THE MINOOKA TILL RIDGE. 321 



instance nearly 200 feet in thickness, the thickness as compared with that 

 on bordering plains being about as much greater as the measure of the 

 relief of the ridge. In the portion along the Kankakee the thickness 

 seldom exceeds 60 feet, but is greater along the higher parts than on the 

 lower or than on the border plain, as would be expected from the relief 

 which it presents. 



STRUCTURE OF DRIFT. 



In the ridge north from the head of the Illinois the surface portion of 

 the drift to a depth of 8 to 12 feet consists of a yellow till containing many 

 pebbles. It is sandy in places, but as a rule is a stiff, sticky clay, very 

 slowly pervious to water. 



This yellow till is underlain by a grayish blue till containing occasional 

 gravel or sand veins, which furnish water in small cpiantities. This bed of 

 till often has a thickness of 100 feet or more. Beneath it there is in places 

 a heavy deposit of sand. This may not underlie the whole of the ridge, 

 but has been penetrated in several wells along the crest in Kendall County. 

 It furnishes abundance of water for wells. 



The ridge from Minooka northward carries a black surface soil several 

 inches in depth, such as is found on the bordering plains. There are few 

 surface bowlders, and pebbles are rare within 18 inches of the surface. 



The drift along the north border of the Kankakee is mainly till, and 

 surface bowlders are more abundant than in the portion north of the head 

 of the Illinois River. 



CHARACTER OF THE OUTWASH. 



The well-defined portion of the Minooka Ridge has a low part of the 

 Morris Basin on its outer border. Unless the outlet down the Illinois had 

 been opened to a level as low as this part of the basin, the basin would have 

 held a lake, and there scarcely could have been vigorous discharge from the 

 Minooka Ridge. Terraces on the Des Plaines River which have their head 

 in the Valparaiso moraine and were formed in connection with that moraine 

 indicate that a lake stood in this basin at the head of the Illinois while they 

 were forming and that its level was about 560 feet above tide, or 60 feet 

 above the present head of the Illinois River. This lake level is well defined 

 also by beaches which are to be seen both north and south of the Illinois. 

 mon xxxviii 21 



