310 On British Drifts mid the Intcnrhicidl Problvin. 



The next event indicated by the Sewerby section is a slij^^ht 

 elevation of the hmd. Then the traces of an increasingly 

 rig-orous climate become conspicuous, 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' ot 

 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 b\' tliis time had become such 

 that permanent snow-caps could accumulate in the northern 

 parts of our country at elevations not much abo\ e 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 (ilacial Period, and tliat the stage ot" maximum 



Naturalist. 



