384 THE ICE AGE IN NORTH AMERICA. 



considerably more than a hundred feet above the top of the 

 trough at Cincinnati. 



Bringing to mind the other considerations, we think no 

 one can doubt that the trough of the Ohio was mainly 

 formed before the Glacial period. This we infer from the 

 enormous lapse of time during which the river had been at 

 work, viz., from the first elevation of the continent to the ear- 

 liest date assigned by any to a glacial period in North Amer- 

 ica. With this trough formed, the problem of accounting 

 for the terrace at Bellevue, below Pittsburg, containing gra- 

 nitic pebbles is a most difficult one, except upon the theory of 

 this ice-dam. The granitic pebbles mark it as connected 

 with the Glacial period, and its height (three hundred feet 

 above the river) renders it impossible of explanation on any 

 other theory than one which assumes an incredible lapse of 

 time since the deposit was made. The small extent to which 

 the material of this terrace has been oxidized and disinte- 

 grated, indicates no such enormous lapse of time ; while the 

 terraces in the Monongahela Kiver, which Professor White 

 describes as containing freshly preserved leaves at great 

 depths, and the terraces in the neighborhood of the Big 

 Sandy, containing pebbles peculiarly liable to disintegration, 

 confirm the inference of the comparatively recent origin of 

 a series of terraces in the upper Ohio, closely correspond- 

 ing to the level of the Cincinnati ice-dam. To these con- 

 siderations, also, are to be added those concerning Beech 

 Flats, between the southwestern angle of Paint Creek and 

 the head -waters of Brush Creek. Altogether this would 

 seem sufficient to give a high degree of probability to the 

 theory, and to justify the wide acceptance which has been 

 given to it by the eminent geologists most familiar with 

 the region. 



In presenting this brief summary of evidence bearing on 

 the existence of the Cincinnati ice-dam, however, I am far 

 from considering the discussion of the theory closed. It 

 remains for local observers, first, to comprehend the facts 

 already presented, and then to test the hypothesis in every 



