32 



by finer sediments, deposited by previous fluvial cycles of the Colorado 

 River. Where there are steep sides and rocky headlands on the east or 

 mainland side of the Gulf, the sediments are again coarse and sandy. If the 

 shelf broadens, the sandy sediments soon disappear, being replaced by 

 clayey silts and silty clays on the outer edges of the shelf (fig. 7). However, 

 two areas where this is not the case are those studied by Curray off 

 Hermosillo and San Bias (figs. 7 and 8). Both of these broad shelves are 

 at least partially a result of the growth of large river deltas during Holocene 

 lowered sea level, plus subsequent inundation, re-working and high rates 

 of deposition during the last rise of sea level during the past 19,000 years 

 (Curray, 1962). The conditions which produced these submerged deltas 

 have therefore complicated the sediment distribution so that both areas 

 are characterized by small patches of varying sediments irregularly 

 distributed. This patchiness of coarse and fine sediments has also produced 

 considerable patchiness in the distribution of the invertebrates. The 

 faunal communities are not located strictly parallel to each other in 

 descending depth as in the Gulf of Mexico, but often run in confused 

 patterns, mostly related to sediment size. 



Bottom Dissolved Oxygen Concentrations. 



Fortunately for this study, there existed a considerable body of in- 

 formation on the hydrography and physical oceanography of the Gulf of 

 California. Beginning with the "E.W.Scripps Cruise", a number of cruises 

 have been undertaken to the head of the Gulf solely for the purpose of 

 measuring various hydrographic factors. Most of these cruises were carried 

 out by Scripps Institution in collaboration with the South Pacific Fisheries 

 Investigations of the U.S. Fish and Wildlife Service and the California 

 Cooperative Oceanic Fisheries Investigations. The results of the first 

 seven cruises were discussed in detail by Roden (1958) and Roden and 

 Groves (1959). Since that time, several more cruises have been carried out 

 under the same program, in addition to which hydrographic data was col- 

 lected by the "Vermilion Sea Expeditions" and various "Tuna Oceanography 

 Cruises" sponsored by the Saltonstall Fund studies at Scripps. Although a 

 number of cross-sections were given in the published studies of oxygen 

 distribution in the Gulf, there was no discussion of the areal distribution of 

 bottom oxygen. The present author compiled the raw data for all known 

 deep casts, averaged the bottom values for all of the stations where data 

 existed (fortunately, the same stations were occupied at almost every 

 season and on several occasions), and plotted them on a bathymetric 

 chart of the Gulf of California (fig. 9). No attempt was made to obtain 



