324 THE ILLINOIS GLACIAL LOBE. 



spreading out a delta similar to that formed in the lower part of the Des 

 Plaines Valley by streams issuing from the same moraine; (2) by streams 

 issuing from the ice at the time the Minooka Ridge was forming, for the ice 

 at that time seems to have extended as far, at least, as the western edge of 

 the Marseilles moraine in Kane County; (3) by outwash from the Marseilles 

 moraine, which sweeps around the eastern and southern border of the plain. 

 In the present state of knowledge it seems difficult to draw inferences con- 

 cerning the date at which this plain was formed, and also concerning the 

 broader question of the character of the outwash from the ice sheet during 

 the Minooka substage of glaciation. 



Whether the Minooka Ridge is to be included in the late Wisconsin 

 series can not perhaps be decided at the present stage of investigation. If 

 it can be clearly established that during the interval between the formation 

 of the Marseilles moraine and the Minooka Ridge the outlet down the 

 Illinois had been lowered from about 640 feet above tide (the level of 

 neighboring outlets through the Marseilles moraine) to a level nearly as low 

 as the 560-foot beach bordering the Illinois in the Morris Basin, a corre- 

 spondingly late date for that ridge may be assumed. But at present there 

 is much uncertainty as to the amount of work accomplished by that outlet 

 prior to the formation of the Minooka till ridge. The bearing of bowlder 

 belts on this question is considered below (p. 327.) 



INNER-BORDER TILL PLAIN. 



On the inner border of the Minooka Ridge, from the vicinity of Elgin 

 southward to the head of the Illinois, there is a narrow till plain nowhere 

 more than 6 miles, and usually but 2 or 3 miles, in width, which separates 

 this till ridge from the Valparaiso morainic system. North from Elgin the 

 Minooka Ridge has not been separated from the Valparaiso system. From 

 the head of the Illinois eastward along the north side of the Kankakee Val- 

 ley there is a similar till plain separating the ridged drift on the border of 

 the valley from the Valparaiso morainic system. Its width is greatest on 

 the meridian of Kankakee, where it is fully 12 miles, the usual width being 

 only 7 or 8 miles. East from Kankakee the till plain north of the river 

 decreases in width and extends only a short distance east of the State line. 

 The Valparaiso system there comes to the border of the Kankakee marsh. 



