G. F. Wright — Continuity of the Glacial Period. 177 



sive a filling up of the trough as we find above the mouth of 

 the Big Beaver. 



The facts brought to light concerning the extensive filling up 

 of the trough of the Allegheny and upper Ohio Rivers must 

 modify to some extent previous theories concerning the still 

 water deposits in the Monongahela River. That the deposits 

 described by Professor White at Morgantown, W. Ya., took 

 place in still water, which was produced by some extraordinary 

 obstruction to the drainage after the main part of the rocky 

 erosion had taken place /s scarcely capable of being questioned 

 by one who has carefully examined them in detail. This Pro- 

 fessor Chamberlin and Mr. Gilbert were not able to do, since 

 their visit to the region was hasty, and they did not have the 

 privilege of the guidance of Professor White, who was absent 

 from home at the time. I have had the privilege of going 

 over much of the ground with Professor White, and need only 

 to reiterate and emphasize the most significant of the facts 

 which he has elsewhere so fully stated. There are at Morgan- 

 town both in the immediate vicinity of the Monongahela and 

 at the Flats (a mile or more back from the river, along one 

 of the small tributaries of the main stream) deposits of clay 

 seventy feet deep containing at all levels the impress of the 

 leaves of existing species of trees and fragments of wood in a 

 good state of preservation. I can see no way in which such 

 depths of clay and gravel could accumulate in the ordinary 

 process of the rock erosion of a stream. There must have been 

 some check to the processes of erosion during which this ex- 

 tensive silting took place. Professor White is inclined to be- 

 lieve that orographic changes may have played more of a part 

 in producing slack drainage in the streams on both sides of the 

 Allegheny Mountains than he at one time thought. The 

 slightly higher level (about 100) feet of these deposits in the 

 Upper Monongahela also compels us to suppose some slight 

 orographic changes of level if the Cincinnati ice dam alone is 

 depended upon for the obstruction. 



In the light of the present discussion it will be seen that a 

 supplementary cause of great significance may now be taken 

 into consideration. The action of the Allegheny River when 

 swollen with its Glacial floods and burdened with its vast loads 

 of Glacial debris would dam up the mouth of the Monongahela 

 by the same accumulations which filled its own channel. As we 

 have seen, these accumulations rose at Pitcsburg to a height of 

 about 300 feet above the present water level, or of 1,000 feet 

 above tide level. As the conditions were so much more favor- 

 able for the sedimentation of the Allegheny during the reign 

 of the Glacial period than for filling up the trough of the 

 Monongahela, such an obstruction was inevitable. Thus also 



Am. Jour. Sci.— Third Series, Vol. XLVII, No. 2*79.— March, 1894. 



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