554 W. M. DAVIS — DATES OP TOPOGRAPHIC FORMS. 



All these elements of form have been baseleveled — the highlands, the 

 AVatchung ridges, the Palisades, and the uplands which lead us back to 

 Massachusetts. Not only so : it is manifest enough from the study of the 

 excellent topographic maps of New Jersey and of the atlas of the Metropoli- 

 tan district around New York city, published by Bien, that all these base- 

 leveled elements are part of one and the same baseleveled peneplain. The 

 upland surfjice determined by one element accords with that determined by 

 its neighbors.-'^ The peneplain is truly no longer either a level lowland, as 

 it was when finished, or a level upland, as it might have been if it had 

 been uplifted evenly after its denudation. It is now a warped surface, from 

 which we conclude that its uplift was somewhat uneven. The inequality of 

 elevation was slight ; it touches sea-level in Long Island sound and along a 

 line lying a little southeast of New York, Trenton, and Philadelphia. An 

 elevation of 1,000 feet is found in Massachusetts east of the Connecticut 

 valley; of 1,200 to 1,400 or 1,500 in the western or Berkshire plateau ; of 

 somewhat more than this where the plateau is cut by the Hudson gorge ; 

 of somewhat less in the northern highlands of New Jersey, and thence de- 

 creasing south westward into Pennsylvania. The warping that it has suffered 

 is gentle. It will be further considered in describing the valleys by which 

 the uplifted peneplain is now dissected and whose depth is determined by 

 its uplift. 



The geological Date of the Completion of the New Jersey Peneplain. — It is 

 an interesting matter to determine the date of the completion of the pene- 

 plain and the processes that may have been chiefly concerned in its produc- 

 tion. 



The date is well defined in New Jersey. If the gently inclined peneplain 

 is followed from the highlands toward the southeast by means'of its remnants 

 now preserved in the crest-lines of the ridges of the hard Triassic lavas, it is 

 found to descend below the oldest of the Cretaceous beds that form the foun- 

 dation of the deposits in the coastal plain. The contact of the two may be 

 seen in some of the clay pits at Amboy, where the lowest members of the 

 Cretaceous lie unconformably on the denuded Triassic strata. The surface 

 of the latter is of moderate relief; the general line of contact between the 

 two formations is comparatively straight as it crosses the state from Amboy 

 to Trenton. Manifestly, the Triassic beds must have been baseleveled be- 

 fore the submergence that deposited Cretaceous beds upon them. So far as 

 the weak Triassic formation is concerned, it is evident from this relation that 

 it had been baseleveled in Jurassic time, when the Atlantic shore-line stood 

 further east than the present line of contact between the Triassic and Creta- 

 ceous formations ; and, as a corollary to this, it may be confidently believed 



*See the Geographic Deveiopmpnt of Northern New Jersey, by W. M. Davis and J. W. Wood, Jr.: 

 Proe. Host. Soc. Nat. Hist., vol. XXIV, 1889, pp. 3G5-423. 



