66 THE ILLINOIS GLACIAL LOBE. 



before entering rock This appears to be outside the glacial boundary, 

 but the material penetrated is probably derived from glacial deposits 

 brought down the valley. 



In southwestern Indiana the drift in the vicinity of the glacial bound- 

 ary frequently reaches a depth of more than 100 feet in the valleys; but 

 on the uplands it rarely exceeds 40 feet, and is usually but 10 or 20 feet. 

 As a rule, a deposit of till several feet in depth appears along the drift 

 border, but in places there are only scattering pebbles for a mile or more 

 back from it. The latter feature is most frequently found in the hilly parts 

 of the border. 



The till in the portion of southwestern Indiana south of East White 

 River contains a remarkably small number of bowlders and coarse frag- 

 ments of rock. In places search is necessary to discover a pebble, though 

 in a neighboring exposure a large bowlder may be found. In a few locali- 

 ties sand is present instead of clay, but, like the clay, it carries very few 

 pebbles. It is thought that these peculiar phases of the drift may be due 

 in part to the character of the underlying rocks (there being a prepon- 

 derence of friable strata which might easily be reduced to clay or sand) 

 and in part to imperfection of drainage conditions, by which the very fine 

 material was all retained in the till instead of being allowed to escape 

 down the valleys, as in regions to the west, where drainage conditions were 

 better. In connection with the first of these causes, it may be remarked 

 that the local rocks throughout the entire region covered by this glacial 

 lobe usually form so large a proportion of the. coarse constituents of till 

 that its character is found to vary in a pronounced degree in accordance 

 with changes in the underlying rocks. The imperfection of drainage in 

 southwestern Indiana attending* the ice invasion was such that several lakes 

 were formed in valleys which lie outside the drift border and whose streams 

 had discharged westward before the ice invaded the territory covered by 

 the ice sheet. These glacial lakes are discussed at some length on later 

 pages, as are also the changes of drainage which resulted from the ice 

 invasion. North from East White River the till usually carries a moderate 

 number of pebbles, and differs but little from the typical till of the interior 

 portion of the district covered by the Illinois lobe. The coarse rock frag- 

 ments are composed largely of sandstone from formations which outcrop 

 in the immediate vicinity of the drift border. Canadian rocks, however, 



