224 The American Geologist. April, 1891 
its range northward extends at least to Great Playgreen lake and 
York Factory, where it has been collected by Dr. Bell. The 
Campbell beach was formed in the later part of the time of the 
lake's southward outflow ; and the Gladstone beach belongs to the 
middle portion of the time of its outflow toward the northeast, its 
south end being then about 85 miles south of the international 
boundary. 
Evidences of man's presence in this region during the departure 
of the ice-sheet have been discovered by Miss Franc E. Babbitt at 
Little Falls in central Minnesota. A stratum containing manj' arti- 
ficially chipped fragments of quartz is enclosed there in the modified 
drift of the upper Mississippi valley, which was deposited by the 
floods supplied from the melting ice-sheet in its retreat while it was 
being withdrawn from northern Minnesota and the Red River val- 
ley. * It seems probable therefore that men lived on the shores of 
lake Agassiz and witnessed the erosion of the channel of the 
river Warren, the gradual lowering of the lake level and reduction 
of its area, and its later northeastward outflow to Hudson bay. 
But this is not left wholby to conjecture, for Mr. Tyrrell informs 
me that in northwestern Manitoba, at an elevation of 1,135 feet 
above the sea, he has found sharp-edged fragments of quartzite, 
chipped by human workmanship, interbedded with the rounded 
gravel of one of the Campbell beaches, f 
If the question be asked how many thousand years ago did the 
recession of the ice-sheet take place, causing lake Agassiz to fill 
the Red River valley and the basin of lake Winnipeg, a replv is 
furnished by the computations of Prof. N. H. Winchell, % that ap- 
proximately 8,000 years have elapsed during the erosion of the 
postglacial gorge of the Mississippi from Fort Snelling to the falls 
* Proceedings of Am. Assoc, for Adv. of Science, vol. xxxii, 1883, pp. 
385-390; American Naturalist, vol. xviii, pp. 594-605, and 697-708, June 
and July, 1884 ; and Proc. Boston Soc. of Natural History, vol. xxiii, 
1888, -pp. 421-449. 
f Preliminary notes ©f this discovery, and of the northwestward con- 
tinuation of the beaches of Lake Agassiz in the district of Riding and 
Duck Mountains, are included by Mr. Tyrrell in a paper, "On the Su- 
perficial Geology of the Central Plateau of Northwestern Canada, read 
before the Geological Society of London, Nov. 7, 1888, of which an ab- 
stract is given in the Geological Magazine, III, vol. vi, pp. 37-38, 
Jan., 1889. 
% Geology of Minnesota, Fifth annual report, for 1876 ; and Final re- 
port, vol, ii, pp. 313-341. Quart. Jour. Geol. Soc, vol. xxxiv, 1878, pp. 
■886-901. 
