370 THE ICE AGE IN NORTH AMERICA. 



above the sea-level. A dam of 568 feet would raise the 

 water in its rear to a height of 1,000 feet above the tide. 

 This would produce a long, narrow lake, of the width of the 

 eroded trough of the Ohio, submerge the site of Pittsburg 

 to a depth of 300 feet, and make slack water up the Monon- 

 gahela nearly to Grafton, W, Va., and up the Alleghany as 

 far as Oil City. All the tributaries of the Ohio would like- 

 wise be filled to this level with the back-water. The length 

 of this slack-water lake in the main valley, to its termination 

 up either the Alleghany or the Monongahela, was not far 

 from one thousand miles. The conditions were also peculiar 

 in this, that all the northern tributaries head within the south- 

 ern margin of the ice-front, which lay at varying distances to 

 the north. Down these northern tributaries there must have 

 poured during the summer months immense torrents of water 

 to strand bowlder-laden icebergs on the summits of such high 

 hills as were lower than the level of the dam. 



The facts leading to this conclusion, together with the 

 theory itself, were first published by me in the " American 

 Journal of Science" for July, 1882.* At the conclusion I 

 added that " it remains to be seen how much light this may 

 shed upon the terraces which mark the Ohio and its tribu- 

 taries in western Pennsylvania." Soon after this I received 

 from Professor I. C. White, of Morgantown, W. Va. (whose 

 long experience and careful work upon the Pennsylvania 

 Geological Survey has made his name a synonym for accu- 

 racy of observation and skill in drawing conclusions), a letter 

 .stating that the theory of an ancient ice-dam at Cincinnati 

 was the key to unlock what had heretofore been a great puz- 

 zle to the Pennsylvania geologists. Briefly told, the progress 

 of the discovery and discussions concerning it are as follows : 



On all the upper tributaries of the Ohio there are high- 

 level terraces in abundance which the local geologists could 

 with difficulty explain. Upon comparison, however, an im- 

 portant portion of the series was found to have very nearly 



* " American Journal of Science," vol. cxxvi, pp. 1-14. 



