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 marsh at a depth not exceeding that of the present river-channels. The rivers, however, are not now cutting into rocky banks or flowing over beds of Secondary strata; they are flowing sluggishly in the middle of alluvial flats, which tend to silt up with every flood or exceptionally high tide. Thus all the evidence seems to show that marshes like those near Lewes and Amberley Wild Brook have originated through the submergence of flat-bottomed valleys cut in soft strata. The ponding back of the muddy tidal water would soon lead to the silting-up of any shallow lakes left after this submergence. When the land stood 70 or 80 feet higher than it does now, the country must have looked very different. The rivers then traversed the chalk downs through V-shaped comparatively narrow valleys; but these valleys opened out in their upper reaches, where they crossed the Gault and Weald Clay. If we could lay bare the true floor of the valley, we should see however that there is always a steady and fairly regular fall seaward, just as there is in the part of its course which lies above the influence of the submergence, which is felt for some 10 or 12 miles from the sea. Except on this theory of recent submergence it seems impossible to account for these curious marshes, with tributary valleys obviously plunging sharply beneath them on either side; they are quite unlike the undulating flats which occur higher up. The flat of Selsey Bill yields evidence of submerged land-surfaces opposite each of the shallow valleys; but here we meet with the same difficulty which confronted us in the Thames Valley and on the east coast. Pleistocene land-surfaces and alluvial deposits of early date are seen on the foreshore side by side with the more modern Neolithic alluvium and submerged forests. Unless great care is taken it may be thought that the well-preserved bones of rhinoceros and elephant, and the shells of _Corbicula fluminalis_, come from the same alluvium that yields Neolithic flint-flakes, or that plants such as the South European _Cotoneaster Pyracantha_ flourished in Britain up to this recent date. Except for the sake of warning against these sources of error the submerged forests of Selsey Bill need not detain us. Still travelling westward we next arrive at the series of tidal harbours opening into Spithead, Southampton Water and the Solent. All of these are obviously continuations of the valleys which lengthen them inland; and this is amply proved by dock excavations and borings.