POST-GLACIAL LAKES IN CANADA 353 



only slightly since that time. The level of Athabaska Lake is 

 now 699 feet above sea-level at average water conditions. 



Northward, however, conditions are very different. The 

 outlet of the basin lies at its western corner, and differential eleva- 

 tion to the north and east has only had the effect of spilling more 

 water down the Mackenzie River, with the result that the basin has 

 been rapidly drained. The rapidity with which the water level 

 fell on Great Slave Lake is excellently well shown in the storm or 

 seasonal beaches found on the forelands, particularly along the 

 north shore. At one place on Windy point over 100 such beaches 

 are observable in an elevation of some 250 feet, and the crest of the 

 hill is marked by a horseshoe beach developed as the ground rose 

 above the water level. The beaches are of fairly uniform depth, 

 and throughout most of the rise occur as a series of very regular 

 waves, indicating that elevation has been at a constant, uniform 

 rate. That movement is still going on is apparent from the beach 

 lines built up in recent times. 



In conclusion, the writer can only admit that these are wide 

 generalities based on very insufficient data, and undoubtedly 

 will bear much correction. He feels sure, however, that the 

 outlines suggested are at least approximately correct. Much work 

 has yet to be done, and detail work on the problem would yield 

 many points of scientific interest. One main problem that presents 

 itself is the study of the Great Slave Lake shore lines with a view 

 to determining the time period since the last glacial age. Dr. A. P. 

 Coleman produced^ much interesting information on this subject 

 by a detail study of raised beaches, delta deposits, and lake bars 

 on Lake Ontario. The seasonal beaches of Great Slave Lake, 

 studied in detail, would give similar information. The Alexandra 

 Falls on Hay River have receded 6.5 miles since their original 

 development — a distance almost equal to that of Niagara Falls. 

 Slave River has completely filled up the long, southward-extending 

 arm of Great Slave Lake, and is still building rapidly into the 

 present lake. The rate at which alluviation is taking place could 

 be readily determined, and an excellent time record seems available, 



December, 1921 



' A. P. Coleman, Proceedings of the Inlernational Geological Congress, Toronto, 

 Canada, ipij. 



