SOME STAGES OF APPALACHIAN EROSION. 521 



as the law, and the variation of the altitudes, hand in hand with the cutting power 

 of the streams, defines the whole series as the result of subaerial erosion. In the 

 Cumberland plateau the appearance of peneplains is often simulated by the out- 

 crop of flat beds of hard rock which may lie at various altitudes and represent no 

 period of reduction. In the Great valley also the peneplains are overshadowed 

 and masked by the great differences of the rocks in point of resistance to wear. 

 When once the criteria are established, however, it needs no extended search to 

 discover successive forms of degradation as distinct as in the Atlantic streams and 

 grouped in the same manner. Difficulties in the paths of the streams are more 

 localized, however, by the diflerences in the rocks, and need consideration in 

 coordinating the forms. 



VARIATIONS OF LEVEL 



During any extended period of degradation minor difficulties of erosion would 

 be reduced, but the major barriers, such as are produced by unusual groups of 

 rocks and afiect entire river basins, can retard reduction so seriously as to produce 

 considerable discordance of elevation in different basins. An excellent instance 

 of this is furnished by the Pigeon and French Broad rivers in North Carolina. 

 These streams flow in concentric curves, and the larger or French Broad meets the 

 least obstruction ; it has accordingly reduced its peneplains and valleys which lie 

 above the barrier 400 feet lower than the corresponding ones of the Pigeon. Re- 

 sults in the same direction would ensue from warping of the surface, which w^ould 

 give added slope and power to one stream and decrease the grade of an opposite 

 flowing stream. This would retard final reduction merely by the added amount 

 of material to be removed, but would leave no permanent differences of altitude in 

 similar forms. An unfaiUng factor in producing differences of altitude at like 

 periods of reduction is the distance from tlie sea. A peneplain produced by a 

 stream 500 miles from its mouth will be higher than one produced only 100 miles 

 from the sea, whether by the same or a different stream and whether in the same 

 or a different region. A certain amount of fall, however slight, is necessary to 

 make the rivers flow and will be expressed in the accumulated altitudes. On this 

 account the Yadkin river, flowing direct to the Atlantic, has formed its peneplain 

 at 2,300 to 2,500 feet close up to the main residual of the Blue Ridge, while the 

 Nolichucky, 30 miles to the west, taking a longer course through the Tennessee, 

 has cut its peneplain along its main valley only down to 2,600, or 200 feet higher. 



Most potent of all factors of variation is the amount and nature of debris fur- 

 nished to the streams. Insoluble rocks clog the streams with debris and large re- 

 siduals furnish great quantities of waste, so that, as residuals in general are composed 

 of the more obdurate strata, the two unite to raise the grades and perpetuate divides. 

 Soluble rocks yield debris which moves with a minimum grade, so that they seldom 

 remain as residuals. Accordingly grades over soluble rocks are low entirely to the 

 divides, which are thus at the mercy of local variations in the rocks and in the 

 stream powers, and which are correspondingly unstable. 



It is to be expected, therefore, that widely dissimilar basins will have peneplains 

 formed at the same time but at somewhat different altitudes. Such expectation is 

 amply borne out by the facts of the field, and is in fact exceeded. The least in- 

 spection of peneplains shows differences of altitude amounting to 3,000 feet. Two 

 explanations can be made of such great differences, either that one or two pene- 

 plains have been warped oiit of their original plane or that many peneplains are 



