Smith.] 



Islands of Southern California. 



225 



as related to the other factors. The lower the cliffs, the softer the 

 rocks, or the greater the exposure to wave action, other things 

 being equal, the sooner the embayment will be destroyed. 



Whether or not a stream valley will be flooded on depression 

 is dependent on (1) the rate of depression as related to the size and 

 grade of the valley, and to the rate of coastal deposition and cliff 

 recession. If the rate of depression is sufficiently slow, the rate of 

 coastal deposition and cliff recession may be such that, for a given 

 coast line, no valleys will be drowned. The broader the valley and 

 the gentler its grade near its mouth, the greater the amount of 

 flooding, and hence the slower the rate of depression necessary to 

 avoid flooding. Thus where valley width and grade vary along 

 a coast line, a given rate of depression, irrespective of rate of cliff 

 erosion and coastal deposition, might drown the valleys of greatest 

 width and least grade, while the narrower and steeper valleys would 

 not be flooded. The flooding of stream valleys on depression is 

 dependent (2) on amount of depression. Where the mouths of the 

 streams are above sea-level prior to depression, — owing to rapid 

 recession of the sea cliffs, or any other cause, — the depression may 

 be insufficient to carry them below sea-level, and in that event there 

 can, of course, be no valley flooding. 



Broad valleys of low grade, so situated that cliff recession is 

 slow and shore drift scanty, are most favorable to valley flooding 

 on depression and to the continuance of the flooded condition. On 

 the other hand, small and narrow valleys of comparatively high 

 grade, opening on a shore line favorable to rapid cliff recession, or 

 in a region of abundant drift, or where much detritus is furnished 

 by the stream occupying the valley, furnish conditions unfavorable 

 to valley flooding or its continuance. Where, then, along such 

 unfavorable shore lines, no flooded valleys are found, it can not be 

 asserted, in the absence of any positive evidence to that effect, that 

 no depression has occurred. 



None of the islands belonging to the group with simple topog- 

 raphy give any positive and unmistakable evidence of depression, 

 through valley flooding, since the conditions along their shore lines 

 are unfavorable to that phenomenon. Their stream valleys, either 

 on account of youth or of the softness of the rocks in which they 



