On British Drifts and the Intcrglacial Problem. 313 



Holderness, such as their rudely linear arrangement, their 

 indifference to the contours, and their relation to the middle or 

 Purple boulder-clays, it appears most probable that they 

 represent the material deposited along- the margin of the 

 ice-sheet by the surface-waters flowing from it and from 

 the adjacent land. From the occurrence of more or less 

 fragmentary marine shells in them, the gravels were, however, 

 originally supposed to be of marine origin, and this view is still 

 upheld by some geologists. It is the same question in which 

 so many of the so-called ' middle glacial ' sands and gravels of 

 the British Islands are involved, and upon which there has been 

 so much discussion. If it be permissible for me to reiterate 

 the well-known argument by which the presence of marine 

 shells in gravels of glacial origin is explained, it may be out- 

 lined as follows. 



Since the basins around our islands are known to have been 

 occupied by the sea at the beginning of the Glacial Period, and 

 since these basins were afterwards filled by ice-lobes, which, as 

 we have seen, moved outward in many places upon the land, 

 dragging with them much of the material of the old sea-floor, 

 it is inevitable that a certain amount of marine detritus will 

 occur in the deposits formed by the ice or derived from its 

 melting. Just as we find shells, and sometimes even trans- 

 ported masses of marine deposits, intact in the Basement Clay, 

 so we find marine relics likewise, though unusually more 

 scattered and less perfect, in the gravels derived from the same 

 ice-sheet. This deduction is consistent with our knowledge of 

 existing glaciers and ice-sheets ; thus, Sir Archibald Geikie has 

 recorded the presence of sea-shells in the moraine of a 

 Norwegian glacier ; * Professors E. J. Garwood and J. W. 

 Gregory have found an excellent illustration of the same 

 phenomenon in one of the Spitzbergen glaciers ; t and Professor 

 R. D. Salisbury, in describing the characteristic upturning of 

 the layers of ice at the end of one of the glacial lobes which 

 descends into a shallow bay in North Greenland, gives the 

 following instructive note on the conditions which he observed : 

 ' Here the upturning of the layers brought up shells from the 

 bottom, of the bay^ and left them in marginal belts where the 

 upturned layers outcropped. These shells were mingled with 



* Geological Sketches at Home and Abroad {X^oniSon, 18S2), pp. 145-6. 

 I ' Contributions lo the Glacial Geology of Spitzbergen.' Quart. Joitrn. 

 Geol. Sac, vol. liv. (1898), p. 210. 



1906 September i. 



