288 THE ILLINOIS GLACIAL LOBE. 



displays its structure from top to bottom. The knoll is capped by a brown 

 clay, containing few pebbles. Beneath this there is at the east side of the 

 knoll considerable sand, but toward the center there is a series of till or clay 

 beds 12 to 20 inches in thickness, inters tratified with gravel and sand beds 

 each 2 or 3 feet in thickness. All the beds dip toward the center of the hill 

 at an angle of 15 degrees or more. The till is very hard and pebbly. The 

 gravel contains a much larger percentage of crystalline rocks of Canadian 

 derivation than is common in knolls and eskers made up entirely of assorted 

 material, though there are many limestone pebbles such as may have been 

 derived from ledges in the neighboring districts on the east. This knoll 

 stands 15 to 20 feet above the bordering plain and about 30 feet above the 

 flood plain of Little Indian Creek, which touches it on the east. 



COVEL EIDGE. 



In the plain drained by Covel Creek, a southern tributary of the Illinois, 

 entering just below Ottawa, there is a low ridge about 7 miles in length 

 extending in a nearly due east-west direction from near the outer border of 

 the Marseilles moraine in sec. 4, Grand Rapids Township (T. 32, R. 4 E.), 

 to the inner slope of Farm Ridge the inner ridge of the Bloomington 

 system in sec. 5", Farmridge Township (T. 32, R. 3 E.). It has a height of 

 15 or 20 feet and a width of 40 to 60 rods, and is interrupted by no gaps 

 of consequence except the one through which Covel Creek passes in sec. 6, 

 Grand Rapids Township, and this gap is only 30 or 40 rods in width. The 

 altitude of the crest of this ridge, as shown by the Ottawa topographic 

 sheet, a portion of which is reproduced in PI. XIX, is mainly between 640 

 and 650 feet above tide. The portion east of Covel Creek is represented 

 to be slightly lower than 640 feet. 



The ridge is capped by a brown silt several feet in thickness which is 

 readily pervious to water. This is underlain by sand which extends to a 

 depth of 15 or 20 feet or more. The few wells which have been made on 

 the ridge are sunk no deeper than 20 feet. The sand also extends out 

 beneath the bordering plain a short distance both north and south of the 

 ridge and affords water for wells at slight depth. The extension beneath 

 the plain, however, seldom exceeds a mile in width or half a mile from the 

 crest of the ridge. Border districts are underlain at slight depth by till. 



The internal structure, the form, and the uniform elevation of the ridge 



