164 THE ICE AGE IN NORTH AMERICA, 



higher one a mile, or so, above the mouth of the Conewango 

 in the narrower valley of the Allegheny. 



A still more interesting and significant dump lies about 

 one-half mile west from Clarendon where there is an extensive 

 gravel deposit running parallel to the valley and resting on 

 the older local slopes. The surface of this deposit is 160 feet 

 above the present level of the valley. Its depth, however, as 

 shown by numerous driven wells is 308 feet, of which the upper 

 sixty feet is gravel containing a noticeable amount of granitic 

 material, underlaid by 148 feet of sand with a small amount 

 of gravel, and this in turn by 100 feet of clean clay. In this 

 clay the drill, in some cases, went through a stratum of logs. 



Anticipating, somewhat, the more comprehensive discus- 

 sions of the chapters on Preglacial and Glacial Drainage, the 

 order of events as indicated by the facts in this interesting 

 locality will be found to be as follows: 



In early and middle Tertiary times the drainage of the 

 Allegheny and upper Ohio basins was northward while the 

 land levels were lower in the north than they were in the south . 

 During the later part of the Tertiary period an elevation of 

 land went on over all the region afterwards covered with 

 glacial ice, being greater in the north than in the south. This 

 differential elevation perhaps had much to do with the reversal 

 of the drainage lines of which we have such abundant evidence. 

 But the advance of the glacial ice front against the northerly 

 lines of drainage completed the reversal, so that unprecedented 

 floods of water poured over the cols between the reversed 

 channels, wearing them down with great rapidity as is shown 

 by the narrowness of the troughs and steepness of the sides 

 of the gorges through various sections of the present drainage 

 lines. The position of the highest col was probably a consider- 

 able distance below the upper end of the Ohio River not 

 unlikely a short distance below Wheeling, W. Va., as suggested 

 by Chamberlin. 



In this view, the rock erosion of the Allegheny and Ohio 



