292 THE ILLINOIS GLACIAL LOBE. 



and of the wooded portion of the ridge, the contours being much sharper in 

 the forest than on the prairie. The ridge is mainly forest-clad from the 

 State line south to the vicinity of Hampshire and mainly prairie from 

 Hampshire to Elburn. This difference in the sharpness of contour may be 

 due in part to the effect of the agencies of degradation, the wooded portion 

 of the ridge being better protected from these agencies. It is probable, 

 however, that the prairie portion was originally possessed of smoother con- 

 tour. It presents a series of billows, often 40 or 50 feet high and 60 to 80 

 rods or more in diameter, whose slopes are usually smooth and regular. As 

 indicated below, the relation of the southern portion of the ridge to the ice 

 margin may be somewhat different from that north of Hampshire. 



There are three gaps in this ridge worthy of mention. The largest is 

 that at Marengo, through which the North Kishwaukee passes. It is fully 

 150 feet in depth and about 2 miles in width. The second gap occurs about 

 5 miles southeast of Marengo. This is nearly a mile in width, but only 

 about 75 feet lower than the neighboring portions of the ridge. It has a 

 nearly plane surface, and has apparently been utilized as a line of discharge 

 for a body of water formerly held between this ridge and the one on the 

 east. A third gap occurs in the north part of T. 40, R. 7 E. It is 60 or 

 70 feet in depth and about one-half mile in width. It has a nearly plane 

 surface and was probably at one time a line of discharge for water held 

 between this ridge and a moraine on the east. It is now utilized by a trib- 

 utary of the South Kishwaukee River. 



THICKNESS OF DRIFT. 



The thickness of drift has been ascertained at only three points, but 

 records of several, deep wells were obtained which show that there is a heavy 

 accumulation along the entire length of the ridge. Of the three borings 

 reaching rock, one is in the village of Harvard and the other two are in the 

 southern part of the ridge. At Harvard the thickness is 102 feet. In the 

 other wells the rock was struck in one instance at 230 feet and in the other 

 at about 250 feet. It is probable that the general thickness of the drift 

 along the crest of the ridge south from Hampshire is not far from 250 feet, 

 for wells on the, plain west of the ridge, at an altitude 100 to 125 feet below 

 the level of its crest, have in several instances struck rock at about 150 feet. 

 One well on the ridge, near Lily Lake reached a depth of 336 feet without 



