1978 



Limestone in the Coast Ranges 



27 



Table 6. Summary of known producers and processors of oyster shells of south San Francisco Bay. 



.NWmc" of 

 operator and years active 



Source of shells 



Remarks 



B»y Shell Co. (was Agricultural Lime & &)mpost Co.) 1924- Purchased from South Bay Dredging; 



previously purchased from Beck Dredg- 

 ing; also mav have dredged near .Mviso 

 and San Mateo Bridge pre-lV^()( = ) 



Dredges shells and mud east of channel 

 near San Mateo Bridge and barges to 

 plant; also buys washed shells from Beck 

 Dredging. 



Reportedly did own dredging. 



1969 



Ideal Cement Co. (was Pacific Portland Cement Co.) 1V25-1%9 

 (active) 



Ortley Shell Co. (W B Ortley) 19!(>-1941 



Pioneer Shell Co. (Capt. I. H Beck, Beck Dredging Co ) 1931- 

 1969 (active) 



South Bav Dredging Co. (Pete Gambetta) 1955 (or earlier)- 

 1969 



Westvaco Chlorine Prixiucts Corp (was California Chemical 

 Corp.; now is Inorganic Chemicals Div., FMCCorp.) 1931-1948 



Dredges near San Mateo Bridge east of 

 channel, may have dredged south of Dum- 

 barton Bridge. 



Dredged north of San Mateo Bridge and 

 east of channel; probably dredged south of 

 Dumbarton Bridge earlier. 



Purchased from Beck Dredging. 



Processed shells for livestock feed and soil 

 conditioning at plant in .Mviso Operations 

 ceased 1969 



Manufactures cement at Redwood City plant. 

 using washed shells to "sweeten"; previous to 

 195(1 w ashed and processed shells for livestock 

 feed and soil conditioning 



Shells processed for poultry feed at Alviso 

 plant. 



Washes shells on dredge and delivers to own 

 plant in Petaluma for livestock feed and soil 

 conditioning; previously sold washed shells to 

 Ideal Cement, Westvaco Chlorine Products 

 and Bay Shell companies; formerly processed 

 shells for commercial sale at plant near .Al- 

 viso. 



Washed shells on barge and sold to Bay Shell 

 C^o. Operations ceased 1969. 



Shells calcined and reacted with bittern to 

 make magnesian compounds at Newark 

 plant; some lime, hydrated lime, and poultry 

 feed sold commercially (see FMC Corp.). 



Younger bay mud deposits. This unit is widely 

 distributed in San Francisco Bay. It consists mainly of 

 soft gray mud (silty clay) with intcrbeds and lenses of 

 silt, sand, peat, and shells. These sediments were de- 

 posited following a progressive rise in sea level after 

 the last glacial episode (Wisconsin Glaciation). Shells 

 accumulated in an environment similar to the present 

 one. The shells exist as numerous thin lenses of varia- 

 ble purity that are interbedded with and grade lateral- 

 ly into mud. Ihe lenses are largely concentrated in the 

 upper 30 or 40 feet of the younger bay mud unit. Test 

 borings show that shell-mud sequences as much as 25 

 feet thick exist in several places in south San Francisco 

 Bay and also at the west end of the Richmond-San 

 Rafael Bridge at the north end of the bay. 



Thick sequences of shells probably represent rela- 

 tivelv persistent environments favorable to the 

 growth and accumulation of shells. Shell lenses also 

 tend to concentrate along certain horizons that are 

 laterally extensive. Such horizons reflect periods of 

 greatly increased populations (population explo- 

 sions) of the native oyster. The most extensive shell 

 horizons known lie east of the main ship channel in 

 south San Francisco Bay. 



In 1962, Ideal Cement Company explored these 

 deposits with 107 drill and core holes in order to eval- 

 uate their shell holdings. According to Story et al. 

 (1966, p. 48), the shell horizon in the vicinity of the 

 San Mateo Bridge is "a fairly extensive layer contain- 

 ing 4 to 15 feet of oyster shells buried beneath 2 to 8 

 feet of mud". The layer is reported to contain 10 to 

 75% shells (dry basis). Radiocarbon age dates indicate 



that this shell horizon was deposited 2300 to 2 500 years 

 ago (Story et al., 1966, p. 48-^9). Because most of the 

 dredged shells are worn and somewhat broken, it is 

 presumed that the shell deposits are at least partly 

 reworked debris from oyster reefs and beds or from 

 older deposits. However, it is likely that eroded rem- 

 nants of ancient reefs are partly preserved. 



Shell deposits of the younger bay mud contain the 

 bulk of the commercial shell reserves and provide the 

 raw materials for cement manufacture by Ideal Ce- 

 ment Company. 



Surficial shell deposits. Although the native oys- 

 ter population has been somevi hat decimated since 

 1900 due to pollution (Skinner, 1962, p. 100). live oys- 

 ters can be found in many places in sheltered parts of 

 San Francisco Bay (J. A. Aplin, 1968, personal com- 

 munication). Extensive beds and cemented reefs, with 

 clusters of live oysters attached, are ttiost extensive in 

 shallov\- parts of the south arm of the bay. Oysters are 

 also reported in a few places along the rocky perimeter 

 of the bay. Debris derived from oyster environs, and 

 perhaps from older deposits, too, has accumulated as 

 veneers and bars. These loose-shell deposits, as a result 

 of >Aave and current action, tend to migrate shoreward 

 to form beaches in San Mateo County. Such beaches 

 are reported to have been rather extensive around the 

 turn of the century. The submerged bars, veneers, and 

 loose debris around the reefs are easy to dredge hy- 

 draulically and are believed to be the principal sources 

 of shells used by Pioneer Shell (.ompany and South 

 Bay Dredging Company. The beaches, although com- 

 posed almost entirely of clean shells, are not known to 



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