GLACIAL DAMS, LAKES AND WATERFALLS. 393 



stems and leaves were found. Its present distribution is 

 from New Brunswick to New Jersey, north of Lake Superior 

 and northward. The occurrence of this species in these beds 

 is, as Professor White remarks, of special interest, since it 

 practically demonstrates that there was during glacial times 

 a movement of water from the edge of the ice near Beaver, 

 Pennsylvania, southward along the Monongahela Valley, 

 through the escape weirs just described, which brought with 

 it this northern plant. 



Since as already shown the preglacial drainage of both the 

 upper and the middle portions of the Ohio basin was to the 

 north, it follows that vast sheets of water were ponded up in 

 front of the advancing ice-sheet from the moment that those 

 northern outlets were closed. One of the most prominent of 

 these would be that occupying the basin drained in preglacial 

 times by the Monongahela and Lower Allegheny rivers, which 

 originally flowed off through Beaver Creek and the Grand 

 River valleys into Lake Erie a little east of Ashtabula. This 

 temporary lake would have its level regulated by the height 

 of the col already referred to at New Martinsville, a little 

 below Wheeling, West Virginia. It was in this glacial lake 

 that the deep still water clay deposits described along the 

 Middle Monongahela accumulated. But after a compara- 

 tively short period the col at New Martinsville was worn down 

 so that a permanent line of drainage was opened in that direc- 

 tion and Lake White, as we may be permitted to name it, 

 no longer existed in the Monongahela Valley. 



But as the ice-sheet continued to advance through central 

 and southern Ohio the other lines of northerly drainage were 

 obstructed and other temporary lakes formed which continued 

 until the col, in the Muskingum above Zanesville, and that in 

 the Ohio below the mouth of the Scioto were eroded suffi- 

 ciently to permanently open present lines of drainage. Then, 

 finally, when the ice reached the junction of Mill Creek Valley 

 and the Great Miami at Hamilton, O., the whole drainage 



