CONDITIONS OF EROSION OF GIANTs' KETTLES 41 



kettles, whether large or small, is abruptly cut in the rock surface, ])er- 

 haps sometimes because of their partial removal by glaciation su))se- 

 quent to the moulin erosion. They seldom have a flaringly curved 

 mouth, such as more frequently characterizes potholes seen at the pres- 

 ent time in the process of erosion by cascades in brooks and rivers. 



Conditions of Erosion of Giants' Kettles 



According to these observations and records of glacial potholes in 

 their best known localities on two continents, it seems to me most prob- 

 able that the time of their excavation in many cases was the earl 3^ part 

 of the Glacial period, or some stage of glacial extension, when the ice- 

 sheet was being formed U[)on the land by snowfall. On any hilly coun- 

 try the ice must have attained an average depth somewhat exceeding 

 the altitude of the hills above the adjoining lowlands before any general 

 motion of the ice-sheet could begin. During the process of slow accu- 

 mulation of the ice-sheet, the summer melting upon its neve surface 

 would produce multitudes of rills, rivulets, and brooks, which might 

 unite into a large stream ; and this, pouring through a crevasse and 

 melting out a cylindric moulin, might fall perhaps 100 or 200 feet or 

 more on a moderately hilly region, but probably sometimes 500 feet or 

 more on a mountainous district, while yet the ice motion, though suf- 

 ficient to permit the formation of the crevasse, might not have gained 

 a definite current to carry the crevasse, moulin, and waterfall away from 

 the spot where they were first formed. We may thus explain the con- 

 tinuation of a glacial waterfall in one place while it was excavating one 

 of these giants' kettles. 



After the ice-sheet acquired a current because of the greater thickness 

 and pressure of its mass, such deep cylindric excavations in the bed rock 

 could not be made, because the ice and moulin were in motion ; and 

 during the final dissolution of the ice-sheet it seems probable that its 

 receding border had generally steeper gradients and consequently even 

 more rapid motion than during the culmination of the Glacial period. 



Moreover, the streams formed on the surface of the ice-sheet by the 

 summer melting before it was so thick as to have motion would be free 

 from drift or any load, excepting what might be derived from projecting 

 hills or mountains, so that their waters could readily find a way through 

 crevasses, forming potholes in the rock beneath by means of detached 

 blocks from the same rock bed, and thence flowing away in subglacial 

 courses. On the contrary, the superglacial streams during the departure 

 of the ice, which then became more or less covered with the previously 

 englacial drift, laid bare by ablation, were heavily freighted with the 



VII— Bull. Geol. Soc. Am,, Vol. 12, 1900 



