THE CRETACEOUS DErRESSION. 419 



Subsequent Degradation. — At present, the Triassic formation presents no 

 trace of its ancient constructional topography ; that was completely obliterated 

 in the long period of erosion that followed the post-Triassic tilting. From 

 evidence found in New Jersey/'^ it may be said with a good degree of prob- 

 ability that the period of erosion ran through Jurassic and into Cretaceous 

 time, and that it endured long enough to reduce the broken constructional 

 surface of the Triassic area and of the adjacent crystallines, which shared 

 in the post-Triassic disturbance to a greater or less degree, to a surface of 

 moderate relief and low altitude ; that is, to a peneplain : a peneplain of 

 much later date than that on which the Triassic beds lie. The same pene- 

 plain may be traced far and wide along our Atlantic border, and for an un- 

 known distance inland. f In New Jersey and further south, the denudation 

 of the peneplain was succeeded by a time of moderate depression, when the 

 ocean advanced over the lowland ; the waste then received from the interior, 

 not at that time submerged, constitutes the Cretaceous strata of the Atlantic 

 slope. Whether these strata ever reached over southern New England has 

 not been determined, but their appearance in Long island makes such an 

 extension eminently possible ; and when the map of Connecticut is com- 

 pleted and the relation of topography to structure is well studied, it may be 

 possible to say something more on this point by means of the location of 

 the rivers, inferring an inland extension of the Cretaceous if the preglacial 

 valleys are generally discordant with the structure. 



The Cretaceous peneplain or lowland of denudation is no longer a low- 

 land ; it was elevated about the beginning of Tertiary time to a greater 

 altitude inland than near the coast, thus forming a gently sloping plateau: 

 *and since then the streams and the processes of subaerial decay have been 

 at w^ork dissecting it. On the crystalline areas they have made but little 

 advance, and here the valleys are still narrow ; but on the softer Triassic 

 rocks a broad lowland has again been opened out at a lower level. The 

 preservation of distinct traces of the old Cretaceous lowland, now a high- 

 land, on the hard crystalline rocks, while the weaker Triassic beds have 

 already been reduced to a second peneplain close to the newer and lower 

 base-level, is an interesting example of the strong difference in the rates of 

 topographic development of masses of different resistance. 



Topographic Expression of Structure. — The trap sheets of the Triassic for- 

 mation, being much harder than the adjacent sandstones and shales, have 

 resisted erosion more successfully; and the main trap sheet still retains a 

 good measure of the height to which the old Cretaceous peneplain was lifted. 



*The Geographic Development of Northern New Jersey; by W. M. Davis and J. W. Wood, Jr.: 

 Proc. Bost. Soc. Nut. Hist., vol. XXIV, 1889, p. 38.5. 



t W J McGee. Three Formations of the Middle Atlantic Slope: Amer. Journ. Sci., 3d series, vol. 

 XXXV, 1889, p. 35. 



B. Willis. Round about Ashevilie: Nat. Geogr. Mag., vol. I, 1889, pp. 299-300. 



W. M. Davis. Rivers and Valleys of Pennsylvania. Ibid., p. 15. 



