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.
