90 



PLEISTOCENE OF INDIANA AND MICHIGAN. 



they occur at various horizons and seem to be of small horizontal and vertical extent. Gravel 

 knolls are scattered along the entire length of the chain of moraines and are conspicuous, though 

 they occupy but a very small part of the surface. 



Surface bowlders are very unequally distributed, being hard to find in some places and 

 conspicuous in others. On the supposed interlobate spur and also south of Sugar Creek in west- 

 ern Montgomery Comity they are numerous, and in southeastern Montgomery and northern 

 Hendricks comities they are rare. The two larger ridges northwest of Danville and the moraine 

 in Johnson Comity are thickly strewn with them, and in the district east of East White River 

 they form a bowlder belt. More than 95 per cent of all the bowlders are of granite. 



In order to compare the classes of rock found in the till and in the gravel with the surface 

 bowlders, samples of pebbles or small pieces of rock an inch or less in diameter were collected 

 in a bowldery portion of the drift in Johnson County and classified with the results shown in 

 the table below. This table, though revealing striking similarity in the rock constituents of 

 the till and of the gravel, shows striking dissimilarity between these and the surface bowlders. 

 Glacial deposits have not been found to contain a large percentage of granite rocks except on 

 the surface, either in this district or in other parts of the region south of the Great Lakes. 



The first set of pebbles in the table is from the bluff of White River from a kame which 

 appears to be of pre- Wisconsin age; the second set is from Donnell Mound; and the third 

 and fourth sets are from an esker and a till plain, all of Wisconsin age. 



Percentage of different rock types in glacial deposits in central Indiana. 



THICKNESS. 



Variations. — Changes in the thickness of the drift are interesting and abrupt. In central 

 and southeastern Montgomery County rock is exposed along many of the small ravines at about 

 25 feet below the uplands, and it is usually struck at moderate depths from Crawfordsville 

 northwestward. In the vicinity of Crawfordsville, in a buried channel, the drift is over 200 feet 

 thick. In central and northern Hendricks County it is so thick that wells 60 to 80 feet in depth 

 do not reach the rock, but in southern Hendricks County rock is exposed in shallow ravines 

 and is reached by wells at 20 feet or less. The knolls and ridges are composed wholly of drift. 



Pre- Wisconsin drift. — It is rather difficult to determine how much of the drift in Mont- 

 gomery and Hendricks counties is referable to the Wisconsin invasion. It may not greatly 

 exceed the amount comprised in the knolls and ridges. In Crawfordsville, however, a buried 

 soil was found between drift sheets at a depth of 80 to 90 feet, as reported by Owen, 1 and similar 

 soil was found at several other places a few miles east of Crawfordsville in Boone County at 

 depths of 45 to 65 feet. 



In Johnson County the moraine begins in the western part of the county where the drift 

 is rather thin, but runs into thick drift in the eastern part. It is probable, however, that only 

 a few feet of the drift below the level of the base of the knolls is referable to the Wisconsin 

 invasion. East of East White River in Rush and Henry counties the moraine is in a region 

 presenting great differences in the thickness of the drift, the depth to rock ranging from 



i Ann. Rept. Indiana Geol. Survey for 1859-60, p. 133. 



