PRE-TRIASSIC PP:NErLAIN IN NEW ENGLAND. 549 



topographic value, minor oscillations must be takeu into account; but in the 

 inner country it is possible to classify our present topographic forms in the 

 three categories thus indicated : First, the uplands and the occasional hills 

 that rise above them, these being the product of long-continued and nearly 

 completed Jura-Cretaceous denudation; second, the valleys and valley low- 

 lands of Tertiary date; and, third, the trenches of post-Tertiary date. 



The resurrected pre-Triassic Peneplain in southern New Eiigland:^ — The 

 most ancient topographic form on the Atlantic slope of which a remnant is 

 known to me as a topographic element of the existing surface is an excep- 

 tion to the general statement just made. It is a small part of the old land 

 surface on which the Triassic monocline of New England was laid down in 

 Massachusetts and Connecticut. The unconformable overlap of the Triassic 

 on the ancient crystalline rocks shows that the latter were deeply eroded 

 from whatever constructional form they may have once possessed before the 

 former were deposited on them. The comparatively even line of the under 

 or western margin of the Triassic beds shows that the eroded surface on which 

 they rest is itself relatively even ; hence the pre-Triassic erosion must have 

 endured long enough to reduce the ancient rocks to a surface of moderate 

 relief — that is, to a peneplain. When the post-Triassic disturbance tilted 

 the sandstones into their faulted monocline, the underlying rocks must have 

 shared in the disturbance, and the average surface of the ancient peneplain 

 must have gained the same dip as that given to the monocline. All the part 

 of this old peneplain that was then tilted above the level of the present up- 

 lands has been worn away ; all that part which still lies beneath the present 

 baselevel is preserved, buried under the remaining Triassic beds ; but a belt 

 of the old surface between these two parts has been disclosed to our sight by 

 the removal of the weak Triassic beds from it, and is now visible on the west- 

 ern side of the Triassic belt, in the slope from the crystalline highland plateau 

 into the valley lowland. I do not wish to imply that this resurrected part 

 of the old peneplain has suffered no loss of material whatever, and that its 

 surface is precisely as it was before it was buried ; but it does seem probable 

 that the general eastern slope of the western highlands is not simply due to 

 a recent wasting of the crystalline mass, but is essentially an uncovered part 

 of the foundation on which the Triassic beds were laid down. As such, it is 

 a representative of the oldest land surface that I have been able to identify 

 on the Atlantic slope. It is probable that similarly uncovered pre-Triassic 

 surfaces exist in Pennsylvania and Virginia wherever the weak Triassic beds 

 lie on hard crystalline rocks. 



The uplifted Cretaceous Peneplain in southern New England. — On ascend, 

 ing the tilted and resurrected part of the pre-Triassic peneplain from the Con- 

 necticut valley lowland to its top, we find a plateau-like expanse stretching 



*See figure 2, p. 420, of this volume 

 LXXXI— Bull. Geoi,. Soc. Am., Vol. 2, 1890. 



