THE CINCINNATI ICE DAM. 197 



bardton Creek, which was not affected by the showers, was carried 

 up stream by the water which set back from the river. Thus it is 

 easy to see that the glacial floods which poured into the Ohio from 

 its northern tributaries would, during their continuance, produce 

 slack water in its southern tributaries. 



A more permanent class of dams is produced when a super- 

 abundant amount of earthy debris is contributed by one tributary 

 of a stream. It is thus that the Chippewa River, in Wisconsin, 

 has brought down an excessive amount of sand and gravel into 

 the Mississippi, where, owing to the gentler gradient and the 

 slower current in the larger valley, a delta has been pushed out 

 across the Mississippi, ponding back the water so as to form the 

 enlargement known as Lake Pepin. Dr. George M. Dawson de- 

 scribes a more striking instance in one of the principal tributaries 

 of the Fraser in British Columbia, where Dead Man's Creek joins 

 the Thompson. Here a sufficient amount of gravel has been 

 brought down to silt up the main stream to a depth of four hun- 

 dred and fifty feet, forming Kamloop's Lake, which is eighteen 

 miles long and two miles wide. It is thus that the glacial silts 

 coming into the channel of the Ohio from its northern tributaries 

 have assisted the Cincinnati ice dam in the work that was laid 

 upon it. 



On the other hand, it is clear that the Cincinnati ice dam must 

 in turn have assisted greatly in the silting process already re- 

 ferred to ; for, as far up the Ohio as slack water was produced 

 by the obstruction at Cincinnati, the deposition of the finer silt 

 must have been greatly facilitated by it. At the same time the 

 deposition of gravel near the mouth of the streams joining the 

 Ohio above Cincinnati, and the obstruction offered by the rock 

 strata, which have since been worn out in the new channel below 

 Cincinnati, combined to relieve the ice gorge there from the sup- 

 posed incredible hydraulic pressure which some have thought to 

 be fatal to the hypothesis. 



In conclusion, it may be said with a fair degree of confidence 

 that the theory of the Cincinnati ice dam still "holds water," 

 though the obstruction itself disappeared many thousand years 

 ago. One may readily admit that some things were at first at- 

 tributed to the dam which were the result of other causes. But 

 fresh considerations have given increased interest to the theory, 

 so that altogether it remains one of the most striking of all the 

 episodes connected with geologic history, and it is all the more 

 dramatic because of its probable connection with human history. 

 There is, therefore, ample justification for the language of Prof. 

 Claypole, in his paper upon the subject, read before the Geological 

 Society of Edinburgh in 1887, and printed in the Transactions of 

 that year. 



