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fraction of the stations where it has been recognized. One of the next most 
common species of the region is Stereodon imponens (Hedw.) Brid., to be found 
in woods everywhere, on humus and logs, or even on rocks. This is called “log 
moss” by our local florists and is collected in quantities for decorative purposes, 
particularly for Christmas work. 
II. Ecological Distribution of the Mosses of the Pittsburgh District 
Upon taking up the study of the distribution of plants in the Pittsburgh 
district it was soon found that the various ecological groups were located in 
habitats which could best be classified upon the basis of the physiography of the 
region. The Pittsburgh district, for the purposes of the following classification, 
has been limited to the hilly country lying within a radius of about thirty miles 
of Pittsburgh, Pa. The district is at the present time hilly, non-glaciated, partly 
forested, ranging in elevation from seven hundred to about thirteen hundred 
feet above the sea, and having in general a rather poor soil resulting from the 
disintegration and erosion of the Carboniferous shales and sandstones. There is 
a general lack of limestones or other rocks than the shales and sandstones men- 
tioned, and this has considerable to do with the determination of the species 
occurring in the district. 
The physiographic features of the Pittsburgh district are the result of a 
development which may be briefly summarized in the following manner: In 
Tertiary times the district was evidently a plain, but little elevated above the sea. 
Later this plain was elevated and the drainage systems began its erosion and dis- 
section. This dissection continued until, at the ’ eginning of the Glacial Period, 
the drainage system had evidently worked c a series of wide valleys about 
3 00- 35 ° feet below the horizon-line of the old plain. The drainage system, 
known as the “Old Monongahela System,” 1 flowed northward into the region 
now occupied by the basin of Lake Erie. 
During the Glacial Period the ice moved southward to a point within about 
35 miles of Pittsburgh, thus blockading and damming up the Old Monongahela. 
The impounded waters south of the ice finally escaped to the west over a low 
divide and, by subsequent erosion, the present Ohio River developed, diverging 
to the west and southwest from the course of the Old Monongahela at a point 
about 25 miles north of Pittsburgh. With the opening up of the Ohio River 
outlet, and possibly also a further elevation of the region, the river began again 
a period of erosion, cutting its channel considerably deeper. This new cycle of 
dissection has now resulted in a deepening of the channels of all of the larger 
streams to about 200 feet below the old preglacial channels. However, if one 
ascends the smaller streams and tributaries it is found almost invariably that 
the valley suddenly narrows into a ravine or gorge with swiftly-flowing and 
actively eroding waters, often forming rapids or falls, while above this region of 
active erosion the streams are still flowing in approximately the same valleys 
as in the time of the old Monongahela System. 
1 Leverett. Frank. “Glacial Formations and Drainage Features of the Erie and Ohio 
Basins.” U. S. Geol. Surv. Monograph 41 : 88: 100. 1902. 
