would probably be a bit less resistant to erosion than a moderately indurated conglomerate -- unless the siltstone has quartz cement and the conglomerate has calcite cement. Then the rates of resistance would be reversed. Fortunately, as I said earlier, the variety of sedimentary rocks in TN is not that extensive. Therefore, the rules of thumb above are pretty useful in understanding the resistance of these rocks to erosion.

Igneous rocks and metamorphic rocks, due to the mechanics of their formation, are usually very well indurated. In addition, many contain significant amounts of quartz. As a result, most tend to be rather resistant to erosion.

Rock types and Tennessee Physiography - Let's begin this discussion of Tennessee's physiography with the eastmost province and then systematically move westward to the Mississippi River.

Unaka Mountains: Because the bedrock here consists of a variety of igneous and metamorphic rocks, it is typically rather resistant to erosion. As you might expect, some rock types erode more readily than others, so significant relief (i.e. rugged topography; steep slopes) is evident nonetheless. However, due to (1) resistance of these rocks to erosion, (2) uplift associated with the orogenies of the past, and (3) subsequent isostatic movements, the elevation throughout this area is generally 1000's of feet above sea level. Note: my cross-section transects an area of the Unakas with relatively low elevations (2000 ft. or so) -- I used this just to keep my drawing short and simple. Mountains in this area are often a couple thousand feet higher than this.

Valley and Ridge: Any examination of this area's physiography must also involve a discussion of its structural geology. The Valley and Ridge 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.

Outcrops that contain mostly resistant rocks -- for example, sandstone or siltstone -- form ridges. Outcrops that consist primarily of less resistant rocks -- for example, limestone and/or poorly indurated 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 and synclines that complicate the geology even more. Elevations are highly variable, but generally 100's to a couple thousand feet lower than those in the Unaka's.

Now you might be wondering -- what formed these thrust sheets? I would hope that the answer would be fairly obvious: continental convergence (i.e. mountain building). Need I say more?

Cumberland Plateau: Here again, structural geology played an important role in the development of this physiographic province. As discussed above, continental convergence triggered orogenesis in the Unakas and thrust faulting in the Valley and Ridge during the construction of Pangaea. Although geologists haven't yet worked out the mechanics to everyone's satisfaction, farther west this compressional tectonic regime resulted in simple uplift of the Cumberland Plateau. Much of the bedrock of this area is flat-lying, well-indurated sandstone -- which as you know, is very resistant to weathering. As a result, erosion has not been particularly effective, and 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 is hardly surprising, when you consider that the plateau is capped by a thick, nearly continuous sheet of resistant sandstone.

Eastern Highland Rim, Central Basin, and Western Highland Rim: As indicated in the geologic column presented on the previous page, uplift of the Nashville Dome accompanied each orogenic episode in TN. 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 TN are preserved in features such as Short Mtn. Isolated, resistant bedrock features like Short Mtn. 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 clastic rocks. As a result, these rocks are more resistant to erosion than the underlying, purer limestones of the Lower (Early) Paleozoic. Hence, the Eastern Highland Rim stands above the Central Basin where Lower Paleozoic limestones crop out and chemically erode quite rapidly. In addition, structural fracturing would have been most intense over the top of the dome; consequently, 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 H.R., resulting in very similar physiography 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 physiographic province in TN. Its areal extent 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 -- for a time -- a low relief coastal plain. This sea deposited numerous interbedded, flat-lying sequences of sand, silt, and mud, which together form a thick blanket of sediment. This blanket is draped over a much older carbonate bedrock (now subcrop) surface consisting of Lower Paleozoic carbonate rock.

These relatively young marine clastic sediments have never been deeply buried, and so are poorly indurated. 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.

Now that we have covered all this material, and you now have some idea of the relationship between surface geology and physiography in Tennessee, I bet I can guess what you are thinking. I bet you are thinking: how can I ever learn all this stuff? Well, first I suggest you review the objectives on the first page of this presentation. Once you are familiar with these, then study the geologic column (on page two) to learn the few basic principles it has to offer. Don't bother learning a lot of specifics -- stick with the principles behind the events.

When you've learned those principles, then move on to this material (page three) and begin to work on it. My suggestion for learning this section is for you to repeatedly make sketches of the figures above -- and label them as well. As you do, more and more of the details will sink in. Then re-read this material. I believe you will find that you have a much better understanding of the topics involved. Repeat this regimen, as you see fit, until you really have a concrete understanding of the material. Throughout it all, however, keep in mind the specific objectives I have laid out for you, and don't try to learn too much -- the objectives are really not that extensive. And, oh yeah -- good luck!