Steveu dlU ng . i,-,, 



ghany and Monongahela could reach the liuah of corrasion al the same al- 

 titude above tide, and that, after that limit bad been reached, the tribu- 

 taries of i luxe streams could continue to eal away the surface until at length 

 a broad and almost horizontal bottom might be formed on each Btream, all 

 having the same elevation. But such a condition is inherently improbable. 

 Aside from that, it would not suffice to explain the phenomena, lor the 

 benches would not be horizontal, they would be river terrace-, with a gen- 

 tle slope down stream. 



If the plains of south-cast Pennsylvania and the adjoining region have 



resulted from any such process as that described by .Major Powell they 

 must be very ancient ; for although the rainfall has always been very great in 

 this region, yet that very excess of rainfall has been the means of prevent- 

 ing too rapid erosion by encouraging the growth of vegetation. In like 

 manner, if the plains had originated as suggested, they would necessarily 

 ante-date the tremendous erosion by which the valleys below have been 

 scooped out ; for often they are shown only by the leveled tops of slender 

 hills ; they must be fragments of once continuous plains, which have been 

 broken up by the erosion producing the valleys as they now exist. It is 

 needful then to thrust back their origin to a time when the drainage was in 

 its infancy. 



In this case the origin might seem to be readily explained. It may be 

 said that when the drainage was in its infancy the fall of the streams was 

 insignificant; that if the drainage existed before tin- mountains rose, ami 

 the corrasive power of the streams was equal to or somewhat greater than 

 the rate at which the axes were elevated, there would be no difficulty in 

 understanding the formation of the benches ; for since the flow was gentle 

 the limit of corrasion would soon be reached and erosion would soon pro- 

 duce the benches, so that at each elevation of the land a new bench would 

 be formed. 



But all this appears to be in discord with the facts. The benches could 

 not have been formed during the elevation of the axes, for the rocks were 

 not lifted vertically ; the elevation was effected by a lateral thrust which 

 wrinkled the rocks. It would have wrinkled in like manner, any stratum 

 resting upon them, and the horizontally of the benches would have been 

 destroyed. 



The phenomena, too, are too recent ; the deposits are too well preserved. 

 If horizontal plains, such as these, had existed so long ago, those plains 

 would not have been in existence to-day. It is incredible that in a region 

 with great rainfall, perfectly horizontal plains should remain unbroken, 

 while valleys, hundreds of feet deep, were being digged out below them. 

 As already stated, the deposits on these benches consist only of loose sand 

 with occasionally a little clay. If the benches were of ancient origin this 

 covering would be absent ; the flat crowns of the hills would be rounded, 

 especially where the rock was a sofl shale ason Chestnut Ridge and in the 

 Ligonier Valley, where, however, the horizontally is complete and the 

 benches particularly well preserved. 



