SPECIFIC DISCUSSION 23 



It is very difficult to make any quantitative comparisons which 

 would be more than mere guesses, but it seems probable that the whole 

 volume of drift left by any ice body would fall far below a fair estimate 

 of the amount of alluvium, geest, soil, and loosened rock which the ice 

 body found on its territory. In any estimate of the volume of drift we 

 should not fail to add that seized by the rivers and borne seaward, the 

 loess mantle over great areas, and the matter carried away in solution. 



It will generally be found that the expert students of the drift make 

 the more moderate claims for its volume. 



(8) The occurrence of tens of thousands of lakes in the glaciated areas 

 is a striking fact when placed in comparison with their rarity in south- 

 ern districts. There is no doubt of their being related to glaciation. 

 The physiographic explanation is that they represent the infantile and 

 youthful stages of the reconstructed drainage left by the interfering ice 

 sheet. The geologic explanation is that they are chiefly morainal — that 

 is, they occupy depressions in the irregularly piled drift or lie in valleys 

 which are blocked by drift fillings. Some of the lakelets are in shallow 

 rock-basins, which will be considered later. 



Some of the larger lake-basins, like those of central New York, are 

 complex in their form, structure, and relations and are not readily or 

 positively explained as to their precise origin or genetic relationship 

 without sufficient deep borings to show the preglacial topography. 

 The slopes of some of these valleys have been smoothed by the glacial 

 rubbing, and it is not surprising that they were once thought to be the 

 product of glacial excavation. Naturally those vallej^s which lie in the 

 direction of the glacial flow have been most modified by the ice action. 

 That the ice has smoothed the valley slopes to some extent and swept 

 away the talus accumulations and other loose materials is very prob- 

 able, but to claim more than this for the ice-work is an assumption 

 without any sufficient basis in geologic facts. The subject is discussed 

 on pages 55-65. 



(9) and (10) Because some of the Swiss and Italian lakes, and the 

 " Finger " lakes of New York could not be at once fully explained in the 

 causality of their attitude, depth, form, relation, etcetera, under non- 

 glacial agencies, they have been regarded as illustrations of deep glacial 

 erosion. Such lake-basins were, indeed, the first suggestion, and have 

 been one main argument, for extreme ice erosion. Even recently some 

 physiographers appeal to glacial action and postulate a thousand feet of 

 cutting by ice because their rules and principles of topographic evolution 

 do not immediately explain the peculiarity of the topography. Tacitly, 

 it has been expected that one denying the theory of ice-cutting must 

 disprove it ; but the burden of proof belongs on the advocates of ero- 



