376 THE ICE AGE IN NORTH AMERICA. 



fifty and three hundred feet above it. The material of this 

 high-level terrace consists largely of gravel and pebbles de- 

 rived from the glaciated region, granitic pebbles being abun- 

 dant in it. It must therefore have been deposited since the 

 ice came over into the head-waters of the Alleghany. Two 

 theories are offered to account for this : One assumes the 

 reality of the ice-dam at Cincinnati, and sees in this terrace 

 a natural result of that obstruction. Bellevue lies in the 

 lower angle formed by the junction of the Ohio and Alle- 

 ghany Valleys ; and the summit of the terrace under consid- 

 eration corresponds closely in altitude with the elevation of 

 the Cincinnati dam ; and here, in the eddy below the mouth 

 of the Alleghany, at the beginning of the larger valley of 

 the Ohio, was the natural place for the accumulation of 

 bowlder-laden masses of ice brought down from the glacier's 

 front by the periodical floods of the Alleghany. 



If we leave the theory of general submergence out of 

 account, the only other way to explain this accumulation is 

 to regard it as a portion of a deserted river valley when the 

 stream occupied a rocky bed more than three hundred feet 

 above its present level. This would require a lapse of time, 

 since the deposit was made, sufficient to allow the Ohio and 

 all its tributaries to lower their rocky beds, for many hun- 

 dred miles, to a depth of more than three hundred feet. As 

 we shall see, later on, it seems to be entirely out of the ques- 

 tion to suppose any such lapse of time since the last glacial 

 period, for the Magara gorge has receded only seven miles 

 since then. But resort may be had to the supposition of a 

 previous glacial period, during which the ice also came over 

 into the head-waters of the Alleghany, and, indeed, every- 

 where came down nearly to the limits of the glaciated region 

 in Pennsylvania and Ohio already delineated. On this sup- 

 position the time intervening between the first and the last 

 glacial period would be equal to that required by the Ohio 

 and its tributaries to lower their rocky beds at least three 

 hundred or four hundred feet. This enormous lapse of time 

 would carry us back, in all probability, well-nigh to the be- 



