394 



NA TURE 



[August i6, 1906 



and on the edge of an area west of the Wolds which 

 appears to have escaped glaciation, sustains me in 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 Holder- 

 ness 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, 

 liad the sea at its margin, has always appeared to me to 

 be probable,' and, I think, supplies an adequate explan- 

 ation of the facts. 



Under this interpretation the complex drifts between the 

 Basement Clay and the Hessle Clay are regarded as the 

 marginal products of the ice-lobe which filled the North 

 Sea Basin during a stage when its eastern 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 rim 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. 



The Upper Bouldcr-Clay. — Concurrently with this shrink- 

 age 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 per- 

 sistently 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 south- 

 ward along the hollow between it and the hilly land, 

 which, in time, they filled again to a somewhat higher level 

 than before, the inosculation of the upper and lower 

 Purple boulder-clays with the stratified drifts marking the 

 gradual stages in this process. The magnificent cliff- 

 sections of the Yorkshire coast north of Flamborough 

 reveal the continuous character of this glaciation, and 

 there is no room anywhere to wedge an inlerglacial period 

 into these sections. South of Flamborough, the interval 

 between the withdrawal of the one mass and the advance 

 <if 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 develop- 

 ment of the stratified drifts. To imagine, with the inter- 

 glacialists, 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 manv in- 

 dications, 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 e.xtent this result may 

 have been brought about simply by changes in the currents 

 as the ice-sheets overwhelmed their erstwhile confining 

 rims of bare land and opened up fresh avenues of dis- 

 charge. 



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 path of the moisture-laden air- 

 currents they must have gained increased effectiveness as 

 condensers, thereby not only augmenting the snowfall in 

 one quarter, but also diminishing the precipitation in the 

 region to leeward. Hence I imagine that there would be 

 a persistent tendency for the great ice-sheets of Western 



' "Drifts of Flamborough Head." Quart. /on, 

 (iSqi), p. 427. 



- Gtacialists' Mag.^ vol. i. No. ii (1S94), p. 

 Snfvc)\ '• Isle of Man " (1903), p. 395. 



NO. 1920, VOL. 74] 



. Geitl. Soc, vol. xlvii. 

 ^31 ; .ind Mew. Geol. 



Europe to thicken and spread more rapidly toward the west 

 than toward the east, until finally the eastern portions were 

 shrunken for want of sustenance, while the westerly lobes 

 were still waxing thicker and stronger. The recent re- 

 searches of Mr. E. W. Harmcr into the probable meteor- 

 ological conditions of the Glacial Period ' are full of sug- 

 gestion in their bearing upon the changes which must have 

 been brought about by the expansion of the ice-sheets. 

 The subject is one of peculiar difficulty, but I believe that 

 the solution of many of the problems connected with the 

 Glacial Period is to be found along the lines of Mr. 

 Harmer's investigatio.is. 



In considering this factor it is also especially interesting 

 to find that Captain R. F. .Scott is of opinion that the 

 great shrinkage in the Antarctic land ice, of which he 

 obtained such convincing evidence during the recent ex- 

 pedition, is due to the present excessive coldness, and con- 

 sequent dryness, of the climate ; and he assigns the former 

 extension of the southern ice-sheets to a period of warmer 

 and moister conditions." It would have been easy, had 

 time permitted, to bring together numerous illustrations 

 from Polar lands to show how strongly localised in many 

 places are the conditions of existing glaciation ; and such 

 conditions must have been still more effective at lovi-er 

 latitudes. Hence we can readily imagine that, during the 

 Glacial Period, differential growth and shrinkage might be 

 brought about concurrently in areas not very wide apart, 

 by local circumstances. 



Waning Ice-sheets. — .So far as the eastern side of England 

 is concerned, I think that the epoch of maximum glaciation 

 was reached, not when the East British lobe pressed 

 farthest westward, but when the Pennine and North British 

 ice advanced southward along its receding flank ; and this 

 stage is, I presume, equivalent to the " Polandian Glacial 

 Epoch " of Prof. Geikie's classification. It was at this 

 time that the ice lapped highest around the slopes of the 

 Jurassic and Cretaceous uplands of Yorkshire, causing that 

 radical diversion of the surface-drainage which produced 

 the remarkable effects first made known to us by the 

 brilliant researches of Prof. P. F. Kendall in Cleveland,* 

 and since traced by him and his fellow-workers at intervals 

 wherever the margins of the ice-sheets have abutted against 

 the slope of the land. 



Farther southward this ice, augmented by the snowfall 

 on its own broad surface, appears to have spread over the 

 lower ground far beyond the bounds of the former invasion, 

 covering most of East Anglia and the East Midland 

 counties with a moving ice-cap, beneath which the Chalky 

 boulder-clay was accumulated. The Upper boulder-clay of 

 Yorkshire I consider to be the product of the same ice- 

 sheet at its waning. 



This final waning of the Brilish ice-sheets, as I have else- 

 where attempted to show,' must have been accompanied 

 by conditions verv different from the waxing stages. It 

 appears from the evidence that the great ice-plateaus still 

 lingered in their basins even after the amelioration of the 

 climate had progressed so far that no permanent snow 

 could remain on hills that rose considerably above their 

 level. Deprived of reinforcement, and wasting ever more 

 rapidlv as their surfaces were brought lower, the lobes 

 must in all their embayments have passed into that con- 

 dition of " dead ice " with which the explorers of Polar 

 regions have made us familiar. The " englacial " load of 

 detritus which the ice was powerless farther to transport 

 was gradually dropped to the ground, and often modified 

 and spread jjy gravitational movement in the saturated 

 mass.' The peculiar features of the upper part of the low- 

 land drifts were thus explained many years ago by the late 

 J. G. Goodchild, in his luminous description of the glacial 



' "The Influence of Winds upon Climate during the Pleistocene Epoch." 

 Quart. Journ. Geol. Soc, vol. hi!, (iqoi), pp. 405-476- 



- "Results of the National Antarctic Expedition.' Geograph. Jouru., 

 vol. XXV. (1905), p. 306. 



"• "A System t)f Glacier-Lakes in the Cleveland Hills." Quart. Joiirn. 

 Genl. .'iflc, vol. Iviii. (1902), pp. 471-571. 



■* "The Geology of the Isle of Man." Mem. Geol. Survey (1903), pp. 

 395-7- 



■5 The flow of loose material at the surface when saturated by water has 

 been recently studied by 1. G. Andersson (Upsala), who cites many remark- 

 able illustrations of the phenomenon, and proposes to apply to it the term 

 "solifluclion." Journ. Geol.. vol. xiv. (1906), pp. 91-112. 



