VALLEY REGION DETAILS; CAHABA VALLEY. 165 
known to usin Alabama. Northwest of this fault, as we 
have already intimated, all the formations between the Cam- 
brian and the Coal Measures are below the surface being 
engulfed in the fault. The beds of the Coal Measures next 
to the fault are very highly inclined, standing mostly nearly 
in vertical position while sometimes they have been pushed 
over beyond the perpendicular. The narrow belt of vertical 
measures borders the Cahaba Field along its entire south- 
eastern and southern boundary. We should naturally ex- 
pect the strata in these vertical measures to correspond 
with those that have not been disturbed further in towards 
the center of the field, since they are only the upturned 
edges of the same beds; but Mr. Squire has generally been 
unable to identify the vertical coal seams, for the reason 
that in the faulting the strata have been so crushed and dis- 
placed that the seams no longer retain their characteristic 
qualities, thickness, relative position, etc, some of the 
measures having been pinched out, and others having been 
correspondinly thickend up. This, in general terms, is the 
structure of the valley from its northern limit to Siluria, 
and even down to Montevallo. 
In more detail, its topographical and geological features 
are as follows: The southeastern rim of the valley is niade 
by the high escarpment of the Millstone grit of the Coosa 
Field known as Big Oak Mountain. This ledge of rock dips 
southeast under the Coal Measures of the Coosa Field, but 
is brought to the surface again in the Double Mountains, by 
a fault that extends through the lower part of the field. 
Between Big Oak Mountain and Little Oak, which is 
formed by the chert of the Sub-Carboniferous formation, 
there is a valley of varying width formed by the Oxmoor 
shales of the same formation. The sandstones which ac- 
company these shales, form one or more small ridges be- 
tween Little and Big Oak Mountains, and in some parts of 
the valley this sandstone extends a good way up the face of 
Big Oak, and then the Millstone grit forms only the capping 
of the mountain. Little Oak is the counterpart of the Red 
Mountain, but the Clinton strata appear to be entirely want- 
ing except in two or three places shown on the map. 
To the northwestward of Little Oak Mountain comes a 
