CHAPTER V 



LAKE CHICAGO AND ITS OLD SHORE LINES 



S THE ice sheet finally melted (Fig. 48) 

 and retreated farther north there formed 

 between the Valparaiso Moraine and 

 the front of the glacier a lake, called 

 now Lake Chicago (Fig. 50). This had 

 its outlet through the gap in the mo- 

 raine known as the Sag. Two streams 

 from the lake one on the north, one on 

 the south of Mount Forest Island (Fig. 53) converged to this 

 point. It was probably just chance inequalities of deposition 

 that determined the location of these two channels, thus form- 

 ing Mount Forest Island, but the Sag was likely predetermined 

 by the fact that here ran the old preglacial valley, a deep rock 

 cut, which though partly filled by glacial drift still made a distinct 

 break in the restraining moraine. Besides Mount Forest Island 

 another isolated mass of drift stood out of the water in the 

 Chicago region, namely Blue Island. This now stands above 

 the level of the Chicago plain, once the old lake bottom, as a 

 ridge several miles long and one-half mile or so wide stretching 

 north from the town of Blue Island to Beverly Hills. The 

 Rock Island suburban line runs close to its east side. It is 

 encountered as an abrupt rise on Western Avenue about Eighty- 

 seventh Street. Anywhere along its crest one looks westward 

 over level country, old lake bottom, to the distant hills of the 

 mainland of the moraine or of the other islands. 



Lake Chicago stood at one level for a long time, long enough 



for its waves to eat into the hills along its shores and deposit 



extensive beaches of sand just as Lake Michigan is doing now 



along its present shore line. The highest of these beaches stands 



78 



