THE CENTRAL BASIN OF TENNESSEE. 69 



It is clear that the structural deformations of the surface, the up- 

 lifts and the downthrows, had nothing to do with determining the 

 2)resent distribution of the plateau (Grand Cafion District) drainage. 

 The rivei'S are whei'e they are in spite of them. As irregularities 

 rose up, the t^treams turned neither to the I'ight nor to the left but 

 cut their way through in the same old places. What then did deter- 

 mine the situations of the present di'ainage channels ? The answer is 

 that they were determined by the configuration of the surface existing 

 at or very soon after the epoch of emergence. Soon afterwards that 

 sui'face began to be deformed by unequal displacement but the rivers 

 had fastened themselves to their places and have ever since refused to 

 be diverted. (Button, Second Annual Report, U.S. Geol. Survey p. 62.) 



The river having made a break in the surface there is room for 

 denudation to go on laterally as well as vertically and by what 

 Powell terms the " Recession of ('liffs " the narrow stream bed of the 

 river is slowly but gradually widened into a great valley or basin, 

 bounded on either side by a wall of the receeding cliffs, with its head 

 lying away up at the source of the stream and its entrance at the 

 place where the river enters the sea or joins some other and gi'e iter 

 stream having a valley of its own. 



There comes a time in the history of all streams in which their cor- 

 rading power reaches a minimum — that is presuming there are no 

 upheavals or no gradual elevation of the land within the course of the 

 stream. This period of minimum corrasion is when what is termed 

 the base level of erosion has been reached. The base level of ei'osion 

 is reached when the channels have reached an altitude in which their 

 declivities are so small, the velocity of the water so feeble, and their 

 transporting power so much reduced that they can do no more than 

 ui'ge along the detritus brought into their troughs from high lands 

 along their margins. Their transporting power is just equal to the 

 load they have to caray, and there is no surplus left to wear away 

 their bottoms. Under these conditions the slopes of their ravines are 

 less rapid ; their nari'ow bottoms widen and flatten until they grade 

 into smooth bottomed meadows stretching from base to base of steeper 

 ascents on either side. The widening of the bottoms is due to the 

 low gradient which has been reached in the progress of excavations of 

 the valleys. The running waters now cut but slightly on their bot- 

 toms because with the low slope at which they have an-ived they are 



