362 THE GLACIAL LAKE AGASSIZ. 



The deposition of this deha, took place during- the highest Herman 

 stag-e of Lake Agassiz It seems to have been very rapid, the .supply of 

 sediments being so great that about the mouth of the Pembina Valley they 

 were accumulated in a fan-like sloping mass to a height of more than 50 

 feet above the lake level. When the recession of the ice-sheet caused the 

 cessation of its supply of modified drift, and perniitted the Souris to flow, as 

 now, to the Assiniboine, the growth of this delta ceased; and its subsequent 

 history is that of the deep channels cut through it by the Little Pembina 

 and the Pembina, and of the steej) escariniient sculptured on its east side. 

 From the erosion of this first Pembina Mountain large amounts of gra\el 

 and sand were swept southward, notably during the Campbell stages of the 

 lake, when they were deposited in a very massive curving beach ridge that 

 crosses the Tongue River in the west part of township 161, range 55, about 

 7 miles west of Cavalier. In the Herman stage, while the delta was being 

 accumulated, much fine clay and silt, brought by the same glacial river, 

 were carried farther and spread upon the lake bed along the central part 

 of the Red River Valley, perhaps extending in appreciable amount nearly 

 100 miles southward to the belt of till that reaches across the valley at 

 Caledonia and forms the Goose Rapids. But on the west side of the 

 lacustrine area this fine sediment is absent, probably because of currents 

 trending oflPshore; and the surface is till both south and north of the 

 gravel and sand delta, as from Park River north to Gai-dar and Mountain 

 and nearly to the Tongue River, and from 2 miles north of the Pembina to 

 the international boundary and onward. 



During the Glacial period the great valley of the Pembina Ri^•er west 

 of its delta was only partially filled with drift, for its reexcavation, with the 

 channeling of Langs Valley, tributary to it from the glacial Lake Souris, 

 would have supplied as large a tribute to Lake Agassiz as the entire Pem- 

 bina delta and the fine silt and clay that are spread over the adjacent laka 

 bed. The volume of the delta, as befoi-e stated, is approximately 2J cubic 

 miles. If an equal amount of fine silt were deposited beyond the delta, 

 both together would measure about the same as Langs Valley and the Pem- 

 bina Valley from the former mouth of Lake Souris to the delta, namely, 

 between 4 and 5 cubic miles.^ But much of the Pembina delta and lacus- 



' See pages 267-272, foregoing. 



