254 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



lllinoian age, though from its general characteristics it appears to be of 

 similar age to the old drift of northwestern Pennsylvania, i. e., Kansan or 

 23re-Kansan. 



The extent of the lllinoian drift having been set forth in some detail 

 in the discussion of the drift border, we may pass at once to the discussion 

 of its features. 



TOPOGRAPHIC EXPRESSION OF THE DRIFT BORDER. 



The border of the lllinoian drift shows definite ridges or knolls at only 

 a few places, as specified below. The drift has, however, along much of its 

 border, sufficient thickness to produce a marked effect on the topography. 

 The valleys and ravines lying just within its border have received much 

 more drift than the uplands or divides between the streams. As a result 

 there is a marked reduction in the depth of valleys in the drift-covered 

 region compared with similar valleys in neighboring driftless tracts. In 

 places where a very thin coating or only scattering bowlders appear on the 

 dividing ridges, the valleys usuallj' carry 50 to 100 feet or n:iore of drift. 

 These deposits in the valleys often terminate ver}^ abruptly at the drift 

 border, both on drainage lines which were blocked by the ice in their lower 

 courses and on lines which were open during the glacial deposition. The 

 abrupt termination affords strong evidence that the ice sheet extended to 

 the extreme limits of the drift. Lakes were no doubt formed in valleys 

 that were obstructed by the ice sheet, but there appears to have been 

 comparatively little transportation of material from the ice margin into the 

 lakes. The rate of removal of material in unobstructed valleys was also 

 somewhat less rapid than the rate of deposition by the ice sheet. The 

 variations in the topographic expression may perhaps be best discussed by 

 following the border from the reentrant in Monroe County, Ind., eastward 

 to the point of disappearance beneath the Wisconsin drift in Holmes 

 County, Ohio. 



On the elevated upland north of- Beanblossom Creek, in northeastern 

 Monroe County, Ind., there are many bowlders but only occasional thin 

 deposits of till; but on passing down into Beanblossom Valley, at Need- 

 more, the drift border changes abrupt!}' to a bulky ridge which rises like 

 a dam 60 to 75 feet above the valley bottom to the west, and blocks the 

 Valley for a space of fully a mile. Its surface is indented by shallow basins 



