Page:Submerged forests (1913).djvu/52

38 base-level must either have been the sea, or some vast alluvial plain then occupying the bed of the North Sea. In either case the plain must then have been fully 60 feet lower than the present sea-level. Not only did the ancient Humber cut to the same depth as the ancient Thames, but in each area the ancient river was flanked by a wide alluvial flat which now lies from 40 to 60 feet below the modern marsh level.

The flat coast of Holderness, which stretches from the Humber northward to Flamborough Head, shows also occasional submerged forests; but the want of excavations beneath the sea-level makes it impossible to say much about them. North of Flamborough Head it seems as though depression gave place to elevation, and when we pass into Scotland the Neolithic deposits seem to be raised beaches instead of submerged forests. We need not therefore devote more time to a consideration of the details connected with the submerged land-surfaces which border the lands facing the North Sea. They evidently once formed part of a wide alluvial flat stretching seaward and running up all our larger valleys. We must now consider how far seaward this plain formerly extended.

Here, fortunately, we meet with a most surprising piece of evidence, which adds enormously to the importance of this plain, and shows that the submergence