310 THE ILLINOIS GLACIAL LOBE. 



also a group of prominent knolls in the vicinity of the line of Kankakee 

 and Iroquois counties, south of Hersher. Some of these reach a height of 

 about 75 feet. At the point where the course of the moraine changes from 

 east of north to southeastward, near Ste. Anne, there is a very prominent 

 group of knolls, called Mount Langum, standing 75 to 100 feet above 

 bordering tracts on the north. These groups of prominent knolls have, 

 however, a combined area of but a few square miles. Toward the inner 

 border throughout the entire length of the moraine the swells gradually 

 decrease in height and become less frequent, until they can not be distin- 

 guished from the gentle undulations of the till plain, which are commonly 

 5 or 6 feet in height. In a few places, aside from those noted above, a 

 somewhat sharp knob-and-basin topography is developed. The most con- 

 spicuous development is in Kendall County along the sharp crest, and this 

 type of topography is characteristic of much of the crest in that county. 

 The knobs seldom exceed 25 feet in height; but as they cover only 2 or 3 

 acres each, they are much sharper than the majority of the knolls. The 

 basins are shallow, saucer-like depressions. It is worthy of note that the 

 portions of the moraine having sharpest expression are usually covered 

 with forest. Possibly the protection afforded by forest growth is partially 

 responsible for the sharper contours, but it seems hardly probable that the 

 difference in contour in forest and prairie can be entirely due to this 

 protection. 



The outer margin of the moraine often extends out in spur-like pro- 

 jections a mile or more beyond a regular border, and usually where streams 

 emerge the margin is indented an equal or even greater amount. These 

 spurs slope down much less abruptly from the morainic crest than the tracts 

 between them, thus giving the appearance of low ridges running out from 

 the crest nearly at right angles to its course. The indentations at the places 

 where streams emerg'e are not referable to erosion by the present streams, 

 but are in all probability due to the removal of material by streams issuing 

 from the ice sheet. 



There are occasional breaks or narrow gaps interrupting the crest. 

 One about 4 miles east of Yorkville, about 75 feet in depth and one-fourth 

 to one-half mile in width, passes entirely across the moraine, permitting 

 drainage from the inner-border plain to pass through the moraine to Fox 

 River. Another gap fully as large is found in the southwest comer of 



