208 G. F. WRIGHT — ^AGE OF DON RIVER GLACIAL DEPOSITS 



the winds to form the dunes which attract the attention of all passengers 

 on the railroad going to Chicago from the east. From the amount of 

 these accumulations and from the rate at which the sand is now known 

 to be moving past Chicago along the west side of the lake toward the 

 south end of it, from 10,000 to 15,000 years is more than ample time to 

 account for the accumulation of the entire mass of the dunes. 



h. The waves of Lake Michigan are constantly eating into the west 

 shore at a rate which, according to the most moderate computations, 

 would produce the 70-foot shelf extending out to deep water in less time 

 than that already allowed for the accumulation of the dunes.^ 



3. The latest glacial deposits on the south side of Lake Erie are found 

 in the beaches that mark the shoreline of Lake Warren, the highest of 

 which is approximately 200 feet above Lake Erie in Ohio. These beaches, 

 I think I have proved conclusively, can not be more than 10,000 or 12,000 

 years old. The evidence exists in the small amount of erosion that has 

 been accomplished by the streams of northern Ohio, which have been flow- 

 ing into the Lake Erie basin ever since the retreat of the ice from that 

 region. In particular, attention is directed to the small extent to which 

 Plum Creek, in Oberlin, Ohio, has enlarged its trough. This trough is 

 50 feet above the highest Lake Warren beach, and only five miles distant, 

 is entirely in glacial deposits, with no rock obstruction, yet it is so narrow 

 and shallow that any calculations making it much more than 12,000 

 years old, and especially those that make it several multiples of 12,000, 

 involve an absurdly low rate of erosion. Moreover, data have been col- 

 lected from the present eroding efficiency of the stream which give results 

 well within the above figures.* 



4. A clue to the length of time during which Lake Warren continued 

 to cover the bordering land on the south side of Lake Erie is furnished by 

 deposits recently uncovered by excavations at Fremont, Ohio. The sedi- 

 mentary plain on which the city of Fremont is built lies below the 100- 

 foot level of the lowest shoreline of Lake Warren. The sedimentary de- 

 posit's consist of the material brought into Lake Erie by Sandusky Eiver, 

 which is spread out as a delta. The depth of these lacustrine beds is at 

 least 25 feet. The thickness of the laminae, according to my measure- 

 ments made in several excavations, is on ap average one-seventh of an 

 inch, making 84 to the foot, making a total of 2,100, which would be the 

 number of years required for the accumulation on the supposition that 



8 See the paper of Dr. Edmund Andrews in Transactions of ttie Chicago Academy of 

 Sciences, vol. ii, 1870, pp. 1-23. This and the later facts bearing on the question are 

 fairly and fully discussed by Leverett in Monograph xxxviil of the U. S. Geol. Survey, 

 pp. 453-459. 



* See Wright's "Ice Age in North America," 5th ed., pp. 564-568 ; also Bull. Geol. Soc. 

 Am., vol. 23, pp. 278-280. 



