Page:Popular Science Monthly Volume 45.djvu/204

190 Ice period. But because of the imperfect knowledge of the glacial geology of the valley possessed at that time, these discoveries attracted little attention. Various causes, however, conspired to give a somewhat extraordinary notoriety to the facts as they were presented at the meeting of the American Association for the Advancement of Science at Minneapolis in 1883. At that time a systematic exploration of the glacial boundary had been conducted from the Atlantic Ocean to Cincinnati, showing that the Ohio River lay for the most part considerably south of the farthest extension of the ice. Also attention was then first called to the full extent to which the ice had crossed the river in that vicinity. For a distance of nearly one hundred miles it was now demonstrated that the ice came down to the north margin of the trough of the river, and for much of that distance crossed it and mounted the hills upon the opposite side, reaching at one point fully ten miles upon the high land beyond the river. This could not well help suggesting the formation of an ice dam at Cincinnati which would set the water back up the Ohio and its tributaries to the level of the watershed between the Licking and the Ohio, thus forming a narrow and tortuous lake several hundred miles long, which would be five hundred feet deep above Cincinnati and two hundred and fifty feet deep at Pittsburg. (See Map I.)

Finally, some of the geologists who had been engaged upon the survey of western Pennsylvania at once came forward and affirmed that such an obstruction as this supposed at Cincinnati helped to explain a great number of facts respecting certain highlevel gravel terraces characterizing the Alleghany and Monongahela Rivers, which were surprisingly near the level of the water of the supposed glacial lake. At the meeting at Minneapolis Prof. Lesley, under whose vigilant eye the recent geological survey of Pennsylvania has been conducted, declared that he had for some time been expecting the discovery of a local obstruction to the drainage of the Ohio River which would account for the gravel terraces on the Alleghany and Monongahela to which reference has been made, and now, says he, Providence has provided it, and Wright's dam clears up the whole problem, or words to that effect.

Such was the boom with which the theory of the Cincinnati ice dam was brought before the public in 1883. During the ten years which have since elapsed, the hypothesis has been subject to much criticism, so that the faith of some has been shaken, and the theory itself is thought by many to be left in rather a damaged condition. The fullness with which the main facts have been already presented makes it possible to tell the remaining part of the story and state the present condition of the theory in few words.