VALLEY REGION ; DISTRIBUTION OF THE ROCKS. 159 
as it has been called, above. Until a few years ago, they 
were universally confounded or.at least not distinguished 
from each other, and the whole of these surface beds were 
thought to be Post-Tertiary, a confusion that very naturally 
followed from the great similarity not only of the material 
but of the mode of distribution, and the stratigraphy. In for- 
mer reports we have called these Drift beds, but it seems 
bestto employ the name originally used by Dr. Hilgard to 
designate them, viz., Orange Sand. 
In his report Mr. Squire speaks of the Drift beds which 
cover so much of the Coal Measures of the Cahaba Field in 
its lower part. These covering beds are in reality both 
Drift or Orange Sand, and Tuscaloosa. 
In the coloring of the map it has not been attempted to 
show the Orange Sand, since its distribution is to all intents 
and purposes identical with that of the Tuscaloosa formation. 
III. DISTRIBUTION OF THE ROCKS OF THE DIF- 
FERENT GEOLOGICAL FORMATIONS 
IN THE VALLEYS BORDERING 
THE CAHABA COAL FIELD. 
In the preceding pages we have endeavored to describe 
in a general way, the foldings, fractures, and displacements 
which the great rock masses of the Appalachian region have 
sustained through the action of the lateral pressure to which 
they have been subjected. This was done for the reason 
that, without some knowledge of the main types of geological 
structure prevailing in this region, it would be impossible 
to account for the present distribution and attitude of rocks 
of the different geological formations which appear in the 
two valleys which we shall attempt to describe. 
We have already referred to the fact that with the flexing 
of the strata the crests of the arches, being lines of greatest 
strain were weakened, and fractured, and thus more easily 
wasted by erosion, and it is not surprising that, in process 
of time through the action of denuding forces, valleys should 
come to occupy the places once held by these arches. It is 
also plain that when the crests of these arches have been 
carried away by erosion, the remnants of the strata com. 
