LAKE SOUKIS AND LANGS VALLEY. 271 



Lake Souris was suddenly lowered about 125 feet to the level of the top 

 of the bluffs of Langs Valley, and a further lowering of 110 feet was after- 

 ward 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 20 feet above the present divide in Langs Valley. 

 Since the waters of the Souris ceased to flow along this course, the sedi- 

 ments of gravel and sand brought by tributaries have filled portions of the 

 Pembina Valley 10 to 20 feet, forming the banners of its shallow lakes; 

 and the divide in Langs Valley has been raised probably 10 feet by the 

 deposits of Dunlops Creek. 



The ice-sheet was forming its moraine of the west part of the Tiger 

 Hills, and of the Brandon and Arrow hills, when Lake Souris began to out- 

 flow by the course of Langs Valley and the Pembina. The extent of Lake 

 Souris was then from the northern part of North Dakota along the Souris 

 and Assiniboine to the lower Qu'Appelle; and the Saskatchewan outflow 

 by its erosion of the Qu'Appelle Valley brought into this lake extensive 

 delta deposits of gravel and sand, which, with similar beds of modified di-ift 

 brought into it from the melting ice-sheet that was its northeastern barrier, 

 reach from the vicinity of Fort Ellice southeast to Oak Lake and Plum 

 Creek. 



After the erosion of Langs Valley had lowered the Lake Som-is below 

 the level (about 1,390 feet) at which its outflow could pass instead to the 

 north and northeast by the way of Oak Lake and the Assiniboine, the ice 

 was withdrawn to the north side of the Assiniboine Valley east of Oak 

 Lake, and the deposition of the great Assiniboine delta of Lake- Agassiz 

 ensued. A width of onlv 3 miles of the morainic belt of the Tiger Hills, 

 extending along the north side of Langs Valley and the elbow of the 

 Souris, intervened between that stream and an expanse of till whose surface 

 is lower than the bottom of Langs Valley and descends with northeastward 

 slope to the Assiniboine. The crest of this moraine rises about 200 feet 

 above Langs Valley, but it had probably been cut through nearly or quite 

 to the level of that valley by drainage southward from a small lake formed 

 between the moraine and the receding ice within the angle between the 



