2i6 The A?nerican Geologist. j April, nm 
transportation is facilitated by the refinement of tiie surface soiT 
(luring its long exposure to the weather. Hence, under ordi- 
nary climatip conditions, normal peneplains mu;:t have deep 
local soils of fine texture at the surface, and grading into firm 
rock at a depth of 30, 50, or more feet. Moreover, it is onh- 
on a lowland surface of small slope that such a depth and ar- 
rangement of local soil can be normally produced. 
In contrast to the deep soil of a peneplain, the steep sides 
of young valleys, whose graded waste sheets are not yet de- 
veloped, must frequently reveal bare, rocky ledges. Only as 
the valleys widen and their side slopes become somewhat more 
gentle, will the ledges disappear; and even then the rock will 
be covered only by a relatively thin and coarse sheet of rapidly 
creeping waste. It therefore follows that the uplands of the 
Piedmont belt, with their deep soil, are of an essentially dif- 
ferent cycle of development from the narrow valleys, with their 
bare ledges. The two elements of form remain mutually in- 
consistent, until reconciled by the postulate of an iuplift of 
the region between their developments. But if this p(estulatc 
is accepted, the plain is shown to have been a lowland of faint 
relief before the existing narrow valleys were cut in it. It 
is this double line of argument, based on deep soil and bare 
ledge, as well as undulating plain and narrow valley, that har 
convinced various observers of the verity of the peneplain 
in the Piedmont belt. 
The Great Plains of eastern Montana include an area of 
nearly horizontal Cretaceous strata on either side of the Mis- 
y souri river, regarding which the evidence of peneplanation 
seems to me beyond dispute. Here and there, volcanic buttes, 
dikes, and mesas surmount the plain by several hundred feet: 
on the south, the Highwood mountains, a network of dikes 
among nearly horizontal shales and sandstones, rise in still 
stronger relief. Hence there can be no question that strata, 
measuring hundreds if not thousands of feet in thickness, have 
been broadly removed from the region by denudation. Yet 
the surface between the various eminences that rise above it is 
a true geographical plain. It is not absolutely level, but 
broadly undulating, with a sky line almost as even as that of 
the ocean itself. In this plain, the Missouri river and its chief 
branches have cut narrow, steep-sided valleys, several hun- 
