88 National Geographic Magazine. 
Kittatinny Valley lowland or peneplain on the wide belt of 
limestones beyond the Highlands; and furthermore for the 
development of a broad baselevelled plain on the weak Triassic 
shales and sandstones, where the old peneplain has been almost 
entirely destroyed. The Cretaceous cover remains only near the 
coast, where it stood too low to be attacked while the valleys and 
lowlands just described were carved out. An interesting pecu- 
liarity in the relation between the newer baselevel plain on the 
Triassic area and the old Cretaceous peneplain is that their sur- 
faces mutually intersect at a small angle along the line which 
now marks the visible contact between the Triassic and Creta- 
ceous formations: the newer plain standing beneath the eroded 
portion of the older one northwest of this line, while it rises 
above the buried part of the older one and obliquely truncates its 
Cretaceous cover to the southeast of the line. Finally, the land 
as a whole has been raised a little since the making of the-newer 
plain, and shallow valleys interrupt its broad surface. It is no 
longer a true plain; it has become a pastplain. A few words 
may be allowed me concerning these terms, peneplain and past- 
plain. Given sufficient time for the action of denuding forces on 
a mass of land standing fixed with reference to a constant base- 
level, and it must be worn down xo low and so smooth, that it 
would fully deserve the name of plain. But it is very unusual 
for a mass of land to maintain a fixed position as long as is here 
assumed. Many instances might be quoted of regions which 
have stood still so long that their surface is almost reduced to its 
ultimate form; but the truly ultimate stage is seldom reached. 
We can select regions in which the valley lowlands have become 
broad and flat, the intermediate “doab” hills have wasted away 
lower and lower until they are reduced to forms of insignificant 
relief ; and yet the surface still does not deserve the name of 
plain as unqualifiedly as do those young lands newly born from 
seas or lakes in which their geometrically level surfaces were 
formed. Ihave therefore elsewhere suggested* that an old region, 
nearly baselevelled, should be called an almost-plain ; that is, 
a peneplain. 
On the other hand, an old baselevelled region, either a pene- 
plain or a truly ultimate plain, will, when thrown by elevation 
into a new cycle of development, depart by greater and greater 
degrees from its simple featureless form, as young narrow valleys 
* Amer. Jour. Sci., xxxvii, 1889, 430. 
