f 



Harsiiberger : Plant formations of Nockamixon Rocks 655 



geologic horizon in the lower part of the Pottstown shales. The 

 whole country hereabouts, elevated between 300 and 400 feet, is 

 a table-land covered with trap rubbish and goes by the name of 

 "the swamp" from the undrained condition of the soil, w^hich is 

 cold, wet, and unproductive. The w^iole series of New Red rocks 

 comprises at least fiv^e subdivisions, beginning at the top, viz., the 

 Pottstown; the Perkasie shales, 2000 feet thick; the Lansdale 

 shales, 4700 feet ; the Gwynedd shales, 3500 feet ; and the Norris- 

 town shales, 6100 feet in thickness. The Nockamixon Rocks con- 

 sist of shales of the second subdivision, viz., the Perkasie shales. 



+ 



These shales are comparatively hard, mostly green, dark red, or 

 gray in color. Fossil traces of plants and animals occurring in 

 these beds and mentioned by Professor Newberry, though few and 

 imperfect, would seem to show that the shales are Jurassic* The 

 land underlaid by the Perkasie shales is generally less fertile than 

 that of the neighboring red shales above (Pottstown) and below 

 (Lansdale). 



PLANT FORMATIONS OF THE CUFFS 



The vegetation of the cliffs may be considered conveniently as 

 it covers the talus slope, the precipitous rock faces, the sides of 

 the ravines, and the crest of the escarpment. As the contour lines 

 of the map will show, the upper part of the cliffs slopes away from 

 the edge o( the precipice until the crest is reached. The vegetation 

 of the talus is a part of that characteristic of the Piedmont Region 

 and it therefore belongs to the 



Deciduous Forest Formation 



w 



The constitution of this formation on the rocks depends upon 

 the historic development of the flora which in all probability occu- 

 pied the ground when the surrounding country was covered with 

 a forest similar to that which now exists farther north in the Cats- 

 kill and Adirondack mountains. So that the facies, as it exists 

 here along the Delaware River, is a fragment or remnant of a 



r 



more extensive forest which exists in a climate somewhat colder 

 than that of the Bucks County of to-day. The tree and associated 

 vegetation of the talus slopes is as follows (figures 2, 3, 4, and 5) : 



^ Lyman, Benjamin Smith. Report on the New Red of Bucks and Montgomery 

 counties. Summary description of the geology of Rennsylvania. Final report Geo- 

 logical Survey of Penna, 3 : 25S9-2638. 1S95. 



