164 GEOLOGICAL SURVEY OF ALABAMA. 
tinuation may be followed as far as Centerville. Like most 
of the valleys of similar nature in Alabama, it is complex, i. @., 
made up of one or more subordinated valleys with ridges be- 
tween them. One of these valleys, lying between the cbert 
ridge of the Knox Dolomite and the edge of the Cahaba Field, 
is known as Possum Valley; the other lying between the chert 
of Knox Dolomite and Little Oak Mountain is in the Cahaba 
Valley proper. In the Cahaba Valley (taken in its widest 
sense) there are the representatives of all the Paleozoic 
rocks above named, from Cambrian to the Coal Measures. 
Its two borders are made by the rocks of the Cahaba Field 
on the one side, and by those of the Coosa on the other, from 
its upper end near Odenville down to Siluria, while beyond 
that the Sub-Carboniferous beds of the Coosa Field make 
its southeastern border, since the Coal Measures of that 
field do not extend further south than the place named. 
Most of the strata in this valley dip towards the southeast 
at varying rates, from which it would be reasonably inferred 
that its structure is that of an anti-clinal fold closely com- 
pressed and pushed over towards the northwest, or of an 
anticlinal fold and thrust fault combined. In the former 
case, we should, in crossing the valley, pass over the strata 
from Coal Measures of the Coosa Field to the Cambrian in 
succession, beyond which should follow the same formations 
again, only in reverse order, to the Measures of the Cahaba 
Coal Field. 
The diagram already referred to, as well as the examina- 
tion of the map will show, that the whole valley is made by 
one-half of a fold only, and the succession of the rocks from 
the Coosa Field is as follows: Coal Measures, Sub-Carboni- 
ferous, Black Shale, Trenton, Chert ridge and red lands of 
the Knox Dolomite and Cambrian, immediately following 
which are the Measures of the Cahaba Field, a great fault in- 
tervening between the Cambrian and Coal Measures. By 
this fault the Cambrian strata on the southeast side have 
been pushed up and over the upturned edges of the Cam- 
brian, Silurian, Devonian, and Sub-Carboniferous on the 
northwest side, into direct contact with the upper measures 
of the Cahaba Field, a displacement of perhaps more than 
10,000 feet vertical, and greater than that of any other fault 
