292 THE ICE AGE AND ITS WORK. . 



its load of stoues and debris to form tlie terminal moraine, while the 

 lower strata cliokedin thedetiles, would move very slo^^ly. And once 

 out ill the open valley of the Po, then a great inlet of the warm jVfedi- 

 terranean Sea, the ice would rapidly melt away in the water and in the 

 warm moist atn^osphere. and therefore have no tendency to erode a 

 lake basin. - - - 



4. The alternative theory and its di^cuUies. — There is really only one 

 alternative theory to that of ice erosion for the origin of the class of 

 lakes we have been discussing, viz, that they were formed before the 

 glacial epoch, by earth-movements of the same nature as those which 

 are concerned in mountain formation ; that is, by lateral pressure caus- 

 ing folds or tiexures of the surface, and where such tiexures occurred 

 across a valley a lake would be the result. - - 



As this theory is put forward with so much confidence, and by geolo- 

 gists of such high rei)utation, 1 feel bound to devote some space to its 

 consideration, and shall, I thiidv, be al)le to show that it breaks down 

 on close examination. 



In the first place, it does not attem])t to exi)lain that wonderful 

 absence ol valley lakes from all the mountain regions of the world except 

 those which have been highly glaciated. It is no doubt true that 

 during the time the lakes were tilled with ice instead of water they 

 would be preserved from filling up by the iiifiux of sediment; and this 

 may be fairly claimed as a reason Avhy lakes of this class should be 

 somewhat more numerous m glaciated regions, but it does not in any 

 way explain their total absence elsewhere. We are asked to believe 

 that m the period immediately preceding the glacial epoch — say, in the 

 newer Pliocene period, — earth -movements of a nature to produce deep 

 lakes occurred in every mountain range without exception that was 

 about to be subject to severe glaciation, and not only so, but occurred 

 on both sides of each range, as in the Alps, or all round a mountain 

 range, as in our lake district, or in every part of a comi)lex mountain 

 region, as in Scotland from the Frith of Clyde to the extreme north 

 coast — all in this very limited period of geological time. AVe are further 

 asked to believe that during the whole period, from the commencement 

 of the ice age to our day, such earth -movements have never produced a 

 single group of valley lakes in any one of the countless mountain ranges 

 and hilly regions throughout the whole of the very much more extensive 

 nonglaciated regions of the globe. This appears to me to be simply 

 incredible. The only way to get over the difficulty is to suppose that 

 earth-movements of this nature occurred only at that one period, just 

 before the ice age came on, and that the lakes produced by them in all 

 other regions have siiu^e been filled up. But is there any evidence of 

 this? And is it probable that all hikes so produced in non-glaciated 

 regions, however large and deep they might be, and however little 

 sediment was carried down by their infiowing streams, should yet all 

 have disappeared. The theory of the ])rcglacial origin of these lakes thus 



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