82 A NATURALIST IN THE GREAT LAKES REGION 



in its alteration of local topography, it changed its level, dropping 

 so as to expose the bottom, hereabout, long enough for vegeta- 

 tion to become rank and form extensive peat deposits. This was 

 probably due to the uncovering, by the retreat of the ice, of 

 some outlet to the north, at a considerably lower level than the 

 Chicago outlet. These peat beds are found in excavations such 

 as some made in Rogers Park and Evans ton several years ago, 

 and they lie on the beaches of the Glenwood stage. They are 

 overlaid, however, by deposits of the next or Calumet stage. 



The lake rose again as possibly the glacier advanced tem- 

 porarily and again covered the northern outlet. It assumed a 

 level about twenty feet below the Glenwood stage, 35-40 feet 

 above present level or 615-20 feet above sea-level. It again 

 outletted down the Desplaines Valley. Undoubtedly during the 

 Glenwood stage the valley had been deepened by river erosion. 

 When the old river came directly from the front of the glacier 

 before the lake was formed behind the Valparaiso Moraine, it 

 was heavily charged with sediment and built up an extensive 

 valley train as already described. But when the river became 

 the outlet of the lake, the debris held by the glacier was dropped 

 in the lake and the outflowing stream was clear, ready to pick 

 up a load in its rapid current instead of making a deposit. So 

 from the time Lake Chicago was formed the outletting stream 

 had been working to erode and carry away the silt, sand, and 

 gravel that it had earlier laid down. So the old valley formed 

 by the preglacial stream that came down the broad valley where 

 Lake Michigan lies and that continued down what is now the 

 Desplaines and Illinois valleys was filled below the terminal 

 moraines by the valley train deposited by the river that carried 

 the outwash from the glacier and still later was again partly 

 uncovered by the outlet of Lake Chicago. 



This latter erosion was apparently checked in the upper 

 valley when the river encountered a rocky ledge below the sand 

 and gravel, a ledge running from the present location of Lock- 

 port to Joliet. This could not be worn away rapidly and so the 



