20 GEOLOGICAL SURVEY OF ALABAMA. 



the fault; no appearance of it exists on the upper surface. 

 The north west side has therefore been denuded more than 

 the other, as much more as the whole depth of the fault, 



The thickness of the Coal Measures on the south east side 

 of the valley cannot be accurately ascertained, as the exact 

 geological position of the rocks appearing in Straight Moun- 

 tain is not yet definitely determined. 



2. Sub- Carboniferous. 



a. Mountain Limestone, Bangor Limestone. Immediately 

 beneath the Coal Measures lies the Sub-Carboniferous or 

 Mountain Limestone. Its thickness seems to vary from place 

 to place. This may be due in part to its more or less per- 

 fect exposure. From measurements of it, made at va- 

 rious places, it was found to be between 200 and 300 feet. 

 At the very head of the valley just west of the fault line, it 

 shows 200 feet. It is prominent all along the north-western 

 side of the valley ; in some places reaching up to near the 

 top of the mountain, in others seen only at the base. Some- 

 times totally hid by talus from the overlying Coal Measures. 

 Sometimes it shows the effects of cross faults, or flexures in 

 the strata. Where these occur it may be completely sunk 

 beneath the surface for a space, half a mile or more, then 

 rise up to or above its normal level. In some places it was 

 observed that these downward curves were associated with 

 upward curves on each side. The most prominent of these 

 flexures is in section 3, T. 13, R. 1, East. At this place, the 

 limestone sinks beneath the level of the valley, while at, or 

 about a quarter of a mile on each side, it rises up over 300 

 feet above it. The eye is often deceived by a curvature in 

 the face of the mountain, w^ich gives the appearance of 

 downward cuive, where the formation is only denuded far- 

 ther back. Careful examination, however, showed that the 

 curvature here was not only apparent, but actual, and that 

 it amounted to a flexure or down- bending from the highest 

 points of the limestone, on each side, of over 200 feet. It 

 was also observed that there was a. visible flexure in the 

 overlying Coal Measures ; and that the same flexure extended 



