128 



Nattiral or artificial changes in the tributary drainage 

 areas and changes resulting from other worKs of man, are 

 indirect evidence that the sedimentation in Santa Monica Bay 

 has varied « In addition, it can easily be shovm that pro- 

 nounced changes in sedimentation have taken place by exam- 

 nation of the sediments in the bay. 



There are a large ntimber of cores, jettings, and 

 borings taken in various parts of the bay and on land 

 close to shore. They show that numerous textural changes 

 occur in a short vertical section of any random core or 

 jetting. Thus, there are distinct beds or laminations of 

 gravel, silt, sand, clay, silt and gravel, beds of shells, 

 and even layers of vegetation. Although cores that are close 

 together may show some correlation, there is in general a 

 poor core to core correlation. The pronounced alterations 

 in texttire can only be explained by changes in the competency 

 of the transporting agents (waves and currents), and to 

 variations in the supply of detritus supplied to the bay, 

 and/or to alternate cutting and filling of the nearshore 

 shelf sediment surface. 



Rain falling in the tributary drainage areas can have 

 several different effects upon the supply of detritus to the 

 bay, depending upon the amount and distribution of the preci- 

 pitation. If the rainfall is small, only silt and clay will 

 be brought to the bay. If there is slightly more precipitation, 

 considerable amounts of coarser sediments, especially material 

 of about fine sand size may be washed to the bay. If a flash 

 flood occurs, material ranging in size from clay to boulders 



