The sky line when viewed from the divides is unbroken by mountain peaks or rugged hills.  The surface of the arm west of Hudson Bay is gently undulating and that of the eastern arm has been roughened to low-rolling hills and dissected in places by such deep river gorges as those of the Ottawa and Saguenay.  This immense area may be regarded as an ancient peneplain truncating the bases of long-vanished mountains and dissected after elevation.

In the examples cited the uplift has been a broad one and to comparatively little height.  Where peneplains have been uplifted to great height and have since been well dissected, and where they have been upfolded and broken and uptilted, their recognition becomes more difficult.  Yet recent observers have found evidences of ancient lowland surfaces of erosion on the summits of the Allegheny ridges, the Cascade Mountains (Fig. 69), and the western slope of the Sierra Nevadas.

The southern Appalachian region.  We have here an example of an area the latter part of whose geological history may be deciphered by means of its land forms.  The generalized section of Figure 70, which passes from west to east across a portion of the region in eastern Tennessee, shows on the west a part of the broad Cumberland plateau.  On the east is a roughened upland platform, from which rise in the distance the peaks of the Great Smoky Mountains.  The plateau, consisting of strata but little changed from their original flat-lying attitude, and the platform, developed on rocks of disordered structure made crystalline by heat and pressure, both stand at the common level of the line ab.  They are separated by the Appalachian valley, forty miles wide, cut in strata which have been folded and broken into long narrow blocks.  The valley is traversed lengthwise by long, low ridges, the outcropping edges of the harder strata, which rise to about the same level,—­that of the line cd.  Between these ridges stretch valley lowlands at the level ef excavated in the weaker rocks, while somewhat below them lie the channels of the present streams now busily engaged in deepening their beds.

The valley lowlands.  Were they planed by graded or ungraded streams?  Have the present streams reached grade?  Why did the streams cease widening the floors of the valley lowlands?  How long since?  When will they begin anew the work of lateral planation?  What effect will this have on the ridges if the present cycle of erosion continues long uninterrupted?

The ridges of the Appalachian valley.  Why do they stand above the valley lowlands?  Why do their summits lie in about the same plane?  Refilling the valleys intervening between these ridges with the material removed by the streams, what is the nature of the surface thus restored?  Does this surface cd accord with the rock structures on which’ it has been developed?  How may it have been made?  At what height did the land stand then, compared with its present height?  What elevations stood above the surface cd?  Why?  What name may you use to designate them?  How does the length of time needed to develop the surface cd compare with that needed to develop the valley lowlands?