32 



California Division of Mines and Geology 



Bull. 197 



1955, p. 416). Shells were dredged north of the San 

 Mateo Bridge causeway during the 1960s (see figure 

 2). Dredging is conducted by making successive 

 "passes" along the cut face of a shallow bench. A 

 bench is thus formed and worked shoreward to the 

 east. Steel barges are loaded with about 700 tons of 

 unxAashed shells and mud and towed to the Redwood 

 City plant by tugs. A clamshell bucket and overhead 

 crane unload the shells and stockpile them on the dock 

 adjoining the plant. Analyses of the unwashed and 

 washed shells are given in table 7. Excessive mud is 

 removed from the shells by washing in a screw clas- 

 sifier, if necessary, but most of the mud is used in 

 cement manufacture. The slurry also can be "sweet- 

 ened" by adding washed shells. Washed shells are ob- 

 tained from Pioneer Shell Company. 



Photo 4. One of two shell dredges operated by Ideal Cement Compan/. 

 Dredge mode successive posses along foce of shallow bench cut in shells and 

 mud. Dredged shells ond mud were screened ond conveyed to odjacent 

 barge (700-ton capacity) and towed to cement plant at Redwood City. 

 Dredging and cement manufacture ceased at the end of 1970. Photo by 

 Edward e. We Ida y, 1970. 



The shells and mud are ground and blended with 

 iron scale or iron ores and other ingredients to make 

 a slurry of proper proportions. This is dewatered and 

 fed to four gas-fired rotary kilns which operate at tem- 

 peratures up to 2, SOOT. Clinkers, which are less than 

 an inch in size, are cooled to 250°F. The clinkers are 

 ground with purchased gypsum and stored in silos for 

 marketing (Bowen and Gray, 1962, no. 8, p. 2). In 

 addition to iron scale, the company purchases large 

 amounts of hydrochloric acid, possibly to reduce the 

 alkalies in the final product. Shale of the Monterey 

 Formation, obtained from the company's quarry at 

 Chittenden, Santa (^lara County, has been used in the 

 raw feed since 1965 and at times prior to that. Only 

 Type I cement is produced due to the high alkali con- 

 tent of the bay mud used. 



About 200 people are employed at the plant, which 

 has a rated capacity of 2,510,000 barrels of cement per 

 year. About 90% of the cement is shipped bulk; the 

 remainder is bagged. Deep-water port facilities enable 

 the company to ship cement to markets in Oregon and 



Washington, although the great bulk of the cement is 

 transported by truck to local markets. 



Other references: Senior, 1929, p. 247-251; Logan, 1947, p. 307-308. 



W.B. Ortley Shell Company- This company 

 dredged and processed small amounts of shells from 

 San Francisco Bay from 1930 to 1941. Dredging was 

 done using a rotary pump mounted on a barge. The 

 shells were barged to a plant near Alviso for drying, 

 crushing, and screening prior to sale as poultry feed 

 (Franke, 1930, p. 9; Logan, 1947, p. 312). Location and 

 nature of the shell deposits worked are undetermined. 



Pioneer Shell Company (L.H. Beck; Beck Dredg- 

 ing Company). The company is owned by Captain 

 L.H. Beck, 2772 Bromely Road, San Carlos. Captain 

 Beck dredged shells from south San Francisco Bay 

 from 1931 to 1947 (or later) under the names L.H. 

 Beck and Beck Dredging Company, and from 1956 (or 

 earlier) to the present time (1967) under the name 

 Pioneer Shell Company. Locations of earlier dredge 

 grounds are unknown, but dredging in recent years 

 reportedly has been conducted east of the ship channel 

 in the vicinity of the San Mateo Bridge, both north 

 and south of the bridge causeway. Some of the shells 

 were processed for livestock feed and soil condition- 

 ing, first at a plant in Alviso and later at the present 

 plant at 100 "D" Street in Petaluma. Large amounts of 

 washed shells also were supplied to Ideal Cement 

 Company, FMC Corporation, Bay Shell Company, 

 and possibly others. 



in June 1962, Pioneer Shell Company obtained 

 shells from the shallow water area north of the San 

 Mateo Bridge causeway near the boundary of Ala- 

 meda and San Mateo Counties (figure 2). The exact 

 nature of the deposit is not certain but probably con- 

 sists largely of surficial accumulations of loose shells 

 associated with soft bay mud. Dredging is conducted 

 with a suction dredge, 1,500-cubic-yard and 800-cubic- 

 yard barges, and two tugboats. The tugboats are used 

 both for dredging and barge transfer. The dredged 

 shells and associated mud are pumped from the bay 

 floor to a trommel where bay water is used to free the 

 shells of mud. The mud and shell fines are wasted 

 overboard and the cleaned shell is conveyed to an adja- 

 cent barge. When loaded, the barge is towed to the 

 company's plant at Petaluma for further processing. 

 Ihe washed shells consist almost entirely of native 

 oyster shells (Ostrea lurida Carpenter). Other mol- 

 lusks (Japanese littleneck clam, bent-nose clam, bay 

 mussel, and an unidentified ornate gastropod), includ- 

 ing some live ones, constitute 5% or less of the shells. 

 A chemical analysis of the washed shells shows 

 53.07% CaO (sample 3, table 7). 



At the Petaluma plant there are dock facilities 

 where the shells are unloaded from the barge by 

 means of a clamshell and stored. The shells are then 

 dried in a gas-fired rotary drier and sent to a set of 

 screens for sizing. Some whole- and half-shell sizes are 



