210 G. F. WRIGHT AGE OF DON RIVER GLACIAL DEPOSITS 



as to open the drainage into Hudson Bay did not occupy more than 2,000 

 years, making the average rate of the retreat of the ice-front one-half 

 mile per annum. '^ This evidence consists largely in the small extent of 

 the dunes at the south end of Lake Agassiz as compared with that of 

 those at the south end of Lake Michigan, and of the small extent of the 

 deltas formed on the western shore of Lake Agassiz deposited by such 

 streams as the Saskatchewan and the Assiniboin. 



So little is known about the cause of the climatic changes which pro- 

 duce and terminate glacial periods that speculation apart from the facts 

 at hand is of little value. 



Origin of the warm Species of Plants and Animals in the Don 



Beds 



The occurrence of warm species of plants and mollusks, referred to, in 

 the Don Eiver Valley takes us back to the earliest stages of the Glacial 

 period, and at first sight seems to prove, as Professor Coleman maintains, 

 that a warm period intervened in that latitude between two successive 

 glacial advances, and implies that the first ice-sheet completely disappeared 

 from North America and was succeeded by a new and independent move- 

 ment. But when the evidence is closely examined in the light of other 

 well established facts it would appear that such a sweeping inference is 

 by no means necessary. 



a. It does not appear that the glacial till below the fluviatile beds in 

 the Don Valley contains any roots of the trees and shrubs supposed to 

 have existed in the inter-Glacial warm period, whereas in front of the 

 Muir Glacier in Alaska my photographs distinctly show such stumps and 

 rootlets projecting from the underlying soil into the fluvial beds that had 

 enveloped them and were subsequently covered, over by the glacier's 

 advance.® 



h. It is entirely possible that the specimens of warm species of plants 

 and animals in the fluvial deposits overlying the lower till were derived 

 from the underlying deposit of late Tertiary age, having been plowed up 

 by a readvance of the ice after a temporary recession and raised without 

 much disturbance to the higher levels, where they are now found. This 

 supposition, which would seem incredible until all the elements involved 

 had been taken into consideration, is rendered easily credible by what 

 has been learned in recent times concerning the deposits of Moel Tryfaen 



■^ See Monograpli xxv of U. S. Geol. Survey ; Bulletin No. 29, U. S. Geol. Survey ; An- 

 nual Report, Canadian Geol. Survey, n. s., vol. iv, for 1889, part E. The facts are suc- 

 cinctly stated by Dr. Upham in Wright's "Ice Age in North America," 5th ed., pp. 400- 

 406 and 543-548. 



* See Wright's "Ice Age in North America," 5th ed., p. 63. 



