National Geographic : 1894 May 23
Areas unreduced. 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 of the 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-NAT. GEOG. MAG., VOL. VI, 1894.
1894 Jun 22
1894 Apr 25