254 W. UPHAM — GLACIAL LAKES IN CANADA. 



the upper part of the Souris river and forms, with overlying drift deposits, 

 the massive terrace of the Coteau da Missouri ou the west. The length of 

 Lake Souris was about 170 miles, from latitude 48° to latitude 50° 35' ; and 

 its maximum width, uorth of Turtle mountain, was nearly 70 miles. 



When the ice-sheet west of Lake Agassiz had receded so far as to uncover 

 Turtle mountain, the outflow from Lake Souris passed north of this moun- 

 tain by the Pembina, perhaps after taking for a brief time the course of the 

 Clearwater river, Lac des Roches, and the Mauvaise coulee to Devil's lake 

 and the Sheyenne. The channel of outlet by the Pembina, extending about 

 110 miles from the elbow of the Souris to the Pembina delta of Lake 

 Agassiz, is eroded 100 to 300 feet in depth, probably averaging 175 feet, 

 along the greater part of its course ; but it is from 300 to 450 feet deep, 

 probably averaging 350 feet, along its last twenty-five miles. Its average 

 width in each of these portions is about one mile. It is cut through the 

 plateau of Fort Pierre shale that reaches westward from the Pembina 

 mountain escarpment. Outside of this valley the shale is overlain by only 

 a thin sheet of till, which varies generally from 10 to 30 or 40 feet in thick- 

 ness ; but the valley itself contains a considerably greater depth of till. 

 From lakes Lome and Louise to its delta the Pembina probably flows in its 

 preglacial and interglacial course, where its old valley became wholly or 

 partly filled with till in each of the glacial epochs. The topographic 

 features of this valley will be more fully shown by the notes of approximate 

 elevations referred to the sea level on page 255 ; those of the first column 

 being in the bottom of the valley, and those of the second along the top 

 of its bluflfs at the general level of the adjoining country. 



At the Mowbray bridge the bottom land is about an eighth of a mile wide 

 and ten feet above the river. About forty feet higher is a narrow terrace 

 of modified drift, an eighth to a fourth of a mile wide, reaching along the 

 southern side of the river for one and a half miles to the east, and also well 

 shown in many places on each side of the river for six miles or more both to 

 the west and east ; but along much of this distance one or both sides of the 

 valley slope gradually from 100 or 75 feet above the river to the bottom land. 

 The higher portions of the sides or bluff's of the valley have steep slopes, 

 rarely interrupted by terraces. But a remarkably broad terrace or plateau, 

 evidently formed during the preglacial or interglacial erosion of this valley, 

 extends on its southern side three miles to the east from the Mowbray bridge 

 and road, with a maximum width of about one and a half miles, and an ele- 

 vation of 1,450 to 1,425 feet above the sea, or about 200 feet above the river. 

 A lakelet half a mile long from east to west lies on the southern part of this 

 plateau at the foot of the bluflTthat rises thence about 100 feet to the general 

 level of the adjoining country. All the way for twenty-five miles from this 

 bridge to the Pembina delta, especially in the vicinity of the fish trap, the 



