144 GEOLOGICAL SURVEY OF ALABAMA. 
greater or less extent resisted the denudation which carried 
away so much material from the intervening crests. It 
may be asked why the strata along the crests of the folds 
were so much more completely removed than from the 
troughs. One reason of this may be found in the fact that 
the strata along the crests would be more or less torn and 
disrupted from the strain of the folding, while those of the 
troughs would be more or less compacted by compression. 
This, along with other causes, has led to the formation of 
anticlinal valleys, that is, of valleys which have been eroded 
out of the tops or crests of anticlinal folds, and of this 
character, more or less masked by faults, overlaps, and 
other complications, are the valleys above named which 
border the Cahaba field. In all these valleys, the strata 
were raised up first into ridges with perhaps originally 
somewhat equal slope both ways, northwest and southeast 
from the central line (anticlinul); with increase of pressure 
the folds were pushed over towards the northwest; com- 
pressed together and lapped over to the northwest; broken 
apart and slipped; and finally by erosion, worn down into 
valleys in which now only the projecting edges of the strata 
are seen. These, by their relative position, give us the 
clew to the structure. When the strata were thrown into 
waves by the compressing force above spoken of, the crests 
of these waves were raised much above the level of the in- 
tervening troughs, and when, by subsequent denudation 
these arches were worn down to the general level or nearly 
to it, the lower strata of the arches were uncovered and ex- 
posed to view, usually in the form of projecting ledges in 
the case of the harder rocks, and of trenches in the case of 
the softer and more easily eroded ones. 
In this way the strata of the different geological forma- 
tions down to the lowest, have come to occupy the surface 
in these valleys, usually in strips or belts which run ap- 
proximately parallel to the length of the valley, and which, 
in consequence of the anticlinal structure are normally 
duplicated, though as a result of faults they sometimes ap- 
pear only once in a section across the valley, and sometimes 
where, as in Jones’ Valley, the structure is a double anti- 
clinal cembined with faults, they are repeated a third time. 
