222 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



return to the north side just below Franklin. A few miles farther down, in 

 the vicinity of Brandon and Kennerdell, it is again on the south side, but 

 apj^arentljr crosses to the north side in the vicinity of Kennerdell, and 

 passes southwestward tlirough the southwestern part of Venango County. 

 Near the common corners of Venango, Butler, and Mercer counties this 

 drift border seems to approach closely the limits of the Wisconsin drift. 

 From this point southwestward into Ohio there are but few places where it 

 appears to extend beyond the Wisconsin. Bowlders and thin deposits of 

 drift occur on uplands for a short distance outside the limits of the heavy 

 deposits of the Wisconsin drift in Beaver County, but the writer's examin- 

 ations in that i-egion have not been sufficiently thorough to warrant an 

 opinion as to their relation to the Wisconsin drift. 



Since this drift and the evidences of its great age are discussed in a 

 subsequent chapter, we may pass now to the portion of the border occupied 

 by the Illinoian drift sheet. 



SECTION II. THE BORDER OF THE IliLIlSrOIAN DRIFT. 



The border of the Illinoian drift seldom reaches the degree of attenuation 

 that charactei'izes much of the Kansan drift border; Indeed, one usually 

 passes within a space of less than a mile, and often in a space of a few 

 yards, from a district that seems to be driftless to one in which the di-ift is 

 a well-defined deposit several feet in depth. Yet there is seldom a marginal 

 drift ridge or moraine. The Illinoian deposits are much heaAder in valleys 

 than on uplands, and there is a marked sinuosity of margin to conform to 

 the topographic conditions. But notwithstanding these irregularities the 

 border is easily mapped with a fair degree of precision; at least this is trae 

 wherever the writer has followed it. 



The reentrant angle in south-central Indiana forms a natural line of 

 separation between the Illinois glacial lobe and the 23art of the ice sheet to 

 the east. For this reason, and also because the border of the Illinoian drift 

 of the Illinois lobe has already been discussed (see Monograph XXXVIII), 

 the discussion of its border is taken up at tliis reentrant angle. 



The northernmost point of the reentrant angle in the Illinoian drift 

 border in southern Indiana is on the north side of Beanblossom Creek, in 

 northern Monroe County, a few miles north of Bloomington. From this 

 point the border takes a course slightly south of east, entering Brown 



