i 9 o THE POPULAR SCIENCE MONTHLY. 



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 Cincin- 

 nati which would set the water back up the Ohio and its tribu- 

 taries 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 high- 

 level gravel terraces characterizing the Alleghany and Mononga- 

 hela 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 sur- 

 vey 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 pro- 

 vided 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 dam- 

 aged 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. 



