fREStDENtlAL ADDRESS. 541 



hiiimals inhabited the coilntry, perhaps as seasonal migrants, until the time that 

 it was actually covered by the encroaching ice-sheets. 



And here I may note my opinion, that throughout the discussion of our glacial 

 deposits too much weight has been allowed to the deductions regarding climate 

 based upon scanty indications afforded by the ancient fauna and flora. 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 under 

 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 conviction 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 liippopotamus. 

 But the far-travelled stones in the Sewerby beach and in the beaches of the 

 same age in the south of Ireland are evidence that the British seas were already 

 cold enough to carry ice-floes while these large mammals still tenanted the land. 



The next event indicated by the Sewerby section is a slight elevation of the 

 land. Then the traces of an increasingly rigorous climate become con- 

 spicuous, for the sand-dunes which had been banked against the old cliff' are 

 covered by chalky rubble 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 time that the bed 

 of the North Sea was gradually filled by a great ice-lobe that spread southward 

 and outward 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 pre-existing 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 connection 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, I 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 

 become ice-covered, any open water within them being in time obliterated, 



' Lamplugh, Prnc. Yorlcs. 6eol. Soc, vol. xv. (1903), pp. 91-95. 



