Mat 22, 1908] 



SCIENCE 



829 



to be mentioned in this connection are briefly 

 as follows: The tops of the main hills consti- 

 tute remnants of an old Tertiary peneplain, 

 now forming a horizon line at about 1,200- 

 1,250 feet above the sea. This peneplain was 

 elevated and dissected by drainage systems 

 and, at the beginning of the Glacial Period, 

 an advanced stage in the cycle of dissection 

 had apparently been reached with wide valleys 

 and graded streams, the latter being about 

 300-350 feet below the level of the old pene- 

 plain, in the Pittsburg region. The drainage 

 system at that time consisted of the " Old 

 Monongahela System," the outlet to which 

 was towards the north to the present basin of 

 Lake Erie.^ With the blockading of this out- 

 let by ice during the Glacial Period an outlet 

 for this drainage system was effected westward 

 and the Ohio Eiver was formed, and, evi- 

 dently associated with certain elevation^ of 

 the land in this region, the drainage system 

 was rejuvenated. This new cycle of dissection 

 has now progressed in the region of Pittsburg 

 to a stage marked by well-developed flood 

 plains in the larger valleys at altitudes of 

 about 720-750 feet above the sea, and about 

 200 feet lower than the flood plains which 

 were built up during the Glacial Period, or 

 soon after, on the old pre-Glacial valley floor. 

 In the smaller tributaries, however, the 

 streams are still engaged in actively cutting 

 out a lower channel, the narrow recently- 

 formed channel, often a gorge, being usually 

 encountered in the upper third of the stream, 

 while above this point the little streams are 

 still flowing in approximately the same graded 

 channels occupied by them during the exist- 

 ence of the Old Monongahela System. 



The more important ecological habitats, as 

 determined by their physiographic origin, and 

 the corresponding ecological plant formations, 

 are briefly as follows : 



A. The tops and uppermost slopes of the 

 hills, altitude of 1,050 feet or more above the 

 sea, with a rather thin and infertile soil, are 

 typically characterized in the vicinity of Pitts- 

 burg by the Querciis alia forest formation. 



' Levcrett, Frank, " Glacial Formations and 

 Drainage Features of the Erie and Ohio Basins," 

 U. S. Geol. Survey, Monograph 41:88-100, 1902. 



As one stands upon one of the higher hills, 

 this forest, easily recognized by its facies, the 

 white oak, can be seen to comprise all that is 

 left of the natural climax forest around the 

 horizon. 



B. Old valleys at the headwaters of the 

 smaller streams of this vicinity, approxi- 

 mately remnants of the Old Monongahela 

 System, at altitudes of not much less than 

 1,000 feet A.T., usually higher; soil somewhat 

 deeper, more fertile, and more largely trans- 

 ported. This habitat is almost invariably 

 occupied, in undisturbed positions, by the 

 Fagus-Acer forest formation, the facies being 

 Fagus americana Sweet and Acer saccharum 

 Marsh., the beech consocies, however, being 

 more important and often occurring almost 

 pure in limited areas. 



C. The narrow ravine or gorge with more 

 or less vertical sides, as brought about recently 

 and being continually extended at the end by 

 the erosive activities of the rejuvenated 

 streams; dark, damp, cool and rocky; eleva- 

 tion about 950-975 feet A.T., usually some- 

 where in the upper third of the smaller 

 " runs." This habitat is occupied almost in- 

 variably by the Tsuga canadensis forest for- 

 mation, the facies being the hemlock, Tsuga 

 canadensis (L.) Carr. The transition from 

 this forest formation t-o the Fagus-Acer for- 

 mation is often as abrupt as is the change in 

 the character of the habitats, neither forma- 

 tion being able to successfully invade the other 

 without a corresponding modification of the 

 habitats through physiographic processes. 



D. The flood plains and terraces of the 

 main rivers and, to some extent, continuations 

 of these terraces into the lower parts of the 

 tributaries, of the Old Monongahela System. 

 This is the " 900-foot terrace " of which con- 

 siderable areas are still in existence in Pitts- 

 burg and vicinity. The soil is a deep, fertile, 

 gravelly alluvium, usually so situated as to 

 be well drained. This habitat is well char- 

 acterized by the Quercus velutina-Q. coccinea 

 forest formation, the facies being the black 

 and scarlet oaks, respectively, the black oak 

 consocies being the most important. This 

 formation is considerably more dense and 

 exhibits much more prominent layers than 



