256 B. N. A. BOUNDARY COMMISSION. 
where Carboniferous limestones come to the surface, they are smoothed 
by the action of ice, and show north-east and south-west striation, crossed 
by scratching in other directions.* Similar phenomena would no doubt 
generally present themselves over the area of the plains, were there 
surfaces of hard rock to receive and preserve them. 
591. Reaching the level of the foot of the third prairie steppe, the 
edge of which had already been marked out by sub-aerial denudation, 
the action of the waves on its base no doubt rendered it more 
definite, and the heavier ice following them, and grounding on its lower 
slopes, began the deposition of the Coteau. It may be that the degreda- 
tion of the Laurentian hills was at this time proceeding with increased 
rapidity, but in any case this deposit, formed in great part of their 
material, must have increased equally fast with the depression of the 
land, and preserved the edge of the Lignite plateau from the destructive 
action of the waves. 
592. The average height of the Coteau near the forty-ninth parallel, 
may be taken at about 2,200 feet; and above this the higher hammocks > 
and ridges probably do not rise more than fifty to eighty, or one hundred 
feet. Its southern extensions have an average altitude of 2,000 feet. 
It is not necessary to suppose that the shore line stood for along time 
nearly at this level, for the heavier ice masses no doubt continued 
grounding on the edge of the third plateau, even when the waters stood 
near their greatest elevation. The data are perhaps yet too slight for 
generalization, but there appears to be a tendency in the Coteau to 
occupy a somewhat lower level southward, which may indicate a less 
subsidence in that direction. 
593. East of the Red River, we find the great drift plateau stretching 
southward and westward from the Lake of the Woods, with an average 
elevation varying from 1,600 to 1,000 feet; and which, though doubtless 
everywhere based on the boulder clay, shows in its upper layers a con- 
siderable thickness of roughly stratified sands and gravel, indicating the 
action of rapid and varying currents. On these rest the scattered boulders 
deposited at a still later period. 
594. Probably contemporaneous in origin with the plateau just 
referred to, are the terraces with a level of 1,435 feet, which Prof. Hind 
describes at Dog Portage, two hundred miles east of Lake of the Woods, 
on the Lake Superior side of the watershed. These he afterwards 
YE ¥¥ra___e_TT —— 
* U.S. Geol. Surv. Territ., 1870. p. 99. 

