A phyto-geographic sketch of extreme southeastern Pennsylvania 
JoHN W. HARSHBERGER 
TOPOGRAPHY 
The area considered in this phyto-geographic survey com- 
prises the region of Pennsylvania drained by Wissahickon creek, 
lower Schuylkill river, Cobbs creek, Darby creek, Crum creek, 
Ridley creek and Chester creek, extending to the divide between 
Chester creek and the lower Brandywine. It is part of the well- 
characterized Piedmont plateau in the Atlantic drainage system, 
and comprises the district situated south and southeast of the 
range of hills formed by the Laurentian syenites. All of the 
streams mentioned, with the exception of the Wissahickon, the 
Brandywine, which heads in the low limestone of the Chester 
valley, and the Schuylkill, which rises in the mountains, take 
their rise to the east and southeast of the divide formed by the 
range of hills that owe their origin to the resistance of the Lauren- 
tian syenite rocks to erosion, from the earliest geologic time. The 
Wissahickon, the Schuylkill and the Brandywine flow from the 
low limestone Chester valley and through gneiss gorges on their 
way to the Delaware river. In the case of the Wissahickon, the 
act is startlingly bold, because accomplished by a small stream. 
It may be stated here briefly that when the gorges of the Wissa- 
hickon, the Schuylkill and the Brandywine were made, Chester val- 
ley was at a much higher level. Its soft limestone rocks were dis- 
solved faster than the gneiss, hence the width of the Chester valley 
and other limestone valleys compared with the narrow valleys cutin 
the harder gneissic rock. Cobbs, Darby, Crum, Ridley and Ches- 
ter creeks rise upon, or on the eastern slopes of the Laurentian 
tidge which extends in a general southwestern direction from be-- 
yond the Delaware at Trenton to West Chester, Chester county, 
Pennsylvania. This ridge, reaching a general elevation of 300- 
590 feet, marks the boundary of the limestone Chester and 
Whitemarsh valleys, crosses the Schuylkill below Norristown and 
widens in Chester county into a confused range of hills. The 
Streams, just mentioned, flow generally southeastward into the 
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