270 THE GLACIAL LAKE AGASSIZ. 



At the Mowbraj' bridge the bottom land is about au eighth of a mile 

 wide and 10 feet above the river. About 40 feet higher is a narrow terrace 

 of modified di-ift, an eighth to a fourth of a mile wide, reaching along the 

 southern side of the river for 1^ miles to the east, and also well shown in 

 manv places on each side of the river for 6 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 bluflPs of the valley have steep slopes, rarel}' 

 interrupted by terraces. But a remarkably broad ten-ace or plateau, evi- 

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

 extends on its southern side 3 miles to the east from the Mowbray bridge 

 and road, with a maximum width of about IJ miles, and an elevation 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 bluff that rises thence about 100 feet to the 

 general level of the adjoining country. All the way for 25 miles from this 

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

 river flows in a ver}^ 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 1 mile back from the river, 

 form the high plateau through which the river has cut its way The nar- 

 rowness and depth of the partially di'ift-filled valley indicate that its area 

 of drainage w^as no greater in preglacial time than now. 



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

 the Big Coul(^e and the Sheyenne was approximately 1,600 to 1,500 feet 

 above the present sea-level, being gradually cut down about 100 feet by 

 the stream. But, on account of subsequent changes which are known 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 Langs Valley. Therefore, when its channel of discharge was 

 transfen-ed to the new course by Pelican Lake and along the Pembina, the 



