286 THE ILLINOIS GLACIAL LUBE. 



have a sharp dip toward the sides of the esker, but this does not appear to 

 be a general attitude. The upper part and frequently the slopes consist of 

 coarser gravel and contain less fine material than the deeper portions. The 

 absence of fine particles is perhaps attributable in part to the action of per- 

 colating waters, which probably have sufficient force to cany down the 

 finer particles a few feet from the surface, but it is probable that the surface 

 portion was originally coarser than the deeper beds. The pebbles are mainly 

 limestone and are apparently, in large part, from the Lockport (Niagara) 

 limestone, which outcrops in the neighboring district on the east. The sand 

 is calcareous, but not to such a degree as in many hillocks containing sand 

 and gravel which occur in the neighboring moraines. It seems probable that 

 the material of which the esker is composed is a residue left upon the exca- 

 vation of the till which was removed along the course of the valley or of 

 material embedded in the basal portion of the ice sheet. Examinations of a 

 sample of the till from a neighboring till plain showed that 93 per cent of 

 the pebbles are limestone, which is about the percentag-e of limestone in the 

 esker. 



Bowlders were observed on the esker in but one place, and this was at 

 a slight depression on its crest. There were about 30 of them, and all were 

 of Archean rocks. The greater percentage of Archean rocks on the surface 

 than in the deeper portions, both of eskers and till sheets, apparently indi- 

 cates a different source for the bowlders than that of the main body of the 

 drift. Presumably they were carried at a higher elevation in the ice. 



LITTLE ROCK ESKER OR "DEVIL'S BACKBONE." 



In the northwest part of Squaw Grove Township (T. 38, R. 5 E.), 

 Dekalb County, there is an esker about 1 \ miles in length, known by the 

 residents as the "Devil's Backbone." It is situated a short distance south 

 of Little Rock Creek in sees. 5 and 6, and trends ESE.-WNW. At the 

 eastern end its height is but 4 to 6 feet, but in sec. 6 it is 10 or 15 feet in 

 height. It is ouly 8 or 10 rods in width, including slopes, and consequently 

 presents, very abrupt slopes. It winds considerably, but has a somewhat 

 even crest line. At the western end there is a sandy tract covering about 80 

 acres, which is thought to be a delta formed in connection with the esker. 

 There is no valley or trough excavated in the drift, as in the case of the Kane- 

 ville esker, for the esker stands at about the level of the bordering till plain. 



