Drainage Features of the Upper Ohio Basin. 271 



feet above the old floor at the mouth of the 

 Clarion. It is, therefore, the highest of the united 

 segments in the track of the Allegheny and most 

 nearly coincides with the present river bed, which 

 lies above the old floor in the upper section and , 

 below it in the lower sections, as we read the j 

 phenomena. On the middle Allegheny, or French 

 Creek section, the old floor descends from the col I 

 at Thompson's to the ox-bow at Franklin where \ 

 it stands at less than 1015 feet A. T. From this, £ 

 the floor of the old east Sandy creek rises rapidly 

 to the col between it and the west Sandy creek, 

 where it descends rapidly to the latter, and then 

 ascends it to the col near the mouth of the 

 Clarion, which has an estimated altitude 1300 

 feet A. T. From this there was a rapid descent 

 to the old well-developed floor at the mouth of 

 the Clarion, at about 1050 feet A. T. Thence 

 there is along very gradual descent to Pittsburgh, 

 and thence to the mouth of the Beaver, and 

 probably onward northwesterly to the old Erie . 

 basin. : 



The above represent our interpretations, based 

 upon present data, of the preglacial condition of 

 the track selected by the Allegheny under the 

 compulsion of the ice. It will doubtless require 

 some modifications in detail, at least. 



Into the small upper segments of the united 

 basins, the glacial waters, issuing from the edge 

 of the ice that here lay close to the track of the 

 Allegheny, poured great quantities of glacial wash 

 and filled them to irregular heights according to 

 the volume of the feeding and the capacity of 

 the valleys. The present stream, therefore, runs 

 relatively high on these accumulations, and has 

 not reached even the old floors in the axes of the 

 two uppermost sections, if we interpret aright. 

 But in the Lower Allegheny-Monongahela-Ohio 

 section, we understand it to lie far below the 

 old floor as has been urged elsewhere.* 



Fig. 4. Profile along the present Allegheny and Ohio rivers from Steamburg, 

 New York, to Toronto, Ohio, representing the upper limit of glacial gravel, the 

 gradation-planes, the cols. and the present stream bed, also the general level of 

 the uplands. The gradation-planes and cols are indicated by broken lines, the 

 other planes by continuous lines. Attention is called to the building up of the 

 glacial gravels to great heights at points where contributions were made from the 

 ice sheet, and to the regular slope in the long stretch between the mouth of the 

 Clarion and the mouth of the Beaver where no contributions of glacial gravel 

 were made by tributary streams. 



*H. M. Chance, Penn. Sec. Geol. Survey. Rept. VV, 1880, pp. 17-20. T. C. 

 Chamberlin, U. S. Geol. Survey, Bull. 58, 1890, pp. 24-37. 



