GEOLOGY 



Cretaceous, at which time a great part of the rainfall would collect on 

 the clay and form into streams which would cut deep channels in it and 

 through it into the underlying Chalk, instead of a small part of the rain 

 percolating through the Chalk and the greater part evaporating or being 

 absorbed by vegetation, as at the present time ; or perhaps later, after the 

 Tertiaries had been removed, when the moister climate and consequent 

 higher plane of saturation of the Chalk would suffice, as under the 

 former supposition, to cause rivers to flow in our now dry valleys. But 

 since the removal of the Tertiaries by denudation, or the lowering of the 

 plane of saturation by the drier climate and the artificial abstraction of 

 water by means of wells sunk into the Chalk, these valleys have prob- 

 ably been and are still being deepened by the Chalk being dissolved and 

 carried away by percolation in the manner previously mentioned. In 

 some instances the plane of saturation occasionally rises to the bottom of 

 these valleys, when a little water may be found in them in places, as in 

 one near Watford on the west of The Grove and Cassiobury Park, but 

 as a rule the water-level in the Chalk is now permanently below the 

 lowest points in these valleys. 



Under natural conditions in any Chalk area there must be a slight 

 tendency for the plane of saturation to become lower, owing to the loss 

 of internal heat by our earth allowing of water percolating further into 

 it, and to the gradual reduction in the rainfall from causes which need 

 not be considered here, but these are secular changes which must be 

 altogether imperceptible to us. The removal of the forests which once 

 covered most of our county was the first artificial aid which man, far 

 too prone to interfere with his Maker's provident arrangements for his 

 comfort and welfare, gave to the acceleration of this natural process ; 

 the deep drainage of the land followed, causing water to flow away more 

 rapidly, and thus giving it less time to sink into the Chalk ; but nothing 

 could possibly have a more prejudicial effect upon our enjoyment of the 

 country and upon such of our industries as are dependent upon a plenti- 

 ful water-supply, than the folly of which certain water companies are 

 guilty of taking water out of our underground Chalk reservoir more 

 rapidly than it flows into it. By thus emptying it from the bottom a 

 void is created which must be filled up from the top, and the certain 

 result follows that our watercress-beds, which should yield our most 

 valuable natural produce next to agriculture which is also prejudicially 

 affected, become dry, our water-power decreases, and our rivers cease to 

 flow in the higher part of their courses. The water companies are thus 

 continually adding to the extent, and may eventually add to the number, 

 of the dry valleys of Hertfordshire. 



