NA TURE 



[August i6, 1906 



We know little regarding the range of adaptability 

 possessed by the forms in the past, and can judge only 

 from their present habitat, which is generally governed by 

 many other factors besides climate ; moreover, it is granted 

 that species already established, when subjected to gradual 

 change, will persist for long in circumstances that would 

 have effectively barred their introduction. In the Upper 

 Zambesi Valley last year I was more impressed with the 

 cold of the nights than with the heat of the days ; and 

 even at that latitude the sturdy hippopotamus in his 

 nocturnal raids must experience a temperature occasionally 

 descending below freezing-point. 



It took us long to break away from the established con- 

 viction that the fossil elephant and rhinoceros could not 

 have existed in a cold climate ; and the same conviction 

 still lingers with respect to their companion, the hippo- 

 potamus. But the far-travelled stones in the Sewerby \ 

 beach and in the beaches of the same age in the south of i 

 Ireland are evidence that the British seas were already cold 

 enough to carry ice-floes while these large mammals still 

 tenanted the land. I 



The next event indicated by the Sewerby section is a | 

 slight elevation of the land. Then the traces of an in- 

 creasingly rigorous climate become conspicuous, for the 

 sand-dunes which had been banked against the old cliff are 

 covered by chalky nibble containing a few land shells ' ; 

 and this material, like the corresponding " head " which 

 covers the ancient beaches of the south of Ireland and the 

 south-west of England, appears to represent the frost- 

 splintered rock washed down from the rock slopes during 

 the season of thaw. 



.According to my reading of the evidence, it was during 

 this lime that the bed of the North Sea was gradually 

 filled by a great ice-lobe that -spread southward and out- 

 ward along the basin, slowly but irresistibly churning up 

 and dragging forward the old sea-floor as part of its 

 ground-moraine. When it impinged upon the rising ground 

 of eastern Britain the progress of this sheet was arrested 

 and part of its burden left in the form of the lowest 

 boulder-clay — the " Basement Clay " of Yorkshire and the 

 " Cromer Till " of Norfolk. In Yorkshire this boulder- 

 clay frequently includes huge transported masses of 

 Secondary strata, which still maintain their identity, in 

 some cases even to their bedding planes ; and along with 

 these we sometimes find patches of the material of the 

 old sea-floor which have similarly escaped destruction. 

 More frequently the preexisting deposits from which the 

 boulder-clay has been derived have been thoroughly kneaded 

 together, and fragments of Pleistocene shells are then 

 scattered through its mass, along with fossils derived from 

 the Secondary and older rocks. 



In adopting the hypothesis that the Basement boulder- 

 clay represents the ground-moraine of an ice-sheet we may 

 consider briefly the probable conditions under which this 

 " East British ice-lobe " was accumulated. Whether the 

 elevation subsequent to the stage represented by the infra- 

 glacial beaches was sufficient to drain off the shallow seas 

 around our islands is uncertain, but it must, at any rate, 

 have restricted their area and rendered them still shallower ; 

 and it is unlikely that there was then any southward con- 

 nection of the North Sea with the English Channel. The 

 climate by this time had become such that permanent 

 snow-caps could accumulate in the northern parts of our 

 country at elevations not much above present sea-level. 

 Indeed, 1 am inclined to think that the climate may have 

 been actually colder at this time than during any of the 

 later phases of the Glacial Period, and that the stage of 

 maximum glaciation lagged considerably behind the stage 

 of minimum temperature. Under these conditions, with 

 the snowfall on the uplands always slowly drawing away 

 in ice-streams to the basins, and there accumulating, it is 

 inevitable that the enclosed basins would eventually be- 

 come ice-covered, any open water within them being in 

 time obliterated, either directly by the encroaching 

 glaciers, or indirectly by the packing of bergs and floes, 

 until the basins themselves possessed a surface upon which 

 the snowfall could accumulate. Thus the basins became 



1 Lamplugh, Proc. Yarks. Geol. Soc 



NO. 1920 VOL. 74] 



• (1903), pp. 



great reservoirs of ice, in which the supplies from the 

 surrounding uplands received important augmentation by 

 direct accretion of snowfall ; — reservoirs, moreover, con- 

 taining a substance sufficiently rigid not to require retain- 

 ing walls ; so that, in time, the surface of the ice within 

 the basins rose higher than many parts of the rim. The 

 general movement of the mass within its reservoir then 

 became dependent mainly upon its own configuration, and 

 only secondarily upon the shape of the solid ground. 



These conditions in the North Sea basin had their 

 parallel in the basin of the Irish Sea, in which the " West 

 British ice-lobe" was developed; and on the low interior 

 plain of Ireland, where the similar though smaller 

 " Ivcrnian " sheet held possession. 



Now, the crux of the Interglacial problem, so far as the 

 British Islands are concerned, lies in the question whether 

 these huge reservoirs, after their first filling, were com- 

 pletely emptied during the supposed interglacial epoch of 

 warmth named by Prof. Geikie the " Helvetian," and were 

 afterwards refilled for the later " Polandian " glaciation, 

 in which, on the evidence of the upper boulder-clays, it is 

 generally agreed that ice-sheets from the basins again 

 closed in upon the land. It is this one interglacial or 

 " middle glacial " epoch only that most of the British 

 supporters of the hypothesis have demanded, and have 

 attempted to establish in the East Yorkshire sections. 



For my own part, although I have sought long and 

 carefully for evidence of this great interglacial episode in 

 the Yorkshire drifts, and at first with the belief that such 

 evidence must surely be somewhere forthcoming, my search 

 has not only failed to bring to light any adequate proof 

 of its reality, but has yielded many facts which I cannot 

 explain otherwise than by recognising that the ice-lobe 

 continued to occupy the basin of the North Sea during the 

 deposition of the beds claimed as interglacial, though its 

 margin had for a time shrunk considerably within its 

 earlier limits. 



The " Purple " Boulder-Clays and Stratified Drifts. — The 

 drifts overlying the Basement Clay in East Yorkshire 

 consist of a comple.x and very variable series, in which 

 bands of boulder-clay predominate in some places and lenti- 

 cular sheets of well-stratified material in others. In the 

 cliff-sections of the Holderness plain certain bands of 

 boulder-clay, known as the Upper and Lower Purple Clays, 

 are persistent for many miles ; but when the series 

 approaches the rising ground of the Wolds the individuality 

 of the beds is lost, and they are often replaced entirely by 

 irregular mounds of sand and gravel. 



I began work on these sections with the then-prevalent 

 idea that every separate band of boulder-clay above the 

 Basement Clay might indicate a separate glacial epoch, 

 and that warm interglacial epochs might be represented 

 by the partings of sand and gravel between these boulder- 

 clays ; and the object of one of my early papers ' was to 

 show that more of these divisions were present than had 

 found place in the scheme of classification then in vogue. 

 But after struggling for a time under an ever-increasing 

 load of epochs I was compelled, in tracing the separate 

 bands northwards, to recognise, as my friend Mr. J. R. 

 Dakyns had previously recognised,^ that the whole series 

 underwent protean changes, the boulder-clays sometimes 

 splitting into numerous shreds amid thick sheets of sand 

 and gravel, at other times merging into a single mass to 

 the exclusion of all stratified material, and not rarely pre- 

 senting a passage from uncompromising " till " to stratified 

 gravel, sand, and clay. Hence I was driven to conclude 

 that stratified and unstratified drift must often have been 

 forming simultaneously at places very little distance apart ; 

 and on finding, also, that the whole of the deposits between 

 the Basement Clay and the Upper or " Hessle " Clay were 

 not only knit together in this fashion, but were similarly 

 interwoven with the top and bottom of these boulder-clays, 

 I had finally to abandon the Interglacial hypothesis 

 altogether so far as the coast-sections were concerned. I 

 mention this experience in order to show that my present 



1 "On the Divisions of the Glacial Beds in Filey Bay." P>vr. Vo 

 GcoL Spc. vol. vii. {1879). pp. 167-177 

 •-' "Glacial Beds at Bridlington." Ibid., vol. vii. (1S70). pp. 123-128. 



