196 ILLINOIS STATE ACADEMY OF SCIENCE 



below the falls of a rapid river or in other eddying 

 streams, but the great abundance of concave surfaces on 

 the Killarney granite and gneiss makes any reasonable 

 observer at once give up any such explanation for the 

 thousands of "pot-holes" in this district, unless indeed 

 he believes that the whole area was at one time over- 

 flowed by a huge and very widespread torrent. Streams 

 doubtless followed the glacial period, but such an expla- 

 nation does not fit all the cases because many of the de- 

 pressions have the smooth, polished surface character- 

 istic of glacial polishing. Many of these depressions 

 antedate the last glacial sheet which covered the land. 

 Observations show that a variety of causes has brought 

 about similar products in different manners. 



The granite at Killarney has a nearly horizontal sheet- 

 ing. It tends to break off in rather irregular and shal- 

 low slabs of rock as a result of frost work. Glaciers 

 riding over these granite surfaces broke out some of these 

 slabs and carried them away. The place left, in several 

 cases found, is almost scoop shovel shaped. (Figure 3.) 

 The glacier chipped out a piece of rock, which broke in 

 a vertical direction down to the sheeting plane in a curv- 

 ing surface which was convex toward the direction from 

 which the glacier came, provided the breaking surface 

 was on the sheltered side of the rocky knob or roche 

 moutonnee. In some other cases the granite broke out 

 from the bed rock on the downstream (stoss) side of the 

 outcrop, concave towards the direction from which the 

 ice was coming, as a sort of giant chatter mark. In 

 either case, the passing ice in time smoothed off the 

 edges of the broken rock, and left a polished depres- 

 sion in the midst of an otherwise convex surface, looking 

 something like a pot-hole. (Figure 4.) 



In other cases the primary depression was made by 

 the conjunction of three or more joint planes which 

 loosened upwards a piece of rock of pyramid shape. 

 After frost had heaved this piece loose, and passing ice 

 had carried it away, the place from which it came pro- 

 vided the start of another polished depression which 

 looks like a pot-hole. Near Killarney these joints were 



3 Pot-hole? have been described and classified in detail by E. D. Elston. 

 The Scientific Monthly, Vol. V pp. 554-567 and Vol. VI pp. 37-51 (1918). 



