196 THE POPULAR SCIENCE MONTHLY. 



As lias been said, the first announcement of the Cincinnati ice 

 dam was thought to give a natural and sufficient explanation for 

 certain high-level gravel terraces occurring in the upper Ohio Val- 

 ley. Subsequent investigations have brought to light other con- 

 siderations which must more or less modify the first conclusions. 

 It still remains true, however, that the ice dam accounts most 

 naturally for many of the slack- water deposits which occur in the 

 valley of the upper Ohio and its tributaries, while there are many 

 areas which are yet but inadequately explored, but which promise 

 important light upon the problem when the facts are all obtained. 

 At the same time it appears that some of the terraces in the Alle- 

 ghany and Monongahela Rivers are slightly higher than the ob- 

 struction at Cincinnati, compelling the advocates of the ice-dam 

 theory to suppose some very probable changes of level since the 

 deposition of the terraces which were at first supposed by Prof. 

 Lesley to be so completely explained by it. 



But more important is the bearing of recent discoveries upon 

 the extent to which glacial gravels accumulated in the gorge of 

 the upper Ohio and Alleghany Rivers, as shown in the section in 

 the lower right-hand corner of Map. I. All along the Alleghany 

 and Ohio Rivers there are remnants of gravel accumulations, 

 from fifty to sixty feet deep, resting upon rock shelves about 

 three hundred feet above the present rock bottom of the Ohio. 

 There is now little reason to doubt that during the Glacial period 

 the floods of water which poured into the Alleghany and the Ohio 

 from all their northern tributaries brought along silt, gravel, and 

 bowlders enough to fill up this rocky gorge with great rapidity, 

 down as far probably as Wheeling. As the Alleghany River 

 received glacial floods and glacial debris in great quantities, while 

 the Monongahela did not receive any, it will be seen that the 

 Monongahela must have been dammed by both the silt and the 

 water which came down the Alleghany. 



Instances in which the water of a tributary is dammed by that 

 of the main stream will occur to any one upon a little reflection. 

 "Whenever one large tributary perceptibly rises, it raises the 

 water level of the main stream as well above as below the junc- 

 tion, while a large rise in the main stream may temporarily re- 

 verse the current in a tributary. The Columbia River, for exam- 

 ple, in Oregon, is subject to very extensive floods at seasons of the 

 year when the Willamette is comparatively low. At such times a 

 current sets up stream past the city of Portland. I remember, 

 also, hearing, when a boy, the story of a June freshet on the 

 Poultney River, in Vermont, caused by a succession of thunder- 

 showers about its head waters. The rise in the lower part of the 

 stream amounted to thirty or forty feet. The thing which fixed 

 itself most deeply in my mind was that a milldam upon Hub- 



