292 CAMPBELL — NORTH PEXXSYLVAXIA AND SOUTH NEW YORK 



the State, is by comparison an insignificant feature and represents a 

 very short and partial cycle of erosion. 



OHIO VALLEY 



Throughout southeastern Pennsylvania the conditions are favorable 

 for the extensive development of the Harrisburg peneplain, and at least 

 along the main drainage lines it can be identified -^-ith a fair degree of 

 certainty as far to the northwest as the Allegheny front. AVest of this 

 line the rocks are more resistant, and it can be traced with difiiculty. 

 Moreover the Alleghen}^ front, in a general way, constitutes the divide 

 between the streams flowing directly into the Atlantic and those flow- 

 ing into the Mississippi valley, and it is doubtful whether this divide was 

 reduced during the Harrisburg cj^cle. The Susquehanna and Delaware 

 rivers are exceptions, but the gorges which they have cut in the eastern 

 part of the plateau are through hard and fairly homogeneous rocks, and 

 it is probable that the Tertiary peneplain was not developed in this 

 region. The absence of accurate topographic maps has heretofore made 

 it impossible to correlate the Tertiary features of the eastern part of the 

 state with those west of the Allegheny front, but within the last three 

 years a large area has been mapped in the western part of the state and 

 the topography has been studied carefulh\ so that now it seems possible 

 to interpret the surface features of that part of the state as accurately as 

 has been done with those east of the Allegheny front. 



The feature equivalent to the Harrisburg peneplain is probably best 

 shown in the Monongahela valley.^ From Pittsburg to the West 

 Virginia line the rocks are composed largely of limestones and calcareous 

 shales which were fairly well reduced during the early Tertiary period 

 of erosion. This is shown in the Brownsville, Masontown, Connells- 

 ville, and Uniontown atlas sheets. In this region the surface was not a 

 perfect plain, but the inequalities were slight and the view today from 

 one of the ridges rising to the altitude of this old surface is that of a 

 very gently undulating plain which in the distance produces an almost 

 even and horizontal skyline. The altitude of the surface is now about 

 1,250 feet. The peneplain seems to have been produced only where 

 erosion conditions were most favorable and the strata were soft and 

 easil}^ eroded. 



The date of origin of this feature can not be determined definitely, but 

 it is well developed up to the foot of Chestnut ridge, which is the western- 

 most outlier of the mountainous region of this part of the state. Chest- 



*See description in the Masontown-Uniontown folio, No. 82, of the Geologic Ath\s of the United 

 States ; also in the BrownsTille-Connellsville folio, now in press. 



