Areas unreduced. 87 
Shenandoah and James is but little higher or narrower than the 
valleys themselves. The same’ is true of the divides between 
the James and Roanoke and the Roanoke and New rivers, and 
their valleys are almost as perfectly baseleveled as that of the 
Shenandoah. In the southern portion of the Appalachian val- 
ley the great Cambro-Silurian limestone becomes very silicious 
and its surface was less perfectly reduced than in Virginia. 
Many rounded ridges of residual chert reach slightly above the 
level ofthe Tertiary peneplain, even in the vicinity of the larger 
streams. _The amount of the erosion, however, was even greater 
than on the Shenandoah and James, for the valley in eastern Ten- 
nessee and northwestern Georgia is considerably wider than in 
northern Virginia. In the New-Kanawha basin the Tertiary pene- 
plain was extensively developed ; conditions of erosion appear to 
have been exceptionally favorable, for not only limestones but 
considerable areas of sandstone and shales were very completely 
reduced. Owing to subsequent elevation this Tertiary plain now 
forms a plateau 2,500 feet above sealevel and the present streams 
have cut their channels 1,500 feet or more below its surface. The 
altitude of the peneplain decreases rapidly westward and in the 
valley of the Ohio corresponds with the highest bluffs, below 
which the river has sunk its bed from 400 to 700 feet. 
Plate 6 shows the portions of the surface not reduced to the 
Tertiary baselevel, and from it more easily than from descrip- 
tions may be obtained a general idea of the physiography of the 
Tertiary peneplain at the end of this baseleveling process. These 
areas are seen to be very extensive on both sides of the Appa- 
lachian valley, while only the narrow ridges remain within the 
latter. The area unreduced to baselevel during this period is in 
round numbers 45,000 square miles, and the ratio of this area 
to that of the entire province then above sealevel is 1:4.7.. Dur- 
ing the Cretaceous baseleveling, on the other hand, the unreduced 
portion is only 8,700 square miles and its ratio to the then exist- 
ing province 1: 22. 
A comparison of these ratios affords some idea of the relative 
duration of the two periods. The reduction of a surface to base- 
level, however, does not vary directly as the time, but rather as 
some highly complex function of the time, being a process which 
decreases in its rate as it approaches completion. Hence the 
comparative duration of the two periods cannot be determined 
without considering other factors whose values are at present 
13—Nart. Groa. Maa., von. VI, 1894. 
