In pre-glacial times Yorkshire seems to have stood at a considerably greater elevation than at present, for drift-filled valleys have been proved to descend in many places below sea-level. In the great valley between Newcastle and Don-caster there is an old drift-filled hollow, which at Gateshead is 140 feet below O.D. South of the Tees borings reveal a great depth of drift until the line joining Northallerton and Bedale is reached, where it becomes thin over an old watershed, but thickens again southwards, and the rock floor of the valley is 74 feet below O.D. at Cawood, and 170 feet at Barnby Dun, near Doncaster. In the Vale of Pickering drift deposits reach a thickness of 107 feet, and the rock floor from Filey to Malton lies below sea-level. This evidence points to a long period of pre-glacial elevation, but, on the other hand, borings in Holderness show a plain of marine erosion, with blown sand banked against the old chalk cliff, at practically the present level. Generally over the coast region of Yorkshire two boulder clays have been identified. An upper clay, reddish and almost devoid of boulders, and a lower clay, which is bluish, and contains many erratics. The erratics in the Cleveland area may be grouped into three divisions, according to the place of origin :— I.—Western Group. South-western Scotland. Lake District and Vale of Eden. Teesdale and Pennine Chain. II.—Northern Group. South-eastern Scotland and the Cheviots.[*][1] Eastern Durham. III.—Eastern Group. Christiania region. Gulf... [1]: #fn-1