SUSQUEHANNA VALLEY 287 



shows a general correspondence in altitude, the hills rising to levels 

 which range from 500 to 600 feet above sealevel. 



The meanders of Swatara creek seem to afford evidence of a widely 

 developed peneplain in late geologic time, which extended not only over 

 the limestone belt on the southern side of the valley, but also over the 

 shale belt on its northwestern side. It does not seem possible for these 

 meanders to date back to the formation of the Cretaceous peneplain, for 

 in the immensely long time that has elapsed since the formation of that 

 peneplain the creek should have adjusted itself to the outcrop of the 

 hard and soft rocks, and should have found a resting place on the south 

 side of the valley. An inspection of the map shows that such is not the 

 case, and that the stream flows indifferently on shales and limestones. 

 The same argument may be used for Conodoguinet creek which has 

 developed such extensive meanders on the shale belt west of Harris- 

 burg. Meanders of this kind could have been developed only on a well 

 reduced peneplain, and if, as pointed out previously, they could not 

 have persisted since early Cretaceous time, they afford strong evidence 

 of the existence of an extensive and well developed plain since that date. 

 Hence the peneplain on which these meanders were formed is of more 

 recent date than the Schooley peneplain, and it is older and more ex- 

 tensive than the Somerville plain ; therefore it seems to agree perfectly 

 with the Harrisburg peneplain as here defined. 



The shale hills in the vicinity of Harrisburg, as shown on the atlas 

 sheet, show remarkable regularity at an altitude of about 500 feet. The 

 Somerville plain is not well developed in this region, but there are wide 

 stream valleys on the quadrangle at an altitude of about 400 feet, which 

 presumably were formed in this cycle. These agree very well with the 

 lowland about Reading, and presumably they are the southwestward 

 representatives of the same feature. It can not, however, be followed 

 continuously to the southwest, as the surface of the limestone belts soon 

 rises to practically the same level as the shale hills on the northwestern 

 side of the valley. 



On account of the lack of maps the Harrisburg peneplain can not at 

 present be traced to the Maryland line, but it shows at an altitude of 

 500 feet in the northwest corner of the Carlisle quadrangle, where it 

 appears to have been as well developed as in the type locality. It also 

 shows in the belt of shale hills lying west of Chambersburg, which have 

 remarkably level tops, at an altitude of a little over 700 feet. The Somer- 

 ville plain also seems to be developed in this region along Conococheague 

 creek at an altitude of 550 to 600 feet. 



From the above facts, it seems evident that the Somerville and Har- 

 risburg peneplains approach each other toward the southwest. Along 



