316 LAND SCULPTURE 



with the dip, always keeping at right angles to the latter. The 

 upper surface of the gently inclined hard stratum may be com- 

 pletely exposed by the stripping away of the softer overlying 

 mass, and then the slope of the ground is the same as that of the 

 resistant stratum and is called a dip slope. A series of gently 

 inclined strata, made up of alternating harder and softer beds, 

 will thus give rise to parallel ridges and valleys, or escarpments 

 and dip slopes, according to the completeness with which the 

 softer beds are removed and the harder ones exposed. A mag- 

 nificent example of such escarpments and slopes is displayed in 

 the high plateaus of Utah and Arizona, where the dip slopes are 

 from 20 to 60 miles broad and the escarpments 1500 to 2000 feet 

 high. The amount of denudation involved in the production of 

 these vast amphitheatres staggers belief, though there is no escape 

 from the enormous figures. 



Under the influence of denudation escarpments are continually 

 though slowly receding, being cut back in the direction of the dip. 

 Rain and frost act directly upon the hard beds, but work more 

 effectively by cutting away the softer beds below and thus under- 

 mining the hard strata, causing them to fall. The fallen masses 

 are gradually disintegrated in their turn and washed away into 

 the water-courses. The escarpments may follow a relatively 

 straight or a very sinuous course. Sinuosities, when present, are 

 commonly due to the presence of springs, which undermine the 

 escarpments and, by the recession of their heads, excavate the line 

 of cliffs into bays and amphitheatres. A sinuous escarpment is more 

 rapidly cut back than a straight one, because, in addition to the 

 cooperation of the springs, it offers a larger surface to the attack 

 of the destructive agencies. Every step in the recession of an 

 escarpment lowers the ridge and brings it nearer to base-level, 

 because the direction of retreat follows the line of dip, which 

 carries the beds down to base-level with a rapidity determined 

 by the angle of dip. A steeply inclined bed needs to be cut back 

 only a short distance, when it will be reduced to base-level, 

 whereas a bed dipping very gently remains above base-level for 

 very long distances. Of course the general elevation of the whole 



