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414 STRUCTURAL AND FIELD GEOLOGY 
determined partly by fracture and partly by flexure, such as 
the valley of the Jordan. Dislocation valleys may extend 
for long distances between parallel faults, or they may follow 
the line of one great dislocation alone. Such valleys are 
approximately straight or gently sinuous, and are of not 
infrequent occurrence. Glen App in Ayrshire and the great 
hollow traversed by the Caledonian Canal are examples in 
this country. Another good example is the valley of the 
Rhine between the Vosges and the Black Forest. Synclinal 
valleys are best developed in regions of recently uplifted 
tectonic mountains, the surface features of which not in- 
frequently coincide more or less closely with the underground 
rock-structure. Such valleys naturally trend in the same 
general direction as the mountains amongst which they 
occur. 
Tectonic valleys are, of course, liable to modification by 
erosion. Dry valleys, whatsoever their origin may have 
been, may remain for long periods comparatively unchanged. 
It is true that in desert regions such valleys are subject to 
the action of the wind, which widens and sometimes deepens 
them, but wind-eroded valleys are exceptional. Wherever 
rain falls and water flows, however, we look for evidence of 
erosion. Under ordinary conditions even the most recently 
formed dislocation and synclinal valleys have become modified. 
_ And in regions exposed for a prolonged period of time to 
denudation, such valleys as coincide with dislocations are 
obviously wholly the ,work of erosion. They have been 
worked out along lines of weakness, which affect a great 
thickness of rocks. However much, therefore, a land-surface 
may be lowered by denudation, the same faults will continue 
to guide the agents of erosion. The land may have been 
planed down ‘again and again to base-level—it may have 
experienced more than one cycle of erosion—but with each 
re-elevation, valleys of erosion tend to reappear along the 
same lines of weakness. Synclinal valleys, on the other 
hand, are much less persistent. When we find a river 
following a synclinal hollow, we may usually feel assured 
that the hollow is of relatively recent geological age. For 
the synclinal structure is more durable—less readily reduced 
than the anticlinal folds on either side. The latter are prone 

