STRUCTURE OF ILLINOIAN DRIFT BORDER. 267 



spring, low knolls along the brow of the bluff, whose tops rise above the 

 general level of the uplands. The writer was unable to ascertain whether 

 they have a nucleus of rock or are drift knolls. The slopes of Middle 

 Creek Valley westward from the rock spring are not smooth like those of 

 the river bluffs, but have irregularities of outline that are due to drift 

 deposits. These irregularities have not the sharp outline which morainic 

 knolls commonly present, but their lack of sharpness of outline may be a 

 result of denudation consequent iipon great age. Were they of as recent 

 age as the morainic knolls of the Wisconsin stage their outline should still 

 be sharp. At the junction of the north bluff of Middle Creek with the east 

 bluff of the Ohio, and at the base of the bluffs, there is a group of knolls 

 standing 50 to 70 feet above the valley bottom, which also appear to be 

 drift aggregations, but there are not sufficiently deep exposures to thi-ow 

 light upon their structure. 



A gas boring at Aurora, on ground standing 45 to 60 feet above the 

 river, reached a depth of 92 feet before entering rock. It was largely 

 through sand and gravel. At Rising Sun, Ind., a few miles below Aurora, 

 a well reached a level 60 feet below the river without encountering rock. 

 Only a half mile from this well, on the Kentucky side, rock extends half 

 way across the river at about low-water level. 



At Lawrenceburg, Ind., Orton noted an outcrop of a blackened clay, 

 apparently a soil, along the river bank, a description of which appears in 

 the Geology of Ohio.^ The Wisconsin gravel terrace at that point stands 

 80 to 85 feet above the river, and the gravel appears to be of Wisconsin age 

 down nearly to river level, where this blackened clay outcrops. A gas bor- 

 ing at Lawrenceburg reached a depth of 140 feet before entering- rock. 

 The rock floor is, therefore, about 60 feet below this blackened clay and 

 the drift between the clay and the rock floor may be Illinoian. It is 

 reported to be gravelly. This blackened clay has been noted at a few 

 points below Lawrenceburg at about low-water level. 



In the portion of the Ohio Valley along the Ohio-Kentucky line the 

 only well records obtained are on terraces of Wisconsin gravel, and the 

 wells appear to terminate in that gravel near the piesent river level. A 

 few in Cincinnati are deeper and reach a level 50 to 60 feet below the river 

 before striking rock. 



'Vol. I, pp. 426-428. 



