58 THE ILLINOIS GLACIAL LOBE. 



On the portion of the Mississippi blnff above Hamilton the drift is 

 only 20 or 30 feet in depth and is largely of' sandy constitution. It seems 

 probable that some modification of the glacial drift has resulted through 

 drainage connected with the melting of the Illinois glacial lobe. The evi- 

 dences of a delta-like filling at the lower end of the Des Moines rapids, near 

 Warsaw, are discussed on another page. 



In Adams County the Illinoian sheet has a series of ridges developed 

 near the border. The outermost one lies but a short distance east of the 

 Mississippi bluff, and is interrupted by wide gaps through which the streams 

 find passage into the Mississippi. The inner ridges occupy a portion of the 

 divide between the Illinois and Mississippi rivers in the central and south- 

 eastern part of the county. These ridges differ greatly in structure from 

 point to point. The portion of the outer ridge north from Bear Creek is 

 composed largely of ordinary brownish-yellow till, but throughout its con- 

 tinuation south of Bear Creek, from near Mendon southward past Eubanks, 

 it contains a large amount of sand and gravel. The upper 30 or 40 feet, 

 however, is often of clayey constitution, and this included, probably, the 

 entire Illinoian sheet as it extends about to the level of the base of the ridge. 

 In places the sand and gravel beneath this ridge is thought to be of pregla- 

 cial age, and it seems not improbable that the portions which contain 

 Canadian rocks or other erratics may be composed in large part of slightly 

 modified preglacial material. The material thrown out from wells at Mendon 

 was carefully examined and was found to be a quartz sand, of orange color, 

 entirely free from calcareous material. But exposures of a similar sand 

 near Eubanks were found to include occasional Canadian rocks iu their 

 upper portion, as if the sand had been worked upon by the ice sheet and 

 redeposited, together with some of the material contained in the ice. 

 Upon following this ridge southeastward to Mill Creek a change to till is 

 found, and the valley of Mill Creek, at the point where the ridge crosses, 

 is shown by wells to have been filled to a depth of 150 feet or more with 

 a clayey deposit which from description appears to be till, there being 

 numerous pebbles incorporated in it. A well made at the residence of Mrs. 

 Ihrig, on the west bluff of Mill Creek, in sec. 15, T. 2 S., R. 8 W., reached 

 a depth of 155 feet without encountering rock. It was mainly through 

 clay, except a few feet of- sand at the bottom. A well on the east bluff of 

 the creek, in section 12 of the same township, is 200 feet in depth and is 



