212 



University of Calif ornia. 



[Vol. 2. 



Since, then, the waves will have cut back to F before they have 

 reached F', and since the cutting away of the terraces themselves 

 is at approximately equal rates, and since, further, the terrace CB 

 is much narrower than the terrace C'B' (the ratio of the widths 

 being about 7:15), it follows that the waves will have cut back to 

 the first sea cliff, AB, long before the corresponding point in the 

 other case will have been reached. Thus we conclude that not 

 only are narrower terraces formed where the angle of slope is high, 

 but they are sooner destroyed by wave action, after elevation. 



The angles assumed in this illustration are such as are not at 

 all uncommon on the coastal slopes of California, in moderately 

 hard and soft rocks, respectively. 



As a rule such differences of slope as are shown in these figures 

 would be accompanied by a difference in the resistance of the rocks 

 to erosion. But even if the rocks forming the slope A'C'D' were 

 much less resistant than those forming the slope ACD, the conclu- 

 sions just reached would not be changed. For, regardless of rock 

 character, if the distance C'B', with low angle of slope and low 

 cliffs, is equivalent to the distance CB, with higher angle of slope 

 and higher cliffs, then with cliffs of equal height in both cases, CD 

 will have been cut back to BE long before CD' will have been cut 

 back to B'E'. Or, looked at in another way, while a terrace formed 

 on gentle slopes in softer rocks will be more readily obliterated on 

 elevation than one formed on the same slopes in harder rocks, this 

 will be largely offset by the more rapid cutting in the formation of 

 the terrace, which will give a comparatively greater width of 

 platform to begin with. 



The chief factor in wave cutting, as illustrated above, is the 

 amount of material removed for every foot cut. This depends on 

 the height of the cliff, and this, in turn, depends, normally, on the 

 angle of slope. Hence slope is spoken of as the essential factor, 

 but it must be remembered that it is considered in connection with 

 cliff height. 



An example of the extent to which the elevated terraces have 

 been obliterated by those formed at lower levels may be taken 

 from San Clemente Island, where the 80-foot terrace now ranges 

 from nothing to a maximum width of about one-third of a mile. 



