SOURIS DELTA AND THE BIG SLOUGH. 377 



The Assiniboine delta of Lake Souris has a length of about 85 miles, 

 extending- from the north end of this glacial lake south-southeasterly along 

 the Assiniboine to its eastward bend and lieyond to Plum Creek and the 

 Souris River. Its width ranges from ,5 to 2.5 miles, averaging about 12 

 miles. This delta is doubtless shallower than that of Lake Agassiz, but if 

 its average thickness is 2,5 feet upon this area of 1,000 square miles, its 

 volume is about f> cubic miles. 



An ancient watercoiu'se, now occupied by a liody of water called the 

 Big Slough, 13 miles long and mostly 20 to .^O rods wide, but in its west 

 part about three-fourths of a mile wide, extends from southwest to north- 

 east 9 miles tlu'ough this delta of Lake Souris and thence continues 4 

 miles east through an area of till. Its west end is 2 miles southwest of 

 Griswold and its east end about a half mile east of Alexander, its whole 

 extent being on the south side of the railway. Its elevation in the stages 

 of low and high water ranges from 1,385 to 1,388 feet, and its depth at low 

 water varies from 2 to 6 or 8 feet. The shores of the Big Slough rise in 

 gentle slopes 15 to 20 feet in 20 to 30 rods, to the general level, not having 

 the usual steepness of banks undermined by streams; yet it doubtless 

 marks the course of a stream that outflowed at one time westward into 

 Lake Souris from a small glacial lake north of the Brandon Hills, and of a 

 later stream that flowed in the opposite direction, eastward from the basin 

 of Lake Souris into the Brandon glacial lake, before that became merged 

 in Lake Agassiz by the departure of the ice-sheet. The succession of 

 events indicated by this channel, together with that of the present Souris 

 and with the great glacial watercourse of Langs Valley, is as follows: 

 Lake Souris outflowed eastward by Langs Valley, Pelican Lake, and the 

 Pembina River until the receding ice formed a lake north of the Tiger 

 Hills and east of the Brandon Hills, which, outflowing south to the Somns, 

 cut a deep gorge through the Tiger Hills moraine, where the Souris now 

 flows tkrough it to the north. Similarly, north of the Brandon Hills, a 

 lake was probably held by the barrier of the ice during its recession from 

 Alexander east by Kemnay and Brandon, outflowing Avestward to the Lake 

 Souris by the course of the Big Slough. As soon as the continued glacial 

 recession left the Brandon Hills wholly uncovered from the ice, these lakes 



