1879.] "-*•" [Stevenson. 



" bottom," is sufficiently proved by tbe condition of the TJnio shells on the 

 several terraces. For those on the first are fresher than those on the sec- 

 ond, and those on the second are fresher than those on the third ; while 

 the species on all are apparently the same with those now existing in the 

 river. 



Since the deposits on the first, second, third and fifth terraces are dis- 

 tinctly of recent origin, there would seem to be good reason for supposing 

 that the valleys below the highest of the terraces are also of recent origin. 

 It has been suggested, however, that the terraces resulted only from re- 

 working of the sides of valleys which had been digged out previously. 

 But this suggestion seems to be hardly in accord with the facts. 



It must be remembered that the streams flow on a rock bottom, so that the 

 beds are now at the lowest point ever reached by them ; that there is no evi- 

 dence thus far, going to show that any gravel deposits ever existed along the 

 Monongahela, so as to make its valley like that of the Ohio at Wheeling or 

 Steubenville ; that the deposits on the terrace shelves are not remains of 

 valley gravels, but simply accumulations of material brought down by the 

 streams and distributed as the matter has been distributed on the present 

 "bottom." Rocky banks are shown at the mouths of tributary streams. 

 Had the valley been filled by gravels, it is incredible that even the most 

 comprehensive erosion could have failed to leave some trace of their exist- 

 ence. 



The structure of the valleys below the highest terrace is very different 

 from that observed above it ; for in the upper portion the sides are gently 

 sloping, whereas below the highest terrace they become steep almost at 

 once. Above the Hue of that terrace, the valleys of the smaller streams are 

 broad swales, with smooth sides, while below that line the streams usually 

 flow in gorges. The abruptness of this transition from gentle to abrupt 

 walls shows that erosion had been long at work on the upper part of the 

 valleys and that it began in the lower part at a comparatively recent 

 period; in other words, that the lower portion was eroded after the upper 

 portion had acquired its present form. 



The river terraces are relics of river beds, such are the present "bot- 

 tom ;" and the valleys below the line of the highest river terrace have been 

 eroded since the drainage system was re-established by withdrawal of the 

 submerging flood to below the line of the former stream beds. 



Conclusions. 



The general conclusions which seem to flow readily from the facts 

 recorded are : 



First. That the erosion, to which is due the general configuration of the 

 surface above the line of tlis highest river terrace, began even before the 

 elevation of the anticlinal axes and continued until the region was sub- 

 merged in post glacial time. 



Second. That the horizontal benches are due to the re-working of preex- 

 isting valleys, and that they mark stages of rest during the emergence of the 

 continent from the submersing flood. 



