Upham.] 446 [Dec. 21, 



ice-border continued across Green bay and the north part of lake 

 Michigan ; and farther east I think that it probably crossed south- 

 western Ontario and the central or northern portions of New York, 

 Vermont, New Hampshire and Maine. The Laurentian lakes were 

 dammed by the retreating glacial barrier and overflowed at the 

 lowest points on their southern watershed. The time when the 

 Little Falls stone implements and fragments from their manufacture 

 were covered by the modified drift seems therefore somewhat later 

 than that of the implements 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 be3 r ond the 

 region where the glacial floods could be discharged b} r the Little 

 Miami and Delaware rivers. 



If the question be asked, how many thousand years ago was this, 

 a reply is furnished by the computations of Prof. N. H. Winchell, 

 that approximate^ 8,000 years have elapsed during the erosion of 

 the postglacial gorge of the Mississippi from Fort Snelling to the 

 falls of Saint Anthony ; of Dr. Andrews, that the erosion of the 

 shores of lake Michigan, and the resulting accumulation of dune 

 sand drifted to the southern end of that lake, cannot have occupied 

 more than 7,500 years ; of Professor Wright, that streams tribu- 

 tary to lake Erie have taken a similar length of time to cut their 

 valle\rs and the gorges below their water-falls ; and of Mr. Gilbert, 

 that the gorge below Niagara Falls has required only 7,000 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 retiring 

 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 to occupy this locality, 

 so soon after it was uncovered from the ice, Was their discover}' of 

 the quartz veins in the slate there and on the Little Elk river, af- 

 fording suitable material for making sharp-edged stone implements 

 of the best quality. Quartz veins are absent or very rare and un- 

 suited for this use in all the rock-outcrops of the south half of 

 Minnesota that had become uncovered from the ice, as well as of 



