224 



University of California. 



[Vol. 2. 



roughly, two miles, and an average elevation of about 200 feet. It 

 is generally flat-topped, and at various points it is modified by 

 terraces at different levels. It has also been cut through at a num- 

 ber of points by the present streams of this region, whereas the 

 first structure described has been cut through at only one point, at 

 the base of the isolated hill already referred to. While it is prob- 

 able that this last-described structure, as a whole, is a former wave- 

 or current-built embankment, it is possible that, in addition to the 

 hill just mentioned, there are other outstanding remnants along its 

 length, for example, Los Cerritos, to the east of San Pedro Hill. 



So far as known to the writer, no elevated wave- or current- 

 built features occur along the rugged parts of the coast, but those 

 parts of the mainland showing no elevated coastal features whatso- 

 ever are frequently adjacent to areas presenting well-defined terraces 

 or other coastal forms. This, taken in connection with the evidence 

 afforded by the present coast line, in this respect, leads to the con- 

 clusion that the absence of any elevated coastal features, built as 

 well as cut, in areas of rugged topographic forms, is not in itself an 

 indication of a different history from that of a neighboring region 

 where such features are found. 



Drozvned Valleys. — If a stream valley is flooded on depression, 

 the amount of the flooding and size of the embayment formed are 

 dependent on (1) the amount of the depression, (2) the width of the 

 valley, (3) the grade of the valley near its mouth; the gentler the 

 grade the greater will be the amount of flooding. The length of time 

 during which a flooded valley remains in that condition depends on 

 (1) the rate of deposition of detritus brought into the embayment 

 by streams or through marine action, as related to the factors above. 

 The smaller the embayment formed, and the more abundant the 

 detritus, the sooner will it be filled. Drowned valleys may be 

 barred at their mouths, forming lagoons which can not always be 

 distinguished from lagoons resulting from a heaping up of shore 

 drift at the mouths of streams which have not been depressed. 

 For although the lagoon formed by depression would have had its 

 bottom originally below sea-level, deposition would have led to a 

 gradual shoaling of the water. The length of time during which a 

 valley will remain flooded depends on (2) the rate of cliff recession, 



