6 < J EOLOGY OF ILLINOJ S. 



universally conceded. Their slow, crawling motion and 

 irresistible force ground the rocks to powder, as wheat is 

 ground to flour between the upper and nether millstones ; 

 not only ground them to powder, but rounded and polished 

 the boulders and the gravel, planed and grooved the rocky 

 surface of the earth, and moved the vast masses of drift 

 materials from place to place in a slow procession. Direct 

 evidences of the ice forces of the glacial period are not 

 met with so frequently, as a modified form of these forces. 

 Along the ridges and gravel hills north of Foreston, in 

 Ogle county, the great accumulation of gravel, sand and 

 boulders, presents the appearance of glacial moraines, as 

 if two glaciers had met and deposited their accumulated 

 loads of dirt, sand, gravel and boulders, much of which 

 seems to have been torn from rocky formations of the Si- 

 lurian age, at no great distance from the place of final 

 deposit. But the great mixing and transporting agency 

 which arranged, assorted, and deposited most of our North- 

 ern Illinois drift deposits, was evidently the mixed action 

 of ice and water. 



When the temperature of the glacial winter began to 

 grow warmer, and the great moving fields of ice began to 

 melt, streams of turbid water would rush out and form 

 shallow seas and lakes. The glaciers on the more elevated 

 portions of the land, still fed by perpetual snows, would 

 creep into the neighboring bodies of water, break off and 

 float away in the form of icebergs and floes, bearing with 

 them the boulders, gravel and dirt, torn from the hills and 

 outcropping rocks along their passage. As this floating 

 ice melted, either by an increase of the earth's temperature, 

 or by being borne into a warmer atmosphere further south, 

 the materials with which it was freighted would sink to the 

 bottom, and become subject to the action of a new force, 

 the assorting and transporting force of currents of flowing 

 water. The contraction and expansion of the ice over these 

 shallow lakes or seas, caused by alternate freezing and 



