Fig. 69. View in the Cascade Mountains, Washington
The general level to which these ridges rise may be accounted for by the uplift and dissection of a once low-lying peneplain
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?
Fig. 70. Generalized Section of the Southern Appalachian Region in Eastern Tennessee