HISTORY OF LANDSCAPE 185 



rock becomes saturated with water before an outlet is reached 

 the water will be stored up under hydraulic pressure, and under 

 favourable circumstances perennial springs, often issuing with con- 

 siderable force, will be produced (Fig. 73). 'This is the case when the 

 permeable stratum is both underlain and overlain by impermeable 

 strata, the three strata being either bent into a trough or brought 

 by faults into contact with another mass of impervious rock. 

 In the latter case natural fault-springs are common (Fig. 73, D). 

 Or if the junction remains impervious to water, a well or boring 

 may be sunk on the right side of the fault to obtain the spring 

 water, which will rise to a height dependent on the height of its 

 gathering ground. When a trough or basin in the strata occurs 

 it is generally necessary to pierce the upper impervious bed by 

 boring through it down into the porous rock. The water may 

 then rise up to or even above the surface. Such wells are called 



fiocAs impervious Rocks 



FIG. 73. Fault Spring, D. Artesian Wells, A. 



artesian wells (Fig. 73, A). The arrangement of strata under Lon- 

 don, the London Basin, is of this nature, the Chalk being folded into 

 a basin with impervious Gault below it, and impervious London 

 Clay above it. Large supplies of water have been obtained by 

 wells of this description; indeed, so much that the water level 

 and its" pressure have been lowered, and the water no longer 

 rises to the surface. 



This class of springs is known as deep-seated, springs, because 

 the water has sunk to a considerable depth, has saturated 

 a great mass of rock and travelled a long distance through it, 

 and has then been forced again to the surface by hydraulic 

 pressure. This class of wells and springs affords water supplies 

 which are especially valuable for several reasons. There is a 

 vast reservoir of water to draw upon in times of drought and 

 the wells or springs rarely or never become dry, the w r ater is 



