Oti British Drifts and the Intcrghicial Probhm. 311 



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, 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 

 great reservoirs of ice, in which the supplies from the 

 surrounding uplands received important augmentation by direct 

 accretion of snowfall ; — reservoirs, moreover, containing a 

 substance sufficiently rigid not to require retaining 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 ' Ivernian ' sheet held possession. 



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

 British Islands are concerned, lies in the question whether these 

 hvige reservoirs, after their first filling, were completely emptied 

 during the supposed interglacial epoch of warmth named by 

 Professor Geikie the ' Helvetian,' and were afterwards refilled 

 for the later ' Polandian ' glaciation, in which, on the evidence 

 of the upper boulder-cla3's, it is generally agreed that ice-sheets 

 from the basins again closed in upon the land. It is this one 

 interg'lacial 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 

 recog'nising that the ice-lobe continued to occup}' the basin of 

 the North Sea during the deposition of the beds claimed as 

 interglacial, though its margin had for a time shrunk con- 

 siderably within its earlier limits. 



1906 September i. 



