470 Pärt IV. Chapter 2. 
3. River Bluff Formation. 
Wooded Rock Cliff Formation. This formation is typically developed ac- 
cording to the observations of JOHN W. HARSHBERGER') at the Delaware Nar- 
rows, Bucks County, Pa, where the Delaware River sweeps eastward, so that 
the palisades (Nockamixon Rocks) of red shale occur on the south shaded bank 
of the river and although the soil is extremely dry yet such slopes are always 
more cool than the surrounding country. These cliffs (300 feet high) are 
seamed by deep ravines down which small streams tumble in wet weather. 
A sharp talus slope occurs at the base of the precipitous cliffs and a number 
of rock shelves stand out from the face of the rocks. 
The vegetation of these eliffs is of a more northern character than the nern vegetation. 
The talus slopes are covered by such trees as Acer saccharum, Betula lent ‚ Fagus americana, 
Be to the Maple-Birch-Beech Facies of the FREIE Ti americana, Tsuga 
canadensis, Betula nigra, Acer rubrum, Quercus coccinea, Q. rubra, Ulmus americana, 
U. racemosa (= U. Thomasi), unse tulipifera, Populus iealaasitz: while the secondary 
trees are Rhododendron maximum, Hydrangea Kiitähdens: Acer pennsylvanicum, Hamamelis vir- 
giniana, Taxus ragen (in Ale masses of pure growth on the shingly shale), and the shrubs 
Physocarpus opulifolius, Rubus odoratus, Viburnum acerifolium, Staphylea trifolia, Sambucus pubens, 
SERSRTOR ae Bandes quinquefolia, Rhus radicans and dee herbs as Asarum, Mitella, 
Sanguinaria, Viola and Aquilegia canadensis, Smilacina, Ari ‚ ferns as Adiantum pedatum, 
Nepheodkan, Botrychium, and on the rocks en Sekt Polypodium and Cystopteris 
bulbifera. Quercus Prinus is the prevailing tree on the slo oping top of the cliffs above the 
precipitous portion in the dry soil, together with Juniperus virginiana (occasionally wind swept), 
Ostrya virginiana, Carya poreina [= Hiegeie glabra] and ovata, Rhus typhina, Morus, Sassafras ete. 
dripping water by ascending currents of air through the ravine clefts makes the Ba an 
exceptionally cool one, is found Sedum Rhodiola in pure association, and on other more 
sunny slopes Sedum telephium grows. Asplenium ebeneum, Polypodium vulgare and Woodsia 
ilvensis are rock ferns in these ravines together with such herbs as Arabis lyrata and Cerastium 
talus slopes, Here grow Fagus, Tsuga, Tilia americana, Betula lutea, Morus rubra, together with 
Rhododendron maximum (in thickets along the streams), Hydrangea arborescens and Taxus ca- 
nadensis. 
b) Southern Area. 
The differences in the character of the forests of the Piedmont Plateau 
in the southern states are the result of the influence of variations in the 
quality of the soils, and in the altitude above sea level. The lowlands of the 
Piedmont Plateau, instead of embracing broad swamps, are confined to narrow 
borders of sedimentary origin along the streams. The loamy alluvial lands 
contiguous to the smaller streams have in all parts of the plateau very nearly 
the same kind of growth viz., Fagus americana, Quercus rubra, Acer rubrum, 
ı) The Plant Formations of the Nockamixon, Rocks, Penna. Bulletin of the Torrey Botanical 
Club XXXVI, Dec. 1909 with map and 4 figures in the text. 
