At present, however, the waste mantle is not forming so rapidly as it is being removed.  The streams of the upland are actively engaged in its destruction.  They flow swiftly in narrow, rock-walled valleys over rocky beds.  This contrast between the young streams and the aged surface which they are now so vigorously dissecting can only be explained by the theory that the region once stood lower than at present and has recently been upraised.  If now we imagine the valleys refilled with the waste which the streams have swept away, and the upland lowered, we restore the Piedmont region to the condition in which it stood before its uplift and dissection,—­a gently rolling plain, surmounted here and there by isolated hills and ridges.

The surface of the ancient Piedmont plain, as it may be restored from the remnants of it found on the divides, is not in accordance with the structures of the country rocks.  Where these are exposed to view they are seen to be far from horizontal.  On the walls of river gorges they dip steeply and in various directions and the streams flow over their upturned edges.  As shown in Figure 67, the rocks of the Piedmont have been folded and broken and tilted.

It is not reasonable to believe that when the rocks of the Piedmont were thus folded and otherwise deformed the surface of the region was a plain.  The upturned layers have not always stopped abruptly at the even surface of the Piedmont plain which now cuts across them.  They are the bases of great folds and tilted blocks which must once have risen high in air.  The complex and disorderly structures of the Piedmont rocks are those seen in great mountain ranges, and there is every reason to believe that these rocks after their deformation rose to mountain height.

The ancient Piedmont plain cuts across these upturned rocks as independently of their structure as the even surface of the sawed stump of some great tree is independent of the direction of its fibers.  Hence the Piedmont plain as it was before its uplift was not a coastal plain formed of strata spread in horizontal sheets beneath the sea and then uplifted; nor was it a structural plain, due to the resistance to erosion of some hard, flat-lying layer of rock.  Even surfaces developed on rocks of discordant structure, such as the Piedmont shows, are produced by long denudation, and we may consider the Piedmont as a peneplain formed by the wearing down of mountain ranges, and recently uplifted.

The Laurentian peneplain.  This is the name given to a denuded surface on very ancient rocks which extends from the Arctic Ocean to the St. Lawrence River and Lake Superior, with small areas also in northern Wisconsin and New York.  Throughout this U-shaped area, which incloses Hudson Bay within its arms, the country rocks have the complicated and contorted structures which characterize mountain ranges.  But the surface of the area is by no means mountainous.