GENERAL AND PECULIAR CHARACTERS OF THE AREAS. 



223 



point where it arrests the stream, and the basin is cut entirely in the 

 softer stratum overlying the sandstone. 



In the best developed baselevels in the region, the barrier is formed by 

 the gently upturned edge of the Pottsville series, and the basin above is 

 cut in the Mauch Chunk shales. Plate 20 is a photograph of the margin 

 of this plain on the headwaters of Meadow river, Greenbrier county, 

 West Virginia. Owing to the soft- 

 ness of the shales, the peneplain 

 is wonderfully developed along 

 the larger streams and even ex- 

 tends to the heads of all of the 

 small ravines which drain into 

 Meadow river. 



Figure 2.- 



-Ideal Sketch of the Margin of a local 

 Basin. 



PECULIAR CHARACTERS NOT EX- 

 PLAINED BY PRESENT THEORIES. 



Every basin in this region re- 

 ceives the overflow waters from 

 an area of greater or lesser extent 

 immediate^ surrounding it. In 

 many cases this drainage basin 

 may be of considerable magnitude, but in all cases it consists at least of 

 the immediate erosion slopes which descend toward the plain. No mat- 

 ter how short these slopes may be, they yield at every rainfall a certain 

 amount of material derived from the decay and disintegration of the 

 rocks forming the slopes, and this load of waste is transported from its 

 place of origin to the floor of the basin. 



In the light of previous knowledge respecting the operation of erosion 

 at baselevel, we should expect to find this waste deposited in an irregu- 

 lar band around the margin of the plain, as indicated in cross-section by 

 the dotted lines in figure 1. This deposit of waste would serve to unite 

 and blend the erosion slopes with the floor, giving to the basin a U-shaped 

 cross-section. As a matter of fact, however, the two surfaces are found 

 to be sharply differentiated in most cases, and therefore it seems prob- 

 able that the waste is removed by some process from the floor of the 

 basin as fast as it is supplied. 



Since these level floors are due to erosion and not to sedimentation, 

 and since the streams have too low a grade to transport mechanically 

 this material, its disappearance seems to imply that near baselevel there 

 may be processes of erosion at work of which at present we have but an 

 imperfect knowledge. 



