Water Supply and Irrigation Papers of the United States Geological Survey . s of a topography similar tothat of the adjoining crystalline area. The continuity of the belt is,however, broken by a series of folded Cambrian and Silurian quartzitesand limestones that extend from near Trenton westward across theSusquehanna to the southeast corner of Adams County. The cities ofYork and Lancaster are located in this belt, while Phcenixville, Norris-town, and Trenton lie just outside its borders. The rocks of the middle topographic belt, which is characterized byprominent ridges, consist mainly of str
Water Supply and Irrigation Papers of the United States Geological Survey . s of a topography similar tothat of the adjoining crystalline area. The continuity of the belt is,however, broken by a series of folded Cambrian and Silurian quartzitesand limestones that extend from near Trenton westward across theSusquehanna to the southeast corner of Adams County. The cities ofYork and Lancaster are located in this belt, while Phcenixville, Norris-town, and Trenton lie just outside its borders. The rocks of the middle topographic belt, which is characterized byprominent ridges, consist mainly of strongly folded beds of Cambrian,Silurian, and Devonian age, but the folded rocks of the anthracite coalbasin and of the Broadtop coal basin in the south-central portion of theState are of Carboniferous age. The rocks of the remaining or plateau portion of the State are De-vonian and Carboniferous, the former constituting the surface in thenorthern and northeastern portion of the State and the latter in itswestern part. The rocks of this belt are characterized in general by. Tr=Triassic C-=Carboniferous C5D = Cambrian, Silurian,and Devonian D = DevonianCr=Crystallino Fig. 19.—Geologic subdivisions of Pennsylvania. very moderate folding, although the rocks of Laurel Hill and otherridges in the southwestern portion of the State show high dips. Thefolds generally decrease uniformly in magnitude as their distance fromthe Appalachian Front increases, until in the northwestern part of theState only slight and irregular undulations can be detected. It is alongthe crest of the anticlines that the lower Devonian rocks are broughtto the surface in the northern part of the State. SOURCES OF WATER AND LAKES. By far the larger number of the streams of Pennsylvania rise eitherin the mountainous regions of the Appalachian belt or in the highlands of the Allegheny Plateau. In general they have their sourcesin rugged forest-covered areas remote from habitations, and for con-sidera
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