Page:VCH Kent 1.djvu/67

 GEOLOGY PLEISTOCENE AND RECENT DEPOSITS The Lenham Beds, with their tantalizingly fragmentary evidence of extensive submergence, are the latest marine deposits of Kent, if we except the low-level shingle of recent accumulation at Dungeness and the estuarine silt of the marshes of the Thames. All the other remnants of its later geological history tell of the long persistent waste of a land surface shattered by winter frosts and torn down by the gathering of the rains and melting snows into streams, or steadily lowered by the solution of its limestones from the percolation of the sub-aerial waters through its pores, each muddy stream and lime-charged spring incessantly carrying its load of particles downward to the rivers, that in turn sank their channels deeper and deeper into the land as they swept powerfully onward to the sea. The Denudation of the Weald. — Let us now consider more fully the eloquent testimony which these rivers in themselves bear to the vast change that the country has undergone since they began to flow in their present courses. Although the plain of Weald Clay lies open eastward to the sea, the Darent, the Medway and the Stour all flow northward from it to break across the high opposing barriers of the Lower Green- sand and Chalk in deep trench-like valleys that they have excavated at right angles to the present escarpments. This behaviour seems inexplic- able until we realize the geological conditions by which their courses were originally determined. We must picture to ourselves the shape of the land after the uplift of the Wealden dome, when the Chalk still formed a continuous arch across the interior, of which only the opposite buttresses now remain in the North and South Downs. From this surface the drainage would necessarily flow northward and south- ward on the opposite sides of the dome, which are precisely the present directions of the principal rivers of Kent on the one side and of Sussex on the other side ; and thus the problem no longer presents any difficulty. Since the courses of these streams were established, the crest of the dome has crumbled away ; formation after formation has been stripped off ; the softer strata have been everywhere lowered relatively to the harder, and longitudinal depressions formed in which tributary streams have been nourished, thereby further accelerating the trenching of the surface ; but still the main rivers have held their original direction and deepened their channels across the broken shell of the land, and they will continue to do so until they have sunk so deeply as to become powerless, or until the country sinks again for renovation beneath the ocean. Some relics of this period of erosion — mere shreds of waste left scattered here and there for a while until the elements find time to round off their work — will now claim our attention. Clay-'with-F lints and other Hill Drift. — It is in the river valleys that such traces are most abundantly found ; but they are not wanting even on the hills. Thus, as already mentioned, the surface of the Chalk on the Downs, where the ground is not too steep, is very generally over- spread with an irregular sheet, from a few inches to several feet in