ESKERS OF NORTH WESTERIn ILLINOIS. 77 



breads through the ridge, 2 miles west of Adeline. The crest line is very 

 uneven and the height of the ridge varies greatly. The highest points 

 slightly exceed 100 feet above the plain bordering Leaf River, but their 

 elevation is no greater than that of the uplands on either side of Leaf River 

 Valley. The lowest points are scarcely 20 feet in height. The esker con- 

 sists usually of but a single ridge, ranging in breadth from 100 feet or less 

 to probably 1,000 feet. At its eastern end, in the vicinity of Adeline, there 

 is a series of nearly parallel ridges and hillocks covering a breadth of per- 

 haps one-half mile and almost filling the valley. At the western end of 

 the ridge there is no delta, or fan-shaped gravel deposit, such as sometimes 

 occurs at the terminus of an esker. There are a few small gravel knolls 

 within a mile north from its terminus, but they do not appear to be definitely 

 connected with it. 



Several extensive excavations have been made in the esker, some 

 of which expose its structure from top to bottom. It is made up largely 

 of coarse gravel well rounded, but contains also beds of fine gravel and 

 sand. The coarser material is most abundant in the upper portion and 

 it is not rare to find bowlderets, and even moderate-sized bowlders, 

 embedded in it. The lower portion displays much cross bedding. The 

 direction of flow of the stream which formed this esker is clearly shown to 

 be toward the west, or the reverse of the present drainage of the Leaf River 

 Valley. As the esker now stands, its western end is nearly 100 feet higher 

 than its eastern. In case the esker was formed under the ice in the valley 

 which it occupies, considerable hydrostatic pressure would have been 

 required to force the water, with its burden of gravel, up this slope. It 

 does not, however, seem necessary to restrict the formation of the esker to 

 the under surface of the ice sheet, since it is found that nearly stagnant ice 

 is traversed by tunnels at some distance above its base. 1 The greater part 

 of the gravel is composed of limestone such as occurs in the neighboring 

 ledges, but there is also a liberal admixture of rocks of distant derivation. 

 In order to compare the rock constituents of the esker with those of the till 

 of the adjacent districts, pebbles were taken, without attempt at selection, 

 from the esker and from a dump at the mouth of a well at Forreston, which 



1 See I. C. Russell, Jour. Geol., April-May, 1893, Vol. I, No. 3, pp. 240-242. See also Hershey's 

 discussion iu Am. Geologist, April, 1897, pp. 238-239. 



