120 THE ICE AGE IN CANADA. 



ill other parts of the Laiirentian region is very uniformly 

 between about 1,600 and 1,200 feet. Allowing, then, 

 1,600 feet as a maximum for the region north-east of the 

 Lake of the Woods, and taking into account the height of 

 that lake and the distance, the general slope is not greater 

 than about three feet per mile — an estimate agreeing 

 closely with the last, which is for a smaller area and 

 obtained in a different way. Tliis slope cannot be con- 

 sidered sufficient to impel a glacier over a rocky surface, 

 which Sir William Logan has well characterized as 

 ' mamillated,' unless the glacier be a confluent one pressed 

 outwards mainly by its own weight and mass. 



" Such a glacier, I conceive, must have occupied the 

 Laurentian highlands ; and from its wall-like front were 

 detached the icebergs which strewed the cUhris over the 

 then submerged plains, and gave rise to the various 

 monuments of its action now found there. 



" The sea, or a body of water in communication with it, 

 which may have been during the first stages of the 

 depression partly or almost entirely fresh, crept slowly 

 upward and spread westward across the plains, carrying 

 with it icebergs from the east and north. During its 

 progress most of the features of the glacial deposits were 

 impressed. In the section described at Long river, we 

 find evidence of shallow current-deposited banks of local 

 material, afterwards, with deepening water, planed off by 

 heavy ice depositing travelled boulders. 



" The sea reaching the edge of the slope constituting 

 the front of the highest prairie-level, the deposition of the 

 Coteau began, and must have kept pace with the increas- 

 ing depth of the water, and prevented the action of heavy 

 ice on the front of the Tertiary plateau. The water may 



