Smith.] Islands of Southern California. 209 



is a moderate off-shore slope. Too low an angle of slope, how- 

 ever, will act as a complete, though temporary, check to cliff 

 cutting, the force of the waves being dissipated over a broad and 

 shallow zone, which must be worn down before cliff cutting can 

 take place. Wherever continuous cliff cutting is taking place, the 

 off-shore slopes must be worn down at the same time, in order that 

 the waves may continue to do effective work on the cliffs. 



Where the original slope of the bottom is at a high angle, the 

 energy of the waves at first must be devoted to the task of 

 building up the off-shore slopes, before active cliff cutting can take 

 place. Abundant detritus (however derived) is essential to this 

 process. Where such detritus is comparatively scanty (as is the 

 case along the California coast in general), the effective work of 

 the waves in cliff cutting, on a steep slope, would be, of course, 

 very greatly delayed. The conditions might be such, in some 

 cases, that while a comparatively broad terrace would develop on 

 a low slope, no terrace at all would be formed in the same time on 

 a steep slope. 



It has already been shown that, accidents excepted, the general 

 on- and off-shore slopes are roughly the same, at the outset of 

 terrace development at any given level. 



With low on-shore angle of slope the sea cliffs are necessarily 

 lower than where the subaerial angle is high — the angle being, as 

 stated, normally dependent on the resistance of the rocks, in regions 

 of mature topography. With waves of equal strength, cliff cutting, 

 even after the necessary conditions are attained (see above), will be 

 slower on the high than on the low cliff, not only on account of 

 the greater hardness of the rocks normally forming such slopes,, 

 but also because, with the high cliff, more material must be cut 

 away and transported for every horizontal foot of cutting; and 

 since, on the whole, erosion can not exceed the removal of the 

 material cut, the lower the cliffs the greater the advantage in this 

 respect. As a natural result of slower erosion, terraces developed 

 on a steep slope will be narrower than those on a gentler slope, 

 other things being equal. High cliffs with abrupt off-shore slopes, 

 even in the case of soft rocks (an exceptional condition in a 

 region of mature topography), will be cut back very slowly, as 

 would follow from the statements already made. 



