HARSHBERGER: PHYTO-GEOGRAPHIC SKETCH 133 
PLANT FORMATIONS AND ASSOCIATIONS 
Southeastern Pennsylvania is a region of hills, of valleys, of 
meadows and of rocky ravines sloping down in general from the 
Laurentide hills to the Delaware river. 
It represents an original table-land whose general elevation 
was about 500 feet above tide-level. Originally the surface, 
hills as well as river-plain above the fresh-water marshes which 
line the Delaware river, was covered by a dense forest of trees. 
Since the settlement of the country the most desirable land has 
been under cultivation, and many flourishing manufacturing towns, 
as Philadelphia, Chester, Conshohocken and Norristown, are found 
partly on the river-plain and partly situated on the rolling hills 
formed by the newer gneissic rocks. Culturally speaking, several 
kinds of land may be distinguished, such as river-land, marsh-land, 
city- and town-land, farm- and cultivated land, uncultivated wood- 
land, uncultivated barren land, and abandoned farm-land. 
Botanically considered, the following ecologic plant formations 
and associations may be distinguished, and these are determined 
approximately by the character of the areas above mentioned. 
A. UNCULTIVATED. 
AQUATIC-PLANT FORMATION.—The rivers and creeks of our 
region, especially in their lower courses, have smooth stretches of 
water in which grow a number of aquatic species. These species 
in the tidal estuaries are usually of the larger sort and are well 
adapted to grow in water where there is a change of level between 
low and high tide of about three feet. In many of the streams, 
the tidal flow is of considerable strength and the current estab- 
lished, therefore, influences the distribution of the vegetation to a 
marked extent. Where the flow is less strong and swift, there the 
Material in the form of mud and silt is deposited and upon this allu- 
vial material aquatic plants take root and gradually raise the level 
of these areas by catching and holding fresh deposits of silt. “The 
result is a tidal marsh intersected by numerous meandering chan- 
nels through which the tidal water ebbs and flows. 
In the Schuylkill river above the dam at Fairmount is such a 
deposit of silt. Vadlisneria spiralis L. forms a pure association of 
Such extent as seriously to interfere with the navigation of the 
