9 
Lowlands, extending at its maximum to approximately the fifty- 
fifth parallel and covering the upper part of the present Nelson 
River system. This forerunner of the present-day lakes Winnipeg, 
Manitoba, and Winnipegosis is known as Glacial Lake Agassiz. 
It was preceded by the much smaller Glacial Lake Souris on the 
Cretaceous plateau to the southwest, this lake originally drain- 
ing southward by way of the lower levels of the present Souris 
Basin west of Turtle Mountain, but later draining eastward into 
youthful Lake Agassiz by way of the present-day Pembina River 
channel. The flat, smooth topography of the Manitoba Lowlands 
is the result of the deposition of silts and clays in Lake Agassiz, 
which, during its various phases of drainage, established the 
many beaches now traceable along the Manitoba Escarpment as 
gravel ridges or wave-cut terraces. According to Upham (1890), 
the waters of Lake Agassiz at the time of the formation of its 
highest beach covered the present site of Winnipeg to a depth of 
about 600 feet. 
Until recently it has been generally assumed that the time 
elapsed since the last glacial maximum of the Wisconsin ice- 
sheet is in the neighbourhood of 25,000 years, and that the north- 
eastward drainage system of Lake Agassiz into Hudson Bay, 
upon the melting of the northern ice barrier, was established 
approximately 9,000 years ago. Flint and Deevey (1951), how- 
ever, report that radiocarbon measurements of wood from the Two 
Creeks peat formation immediately underlying the Mankato drift 
in Wisconsin give the age of the wood as only about 11,000 years, 
a result that “seems to have been anticipated by the opinions of 
some geologists and soil scientists, who had come to believe 
that the degree of soil development and erosion on the Mankato 
drift are inconsistent with an age as great as 25,000 years... The 
whole process of deglaciation seems to have been more rapid 
than had been supposed.” In Manitoba, then, the period available 
since Pleistocene times for colonization of extensive parts of 
the land by plants may eventually prove to be of the order of only 
four or five thousand years. The presence of a former great lake 
in the Red River basin of Manitoba was first noted by Palliser 
(1863, p. 41), in the following words: “This plain, no doubt, 
had formed at one time the bed of a sheet of water, and Pembina 
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