256 W. UPHAM — GLACIAL LAKES IN CANADA. 



river flows in a very picturesque valley, whose sides, rising steeply 300 to 

 450 feet, are roughly seamed and cleft by tributary ravines and gorges, with 

 here and there hills and small plateaus that have been left isolated by the 

 process of erosion. This valley has frequent exposures of the Fort Pierre 

 shales, which also within a half mile to one mile back from the river form 

 the high plateau through which the river has cut its way. The narrowness 

 and depth of the partially drift-filled valley indicate that its area of drain- 

 age was**uo greater in preglacial time than now. 



The mouth of Lake Souris where it first outflowed to Lake Agassiz by 

 the Big coulee and the Sheyenne was approximately 1,600 to 1,500 feet above 

 the present sea level, being gradually cut down about a hundred feet by the 

 stream. But on account of subsequent changes which are kuown to have 

 taken place in the relative elevation of the land and water surfaces in this 

 district, the shore line of the northern part of the lake at the end of its time 

 of outflow to the Sheyenne would now have an elevation of about 1,600 feet 

 at Lang's valley. Therefore, when its channel of discharge was transferred 

 to the new course by Pelican lake and along the Pembina, the Lake Souris 

 was suddenly lowered about 125 feet to the level of the top of the bluflTs of 

 Lang's valley, and a further lowering of 110 feet wa^ afterward effected by 

 the gradual erosion of this valley. The lake was wholly drained by this 

 outlet, for the general level of the land adjoining the Souris in the vicinity 

 of the mouth of Plum creek, which is the lowest portion of the lake bed, 

 is about twenty feet above the present divide in Lang's valley. Since the 

 waters of the Souris ceased to flow along this course, the sediments of gravel 

 and sand brought by tributaries have filled portions of the Pembina valley 

 10 to 20 feet, forming the barriers of its shallow lakes ; and the divide in 

 Lang's valley has been raised probably ten feet by the deposits of Dunlop's 

 creek. 



Seventeen shore lines of Lake Agassiz are recognized in descending order, 

 which were formed while this great glacial lake outflowed southward by 

 Lakes Traverse and Big Stone; aud eleven lower shore lines record later 

 pauses in the declining lake level, while it outflowed northeastward across 

 the south part of Keewatiu. Perhaps at first this northerly outflow was 

 turned eastward and then southward, passing along the border of the re- 

 ceding ice-sheet which still covered the area of Hudson's bay, and event- 

 ually flowing through lakes Superior and Michigan to the Mississippi. 

 When the ice upon Hudson's and James's bays and the adjoining country 

 was so far melted as to admit the ocean there, it at first covered the land 

 west of James's bay 350 to 500 feet above the present sea level. Lake 

 Agassiz, and the numerous glacial lakes which had existed in the basins of 

 the Moose, Albany, and other rivers tributary to James's bay, were then 

 drained northward into this inland sea. An early northeastward outflow 



