72 PROCEEDINGS OF THE PHILADELPHIA MEETING 



The question arose whether these solution effects had been produced before 

 or since the invasion of the region by the glacier. The perfectly fresh appear- 

 ance of the striae, as compared with the weathered, etched, and pitted surfaces 

 of the joint cracks and caverns, pointed toward the preglaciaP origin of the 

 latter. So also did the fact that in several places caverns were cleanly cut 

 across by the glacial surface (plate 7. figure 1). In one instance a work- 

 man, noticing that the rock sounded hollow, drove his crowbar throiigh about 

 one inch of striated limestone into a good-sized cavern. Decisive evidence on 

 the point was secured when it was found, in one of the higher parts of the 

 qiiarry, that glacial strife descended to the depth of about a foot into one of 

 the enlarged joints which happened to lie in a direction parallel to that of the 

 ice-movement (plate 7, figure 2). 



The pre-Wisconsin age of the solution phenomena having been established, 

 it becomes possible to use these in estimating the character and amount of 

 ice erosion which the limestone has suffered. When this line of investigation 

 is followed it leads at once to the conclusion that erosion has been very un- 

 equal in different parts of the quarry. The surface of the limestone varies 

 slightl.y in elevation, possibly as much as 10 feet within the area exposed. 

 On the higher parts erosion has been intense enough to plane away all irregu- 

 larities and to leave an almost perfectly smooth surface. It has not, however, 

 been sufficient to obliterate the caverns or the solution channels along the 

 joints. A conservative estimate would place the thickness of rock scoured 

 away at not more than 2 to 4 feet. Over those parts of the quarry lying at 

 intermediate levels a large part of the surface has been planed smooth, but 

 the bottoms of the solution hollows have not been touched. Such a condition 

 indicates very moderate erosion — not over 1 or 2 feet at most. In the lowest 

 parts of the quarry the ice scarcely reached the rock. Ozily the highest pro- 

 jections were planed off. The surface, as revealed by stripping in one of the 

 lowest parts of the quarry, resembles in every way that of weathered lime- 

 stone in an unglaciated region (plate 7. figure 3). 



In detail, as well as in broader features, the distribution of the eroded sur- 

 faces reveals the inability of the ice to descend into hollows, even where 

 broad and shallow. Depressions 20 or more feet in diameter and less than 

 1 foot deep have escaped erosion, while the surroimding rocks have been planed 

 off. Even the far sides of such depres.sions, where the erosive action should 

 have been greatest, are commonly unstriated. In passing over smaller de- 

 pressions, such as caverns or solution-enlarged joints, the ice seems not to 

 have sagged downward in the least. 



Such conditions show clearly that the ice behaved like a rigid plane, which 

 actively eroded the higher projecting points, but was totally unable to descend 

 into the depressions, even though they were broad and shallow. In the higher, 

 more exposed parts of the quarry it bulged down slightly into solution channels, 

 whose direction happened to be parallel to that of its movement ; but it seems 

 never to have descended into the transverse joints. 



The ice doubtless rode over all these depressions on its own debris, but the 

 significant feature is that it failed to clear this away and to scour down every- 



- The word preglacial is used here in the sense of certainly pre- Wisconsin and possibly 

 even pre-lllinoisan or pro-Kansan. No data are at hand to show how innch of the solu- 

 tion should be attributed to interglacial and how much to strictly preglacial weathering. 



