PHYSIOGRAPHIC FEATURES 283 



late Tertiary or Somerville plain, and which presumably was produced 

 in the early stages of the Tertiary epoch. 



Somerville Peneplain 



According to Davis, this peneplain was developed only on the weakest 

 rocks of the region. It is best preserved in the area of Newark rocks 

 about Somerville, New Jersey, and consequently derives its name from 

 that point. Along this belt Davis recognized the peneplain across New 

 Jersey and into Pennsylvania. He also identified it in the Kittatinny 

 or Lebanon valley, where it is developed on the softer part of the Cambro- 

 Silurian limestone along the Delaware and Lehigh rivers. Its present 

 altitude in this locality ranges from 350 to 400 feet. The time during 

 which the crust of the earth remained stationary appears to have been 

 too short to permit of the reduction of the limestone in the interstream 

 areas, for in passing to the west of Allentown the low valley disappears, 

 and the land rises to an altitude of 500 feet in a distance of 15 miles. 



On the Susquehanna river this peneplain seems to have a wide de- 

 velopment on the limestone, in the vicinity of Lancaster at an altitude 

 of about 350 feet and below Harrisburg at an altitude of 400 feet. On 

 the Potomac river it is probably present in the vicinity of Harpers 

 Ferry at an altitude of 500 feet. 



Harrisburg Peneplain 



shenandoah valley 



Throughout the great valley which locally bears the names Shenan- 

 doah, Lebanon, and Kittatinny, there are two wide bands of outcrop- 

 ping rocks of different characteristics, and consequently possessing dif- 

 ferent degrees of resistance to erosion. The rocks on the southeastern 

 side of the valley are easily eroded limestones, and on these soft rocks 

 the Somerville plain, as previously described, is developed in the vicinity 

 of the larger streams. The rocks on the oj^posite side of the valley are 

 shales of the Hudson formation, and the}^ possess a moderate amount of 

 resistance to erosive agencies. As a result of their composition they are 

 not reduced to the level of the limestone part of the valle}^ but stand 

 distinctly above it as a sort of hilly upland. Such a relation of the sur- 

 faces found on rocks of different degrees of resistance is not surprising. 

 Indeed, it is perfectly normal, and should occur in any partial cycle of 

 erosion ; but when the hill country is examined more in detail, it is 

 found to be remarkabl}^ uniform in altitude, and it is this feature which 

 calls for a special explanation. The regularity of the hilltops along this 



