TERTIARY HISTORY OF THE TENNESSEE RIVER 207 



two cases is of very different thickness. An examination of the 

 Sewanee, Stevenson, and Gadsden folios of the United States Geo- 

 logical Survey shows that the sandstone covering thins out toward 

 the Scottsboro region, this thinning being due in part to an original 

 westward thinning of the beds, but more especially to the beveling 

 of the successive formations by the Cretaceous peneplain, represented 

 by the plateau tops. The Walden sandstone disappears entirely 

 over much of the Sewanee area, northeast of the Scottsboro sheet, 

 so that the underlying Lookout sandstone alone is represented on 

 many of the plateau remnants; and while in the Stevenson and 

 Gadsden regions the Walden sandstone covers a greater area, it is 

 markedly thin, even on Sand Mountain, where the underlying 

 Lookout sandstone comes to the surface over considerable areas, 

 probably in part the result of a shallow synclinal fold. Over that 

 part of the Stevenson region immediately northeast of Scottsboro, 

 the Walden sandstone is wholly absent, and only a thin layer of 

 the Lookout sandstone caps the plateau remnants. So, while we 

 have no folio of the Scottsboro region itself, the conditions over 

 the adjoining districts are such as to lead us to expect a much 

 thinner cap of the sandstone over the lime about Scottsboro than 

 in the vicinity of the Tennessee gorge. An examination in the 

 field proves this to be the case. Just what the difference in thick- 

 ness of the sandstone cap is in the two cases is difficult to determine 

 with accuracy, owing to the fact that the limestone readily weathers 

 out from under the overlying sandstone, allowing fragments of the 

 latter to drop down and mantle the mountain slopes, thus concealing 

 the limestone and making it harder to ascertain its true thickness. 

 The tendency is to underestimate the thickness of the limestone, 

 and correspondingly to overestimate that of the sandstone. But 

 even if we assume the maximum possible thickness of sandstone 

 over the Scottsboro region, and the minimum thickness over the 

 region of the Tennessee gorge, there is still shown to be a markedly 

 greater thickness at the latter place than at the former. 



Ten miles southwest of Scottsboro the valley has its normal devel- 

 opment, and the adjacent plateau remnants are well shown. Here, 

 as elsewhere, the valley is seen to be cut largely in limestone, which 

 shows conspicuously on the mountain slopes. Limestone float is 



