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VI] 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