

352 STRUCTURAL AND FIELD GEOLOGY 
rence in horizontally bedded rocks of varying character, some 
being pervious, others impervious, for every porous stratum 
is likely to contain a store of water which will ooze or flow out 
wherever the beds are truncated, as on hill-slopes and in 
valleys (Fig. 127), 
Generally, however, strata are more frequently inclined 
than horizontal, and through these water flows under the 

Fic. 127.—DRAINAGE IN HORIZONTAL STRATA. 
combined influence of gravitation and hydrostatic pressure. 
When such strata are traversed by a valley running in the 
direction of strike, underground water tends to be discharged 
only on one side, namely, on that side from which the 
truncated beds dip into the valley. If the beds dip at a 
high angle, the springs will usually be insignificant, since 
with a high dip the outcrops will be narrow; with a low dip, 
on the other hand, the discharge will be proportionately 
_ greater, for the simple reason that the outcrops of the perous 
beds will spread over a wider area, and be thus capable of 
imbibing a larger proportion of the rainfall. 
Synclinal valleys are of rare occurrence, especially in 
regions which have experienced much denudation ; but when 

FIG. 128.—DRAINAGE IN SYNCLINAL STRATA, 
they do occur amongst water-bearing and impervious strata, 
springs may abound on both sides of a valley (Fig. 128). 
When a valley coincides with an anticline, however, the 

