36 FIELD GEOLOGY. 



have been able for the longest time to resist erosion. 

 Were it otherwise, we should probably now find the older 

 hills only capped with drift, and the valleys again cleared 

 out by the agents of denudation but in all rocks hard 

 or soft, old or recent, the anticlinal line is the most 

 readily denuded. Partly, it may be, from the bend 

 having somewhat opened out its particles by tension, 

 thus producing in some cases fractures, and in others 

 weakening its power of resistance. Partly, or it may be 

 entirely, to the inward and downward dip of the beds 

 on each side of an anticline, which would make them 

 relatively stronger than the intervening flat part to re- 

 sist the attacks of erosion. Another and not unimpor- 

 tant reason may be, that a considerable portion of the 

 rain falling thereon percolates down into the rocks with 

 such inclination and is thrown out elsewhere, while along 

 the axis of elevation it all flows over the rock and re- 

 moves a proportionately larger share of its material. 



It may be well here* to remind the student, that in 

 "drifts," the lower as regards level is not necessarily 

 the older of two or more deposits ; take, for instance, the 

 gravels occurring in a large drainage area. In past time 

 the river has spread a large sheet of gravel, through 

 which, as denudation went on, its course was again cut 

 with a lowering of its level, perhaps 20 feet. It de- 

 posited more gravel, which was in turn cut through and 

 the river's course was again lowered, it may be other 20 

 feet. In such a case the gravel at the higher level is 

 the older, and the various deposits form terraces at 

 heights 20 feet apart, marking as many stages in the 

 formation of the valley. Such are the higher, lower, and 

 intermediate terraces of gravel in the Thames and other 



