Smith.] 



Islands of Southern California. 



221 



Given a suitable embay ment to be barred, the most important 

 factor, as in the case of the barrier, is an abundance of material, of 

 a size for ready transportation by the shore currents. The condi- 

 tions with regard to this factor are similar to those already given 

 for the barrier. 



The formation of the bar is further dependent on the character 

 of the coastal currents, their direction and strength, in relation to 

 each other and to the other factors concerned. Witli a strong 

 littoral current, sufficient material for the formation of a bar (hook, 

 spit, &c.) may be transported, when with a weaker current the 

 material might be ground up by the waves and disposed of 

 off shore. 



The bar is essentially a current-built feature, while the barrier is 

 essentially wave-built. We may have a structure built across a 

 very shallow embayment,' which, though functionally a bar, is gen- 

 etically a barrier, being due to wave action in shoal water. Though 

 these two forms of coastal deposit are typically different, there are 

 all gradations between the two extremes, through different com- 

 binations of the essential conditions on which each depends. 



A region of mature topographic forms in generally hard rocks, 

 yielding narrow and comparatively steep-graded valleys, and steep 

 on- and off-shore slopes, gives in general very unfavorable condi- 

 tions for the formation of bars. Beaches for the transportation of 

 material will not be numerous or continuous. Detritus will not be 

 readily or rapidly formed, and most of what is furnished will be 

 disposed of off shore. If such a region is depressed, bars may be 

 thrown across the mouths of many of the narrow canyons or of the 

 streams, forming lagoons. These will not be long-lived, being 

 filled sooner or later with sediments brought down by the high- 

 grade streams; or else the embayments may be destroyed through 

 the rapid cutting back of the sea cliffs. Even on such a coast, 

 however, there will probably be exceptions, and favorable condi- 

 tions may be found here and there for the formation of compara- 

 tively extensive bars. 



It is to be noted that, along a sea coast, a moderate depression, 

 general or local, is usually a condition necessary to well-formed 

 bars, and that during a general movement of elevation, even after a 



