AND THE FORMATION OF VALLEYS. . 95 



consideration it is evidently a natural position, as 

 when the strata were folded up tension would form 

 open breaks thereabouts. The upper rocks in a 

 synclinal trough may be wedged and compressed one 

 against another, but the upper rocks on an anticlinal 

 are liable to be separated one from another by stretch- 

 ing, and broken up by the joint-lines opening, leaving 

 the rock more or less loosened, and ready to be 

 carried away by any denudant, whether it be a river, 

 ice, or the sea. In some cases, however, there is 

 probably less material carried away than might be 

 supposed ; for if we take a given length of horizontal 

 strata and bend it into an anticlinal curve, the[upper- 

 most beds, under some circumstances, must either 

 stretch or break ; and if the latter, a V-shaped valley 

 would form (fig. 14, PI. II.), prior to any denuda- 

 tion having taken place. The horizontal width of 

 the valley at the top, b, being nearly equal to twice 

 the versed sine, to radius ab, of the angle formed 

 between the bed ab and the horizontal line at the 

 bottom of the valley ; (fig. 23, PI. IY. horizontal 

 width = 2 db = 2 versed sine A, to radius ab.) 



The folding of strata is worthy of study. If the 

 strata are thin and each bed is capable of moving 

 every way, over and under the associated beds, such 

 strata may be bent up sharply without fracture, the 

 only effect, apparently, being a little tension above and 

 compression below on each individual bed. Excellent 



