Smith.] 



Islands of Southern California. 



Wave- and Current-Built Features. — The essential factors in the 

 formation of barriers — aside from the necessary waves and current 

 — are (i) abundance of drift, (2) gentle off-shore slopes, (3) nearly 

 unbroken shore contour. (1) The chief of these three factors is 

 an abundance of drift, of such size as to be readily transported by 

 shore currents, the supply being continuous and greater in amount 

 than may be readily disposed of off shore by wave and current 

 action. This supply may come from neighboring sources, or it 

 may come from a distance, transported to the point of deposition 

 by shore currents. When so transported, a practically continuous 

 road-bed, either beach, barrier, or embankment, from the point of 

 supply to the point of deposit, is also necessary. This supply of 

 drift may come from the shores through wave cutting, or it may 

 be furnished by rivers, or it may come partly from both sources. 

 In no case can the supply of drift come from the part of the shore 

 where deposition is taking place; for active cutting and deposition 

 can not occur at the same place at the same time. The barrier can 

 persist only so long as the supply of drift is continuous. If this 

 fails, through any cause, the barrier will be cut away, the submarine 

 shelf worn down, and the shore line itself attacked. 



(2) The steeper the subaqueous slopes the nearer the shore 

 will any wave-built deposit form. For off-shore barriers, therefore, 

 a very gentle off-shore slope is necessary, so that the wave attack 

 may be at a distance from the shore. Such gentle slopes may be 

 normal, or they may have been built up by abundant drift. Where 

 such very gentle slopes occur they can be preserved only by an 

 abundance of drift; for without this the subaqueous slope will be 

 gradually worn down, and the waves will cut back shoreward. 



(3) A nearly unbroken shore contour for considerable stretches 

 is essential for the development of extensive barriers. Such a shore 

 contour may be formed originally, as by the elevation of the sub- 

 marine platform, or it may be developed by modification of an 

 uneven shore line through the accumulation of shore drift, where 

 this is abundant. 



The conditions along the California coast appear to have been 

 very unfavorable to the formation of barriers during the period in 

 which the terraces were formed. The rocks along this coast do 



