22 H. L. FALRCHILD — ICE EROSION THEORY A FALLACY 



several reasons. The waters of Muir inlet have a depth of 300 feet and 

 a tidal range of 23 feet; this, with the disturbance of the waters and of 

 the bottom sediment, due to the calving of icebergs, makes the amount 

 of sediment held in suspension at the front of the glacier no criterion for 

 the sediment in the glacier drainage. Moreover, even if we had the true 

 measure of the gletschermilch, it would be of no value as a measure of 

 the Muir glacier erosion, since the lower part of the glacier for an unknown 

 distance is overriding gravels. Besides all this is the fact that the sedi- 

 ment washed from this or any other glacier is derived from the grinding 

 of the stones within the glacier as well as the scratching on the bottom, 

 and also from whatever rain and stream-wash there may be on the valley 

 walls. 



(6) The lime carbonate in the drift is doubtless due to mechanical 

 action on calcareous rocks ; but it is inconsiderate and unwarranted to 

 attribute it entirely to abrasion of the channel walls. It represents in 

 the main, probably, the trituration of the limestone fragments which the 

 glacier was transporting, and which had been made ready for the ice-mill 

 by the millions of years of pre-Pleistocene weathering. The great quan- 

 tities of striated limestone fragments in the drift immediately southward 

 of limestone outcrops show the main source of the carbonate in the till. 



(7) The volume of the glacial drift has been cited as the measure and 

 proof of great glacial erosion. This is decidedly untrue and misleading. 

 More justly it is to be regarded as merely the measure of the transporta- 

 tion by the glacier. The mass of drift, no matter how great, is no proof 

 whatever of any erosion at all of sound rock. The assumption fails to 

 recognize the enormous supply of loose material which the ice found 

 ready to its grasp. During the millions of years of the Mesozoic and 

 Tertiary the northern lands were exposed to active weathering agencies 

 under climatic conditions probably similar to those prevailing today in 

 the middle and southern United States. We may well believe that the 

 mantle of geest over all the glaciated areas of the northern hemisphere 

 was fairly comparable to that which covers our southern lands today. 

 This mantle was largety removed from the central areas of glaciation 

 and was piled over the regions where the ice margins lingered. The 

 amount of drift over any glaciated territory is very easily overestimated, 

 and the mental bias of the writer must be noted. For example, the 

 estimates of vast glacial erosion of Scandinavia based on the enormous 

 amount of drift over the German lowlands and elsewhere, and all as- 

 sumed to be derived from Scandinavia, is most certainly valueless. 

 Morainal belts and valley fillings are the conspicuous masses of the 

 drift and strike the attention, but they are relatively local and should 

 not be taken as typical of the general drift sheet. 



