LOCAL GEOLOGY OF PITTSBURGH 721 



Above the Nowata shale different formational limits are necessary for the 

 northern and southern parts of the region. 



Northern part : 



Feet. 



Lenapah limestone 20 



Curl formation, shales, sandstones, and thin beds of coal 250 



Wann formation, embracing: 



Hogshooter limestone member 10 



Copan member, shale, sandstone, and two limestone lentils 250 



Stanton limestone member 8 



Southern part: 



Skiatook shale 500 



Ramona formation, two limestones, and an intervening clastic bed. . 100 



The limestones usually abound in remains of marine organisms. There is a 

 dearth of fossils in the clastic beds. 



The thinning of limestones and shales, the thickening of sandstones and of 

 the entire series, to the southward, points to a former land area in that direc- 

 tion. 



SKETCH OF THE LOCAL GEOLOGY, CITY OF PITTSBURGH 

 BY PERCY E. RAYMOND 



(Abstract) 



The city is situated along deep trenches cut by the present rivers and in 

 abandoned valleys of pre-Glacial streams. Both were cut in a nearly level 

 plateau, whose surface is from 500 to 600 feet above the present water level. 

 Terraces and river gravels are well exhibited at many places along the aban- 

 doned valleys. 



The strata underlying the city are of middle and late Pennsylvanian age, 

 almost the whole of the Conemaugh and Monongahela series being well ex- 

 posed. The nearly continuous exposures along the rivers afford an excellent 

 opportunity for tracing the various beds, and many interesting changes in 

 sedimentation, including erosional unconformities, may be seen. 



Discussion 



Mr. E. W. Shaw : The high terraces and abandoned valleys of western 

 Pennsylvania are gravel-covered rock shelves lying 200 feet or so above present 

 stream channels. Their impressiveness is attested by the long list of names 

 of eminent men who have studied and described parts of them. The features 

 have been ascribed to submergence and marine erosion, to a large ice dam at 

 Cincinnati or Beaver, to normal stream-work, and to huge local dams of ice. 

 At the Boston meeting, last year, I presented a paper in which I attempted 

 to show that the high terraces and abandoned valleys, instead of being due to 

 any of the above causes, developed as a unit through the overloading of the 

 Allegheny in early Glacial time. T hope that within a few months my paper 

 on this subject will appear in the Journal of Geology. At present I can not 

 take time to present the evidence, but in brief the view is as follows: The 



