DELAWARE VALLEY 285 



the valley or in close proximity to the major drainage lines, and so have 

 been exposed more severely to erosion than the more protected localities, 

 and consequently are reduced below the general upland surface. From 

 these facts it seems probable that the general platform on which the 

 higher knobs stand and from which the projecting spurs were reduced 

 has an altitude of about 700 feet. 



The production of such a platform might easily be accounted for if 

 the geologic structure of the region were simple and the top of the plat- 

 form corresponded in position with a particularly hard bed of the series. 

 But, generally, the rocks are greatly crushed and crumpled and the 

 surface of the platform as described above cuts these beds at all angles 

 from 90 degrees to 0. Manifestly it is impossible to explain the exist- 

 ence of this surface by the effect of the underlying rocks on the erosion 

 of the region. 



Although the surface of the shale belt is far from flat, there is such a 

 general correspondence in altitude that it seems to the writer that it 

 must be regarded as the result of subaerial erosion when the land in 

 this locality stood nearly 700 feet lower than it does at the present time. 

 From the nature of the material on which this topographic feature is 

 developed, it is probable that a plain would not be formed except under 

 the most favorable conditions of long-continued and uninterrupted 

 erosion. The erosion period which resulted in the formation of this 

 feature was evidently a partial cycle only in which the harder rocks of 

 Blue mountain on the northwest and the Archean ridges on the south- 

 east were not materially aff'ected, but the softer rocks were well reduced, 

 resulting in a rolling or somewhat hilly surface on the outcrop of the 

 shales and probably a nearly perfect plain at about the same level on 

 the outcrop of the Cambro-Silurian limestone. Along the limestone 

 belt the plain has been dissected and generally removed in subsequent 

 periods of erosion, and during one of these short sub-cycles the Somer- 

 ville plain was produced at a considerably lower level. 



Lesley* recognized these two distinct surfaces, and it is interesting to 

 note that while regarding them as products of subaerial erosion, he 

 attributes the low altitude of the limestone area solely to the greater 

 activity of chemical erosion. He speaks of the features as follows : 



** The old idea that the land surfaces have been produced by sea waves has 

 been discarded ; and with regard to the two plains of Northampton county, com- 

 posing side by side the floor of the great Kittatinny valley, but the one 200 feet 

 higher than the other, it is evident that the ocean has had no hand in their for- 

 mation." 



*0p. cit..,p. 37. 

 XLI— Bull. Geol. Soc. Am., Vol. U. 1902 



