NON-MARINE PROGRESSIVE OVERLAP 633 



ghenies, the formation is 1,107 feet thick and consists at the base of 227 

 feet of Greenbrier limestone resting directly on the Pocono, followed by 

 800 feet of shales, mostly red. Southward, in Pendleton county, West 

 Virginia, the Greenbrier limestone at the base is 325 to 400 feet thick, 

 and is followed by gray and brown sandstones and shales and red shales 

 (Canaan shales), with a thickness of 1,250 feet. These shales, however, 

 are in all probability only partly non-marine. 



The Mauch Chunk thus seems to present two periods of non-marine 

 fan-building separated by a period of partial subsidence. Non-marine 

 (fluviatile) sedimentation appears to have been continuous in eastern 

 Pennsylvania throughout. In both periods the greatest accumulation of 

 non-marine sedimentation was in the east, and the members overlapped 

 westward and northward. A diagrammatic section will make this clear. 



Figure 15. — Relation of the Upper and the Lower Mauch Chunk and the Greenbrier. 



The upper Mauch Chunk fan represents the recovery of the land after 

 the Greenbrier subsidence. With this recovery corresponds the presence 

 of coarser sands in the upper Mauch Chunk in the eastern region, where 

 non-marine sedimentation was uninterrupted. That the land was low 

 and streams sluggish is indicated by the fact that the surfaces of the beds 

 are marked by ripple-marks, sun-cracks, rain-drop impressions, and foot- 

 prints of vertebrates — all signs of floodplain deposits. 



The fossils of the Greenbrier in southern Pennsylvania and Maryland 

 correspond to those of the Maxville of Ohio. The Maxville is separated 

 from the Logan by an interval of erosion, which may correspond to early 

 Mauch Chunk sedimentation in the east; for the beginning of a new fan 

 on an older one indicates either an increased supply of detritus or a period 

 of elevation. The fineness of the lower Mauch Chunk is, perhaps, more in 

 harmony with the theory of a renewed elevation of the region, which in 

 the western area permitted the post-Logan erosion. A part of the upper 

 non-marine Pocono probably also suffered erosion during this time. It 

 may perhaps be further surmised that the change indicated was not due 

 to an elevation in the east, but rather to an elevation of the western region, 

 which gave the surface of the Pocono fan so gentle a slope that the 

 streams no longer were able to carry to this western area the detritus 

 derived from the east ; and so they dropped it nearer the source, building 



