ABSTRACTS 767 



the westernmost of the sharp folds which characterize the central or 

 valley division of the province. The larger part of the district is 

 occupied by the Cumberland Plateau, which has a gradual ascent toward 

 the north, rising from an altitude of between 1700 and 1800 feet on the 

 south to 1900 or 2000 feet on the north. The plateau is limited by a 

 steep escarpn:ent from iioo to 1500 in height on the east and about 

 1000 feet in height on the west. Many streams have cut their channels 

 backward into the plateau, forming deep, narrow caves, so that the 

 escarpment forms an extremely irregular line. Small portions of 

 Walden Ridge and Sand Mountain appear in the extreme southeastern 

 corner of the district, these being plateaus similar to the Cumberland 

 Plateau farther west. A small portion of the Sequatchie Valley 

 occupies the southeastern part of the district, with an altitude of about 

 600 or 700 feet, -while its northwestern portion is within the "highland 

 rim " a broad terrace surrounding the lowlands of middle Tennessee 

 and separating on the east from the Cumberland Plateau. 



Geology. — The rocks appearing at the surface are of sedimentary 

 origin, and include representatives of all the geologic periods from 

 Silurian to carboniferous. The Silurian formations, consisting of the 

 Knox dolomite, Chickamauga limestone, and Rockwood shale, occur 

 only, as narrow belts in the Sequatchie Valley. The same is true of 

 the Devonian, w^hich is represented by a single thin formation, the 

 Chattanooga black shale. The Carboniferous formations occupy by 

 far the larger part of the district, the Fort Payne chert and Bangor 

 limestone forming the lower portions of the plateau escarpments and 

 the highland rim, while the Lookout and Walden sandstones, belong- 

 ing to the Coal Measures, form the summits of the plateaus. 



The geologic structure of the region is in general extremely simple. 

 The plateaus and the highland rim to the westward are underlain by 

 nearly horizontal strata, while the Sequatchie Valley is upon a sharp, 

 narrow fold, the beds dipping downward on either side beneath the 

 adjoining plateaus. If the rocks which have been eroded from the top 

 of this arch were restored, there would be a ridge several thousand feet 

 in height in place of the present valley. In addition to the folding 

 which the strata have suffered along this line, they have also been 

 fractured, and the beds on the east have been thrust upward and across 

 the edges of corresponding beds on the west of the fracture, so that 

 along the western side of the valley the formations do not appear at 

 the surface in their normal sequence. 



