Effects of tbe Olacfal period 231 



away the rock underneath until round holes 

 were formed to the size and depth heretofore 

 mentioned. Where there was only a single 

 bowlder the holes were almost perfectly round, 

 but where there was more than one bowlder 

 the holes were sometimes in an oblong shape. 

 The bowlders were worn down to a very small 

 size in most cases, and were round and smooth. 

 The probabilities are that when the action first 

 began these bowlders were large and of irregu- 

 lar shape. They must have been, in order to 

 do the enormous amount of grinding that 

 some of them did to produce excavations in 

 the solid rock with a diameter of thirty feet 

 and a depth about the same. The bottoms 

 were round like an old-fashioned pot, and the 

 insides polished perfectly smooth. This was 

 purely an effect of the tumbling about of 

 the bowlders by the running water from the 

 melting ice of the great glacier that covered 

 that region some time in the long ago. 



There are other effects produced in rocks 

 during the ice flow in North America that are 

 very interesting. Great grooves are formed 

 in the rocks, in many cases running for long 

 distances, that have been worn in by the cut- 

 ting power of the great ice sheet during the 

 progress of its movement. There is a great 

 groove to be seen at Kelly's Island in Lake 

 Erie. It will be remembered that this lake is 

 supposed to have been formed entirely by the 



