GEOLOGICAL MAPS 



165 



the other hand, where the strata dip steeply, the distinction in 

 slope between the two sides diminishes until hogs' backs and ridges 

 with front and back slopes at about the same angle are produced. 



FIG. 57. Incipient longitudinal valleys. 



It has already been shown that the dip of rocks is the result 

 of the originally horizontal strata being thrown into folds, or 

 alternating arches and troughs. As the folding was taking place, 

 the arches must have been those parts of the crust which were 

 rising, and the troughs the sinking parts. Thus the chief slopes 



FIG. 58. Section along transverse stream, ac, of Fig. 57. Lm is the floor of this 

 stream, and under / and c are given the successive enlargements of two of the 

 longitudinal valleys. 



must have at this time led directly from the arches to the troughs. 

 When the strata were elevated in this way from the sea bed, rain 

 water would immediately begin to flow down the slopes produced, 

 that is, directly from arches to troughs. Thus the first valleys 

 would be naturally of transverse character, crossing the direction 

 of strike, and running down the flanks of the arches in the 

 direction of the dip. This would be the case even if the strata 



FIG. 59. Outline produced by processes at work in Fig. 58. 1 



were to some extent denuded by the sea during the process of 

 emergence, indeed the process would be rendered more simple 

 and direct if this were the case. During emergence there would 

 be a battle between the denuding sea tending to plane the rocks 

 down, and the rising fold tending to form an arch. If the former 



1 The blocks on this and the preceding page are from the Author's Geology for Beginners 

 (Macmillan). 



