DEVELOPMENT OF THE NEW JERSEY PENEPLAIN. OOO 



that the Jurassic strata absent from our Atlantic slope lie concealed under 

 the ocean waters in the deposits of our continental shelf. 



The tilted beds of the Triassic formation having been baseleveled, a 

 gentle submergence caused a transgression of the shore toward the crystalline 

 highlands. When these were reached the waters probably halted ; for, being 

 harder rocks, they could not have been reduced to baselevel so soon as were 

 the weaker shales and sandstones of the»Trias. Playing against the high- 

 land shore, with slight oscillations during Cretaceous time, the waste of the 

 crystallines formed the sands and pebble beds, the marls and greensands of 

 the Cretaceous series, the deposits of a shallow sea, not far from shore for 

 the most part. As the sea-floor w^as spread over with these varied strata, 

 the highlands were worn dow^n to a less and less relief, and when the whole 

 of Cretaceous time had elapsed the highlands must have reached the even 

 surface now so conspicuous in the uniform altitudes of the uplands. Erosion 

 of the surface may have continued into Tertiary time ; for we must not 

 imagine that changes of elevation took place at the even hours of our geo- 

 logical clock, as if they were railroad trains starting at even hours of the 

 day. But we shall see reason to regard Tertiary time, as a whole, as one of 

 erosion of the uplifted peneplain ; and, therefore, rather as a matter of con- 

 venience than with intention of defining geological dates precisely, I shall 

 call the peneplain, of which so much mention has been made, a Cretaceous 

 peneplain — the product of denudation in Jurassic and Cretaceous time, pre- 

 sumably with forms of bold relief in the former period, but of faint relief 

 near the end of the latter. The mass on v/hich the denuding forces acted 

 was the combined crystalline and Triassic area, deformed and uptilted by a 

 post-Triassic — that is, early Jurassic — disturbance. The disturbance was of 

 the peculiar overpushing kind that gave rise to the faulted monocline of the 

 Triassic belts, and that undoubtedly faulted and uplifted the crystallines 

 beneath and for some unknown distance on either side. The constructional 

 forms thus produced may have determined for a time the relief of the region ; 

 early Jurassic time may have witnessed a topography of the Sierra Nevada 

 pattern on a small scale. This was followed by erosion forms of greater 

 variety, more accented by peaks and valleys ; middle Jurassic time may 

 have been thus characterized. But all the constructional forms and all the 

 subsequent variety of relief were finally obliterated in the lowland of denuda- 

 tion of Cretaceous time. The topographic forms of to-day were nearly all 

 carved in the Cretaceous lowland after its Tertiary elevation. It is for this 

 reason that I regard the Cretaceous peneplain as of so great importance in 

 the study of our topography. 



I may here briefly consider a question that has sometimes been put to me 

 in regard to the necessity of supposing that the even crest-lines of the Tri- 

 assic lava sheets necessarily call for a denudation at a low'er stand of the 



