THE PEMBINA DELTA. 363 



trine silt was doubtless supplied from the melting- ice-sheet at the same time 

 with the deposition of the tracts of modified (h'ift that border the valley 

 north of Rock and Swan lakes; so that the material derived from ei'osion 

 in this valley was considerably less than would be required to fill it. More- 

 over, it seems likely that the entire erosion of Langs Valley — that is, of the 

 portion of this watercourse extending- from the Souris to Pelican Lake — 

 together with most of the valley along- the extent of that lake, was effected 

 by the outflow from tlie T^ake Souris during- the time of formation of the 

 Pembina delta; and this larg-e supply froni erosion in the upper part of 

 the valley still further diminishes its probable amount along the course 

 of the river below. Thus it is clearly indicated that the Pembina Valley, 

 like the vallevs of tlie Minnesota, Sheyenne, and Assiniboine rivers, was 

 eroded during- preg'lacial time and was not entirely filled by the drift. 

 Comparing this delta with all the other conspicuous deltas of Lake Agassiz, 

 it seems indeed probable that more than half of its mass was supplied 

 directly from the englacial di-ift of the ice-sheet, and that less than half 

 cjvme fi-om erosion of the valley, which, therefore, along the lower and 

 deeper portion of its course a])pears not to have been nuich obstructed by 

 the o-lacial (h-ift. From this it follows that the extensive high terraces 

 observed on the sides of the Pembina Valley in the vicinity of the Mow- 

 bray bridge and -westward (page 270) are due to preglacial erosion in the 

 Cretaceous shales, owing to the action of the ice-sheet only their minor 

 features, together with the drift forming their surface. 



THE UPPER OR IIERMAX BEACHES ANT) DELTAS IN MANITOBA. 



FROM THE INTERNATIONAL BOUNDARY TO THE VICINITY OF NEEPAWA. 

 (PLATES XXX, XXXI, AND XXXII.) 



The west shore of Lake Agassiz enters Manitoba 2 miles west of the 

 east line of range 5, at a distance of 36 miles from the Red River. On 

 the international boundary and for the next 10 miles northward the shores 

 of the highest stages of the lake were on the steep wooded escarpment of 

 the Pembina Mountain, the base of which here is LI 00 to L150 feet 

 above the sea, rising slightly northward, and the verge of its top, 1,300 to 



