52 CHICAGO ACADEMY OF SCIENCES, 



STRIAE. 



Exposures of striae are numerous and indicate that the axial 

 movement of the ice sheet in this district was decidedly west of 

 south rather than due south. This should be borne in mind in 

 estimating- the amount of divergence in the lateral flowage. The 

 striae in this region are remarkably well preserved, and that, too, 

 where conditions for obliteration seem favorable. In some places 

 there appears to have been no drift protecting the ledges since the 

 withdrawal of the lake, if not since the time when the ice sheet 

 withdrew, and yet but few exposures do not preserve the striae. 

 In similarly exposed situations outside the Valparaiso moraine 

 striae are seldom preserved. This difiference may be due partly to 

 the greater age of the striae outside the moraine. But aside from 

 this there appears to have been an original difference in the in- 

 tensity of glaciation by which ledges more often escaped the rasp- 

 ing action of the ice sheet outside the moraine than in the district 

 between the moraine and the lake. 



The following embrace the localities where bearings were 

 observed : 



1. Stony Island in south part of Chicago. The rock where 

 glaciation was observed dips toward the southeast, with an angle 

 of nearly 40 degrees, and heavy scorings follow the line of strike, 

 with a bearing S. 44 degrees W. (See Fig. 5.) Associated with 

 these are striae of feebler development, which vary in direction 

 fully 10 degrees to the east and west of the heavy scorings, thus 

 ranging from S. 34 degrees to S. 54 degrees W. An escarpment 

 of the dipping layers rising about six feet above the remainder of 

 the quarry is glaciated not only on the upper surface and nearly 

 vertical front, but also beneath one of the lower layers, its dipping 

 under surface being smoothly polished for about 18 inches back 

 from the front of the ledge. (See Fig. 4, where the glaciated under 

 surface appears near the sachel.) 



2. On Thorn Creek near Thornton. The prominent portions 

 of arching layers of the Niagara limestone are roche moutonneed 

 and a few striae have a bearing S. 31 degrees W. 



3. Quarries one mile southwest of Blue Island. A few faint 

 striae were observed bearing S.- 54 degrees W. 



