348 D. Bell — The Great Ice Age and Submergence. 



IIL — Notes on "The Great Ice Age" in Relation to the 

 Question of Submergence. 



By Du&ALD Bell, F.G.S. 



{Continued from p. 326.) 



III. The Second Glaciation. 



^^"TIVEN if marine life had been prolific, and the old sea-bottom 



X_J more or less well covered with sedimentary deposits, it does 

 not follow," says Dr. Geikie, "that the Boulder-clay of the suc- 

 ceeding mer de glace should now contain any shells" (p. 141). 



This is what we have now to consider —that the marine deposits 

 of the submergence, however abundant, may have been swept away 

 by the succeeding glaciation so completely as to leave no trace in 

 the Upper, or post-submergence. Boulder-clay. 



Now, we may assume that, in the gradual subsidence and re- 

 emergence of the land ("leaps and bounds" being discarded), the sea 

 would for some time occupy all possible levels up to the supposed 

 maximum limit, and would, on the premises now granted, leave 

 traces of its presence, and of its " prolific " organic life, more or less 

 abundantly at them all. Indeed, such traces would, so to speak, 

 have a double chance of being left — first, during the submergence, 

 and again, during the re-emergence. Further, at every successive 

 level the sea would go into innumerable bights and bays, ravines 

 and sheltered hollows of the land where, from their very nature, 

 subsequent ice-sheets could not follow it.^ This is obvious enough 

 in itself, but we are glad to have it also on Dr. Geikie's authority. 

 Speaking of the beds of silt, sand, and gravel which in many places 

 are intercalated in the "Till" — indicating a time when the great ice- 

 fields had receded " so far at least as to uncover the lowland tracts 

 and valleys " — he shows that " during the Glacial period the ice- 

 sheet, which followed the lines of the principal valleys, must 

 frequently have crossed the lateral and tributary valleys nearly 

 at right angles. In the main valleys," he continues, " the glacier 

 would exert its full influence, but it would not be able to do so in 

 the narrow lateral valleys and ravines ; the ice and Till would 

 merely topple into the glens referred to, and gradually choke them 

 up, and the main mass of the glacier would then pass over the 

 whole .... In such narrow glens, then, any silt, sand, or gravel 

 that had gathered during the absence of the ice-sheet would not 

 be so likely to be ploughed out" (pp. 108-9). 



Accordingly, in many hundreds of such little tributary glens all 

 over the country, and at many different levels in them all, marine 

 deposits, if the sea had ever been there, would have a good chance 

 of being preserved. 



And that chance, we have to add, would be considerably increased 

 by the very circumstance of these deposits being mainly spread out 

 over the low grounds, where the erosive power of ice-sheets is 

 least, and where, instead of "sweeping out," they tend rather to 



1 See Trans. Geol. Soc. Glasgow, vol. is, pp. 109, 110. 



