SOME SUBORDINATE RIDGES OF PENNSYLVANIA 123 



Again, in another article, he' says: "The weaker Siluro- 

 Devonian beds are generally reduced to a lowland farming country, 

 except where the Oriskany sandstone or a Chemung conglomerate 

 of more resistance than the adjacent beds rises in ridges of moderate 

 height. The hard Medina and Carboniferous sandstones hold their 

 crests close to the Cretaceous peneplain." 



Professor Davis assumes that the original height of the Oriskany 

 and Chemung ridges equaled that of the Cretaceous peneplain 

 surface but offers no confirmatory data. Apparently he does not 

 distinguish clearly between the heights of the Oswego and Tus- 

 carora ridges, but he may have this discordance in mind when 

 stating^ that the Medina crests accord with "geographic "rather 

 than "geometric" exactness. 



Years later Stose,^ in comparing the high, even crest of Cross 

 Mountain with the low, "comby" top of Cove Mountain in the 

 Mercersburg-Chambersburg region in southern Pennsylvania, says: 

 "It is apparent that Cove Mountain once stood at approximately 

 this altitude, but has been lowered by the active erosion of the 

 relatively narrow exposure of upturned rocks." Cross Mountain 

 is supposed to be part of the old peneplain surface. Farther on in 

 the same paper he says: "These two level tracts [referring to 

 areas on South Mountain] are undoubtedly remnants of the old 

 peneplain, preserved at the height of 2,000 feet. The monoclinal 

 ridges along the front of the mountain, which once stood at this 

 same altitude, have been reduced by erosion to 1,700 and 1,900 

 feet.""* In these two quotations the height of a lower ridge is 

 explained by reduction from the original peneplain. 



Reverting to the Nittany valley and the Seven Mountains 

 district, the following are altitudes^ (obtained by barometer) on 



'W. M. Davis, "The Geological Dates of Origin of Certain Topographic Forms 

 on the x\tlantic Slope of the United States," Geol. Soc. Am., Bull., II (July 2, 1891), 572. 



^ Ibid., p. 560. 



3 G. W. Stose, "Physiographic Studies in Southern Pennsylvania," .Tour. Geol., 

 XII (1904), 476. 



'• Ibid., p. 478. 



s Charles E. Billin, "Map of Adjoining Portions of Huntingdon, Mifflin, Centre, 

 and Union Counties," Sec. Geol. Surv. Pa., Kept, of Progress S, 1878. 



