GLACIAL DAMS, LAKES, AND WATERFALLS. 371 



the same absolute elevation above the sea-level with that of 

 the assumed top of the glacial dam at Cincinnati. At ray 

 request, Professor I. C. White prepared a paper to be read 

 at the meeting of the American Association for the Advance- 

 ment of Science at Minneapolis in the summer of 1883. In 

 this paper the facts concerning the terraces in the valley of 

 the upper Ohio were set forth, together with their bearing 

 upon the existence of the supposed glacial dam. The Min- 

 neapolis meeting was remarkable for the number of distin- 

 guished geologists present who had given special attention to 

 glacial phenomena. Among them was Professor Lesley, who 

 has been for so many years the organizing mind of the Penn- 

 sylvania Geological Survey. When the subject of the Cin- 

 cinnati ice-dam was brought up by the reading of Professor 

 White's paper. Professor Lesley at once gave his adhesion to 

 the theory, and frankly stated that what he had written some 

 years before in explanation of the terraces in western Penn- 

 sylvania was entirely superseded. He had then, in order to 

 account for the terraces, resorted to the hypothesis of a gen- 

 eral subsidence of the region to the extent of several hundred 

 feet. But later he had himself perceived the inadequacy of 

 such an hypothesis, since there were not, as upon this suppo- 

 sition there should be, corresponding terraces upon the east 

 side of the Alleghany Mountains. He had therefore ex- 

 pressed the belief that some local obstruction would be dis- 

 covered in the lower part of the Ohio which would account 

 for the facts. " And now," said he, " Providence has pro- 

 vided one, and Wright's dam will explain it all," and went 

 on to show that the absence of similar terraces on the east 

 side of the mountains was fatal to the theory of a continental 

 subsidence, while it was just what would be expected on the 

 theory of an obstruction of the drainage of the upper Ohio. 



A theory of such wide significance is not, however, to be 

 too hastily accepted, and much attention has since been di- 

 rected toward its verification or disproval. 



In presenting the evidence upon this subject it is well to 

 remark that the study of river terraces brings to light a com- 



