20 DRIFT DEPOSITS OF ILLINOIS. 



this there may have been other hundreds of feet of snow. 

 Glaciers of so vast extent and thickness, would have moved 

 downward wherever the conditions would permit, like the glaciers 

 of the Alps, and all the more readily for their enormous weight." 



A glacier is usually defined as "an immense mass of ice, or 

 snow and ice, moving slowly down mountain slopes or valleys," 

 and it is probable that the earliest advocates of glacial theory 

 in this country formed their conclusions mainly from an exami- 

 nation of the phenomena presented in New England, and other 

 mountainous regions, where the conditions necessary for the 

 formation of typical glaciers were prevalent, but as such condi- 

 tions were not found in the broad plains of the northwest, the 

 "ice sheet" or "ice cap" was substituted for the glacier, to ex- 

 plain the occurrence of foreign material in the drift deposits of 

 the northwest. 



The glacial theory requires for its support, a supposititious 

 elevation of the surface, to the extent of several thousand feet, 

 of all that portion of the continent over which the drift is dis- 

 tributed. This was necessary in order to obtain the conditions 

 of an arctic climate in a temperate zone, a supposition with no 

 evidence to sustain it except what is afforded by the so-called 

 glacial phenomena. 



As has been clearly shown by the preceding sections of the 

 drift deposits of Illinois, the boulder clays invariably overlie 

 an old soil, which covers such a wide area as to justify the con- 

 clusion that it was originally spread over the whole surface of 

 the State, and it seems highly improbable that a vast ice sheet, 

 of enormous thickness, could push its way over a nearly level 

 surface, and leave the soil intact beneath. Moreover, wherever 

 a good section of the drift clays, overlying the black soil, is 

 well exposed, the evidences of a more or less distinct stratifica- 

 tion is observable, a fact which does not accord with the "gla- 

 cial theory," but indicates clearly that water was an active 

 agent in their accumulation. 



Foreign boulders are comparatively -rare in the central portion 

 of the State, except in the vicinity of the old valleys, where they 

 are more abundant than elsewhere, though seldom exceeding a 

 diameter of two feet, and in the extreme southern portion of 

 the State, those exceeding a few inches in diameter are seldom 



