The clays of the Lake Agassiz basin
Maynard, James Edward
Lake Agassiz covered an area of great extent during the retreat of the last ice sheet towards Hudson Bay. Its area at any stage has not been defined with accuracy; for its northern boundary was the retreating ice wall. At its greatest height it covered almost the whole of southern Manitoba, and part of North Dakota and Minnesota; and the place where Winnipeg now stands was over 500 feet below the surface of the lake. The lowering of the lake was not a gradual process, but was interrupted by many breaks, of sufficient duration to permit of well defined beaches being formed on the shores of the lake. These beaches are clear cut topographical features of the present surface of southern Manitoba, more particularly along the foot of the Manitoba escarpment on what was the western shore of the lake. The area is not yet completely drained out, for Lake Winnipegosis, Lake Manitoba and Lake Winnipeg are the present-day representatives of this great inland sea; the obstacle to further lowering of the waters being not a receding ice-sheet, but the Pre-cambrian ridges which only slowly give way before the erosive force of the Nelson river, through which the system now empties into Hudson Bay. In a freshwater basin of such considerable extent conditions are favorable for the study of freshwater sedimentation. At the low temperature of a glacial lake the salt content in the water is low, for rock disintegration has been limited. The action of electrolytes in precipitating sediment is therefore reduced to a minimum. During the melting season glacial rockflour and other rock sediment is poured into the lake by fluvioglacial streams and from land rivers: in the winter such contributions cease, and time is given for the finer sediments to settle undisturbed. Particularly in the deeper parts of the lake, as where Winnipeg now stands, undisturbed by local fluctuations the sedimentation of the summer and the winter periods should register itself in the clays now exposed to view...