298 National Geographic Magazine. 



been revived ; and the wide ramification of the brooks is the 

 characteristic of approaching maturity. 



We have also glanced at the topography of the valley and 

 have found the rivers flowing in deep-cut simple channels which 

 are young, and the smaller streams working on an undulating 

 surface that is very sensitive to processes of degradation. 



The minor stream systems are very intricate and apparently 

 mature, but they have not yet destroyed the evidence of a gen- 

 eral level to which the whole limestone area was once reduced, 

 but which now is represented by many elevations that approach 

 1,600 feet above the sea. Here then in the valley are young 

 river channels, mature stream systems and faint traces of an ear- 

 lier base level, all of them more recent than the Asheville level, 

 which is in turn less ancient than the dome-like summits of the 

 TJnakas. 



What history can we read in these suggestive topographic 

 forms and their relations ? 



The first step in the evolution of a continent is its elevation 

 above the sea. The geologist tells us that the earliest uplift of 

 the Appalachian region after the close of the Carboniferous 

 period was preceded or accompanied by a folding of the earth's 

 crust into mountainous wave-like arches ; upon these erosion at 

 once began and these formed our first mountains. Where they 

 were highest the geologist may infer from geologic structure and 

 the outcrops of the oldest rocks ; but the facts for that inference 

 are not yet all gathered and it can only be said that the heights 

 of that ancient topography were probably as great over the val- 

 ley of Tennesseee as over the Unaka chain. The positions of 

 rivers were determined by the relations of the arches to each 

 other and, as they were in a general way parallel, extending 

 from northeast to southwest, we know that the rivers too had 

 northeast-southwest courses. From that first drainage system 

 the Tennessee river, as far down as Chattanooga, is directly de- 

 scended, and when the geologic structure of North Carolina and 

 East Tennessee is known, we may be able to trace the steps of ad- 

 justment by which the many waters have been concentrated to 

 form that great river. At present we cannot sketch the details, 

 but we know that it was a long process and that it was accom- 

 panied b}^ a change in the raison d'Ure of the mountain ranges. 

 The first mountains were high because they had been relatively 

 raised ; they gave place to hills that survived because they had 



