THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 55 



only to an extent sufficient to " close in again upon the lower flanks of the hills." In 

 his Survey Memoir, on " The Geology of the Isle of Man " (1903), p. 395, Mr Lamplugh 

 has adduced reasons to show that " the shrinkage of the ice-sheet covering the Isle of 

 Man is likely to have commenced while the Welsh and Ivernian sheets were still in- 

 creasing." "But," he adds, "it was not until there had been a great amelioration of 

 climate that the island began to emerge from the waning mass " (p. 395). So this 

 "great amelioration of climate" must have taken place at the time in which the 

 stratified Drift deposits were accumulating between the receding ice-sheet and the 

 emerging land. Does not this change in the climate indicate the gradual passing away 

 of glacial conditions at a time prior to the deposition of the Upper Boulder Clay ? Mr 

 Lamplugh refers again and again to the fact that a very considerable alteration in the 

 climate must have occurred to account for the shrinkage of the ice from the island. 

 " The shrinkage," he observes (p. 396), "may have begun from a diminution in the 

 amount of snowfall alone ; but before it could proceed far there must have been a wide- 

 spread change of climate and an essential difference between the conditions of this stage 

 and of the initial stage of the period. At the beginning of the period the climate was 

 such as to permit the permanent snow-line to descend nearly to sea-level ; while, when 

 the ice-sheet had reached its maximum, it was requisite that the climate should be warm 

 enough to prevent snow lying permanently at over 2000 feet above sea-level before a 

 positive lowering of the surface by melting could take place. Thus, an ice-sheet 

 already in existence may be able for a long time to withstand a climate in which it 

 could never originally have accumulated ; and ameliorating changes may attain an 

 advanced state before their effect on such an ice-sheet becomes marked. But when the 

 permanent thawing of the surface once commences, it must go on with accelerated 

 rapidity as lower levels are reached." Again, a little further on he says : " It is clear 

 that by the time the hill-tops had reappeared the Arctic conditions of climate had 

 passed away, and permanent snow was no longer possible on rock-surfaces, even at the 

 higher levels." If the Arctic conditions of climate had thus passed away by the time 

 the hill-tops had reappeared, it is likely that the amelioration in the climate continued, 

 and that something more than a mere temporary and short-lived recession of the ice-lobe 

 took place. Mr Lamplugh accounts for the deposition of the Upper Boulder Clay by 

 a re-advance of the ice which followed closely again the local depositions of the stratified 

 drift. In view of the great change of climate which is admitted, it seems more probable 

 that at this time also the ice-plateau, though lingering for a considerable time in the 

 Irish Sea basin, went on wasting and shrinking, passing into a condition of " dead ice," 

 and eventually disappearing entirely in much the same way as Mr Lamplugh thinks the 

 final waning of ice-sheets from the British area to have taken place. 



Though the mass of ice may not have attained its maximum development at the 

 same time in all parts of the West British region, and shrinkage may have started in the 

 ice covering the Isle of Man sooner than it did further south and west, one cannot 



