268 THE GLACIAL LAKE AGASSIZ. 



way of the Wascana River, after passing tlu-ough a glacial lake which 

 probably extended from Regina 60 miles to the west in the upper Qu'Ap- 

 pelle basin. When the Dakota ice-lobe was melted back to the vicinity 

 of Devils Lake the drainage of Lake Som-is passed southeast by the Big 

 Coulee, one of the head streams of the Sheyenne, flowing thence for some 

 time southward by the James River to Lake Dakota, but later eastward 

 and southward by the Sheyenne into Lake Agassi z. A manuscript report 

 of a reconnoissance in North Dakota by Maj. W. J. Twining, in 1869, 

 describes the valley of the Big Coulee as 125 feet deep and a third of a 

 mile wide, inclosing several shallow lakes along its course. " This great 

 valley," he writes, "preserves its character to within 12 miles of the Mouse 

 River, and connects through the clay and sand ridge with the open valley 

 of that stream." 



The glacial Lake Souris (PL XXI) occupied the basin of the Souris or 

 Mouse River from the most southern portion of this river's loop in North 

 Dakota to its elbow in Slanitoba, where it turns sharply northward and 

 jjasses through the Tiger Hills. North of the Souiis basin an arm of this 

 lake extended along the Assiniboine from Giiswold and (_)ak Lake to 

 some distance above the mouth of the Qu'Appelle; and the main body of 

 the lake was deeply indented on the east by the high oval area of Turtle 

 Mountain, an outlier of the lignite-bearing Laramie formation, which is 

 Avell developed on the upper part of the Souris River and forms, with over- 

 lying drift deposits, the massive terrace of the Coteau du IVIissomi on the 

 west. The length of Lake Souris was about 170 miles, from latitude 48° 

 to latitude 50° 35', and its maxinuim width, north of Tm-tle Mountain, was 

 nearly 70 miles. 



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

 uncover Turtle Mountain, the glacial lake in the Souris basin continued to 

 outflow hv the Sheyenne and Iniild up its delta. Next its outflow passed 

 north of Turtle Mountain liy the Pembina, perhaps after taking for a brief 

 time the course of the Badger Creek, Lac des Roches, and the Mauvaise 

 Coulee to De^^ls 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, proba- 

 bly averaging 175 feet, along the greater part of its course, but it is from 



