Page:Popular Science Monthly Volume 45.djvu/211

Rh 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 superabundant amount of earthy débris 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 describes 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 hundred 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 referred 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 supposed 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 attributed 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.