284 CAMPBELL — XOBTH PEXXSYLVAXIA AXD SOUTH NEW YOKK 



belt is such that it was recognized by Lesley,^ who described it as fol- 

 lows : 



" It (the shale belt) is a region of low, flat-topped hills, trenched by a multitude 

 of small valleys, and, when looked down upon from the mountain, appears like 

 a great plain, which it really is." 



Since in a broad way the surface of the shale belt resembles a plain, 

 but in detail is irregular, it may be regarded as a peneplain, and for con- 

 venience in discussion the writer designates it the Harrisburg peneplain 

 from its development near the capital of the state. 



DELAWARE VALLEY 



The Harrisburg peneplain is well developed in the vicinity of the 

 Delaware river, and since this is one of the localities mentioned b)" Davis, 

 in which the Somerville plain is well shown, it will be described in con- 

 siderable detail. 



As previously mentioned, the limestone part of the valle}^ from Belve- 

 dere to AUentown is reduced to a fairly uniform surface, having an alti- 

 tude of from 350 to 400 feet. This valley is bounded on the southeast by 

 irregular ridges of Archean rocks, which do not appear to have been 

 reduced to any particular level. On the northwest the valley floor rep- 

 resenting the Somerville plain is bounded by a belt of hilly country 

 from 10 to 12 miles in breadth, developed on the somewhat resistant 

 shales of the Hudson epoch. Although the line separating these two 

 topographic features is not so distinct as that which bounds the lime- 

 stone valley on the southeast, still, in a general way, the shale hills stand 

 sharp and distinct above the surface of the Somerville plain. Fortu- 

 nately the Slatington quadrangle, which includes the western part of the 

 area under discussion, recently has been carefully mapped, and a study 

 of this atlas sheet shows clearly the character of the topography devel- 

 oped on the shale belt. The map shows almost no level land in this area, 

 but, what is more important, it shows great regularity in the heights of 

 the hilltops over most of the region. True there is a range in altitude 

 from 600 to 900 feet above sealevel, but the hills, which rise to 800 or 900 

 feet, are few in number and generally isolated, and it is apparent that 

 they rise above what may be called the general platform of the shale 

 belt. They are residuals composed of harder rock which have more 

 successfully withstood erosion than the surrounding region, or they 

 occur in sheltered localities away from the main drainage lines, and so 

 have been preserved from the general degradation of the region. The 

 tops, which stand at an altitude of 600 feet, are on points projecting into 



* General Introduction to Report D3, p. 27. 



