THE GLACIAL PERIOD 59 



It is now impossible to be certain of the topography of the 

 country hereabout before the glacier came to make its changes, 

 for that old suriace was undoubtedly greatly altered, its rock 

 foundation pretty completely covered by the debris the glacier 

 left. Still, knowing the general lay of the old rock strata and 

 their nature, and learning something from the rock hilltops that 

 crop out of the glacial deposits and from well borings that pene- 

 trate to the rock surface, it is judged that the old valley now 

 occupied by Lake Michigan extended past the site of Chicago, 

 through rugged hill country, and that in it flowed a river, part 

 of the preglacial river system, a more or less conjectural diagram 

 of which is shown in the accompanying figure (Fig. 39). Fortu- 

 nately there is one region not far removed from Chicago which 

 presents no evidence of the recent glacier, and it is believed it 

 escaped the late ice covering — ^an island in the broad ice flow. 

 This is the unglaciated region of northwestern Illinois and south- 

 western Wisconsin. It is a well-drained region with much 

 branched rivers flowing in deeply cut valleys between the 

 rounded hills. It is not a region of lakes or ponds. The soil 

 grades insensibly into the rock — fine soil on top, coarser below, 

 finely broken rock, coarser fragments, then the rock ledge just 

 beginning to disintegrate. It is probably quite typical of the 

 Chicago region in preglacial times except that the processes of 

 erosion had not gone so far here before they were interrupted 

 by the onset of the glacier. 



As the great glacier came pushing its way southward it 

 shoved ahead of it some of the soil scraped from the land, soil 

 that had been accumulating through many centuries of rock 

 decay; or else the frozen soil, together with the rock fragments 

 below it, was over-ridden by the glacier, frozen to its base, and 

 so became a part of the onward-moving mass and ultimately was 

 incorporated in the plastic ice. Then this same material became 

 in the grip of the glacier, an efficient grinding and polishing tool, 

 This is found to be true of present-day glaciers. They are 

 mighty agents of erosion. While the ice itself glides over the 



