TITLES AND Ai;STRA(!TS OK I'Al'KllS 145 



RECORDS OF LAKE AGAS8IZ, IN SOUTHEASTERN MANITOBA, AND ADJACENT 

 PARTS OF ONTARIO, CANADA i 



BY WILLIAM ALFRED JOHNSTON 



Recent explorations by the author in southeastern Manitoba and adjacent 

 parts of Ontario have brought forth evidence which confirms Mr. J. B. Tyrrell's 

 view that Lake Agassiz had a rising stage owing to the blocking of the north- 

 ward drainage by an advance of the I^abradorean glacier after the partial 

 withdrawal of the Keewatin glacier. The life history of Lake Agassiz was, 

 however, more complicated than was supposed by Tyrrell or by Mr. Warren 

 Upham, through whose work Lake Agassiz is best Imow^n. The evidence shows 

 that the Red River basin has been twice flooded by glacial marginal lakes — 

 an earlier one which was associated with the retreat of the Keewatin ice-sheet 

 and a later one associated with an advance of the ice from the northeast. 

 The early lake was largely drained before the inception of the later lake. 

 The advance of the ice from the northeast, which brought the later lake into 

 existence, was not, as Tyrrell supposed, the first advance of the Labradorean 

 glacier, but a readvance of the ice-sheet at a late time during the Wisconsin 

 stage of glaciation; for, as Mr. Frank Leverett has also shown, a till sheet 

 derived from the northeast underlies a till sheet deposited by the Keewatin 

 glacier. 



The evidence on which this modification of the life history of Lake Agassiz 

 is based was in part presented by the author in a paper published in the 

 Journal of Geology, volume XXIV, 1916. Further evidence found in south- 

 eastern Manitoba and adjacent parts of Ontario during the past season con- 

 firms this view. The evidence consists largely in the fact that a marked 

 depositional break or unconformity occurs at the base of the later Lake 

 Agassiz sediments at various altitudes in the Red River basin. Sections ex- 

 posed along the Greater Winnipeg Water District Railway between Shoal Lake 

 and Winnipeg show this depositional break at the base of the upper lacustrine 

 clays, which have in places a maximum thickness of 20 feet. Sections exposed 

 along the Roseau River in southeastern Manitoba also show this break. The 

 evidence shows that the waters of the later lake rose from the lower parts of 

 the Red River Valley to at least the altitude of the main Campbell beach, 

 which was connected with the southern outlet of the lake, and probably some- 

 what higher. 



A moraine which is well developed at Ignace, Ontario, and which extends 

 northwestward for a considerable distance, appears to mark the limit of the 

 latest advance of the ice-sheet in this region, and also to mark the border of 

 the second Lake Agassiz at the time of its maximum extension. 



The acceptance of the present view regarding the life liistory of Lake Agassiz 

 has an important bearing on the question of the character and cause of the 

 difeerential uplift, which is shown to have affected the region by the deforma- 

 tion of the shorelines. The direction of maximum uplift agrees very nearly 

 with the direction of advance of the ice from the northeast, but not with that 

 of the Keewatin ice-sheet. The whole southern portion of the lake basin, as 

 well as the northern portion, was affected by uplift, as was shown by Upham. 



Presented with the permission of the Director of the Geological Survey of Canada. 



