576 E. W. SHAAV AGES OF APPALACHIAX PENEPLAINS 



the peneplain with an nnconformity. The second makes use of deposits 

 of known age by correlating coarse deposits believed to have come from 

 the peneplained area in the initial stages of erosion and fine deposits and 

 limestones with final stages. The third takes account of general prin- 

 ciples as to rate of erosion and involves projecting the present rate back 

 and comparing it with degree of dissection of the peneplain. 



Deductions more or less obviously based on the first method of attack 

 appear in many places in the literature of the Appalachian jorovince, but 

 seem to need critical examination because of an apparent lack of harmony 

 and exactness. The Cretaceous penej^lain, for example, is in many places 

 spoken of as passing beneath Cretaceous sediments, although other state- 

 ments imply that its development continued during part or all of the long 

 Cretaceous period. 



The second kind of evidence seems to have been first used extensively 

 by Hayes and Campl^ell,^ who correlate the finishing of the Cretaceous 

 peneplain with the Selma chalk and that of the Tertiary peneplain with 

 Oligocene limestone. A danger in using this method is that a peneplain 

 may be correlated with the wrong limestone. 



The third line of evidence, involving deductions based on the rate and 

 amount of erosion, seems to have been little used, probably because gen- 

 erally regarded as extremely uncertain. 



Times op Foematiox op Appalachian Peneplains 



The quotations given below have been selected for the pur^^ose of set- 

 ting forth current views as to the age of the Appalachian peneplains, and 

 also the basis of these views so far as can be indicated by short quotations. 

 It will be argued later that the evidence is not sufficient to warrant the 

 conclusions that have been drawTi, and that they harmonize quite as well, 

 if not better, with a different set of conclusions. 



Apparently the earliest discussion of the ages of the Appalachian pene- 

 plains is the classic paper by Davis^ on the rivers and valleys of Pennsyl- 

 vania, in which he speaks of "Jura-Cretaceous denudation" which "ended 

 in the production of a general lowland'' ; . . . and of "Tertiary base- 

 level lowlands,'' thus giving the impression that both the Jurassic and 

 Cretaceous periods were occupied in the production of an older peneplain 

 and the Tertiary period in the development of a younger. In a later 



- C. W. Hayes and M. R. CampbeU : Geomorpliology of the southern Appalachians. 

 Nat. Geog. Mag., vol. 6, 1894, pp. 63-126, pis. 4-6. 



' W. M. Davis: The rivers and valleys of Pennsylvania. Nat. Geog. Mag., vol. 1, 1889, 

 pp. 197 and 199. 



