The general lay of the rock beds beneath the Cumberland Plateau is that of a syncline or trough, with gradually sloping sides, but this simplicity of structure is broken in the south half by a pronounced arched or anticlinal struc ture. It is along this arch that Sequatchie River has carved out the valley of the same name.
The general lay of the rocks beneath and bordering the Central Basin is that of an ellipti cal flat dome with its highest part in the centre of Rutherford County. The direction of the major axis of this dome is northwest-southeast, or roughly parallel with the folds of East Tennessee and the direction of Cumberland Plateau. The sides do not slope uniformly, but are billowy. This dome is the counterpart of a similar one in Kentucky and adjoining territory in southwestern Ohio and southeastern Indiana. In the southeastern corner of Stewart County there is a small structural dome upon which a depression, known as Wells Creek Basin, some two miles wide, has been excavated by the same natural processes that formed the Central Basin. The rock layers on the sides of this dome stand at a much higher angle than those beneath the Central Basin, of which it otherwise is a miniature duplication. The lay of the hard rock beds in West Tennessee, which there are far beneath the surface, is not known, but they probably dip at a low angle to the west, as do the unhardened beds of sand .and clay, above them.
The oldest rocks of the State are of Archeo zoic Age, and occupy three small areas along the border in Johnson, Carter and Unicoi counties. They include several formations and consist, for the most part, of gneiss, schist and granite. The remainder of the Great Smoky Mountains is composed of rocks of Cambrian Age of which there are several formations. They consist of sandstones, quartzites, conglomerates, lime stones, shales and slates. Without the disturb ance that has affected the region, these rocks would be overlain in the valley of East Ten nessee by the younger Ordovician ones, but the folding, faulting and denudation the area has suffered has in places brought these to the sur face in long, narrow belts and smaller irregu lar patches. Of the Ordovician formations in East Tennessee there are several, the oldest being the upper part of the Knox dolomite, a formation that in places exceeds 3,000 feet in thickness. The lower part is of Cambrian Age.
Like the Cambrian rocks, and for the same reason, those of Ordovician Age in the valley of East Tennessee lie for the most part in northeast-southwest belts. Because of the Se quatchie anticline and the erosion upon it, rocks of Ordovician Age are exposed in the floor of the valley as they are in that of the Central Basin, and for the same reasons. Those of East Tennessee are limestone, marble and shale, while those in the Central Basin, which include 12 formations, are nearly all limestone. The Knox dolomite occurs nowhere at the sur face in the Central Basin, but owing to greater uplift and probably to reduction in thickness of the overlying formations, it does occur in Wells Creek Basin. Limestone of Silurian Age occurs in narrow belts in the valley of East Tennessee, along the eastern escarpment of Cumberland Plateau, on the western side of the Central Basin, along the streams that flow through the western part of the Highland Rim and along Tennessee River. Black shale of Devonian Age, known as the Chattanooga shale, overlies the Silurian limestone and has the same distribution, except that it occurs in cer tain places where the former is absent, as on the northern, eastern and southern borders of the Central Basin. The rocks of the Highland Rim and the lower ones of the Cumberland Plateau are of Mississippian Age and consist of chert, shale and limestone. Those of the upper part of the Cumberland Plateau are of Pennsylvanian Age and are composed of shale, sandstone, conglomerate and coal beds.
West of the Tennessee, most of the for mations -are unconsolidated sand and clay. The lowest and oldest is of Cretaceous Age. Of these there are four formations, which out crop in north-south belts, within 30 miles of the Tennessee. Westward from the Cretaceous border the surface formations are of Eocene Age, with the exception of the loess and the alluvium bordering the streams. The former, which covers a belt of 30 miles wide next to the Mississippi bluffs, is of Pleistocene Age, and the latter is Recent. The thickness of the Cre taceous and Eocene beds is not known, but a well reported to be 2,000 feet deep on the edge of Reelfoot Lake did not reach through them.