TIMES OF FORMATION OF THE PENEPLAINS 579 



completed early in the Cretaceous period^ and lience it is sometimes known 

 as the Cretaceous haselevel/' 



Willis/^ in his monograph on the northern Appalachians, describes the 

 Kittatinny peneplain, but passes over its age with the statement on page 

 189 : "Geologists date tliis Kittatinny plain as of the so-called Cretaceous 

 period of the earth's history." In a later paper/^ however, he says : 



. . . "The surface of the crystalline rocks beueath these (Mesozoic) sedi- 

 ments ... is a plain sloping beneath the sediments toward the Atlantic, 

 rising from under the sediments westward toward the Appalachian Moun- 

 tains. ... By filling the valleys in such a manner as to connect all ridges 

 whose crests fall into the general slope, there is restored the plain, which was 

 eroded nearly to sealevel during the Cretaceous period. 



"That old plain, now elevated and dissected, has been traced over New Eng- 

 land, over the Middle States, and over the South Atlantic States. It coincides 

 with the summits of the highest ridges, which in Maryland are represented by 

 the Catoctin, the Blue Ridge, the Alleghany ridges, and the Cumberland plateau. 

 Only in North Carolina and the interior of New England are surviving moun- 

 tain summits of that date. 



"Recognition of the old Cretaceous plain, surviving in the ridge summits of 

 the present time, is the first step in reading the Cenozoic history of Appalachia." 



It will be noted that Willis, like others, speaks of the plain as passing 

 under Cretaceous deposits and yet as having been formed during Creta- 

 ceous time. 



The Appalachian folios of the Geologic Atlas of the United States treat 

 the subject of peneplains in various degrees of detail. For example, 

 Hayes' ^^ discussion of peneplains in the Columbia, Tennessee, folio seems 

 surprisingly brief. It is found under the heading topography, the chap- 

 ter on history treating only the sedimentary record. It is stated that 

 "the elevation of the region to its present altitude (after the formation 

 of the Highland Rim peneplain) was not continuous, but occurred at 

 several periods separated by intervals of repose." It is interesting to 

 note from this quotation that Hayes recognized more than one erosion 

 stage after the formation of the Highland Rim peneplain, this position 

 seeming to accord more closely with Keitlrs idea of multiple peneplains, 

 expressed in 1895, than with his own of two peneplains, expressed in the 

 same year. 



In most of his folios Keith speaks of peneplains which evidently are 

 not equivalent to the two described by Hayes and Campbell, or the three 



15 Bailey Willis: Nat. Geog. Soc. monographs, vol. I, ISIX;. itp. l(;j)-ii(>2. 



i« Bailey Willis: I'aleozoic Appalachia, or the history oL' INlarylaiul diirinj; Paleozoic 

 times. Geol. Survey Special Pub., vol. iv, i)t, 1, 1902, pp. 01 and 02. 



1^ C. W. Hayes and E. O. Ulrich : IJ. S. Geol. Survey Gool. Atlas, Columbia Folio, No. 

 95, 1903, p. 1. 



