STJKFACE GEOLOGY. 447 



This I suppose to be the law of erosion in rocks of unequal hardness, and 

 has innumerable illustrations. 



Any elevation of the continent would not tend to change this law, 

 although such elevation is required for the explanation of the erosion of 

 channels and of bays now below the level of the sea. But inland, and in 

 regions so high that the streams could not be affected by any back or 

 dead-water, the drainage has always been the same as now. The erosion 

 of the pools in the softer rocks would, after a time, reach its maximum, 

 and the pools would gradually fill up with sand and mud, and then the 

 chief erosion would be on the hard strata of the intervening ledges, re- 

 ducing them slowly to the common level of the bed of the stream, to be 

 covered in time with alluvial materials. In our larger streams only a 

 few of the old rock barriers are now to be seen, but there are doubtless 

 large numbers which are covered with only a few feet of sand or mud. 



The work of erosion in southern Ohio has been going on ever since the 

 Carboniferous era, and it has been, consequently, very great. Standing 

 on the summit of one of our high hills, we may look for miles across in- 

 tervening valleys to some distant knob, and realize that by the slow pro- 

 cess of surface drainage the rock strata which once connected the two 

 points have been removed. The tops of anticlinals, such as the Newell's 

 Run uplift in Washington county, have also been removed by the same 

 slow agencies. A few miles south of the Ohio River, in West Virginia, 

 the continuation of the Newell's Run anticlinal ridge was once a narrow 

 mountain a thousand feet high above the present streams. It has been 

 eroded away, leaving hills no higher than the others in the neighborhood, 

 and these are intersected in all directions by valleys. 



In these ancient valleys of southern Ohio, and doubtless over the hills 

 as well, there was a growth of vegetation, and trunks and branches of 

 trees indicate a forest growth. These remains are found both in the 

 alluvial materials at very considerable depths, and also in the blue clays 

 of the Drift. It is, however, improbable that these valleys were ever 

 occupied by moving glaciers, for such glaciers would entirely sweep 

 away all the local vegetation. The short, sharp curves of many of these 

 valleys would apparently entirely prevent any glacial motion in such 

 deep and crooked river beds. In the subsidence by which the land was 

 lowered so that the waters could bring in and deposit as sediment the 

 blue clays, the overthrow and burial of the old forest trees of the valleys 

 took place. This was the first work of the Drift period, as recorded in 

 south-eastern Ohio. These waters were connected with a great northern 

 subsidence, and in the waters of this sea was floating northern ice, from 

 which bowlders were dropped into the same mud, which buried the old 



