GLACIAL EROSION AND TRANSPORTATION. 263 



an area of several square miles, and none of them rising 100 

 feet above the surface of the lake. They all consist of the 

 hard limestones of the Niagara series. In every instance, as 

 one approaches them from the eastern side, his attention is 

 attracted by the remarkable depth and continuity of the 

 glacial grooves running nearly east and west upon them, and 

 which rise out of the water, and continue to the summit of 

 the islands, or until they are covered by the ground moraine 

 which has not been washed away by the waves. In some 



Fig. 78 



s.— Glacial grooves on east side of South Bass Island. Lake Erie, running west 10 c 

 south. 



instances, these grooves are two or three feet dee]), and ex- 

 tend many rods in plain sight. Xor are they in all cases 

 straight, hut sometimes are extremely tortuous, winding along 

 in their course like the channel of a sluggish stream. It is 

 evident, in some cases, that the main features of these deep- 

 est grooves have been determined by preglacial or subglacial 

 water-action, and that the ice, or the ground moraine under 



