

172 STRUCTURAL AND FIELD GEOLOGY 
we have direct evidence to show that such faults are occasionally so 
intimately connected with folds and flexures that we can have no doubt 
that they are contemporaneous, and due to one and the same crustal 
movement. Again and again, for example, large strike-faults, when traced 
continuously, have been found to die out in a flexure. In the case of 
monoclinal flexures it is not hard to see how that should be. Strain 
or tension must be set up along the margin of a sinking area. If 

FIG. 50.—PARALLEL FAULTS WITH DISTORTED STRATA BETWEEN. 
subsidence should take place within a region built up of horizontal strata, 
the horizontal position of the rocks along the boundary or margin of the 
sinking area will be interfered with. The pull or drag of the descending 
mass will cause the strata of the adjacent stable area either to bend over 
or to snap across. Should the movement be slow and protracted, the 
rocks will probably at first yield by bending. They will be turned 
downwards and compressed, it may be, by stretching. But should the 
movement continue, they must eventually give way, and a fold will thus 
be replaced by a fracture. Towards one or both ends, therefore, we 
should expect such a fault to die out into a simple flexure or monocline 
(see Fig. 51). 
Although faults of the kind described may be considered the result 
of direct subsidence, it is obvious that they might equally well have 
resulted from movements of elevation. During the slow uplifting of a 
broad plateau, tension will come into play along the margin of the 
rising area. Flexures will then be formed, and these will eventually be 
replaced by rents and dislocations. The resulting structure would thus 
be the same as if folding and faulting had been caused by a movement 
of subsidence. Thus, in Fig. 51, the fault / might have been caused 
either by the subsidence of the strata at x, or by the upheaval of the 
strata at a, But as the dominant movement of the earth’s crust must 

