VALLEY REGION DETAILS; CAHABA VALLEY. 167 
especially the former, attain very considerable thickness and 
underlie a wide area. Beds of limestone are very generally 
interstratified with these shales. 
By referring to the map it will be seen that the Cahaba 
Valley in its upper part runs nearly northeast and south- 
west, but below Helena it turns nearly southward to Monte- 
vallo, while beyond the latter point the edge of the Cahaba 
Field turns nearly west, as does also to some extent the 
Sub-Carboniferous border of the valley on the other side. 
These changes in the direction of the folds, bring about com- 
plications of the structure, as may be seen in the formation 
of a great number of subordinate basins in this part of the 
Cahaba Field ; in the faulted and overturned measures west 
of Montevallo; in the faulting and duplication of the Cam- 
brian and Silurian strata in the valley between Montevallo 
and Centerville; in the formation of a synclinal of Trenton, 
Red Mountain, Devonian and Sub-Carboniferous strata in 
the vicinity of Pratt’s Ferry. 
The uppermost beds exposed in this synclinal are the 
Oxmoor shales which are seen in the basin of the “Moun- 
tain,” which, beginning a mile below Pratt’s Ferry, extends 
in V shape for several miles towards the southwest. The 
point of this mountain or apex of the V overhangs the river: 
The dip of the northwest side is moderate towards the south- 
east, while the strata on the southeast side are alngost ver- 
tical, following the usual law. The crest of the tain is 
formed by the Fort Payne Sub-Carboniferous chert, while 
upon its flanks are to be seen the underlying en down to 
the Trenton. Below the chert there are sand®tones and 
shales that we have referred to the Clinton or Red Moun- 
tain, though we have no fossils nor any of the red iron ore 
to determine the matter. a 
The Trenton rocks underlying this Sub-Carboniferous 
basin emerge from below it, both towards the northeast and 
to the southwest, but more rapidly in the last named direc- 
tion. Towards the northeast the Trenton belt may be fol- 
a Some red ore which occurs a mile or two to the northwest of the 
“‘Mountain,’’ appears to belong to a bed lying between the uppermost 
of the Knox Dolomite and the lowermost of the Trenton. At least 
such is its position at one locality where all these beds may be clearly 
made out, and their ages distinguished by the fossils which they hold. 
