G. W. Lamplugh — The Yorkshire Boulder-clay. 67 



attain to this position in the ice. I cannot understand. I am 

 sometimes inclined to speculate on the effect of " anchor-ice " 

 forming in water of some depth at or under the edge of an 

 advancing or gradually thickening ice-sheet, which, buoying up 

 portions of the sea-bottom, might become affixed to the glacier as 

 part of its mass and be driven forward with it; but this is, of course, 

 mere guesswork. 



But however carried, the curving lines and scattered shells of the 

 included bed show that it has, either during or after its trans- 

 portation, been rolled out and sheared by the passage of a heavy 

 mass above it, and we can see that had the movement gone on 

 a little longer, the clays and sands which now appear as streaks 

 and patches would have become completely merged into the mass 

 of the Boulder-clay. In fact, at this spot we study the Basement 

 Boulder-clay in a state of arrested development. 



It has frequently been noticed that the shells found in the drift 

 deposits of the Western side of England indicate a warmer climate 

 than those found on the Eastern side, and this has sometimes been 

 quoted as evidence that the beds were not synchronous. But if it 

 be taken as proved that at a certain period the Scandinavian ice- 

 sheet coalesced with that of Scotland, then during that period what- 

 ever portion of the North Sea remained to the south of the ice 

 must have been practically severed from the warm Atlantic and 

 converted into an inclosed basin almost surrounded by ice-cliffs, 

 wherein we may well imagine an Arctic fauna could establish itself. 

 But during the same period the currents of the great Atlantic would 

 preserve the more temperate fauna comparatively unaltered on our 

 western shores. 



This fact of the destruction of a glacial sea-bottom, and the 

 incorporation of its material in the mass of the Boulder-clay, must 

 not be lost sight of, as Clement Reid points out, 1 in studying the 

 distribution of boulders in our Eastern Counties drifts. Under such 

 circumstances the mere presence of fragments of Scotch, Scandinavian, 

 or Baltic rocks in the clay cannot alone prove, as it seems sometimes 

 to be taken as doing, that an actual flow of the ice has passed from 

 those localities to the spot where the boulder is found. 



During the encroachment of the glaciers there must have been a 

 long period when the North Sea was crowded with ice-bergs and 

 floe-ice, and these, floating hither and thither at the mercy of wind 

 and tide, would scatter their blocks indiscriminately, and bring about 

 much " intercrossing of erratics." 



Thus at any one spot there might be brought together boulders 

 from all parts of the catchment-area of the basin, and these, when 

 the land-ice reached the spot, might be incorporated in the resulting 

 Boulder-clay just as the shelly sands have been. Indeed the presence 

 of innumerable Saxicava- and Clioua-hored stones and well-rounded 

 beach-pebbles in the clay proves that many of its boulders have thus 

 been derived. 



Another point of interest in connection with this section is that 

 1 Survey Memoirs : " Cromer," p. 90, and " Holderness," p. 43. 



