AND TRANSPORTING AGENT PREST. 337 



Drift Ice as an Eroding Agent. 



A great part of the erosion now acknowledged as due to 

 other causes has often been ascribed to drift ice. Formerly 

 great stress was placed on erosion by drift ice, particularly by 

 icebergs as in opposition to drift ice. No doubt some erosion 

 was actually effected, but that its traces in the form of striations 

 are still retained above the sea level is very doubtful. 



In the official reports of some of the Canadian Geological 

 Survey staff, and also in the writings of other geologists, we can 

 trace a gradual conversion from the old theory to the new, in 

 which ice-action is confined almost solely to the polishing out of 

 former inequalities and striae In some of the latest reports, 

 erosion by drift ice is considered possible only under exceptional 

 circumstances. The cause is often proved by circumstantial 

 evidence, or entered with a mark of interogation. It is also 

 admitted that only where a low point or ridge is exposed to an 

 ice jam forced over it by a storm, is striation possible, and then 

 only when the ridge can also be reached by stones to act as 

 graving tools. 



Some of the results of my observations on ice action are as 

 follows: Ice action on a steeply sloping shore occurs with an 

 onward rush of water carrying immense masses of ice 5 to 15 

 feet in thickness. When reflex action begins the ice is poised 

 for a few seconds on the rocks until the water drains partly 

 away. Then, being deprived of support, it slides back with a 

 tremendous plunge into the next advancing wave, dragging with 

 it into deep water such rock fragments as it may have been able 

 to reach. And what is very important, these rock fragments 

 are never carried forward again ; for the next wave lifts the ice 

 pans forward, high over every obstruction. The scoring, if any, 

 in this case is done while the ice mass is sliding into the water 

 with stones beneath it, as it exerts little downward pressure 

 when rising with the rush of water. Where exposed to the 

 Atlantic swell, ice pans 15 feet thick and 50 feet in diameter 

 are often carried forward through a perpendicular distance of 



