cutting through the high chalk-hills of the South Downs. We need not discuss the origin of these peculiar courses, which date back to the period when the central axis of the Weald was uplifted ; that discussion would take too much time, and is here unnecessary. We are now only concerned with the later stages of the evolution of these river-valleys, each of which yields striking confirmation of the view that a sinking of the land has taken place in comparatively modern times.
At the present day the tidal part of each of these rivers extends right through the Downs into the lower Wealden area, and it is obvious that their valleys tend to silt up, not to deepen, and scarcely anywhere to become wider. When we examine further we find that the true valley-bottom lies far below the present alluvial flat; though the scarcity of borings and the uncertainty of many of the records make it difficult to say exactly how deep it lies.
If we follow these rivers upwards we find that in each case the alluvial flat widens out greatly after we have passed the chalk-hills and reached the clay lands beyond. These wide flats, according to old ideas, were formed by the swinging from side to side of the stream, which thus gradually widened its valley in the softer strata. If this were the case in these instances, we should find a solid floor beneath each
