70 ILLINOIS STATE ACADEMY OF SCIENCE 



union with the Illinois. The valley train, however, does 

 not exist today in its entirety, but only as remnants of 

 terraces. 



In addition to the Fox Eiver valley train, other out- 

 wash deposits were made in the form of outwash plains. 

 A remarkable example of this occurs east and southeast 

 of Elgin. Here these enormous gravel resources are be- 

 ing drawn upon for our railroad and highway building, in 

 addition to other phases of the concrete industry, and the 

 topography is being markedly changed by the activities 

 of man so well displayed in the pits of the Chicago 

 Gravel Company. 



This gravel outwash occurs at a distinctly lower level 

 than that at Dundee, Carpentersville, and Crystal Lake, 

 and is later in origin than the cutting of the Fox Eiver 

 Valley, whereas the Crystal Lake gravel deposits just 

 mentioned are older. This conclusion is based not only 

 upon the difference in level at which the two deposits 

 ccur, but upon the fact that the higher gravels are found 

 to pass and to continue eastward beneath the Valparaiso 

 moraine for a distance of approximately six miles in Mc- 

 Henry County, whereas the lower gravel plain has its 

 source in the Valparaiso moraine. 



THE CLOSING STAGES OF THE GLACIAL PEEIOD 



After the ice had readvanced to within two miles of 

 Elgin, and built the outer Valparaiso moraine, the climate 

 became such as to cause the recession of the ice by stages, 

 with the resultant building of recessional moraines to the 

 east until not only had the entire Valparaiso system been 

 developed, but also the Lake Border system, and the ice 

 had receded to within the basin of the Great Lakes. From 

 this time forth, the average severity of the climate de- 

 creased, the lake levels shifted from time to time as the 

 lower outlets were uncovered by the receding ice or 

 shifted by the warping of the land surface, until finally 

 the North American continent was freed of this active 

 and colossal geologic agency which had so thoroughly 

 revolutionized the topography of the land surface upon 

 which it encroached. 



