THE BLOOMINGTON MORAINIC SYSTEM. 253 



gently undulating and dotted with occasional knolls of considerable promi- 

 nence, the highest knolls rising 30 to 40 feet above border districts. 



The outer ridge crosses Mackinaw River immediately above Mack- 

 inaw village and leads southeastward to Bloomington and thence eastward 

 to its point of junction with the inner ridge near Padua. It has a breadth 

 of about 3 miles. It is crossed by several streams which head in the 

 inner ridge, among which are three of the headwater branches of Sugar 

 Creek, two headwater branches of Kickapoo Creek, and one headwater 

 branch of Salt Creek. Sangamon River also leads through it, east from 

 the junction with the inner ridge. There are thus seven streams crossing 

 it within a space of 35 or 40 miles, admitting only about 5 miles average 

 distance between streams. The gaps through which these streams pass, each 

 cause a break in the crest of the moraine nearly a mile in width. They do 

 not appear to be entirely erosion gaps, for the morainic swells occupy them 

 down nearly to the level of the streams, or about 75 to 100 feet below the 

 level of the neighboring crests. It is probable that streams issuing from the 

 ice sheet at the time the moraine was forming prevented the accumulation 

 of heavy deposits in the vicinity of their points of departure from the ice. 

 Between these gaps the moraine usually has a well-defined crest and gently 

 undulating surface. The crest lies near the outer border of the moraine, 

 the outer face being, as a rule, much more abrupt than the inner. As the 

 moraine, where not interrupted by gaps, has a relief of 100 feet, the outer 

 face often presents the abruptness of a river bluff. It differs, however, 

 from a river bluff in the absence of erosion contours, there being instead a 

 billowy slope, such as characterizes moraines. The erosion effected by 

 streams since the withdrawal of the ice sheet is very inconspicuous com- 

 pared with the inequalities of drift aggregation. The crest is usually so 

 gently undulating as scarcely to suggest the strength of the moraine. It is 

 not uncommon to find it so level for a space of one-fourth to one-half 

 mile in width and for several miles in length that artificial ditching is neces- 

 sary to give it good drainage. In such places there are usually shallow 

 basins, 2 to 5 feet in depth, occupying an acre or more each, which add to 

 the imperfection of drainage. The crest varies considerably in altitude 

 independent of the gaps just mentioned, its highest points being about 900 

 feet above tide and its lowest about 800 feet. The range of 100 feet in 



