PRESENT ALTITUDE OF THE CRETACEOUS PENEPLAIN. 5G7 



Pennsylvauia stauds about 2,000 or 2,200 feet. In the geological reports 

 and other accounts of Pennsylvania, crest-line altitudes are seldom recorded; 

 but in a general way an elevation of 1,500 feet may be quoted for North 

 mountain, 2,000 to 2,200 for the inuer zigzag ridges, and 2,200 to 2,500 for 

 the plateau.^ The crest-lines of the Medina ridges of Blair county are given 

 in the elaborate contour survey of that district as about 2,100 to 2,200 feet.f 

 The sketch topographic map of McKean county X makes the plateau there 

 2,200 feet above tide. In Virginia and West Virginia, according to the topo- 

 graphic maps of the United States Geological Survey, the dissected plateau 

 possesses an average altitude of 2,500 to 3,000 feet, or even more. In Ten- 

 nessee, according to tlie same authority, it averages about 1,800 to 2,000 feet 

 on the Chattanooga sheet ; in northern Georgia about 2,000 feet, and in 

 northern Alabama about 1,200 to 1,500 feet, as shown on the Cullmann, 

 Fort Payne, Gadsden, Stevenson and Scotsboro sheets. 



It is clear from this rapid survey that the peneplain was distinctly tilted 

 and somewhat warped w^hen it was uplifted ; but the tilting weakened and 

 changed to a simple elevation in the plateau belt next west of the folded 

 Appalachians; and still further westward a lower upland surface prevails ; 

 but whether this is due to a weaker Tertiary elevation or to a more general 

 degradation of the softer rocks of the Ohio basin, I cannot now determine. 

 In Iowa and Minnesota, however, ^vhere remnants of a Cretaceous overflow, 

 advancing from the west, are discovered in thin Cretaceous deposits at an 

 altitude of about 1,000 feet, the elevation was certainly less than along 

 the Appalachian axis. 



When the altitude of the Cretaceous peneplain is carefully determined 

 over a large area, contour lines may be drawn upon it to indicate the form 

 that it had before the present valleys were cut out ; but it must be remem- 

 bered that this altitude includes not only the main uplift after the Cretaceous 

 denudation, but all the oscillations of later date. Judging by the valleys, 

 these later oscillations have not been very great, or if great, they have been 

 short lived. The chief one of them, so far as existing topography is con- 

 cerned, is a late Tertiary or early Quaternary elevation of moderate amount, 

 as a result of which the valley lowlands are trenched by young valleys. 

 This Avill be briefly considered under a later paragraph. 



Tertiary Excavations in the Cretaceous Peneplain. — It will have been 

 noticed that the search for the remnants of the Cretaceous peneplain has 

 been conducted entirely in the belts of hard rocks ; in the crystallines, the 

 Medina and Carboniferous ridges, and the sandstone plateaus. In a general 

 way, the reverse is true in the examination of the Tertiary valleys ; they are 



* These figures for Pennsylvania nre in large part taken from a collection of crest-line altitudes 

 made in a thesis by Mr. Collier Cobb, class of 1889, Harvard College. 

 tSee atlas to Report T, 1887, Second Geological Survey of Pennii. 

 t Atlas to Report R R. 1885. 



