142 GEOLOGICAL SURVEY OF ALABAMA. 
and a few other places good illustrations in point. After 
the folds have been pressed together and lapped over to one 
side, no further yielding to the compressing force can take 
place except by the giving way of the strata and the sliding 
of one part over the other, in other words, by the breaking 
apart and piling up of the beds. Now when a break occurs 
in a fold of the usual type, i. e., one which has been pushed 
over to the northwest, it is along the crest of the arch where 
the strain has been greatest, and the southeastern side slips 
up over the northwestern. Faults of this kind are usually 
designated as thrust faults, and the displacement sometimes 
goes so far as to shove a great body of strata over other beds 
for many hundreds of feet, and in some countries for miles 
even. In folds of the other class named, i. e. where the 
troughs have been shoved under the arches, the break oc- 
curs near the bottom of the trough, and the strata on the 
southeast of the line of fault are slipped under those on the 
northwest. The general effect of this kind of slip or fault 
is the same as if the compressing force had come from the 
opposite direction and had produced a thrust fault of the 
ordinary kind. These are also thrust faults, but to dis- 
tinguish them from the normal type of thrust faults they 
might perhaps be called reversed thrust faults. In Mur- 
phree’s valley and west of McAshan mountain, we have fine 
illustrations of this type of structure. In all these thrust 
faults we have either the older beds slipped up over newer 
ones, or newer ones shoved under the older, in either case 
bringing about a reversal of the natural arrangement. 
But there is another kind of reversal. We have seen that 
all our Alabama thrust faults are, in their origin, folds in 
which the strain of the compression has been carried beyond 
the limits of endurance of the strata, and hence when the 
break occurs along the crest of an arch of the typical sort, 
the gently sloping beds of the over-riding site will slip up 
over the steeply inclined or even overturned edges of the beds 
of the overridden side, the inclination of the edges of this 
side depending upon the degree of overpush or over-lap of 
the fold, and it may be quite possible that in the movement 
of tne one series of beds over the other the edges of the 
underlying series may by friction be bent still further in the 
