PRESIDENTIAL ADDRESS. 545 



The position of the deposit, at the fringe of the great sheet of drift which 

 covers the lowland east of the Wolds and on the edge of an area west of the 

 Wolds which appears to have escaped glaciation, sustains me iu the opinion that 

 it was accumulated during that temporary recession of the East British ice-lobe 

 of which we have other evidence. Its proposed correlation with the Holderness 

 gravels seems hardly tenable in the light of the fuller information which we 

 now possess regarding the section. That the East British ice-lobe, during one of 

 its phases, had the sea at its margin, has always appeared to me to be probable,^ 

 and, I think, supplies an adequate explanation of the facts. 



Under this interpretation the complex drifts between the Basement Clay and 

 the Hessle Clay are regarded as the mai'ginal products of the ice-lobe which filled 

 the North Sea Basin during a stage when its western border began to lose ground 

 by rapid wasting. By this recession a broad hollow was left between the hills 

 and the ice-sheet, and into this hollow were swept the abundant washings from 

 the glacier on the one side and from the bare land on the other, thus forming the 

 irregular mounds and broad fans of stratified material which run parallel with the 

 receding ice-border. The sea at this time encircled the southern end of the ice- 

 lobe, but its waters were restricted, in the area under consideration, to narrow 

 estuarine inlets between the ice and the land. 



Jhe Upper Boulder-Clay. — Concurrently with this shrinkage of the East 

 British ice-lobe there appears to have been a steady increase in the ice-caps which 

 covered the broader upland tracks of the northern English counties. But all the 

 evidence tends to show that the tongues descending eastward from these caps, 

 from the time of the Basement Clay onward to the close of the glaciation, were 

 persistently prevented from passing freely outward by the presence of the main 

 lobe in the North Sea Basin. Upon the shrinkage of the main lobe they were 

 deflected southward along the hollow between it and the hilly laud, wliich, in 

 time, they filled again to a somewhat higher level than before, the incsculation of 

 the upper and lower Purple boulder-clays with the stratified drifts marking the 

 gradual stages in this process. The magnificent clifiP-sections of the Yorkshire 

 coast north of Flamborough reveal the continuous character of this glaciation, 

 and there is no I'oom anywhere to wedge an intevglacial period into these sections. 

 South of Flamborough, "the interval between the withdrawal of the one mass and 

 the advance of the other was longer, because the passage of the new invader to 

 the eastward of the Oolitic hills was only gradually effected ; and consequently it 

 is in the interior of the Holderness recess that we "find the greatest development 

 of the stratified drifts. To imagine, with the interglacialist.", that the North Sea 

 Basin was emptied of its ice-sheet, and was then filled again just far enough to 

 influence the flow of the local ice, without extraneous re-invasion of our coast, 

 seems to me an unwarranted sacrifice of the evidence to the idea. 



Local Shrinkage in the Ice-sheets. — There are many indications, especially in 

 the Midland Counties and along the southern margin of the glaciated region, 

 that the several lobes and tongues of ice of the Glacial Period in Britain did not 

 all attain their maximum development at the same time, but that while some 

 were creeping forward, others were shrinking back. To a certain extent this 

 result may have been brought about simply by changes iu the currents as the ice- 

 eheets overwhelmed their erstwhile confining rims of bare land and opened up 

 fresh avenues of discharge. 



It appears to me, however, that the prime factor lay in the displacement of the 

 areas of greatest precipitation during the course of the Glacial Period.- As the 

 plateaus of ice rose higher in the patli of the moisture-laden air-currents they 

 must have gained increased efl'ectiveness as condensers, thereby not only augment- 

 ing the snowfall in one quarter, but also diminishing the precipitation in Ihe 

 region to leeward. Hence I imagine that there would be a persistent tendency 

 for the great ice-sheets of Western Europe to thicken and spread more rapidly 



' ' Drifts of Flamborough Plead.' Quart. Jimrn. Geol. Soc, vol. xlvii. (1891), p. 421. 

 "^ Glacialuts' Mag., vol. i. No. 11 (1894), p. 231 ; and Mem. Geol. Survci/, ''isle of 

 Man'(1903), p. 395. 



1906. N N 



