3J4 MAN AND THE GLACIAL PERIOD. 



listed similar calculations concerning Plum Creek, at 

 Oberlin, in Lorain County, Ohio.* Like Raccoon Creek, 

 this has its entire bed in glacial deposits, and has had 

 nothing else to do since its birth but to enlarge its bor- 

 ders. The drainage basin of the creek covers an area of 

 about twenty-five square miles. Its main trough averages 

 about twenty feet in depth by five hundred in width, along 

 a distance of about ten miles. From the rate at which 

 the stream is transporting sediment, it is incredible that 

 it could have been at work at this process more than ten 

 thousand years without producing greater results. 



Calculations based upon the amount of sediment de- 

 posited since the retreat of the ice-sheet point to a like 

 moderate conclusion. When one looks upon the turbid 

 water of a raging stream in time of flood, and considers 

 that all the sediment borne along will soon settle down 

 upon the bottom of the lake into which the stream 

 empties, he can but feel surprised that the " wash " of 

 the hills has not already filled up the depression of the 

 lake. It certainly would have done so had the present 

 condition of things existed for an indefinite period of 

 time. 



Naturally, while prosecuting the survey of the super- 

 ficial geology of Minnesota, Mr. Upham was greatly im- 

 pressed by the continued existence of the innumerable 

 lakelets that give such a charm to the scenery of that 

 State. Every day's investigations added to the evidence 

 that the lapse of time since the Ice age must have been 

 comparatively brief, since, otherwise, the rains and streams 

 would have filled these basins with sediment, and cut out- 

 lets low enough to drain them dry, for in many instances 

 he could see such changes slowly going forward. f 



Some years ago I myself made a careful estimate of the 



* See Ice Age in North America, p. 469. 



f Minnesota Geological Report for 1879, p. 73. 



