85 Hayes and Camphell—Appalachian Geomorphology. 
piedmont plain, the Tertiary peneplain is well develoned and 
only occasional monadnocks show the position of the Cretaceous 
plain. Although erystalline rocks are generally regarded as 
offering great resistance to erosion, they are, under baseleveling 
conditions, subject to very deep decay and probably at the close 
of the Cretaceous cycle were softened to a far greater depth than 
at the present time. As the elevation succeeding the Cretaceous — 
period of baseleveling was not great, the streams quickly swept 
away this mantle of residual material down to baselevele Under 
such conditions the Tertiary peneplain was very perfectly devel- 
oped throughout the whole of the piedmont plain. The subse-- 
quent erosion of this peneplain has been comparatively slight 
and in many parts, especially in the vicinity of the James and 
Potomac rivers, it is almost perfectly preserved. 
Interior Valley Type—Asg stated above, this period was not suf- 
ficiently long for hard rocks to be reduced except under pecu- 
liarly favorable conditions. In the interior of the province only 
areas of limestone and shale were lowered to the newly estab- 
lished baselevel. These-rocks formed the surface chiefly in the 
zone of folded rocks known as the Appalachian valley. Upon 
the elevation of the region the streams sank their channels 
mainly within these belts of easily erodible rocks, although in 
some cases their wanderings during the preceding period of 
baseleveling had led them across hard rocks upon which they 
thus became superimposed. The greatly stimulated erosion 
rapidly reduced the soft rocks to baselevel in the immediate 
vicinity of the large streams; the valleys were broadened until 
checked by hard rocks which remained at the level of the, old 
peneplain, either as the valley ridges, the plateaus upon the 
west, or the present mountain valleys upon the east. This 
removal of the soft rocks progressed well toward the head 
branches of most of the rivers within the Appalachian yalley. 
In many cases the divides between adjacent river basins were 
almost perfectly baseleveled, though in some cases (explained in 
Part IL of this paper) the present divides were then crossed by 
large streams whose courses were subsequently changed. The 
Shenandoah valley may be taken as the type of this portion of 
the Tertiary peneplain. Its level floor, cut in the soft limestone 
and shale, is abruptly terminated on either side by steep slopes, 
composed of more resistant strata. The divide between the 
