The salient facts in the history of the Appalachian peneplains are (1) irregular depression and sedimentation during all of Paleozoic time, the sediments reaching a thickness in the area of greatest depression of about 40,000 feet; (2) the folding of these sediments, as if by compression from the southeast, involving a shortening of many miles, possibly 100 or more, and the uplift of the folds (as restored) of 30,000 to 40,000 feet, as measured by the thickness of the beds above the crystalline rocks now exposed in the anticlines; (3) (after block faulting in Upper Triassic time) the erosion of the upfolded strata until, possibly, a peneplain was formed.
The surest way to determine the age of a peneplain is to trace it, if possible, into a plane of contact between sediments of known age. The lack of recognizable . . .