MAN AND THE GLACIAL PERIOD. 665 



Mississippi Rivers by the way of the Bois Brule River and Upper 

 St. Croix Lake. It seems nearly certain also that the ice- 

 border continued across Green Bay and the north part of Lake 

 Michigan ; and farther east I think that it probably crossed 

 southwestern Ontario and the central or northern portions of 

 New York, Vermont, New Hampshire, and Maine. The Lau- 

 rentian lakes were dammed by the retreating glacial barrier 

 and overflowed at the lowest points on their southern water- 

 shed. The time when the Little Falls stone implements and 

 fragments from their manufacture were covered by the modi- 

 fied drift seems therefore somewhat later than that of the im- 

 plements found in southern Ohio and in New Jersey ; for, if 

 this was the course of the ice-boundary east from the Leaf 

 Hills of Minnesota, it had already receded beyond the region 

 where the glacial floods could be discharged by the Little 

 Miami and Delaware Rivers. 



If the question be asked, How many thousand years ago was 

 this ? a reply is furnished by the computation of Professor N. 

 H. Winchell, that approximately eight thousand years have 

 elapsed during the erosion of the post-glacial gorge of the 

 Mississippi from Fort Snelling to the Falls of St. Anthony ; 

 of Dr. Andrews, that the erosion of the shores of Lake Michi- 

 gan, and the resulting accumulation of dune-sand drifted to 

 the southern end of that lake, can not have occupied more than 

 seventy-five hundred years ; of Professor Wright, that streams 

 tributary to Lake Erie have taken a similar length of time to 

 cut their valleys and the gorges below their waterfalls ; and of 

 Mr. Gilbert, that the gorge below Niagara Falls has required 

 only seven thousand years or less. These measures of time 

 carry us back to the date of the Little Falls quartz- workers, 

 when the ice-sheet of the last Glacial epoch was melting away 

 from the basins of the upper Mississippi and of the Laurentian 

 lakes. 



Plants and animals doubtless followed close upon the retir- 

 ing ice-border, and men living in the region southward would 

 make journeys of exploration to that limit, but probably they 

 would not take up their abode for all the year so near to the 

 ice as Little Falls at the time of the Fergus Falls and Leaf 

 Hills moraines. It may be that the chief cause leading men 



