THE CENTRAL BASIN OF TENNESSEE. 65 



Valleys may be due to the flexing of beds or to the breaking of 

 ■them, the flexing producing one kind of valley and the breaking 

 -another. Although so much, with regard to the extent and form of 

 all valleys, is due to erosion, yet it is doubtful whether this cause 

 ever originated a valley. 



"The examination of the geological structure of valleys j^lainly 

 testifies that almost every great hydrographical basin has derived its 

 form originally from some other agency altliough its outline may 

 have been subsequently altered by the continued action of currents 

 within it." — {Chambers' Encycl. Vol. IV, Art. Erosion). 



Valleys may be divided into the following : 



1. Synclinal Valleys. — These are due to the folding of the earth's 

 crust and always run parallel to the folds, or mountains, upon either 

 side of the valley. As a general rule the dips of the two opposing 

 sides are toward the centre of the valley. While this may be a 

 general rule regarding the dip of the sides of a synclinal valley it is 

 by no means invariably the case. There are valleys which properly 

 belong to this class in which the dips are parallel to each other. 

 Amung the folds of the Appalachians many of them were so folded 

 back upon each other as to give under the action of denudation 

 valleys which although true synclinal valleys have th'^ dip of the 

 strata on both sides parallel. Synclinal valleys never cross the 

 ridges or folds among which they lie. 



2. Monoclinal Valleys.—YMeys of this description always face an 

 escarpment, and have for the other side an indefinitely long dip or 

 slope. Such valleys may be due to the slipping of the upper beds 

 upon the lower ; or to the unequal erosion of beds lying upon the 

 same side of a fault with the same exposure, or they may be formed 

 along a fault by the difference between the uplift on the one side and 

 the down throw upon the other. 



3. Valleys of Elevation. — Paradoxical although it may seem, we 

 have a cla.ss of valleys which are higher than the hills. These run 

 along the summit of anticlinals and are due to the fracturing of the 

 beds at the point of flexure and the admission of water. The water 

 erodes the under-lying beds to such an extent that in a short time, 

 geologically speaking, a valley often of a considerable size is formed. 

 There are several valleys of this description within the territory con- 



