Page:The Ancient Stone Implements (1897).djvu/696

674 upper surface of any such relics of a former condition of things would, of course, be covered with débris and rain-washed clay, brought down from a higher level on the slopes, but on digging into them their true nature might be recognized.

Nearer the sea, and farther up the valleys, the state of things would be somewhat different. At the mouth of the river, as has already been pointed out, the declivity of the stream would have been greater, audits excavating power therefore increased. If, as originally assumed, the bed of the river, when the land was first elevated, was, at a mile distant from the sea, 200 feet above its level, the declivity would be 200 feet to the mile; when the 200 feet level was 4 miles from the sea, the slope would still be 50 feet to the mile; at 10 miles distance it would still be 20 feet, and it would not be until the 200 feet level was 15 miles from the sea that the ordinary slope of the bottom of the Chalk valleys of Hertfordshire, which is about 13 feet 6 inches to the mile, would be attained. In the meantime, however, if the sea were encroaching on the shore, or were, owing to the nature of the rocks, widening and extending that portion of the river subject to tidal influences, the actual point of contact with the sea would be carried far inland, and—assuming the rock traversed to be of one uniform nature and hardness—it would be long before the river towards its mouth ceased to have a greater declivity than nearer its source. We see, then, that the amount of excavation effected by the river, during the time necessary for the deepening of the valley by 100 feet, at a point midway in its course, would, near the sea, have been twice as great, or 200 feet. We should, therefore, expect to find beds of the same age as those which, at the middle of its course, were 100 feet above the river, at relatively twice that elevation near the mouth; and any intermediate beds would also be proportionally higher above the then existing stream, than contemporary beds farther up the valley.

At the heads of the valleys, the excavation would, on the contrary, have been less than towards the middle of the course of the river; partly owing to there always being less water present, partly to the reduced liability to floods, and partly to other causes. The heads of the valleys would, however, be constantly receding in all cases, and their retrogression would in most instances be aided by springs issuing from them. In cases where, from some geological cause, the heads of two valleys running in opposite directions receded in the same line, we can readily imagine their