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174 STRUCTURAL AND FIELD GEOLOGY 
over the other. But faults of this kind, occurring in gently inclined 
strata, are usually on a small scale, and merely of local importance. 
One of the commonest kinds of reversed fault is that known as an 
overthrust. All folds, as we have seen, are the result of horizontal move- 
ment or tangential pressure. When this is excessive, folds are rendered 
more and more unsymmetrical, the middle limb of each fold becoming 
thinner and thinner as the rock is drawn out in the direction of move- 
ment. With continued pressure the limb at length yields, and the highly 
inclined or recumbent anticline is pushed forward—in short, the fold is 
dislocated and a reversed fault comes into existence. All gradations of 
such overthrusts may be studied in most regions of highly folded and 
contorted strata (see Fig. 53). So overpowering has been the horizontal 




Crm 

FIG. 53.—ORIGIN OF REVERSED FAULTs IN HIGHLY FOLDED Rocks. 
movement in some cases, that masses of rock thousands of feet in thick- 
ness have been buckled up and sheared. In other cases, however, 
great reversed faults have been produced without much preliminary 
buckling or folding of the rocks. Many remarkable examples of this 
kind occur in the north-west of Scotland. In that region sheet after 
sheet of rock has been successively sliced off and driven forward, some- 
times for ten miles or more, so that the oldest rocks often overlie the 
youngest rocks (see Plates XL., XLI.). 
Plate XL. is a view of Sgurr Ruadh, a mountain in Ross-shire. It shows 
the north face of the mountain along which the following series of bedded 
rocks crop out :—a, Torridon Sandstones ; 4, Basal Quartzite ; c, Pipe-rock. 
The white lines are thrust-planes which traverse the hill-face in the same 
direction as the outcrops. Plate XLI., for which I am indebted to my old 
colleague and friend, Dr Peach, is a section taken obliquely across the 
mountain, and shows the general structure of the ground. From the 
base of the mountain up to the first thrust-plane the strata occur in their 
true order ; thereafter, it will be seen they are inverted, and have been 
driven forward. ‘The two thrust-planes which appear in section on the 
hill-face are branches of one and the same overthrust, as is shown in the 
section. The thrust-planes of the north-west Highlands are inclined at 
various angles, but the larger ones usually deviate most from the vertical. 
Not infrequently, indeed, they are almost horizontal. In the general 


