THE LA URENTIA N PENEPLA IN 647 



merged valley of the Hamilton River. The Mistassinni depression 

 is bounded by a well-marked scarp on the southeast, and a less 

 definite, but still distinct, margin on the northwest. 



The Lake St. John basin seems to be of a distinctly graben 

 type; its margin is well defined, often cliffed or scarped; and all 

 the streams tributary to the lake spill over the edge of the basin 

 from the adjacent upland each in a series of waterfalls and cas- 

 cades, often visible many miles away from the open lake. 



The sediments in Lake Temiscaming and Lake Nipissing basins 

 are in valleys lying below the level of the plain. The sediments 

 in the vicinity of Lake Nipigon rest directly upon the plain, and 

 rise above its surface. 



In the greater number of cases it seems that there is a well- 

 defined margin to the valley. In the case of some of those in 

 the localities referred to above it may be possible that they ante- 

 date the peneplanation epoch, and that the sediments lying within 

 them have been partly re-e.xcavated since then. In the case of 

 Lake Nipissing and Lake Temiscaming depressions, and possibly 

 in the case of Lake St. John and Lake Mistassinni, the depres- 

 sions may also be of a date antecedent to the peneplanation. 



Barlow notes with regard to the Lake Temiscaming depres- 

 sion, and of others in that vicinity, that they bear no significant 

 relation to the direction of the movement of the glacial ice, in fact, 

 they lie at various angles up to as much as ninety degrees to the 

 general direction of the movement of the ice-sheet. He also 

 notes that many of them cut across hard and soft rocks alike, 

 and are independent of the strike of the structure of the rocks. 



The character of the bounding walls, and the preservation of 

 Paleozoic sediments in the bottoms of many of these valleys, 

 would suggest that they are graben formed after the deposition 

 of the sediments, and that the sediments are preserved in them 

 because they were below the base level of erosion at the time 

 when the balance of the similar sediments on what are now the 

 adjacent uplands were removed. The fact that in many cases 

 streams cascade down the sides of these valleys in ungraded 

 channels suggests that this faulting has been quite recent. It is 

 doubtless possible that in some cases where these valleys open 



