THE DATE OF THE GLACIAL PERIOD. 565 



evidently the remains of a modified glacial deposit formerly 

 .tilling the whole valley to that height. Since the Glacial 

 period the present stream has been occupied with the task of 

 slowly removing this material. The number of cubic yards 

 which it has already carried away can be approximately esti- 

 mated. The rate of removal is more difficult to determine. 

 Assuming the rate to be the same per cubic foot of water as 

 that which is transported by the Mississippi Kiver past New 

 Orleans, which doubtless is far too small, the time required 

 would be, according to the calculation of Professor Hicks, 

 less than fifteen thousand years. 



I have been able to make a more definite calculation in 

 connection with Plum Creek, in Oberlin, Ohio. The situa- 

 tion is peculiarly favorable both on account of its relation to 

 the glacial shore lines around Lake Erie, and of its freedom 

 from disturbing obstacles. The section of the Creek Valley 

 from which the facts are gathered lies ten miles south of the 

 present shore of Lake Erie, and 250 feet above the lake level. 

 It is about five miles south of the highest of the lake ridges, 

 and fifty feet higher than the upper ridge. Its course is 

 wholly in glacial till with no rock bottom anywhere in its 

 course. The average gradient of the stream is twelve feet 

 to the mile, falling 100 feet in eight miles. It is evident that 

 the stream did not begin the erosion of its present trough 

 until the ice-sheet had retreated from the water-shed on the 

 south and had uncovered the outlet of the glacial lake at 

 Fort Wayne, which determined the level of water on whose 

 shores the upper ridge was thrown up by the waves. The 

 Plum Creek trough is therefore older than the Niagara gorge 

 by the length of time that was required for the retreat of the 

 ice-sheet from the south shore of Lake Erie to the Adirondack 

 Mountains, a distance of 200 or 300 miles; for, as already said, 

 Niagara did not begin its work until the Mohawk Valley, 

 south of the Adirondack Mountains was free from ice. 



Now, in 1895, a reservoir was constructed in the village 

 occupying the whole width of the trough of the creek, and 



