1<)78 



Limestone in the Coast Ranges 



25 



In recent (I'MZ) quarr\- operations, the fractured 

 limestone and chert are ripped from the upper 

 benches and bulldozed to the i]uarry face, falling to 

 the quarry floor belou. Only occasional blasting is 

 needed. Some selective quarrying is necessary to 

 maintain a uniform product. Thin soil overburden 

 and occasional soft, noncarbonate interbeds are 

 moved to the west slope of the hill where they are 

 wasted. At the quarry floor, the rock is bulldozed to 

 a grizzly, u here the oversize rock is reduced in a jaw 

 crusher. Both fractions are conveyed to the crushing- 

 screening plant, which has a capacity of 250 tons per 

 hour. At the plant, the minus l'/,-inch material is 

 removed and stored for use as road base material. The 

 plus r/,-inch rock is crushed to five sizes, including 

 sand, by one jaw and three cone crushers and a roll. 

 The material is then stored in open piles for use as 

 asphalt and portland cement concrete aggregates, 

 drain rock, roofing gravel, and other purposes. Proc- 

 essing is drv, although some products are occasionally 

 washed. Large boulders left in the quarry are used for 

 riprap and landscaping. Most of the aggregate pro- 

 duced is utilized at the quarry in an asphalt plant 

 owned by Marks Material, Inc., and a readymix con- 

 crete plant owned by H. E. Casey of San Mateo. 



Available reserves of the Rockaway deposit amount 

 to about 2 million tons of usable rock above sea level — 

 assuming the limestone extends to that depth through- 

 out the deposit and the amount of wastage does not 

 increase significantly. 



Other references: Lawson, 1914, p. 22 ond map; Kelly, 1933, p. 362-363; 

 Walker, 1950b. p. 4, 5, 7. 



San Francisco Bay Shell deposits (Ideal Cement, 

 Pioneer Shell, South Bay Dredging, Bay Shell). 

 Location; South arm of San Francisco Bav; mainly T. 

 3 and 4 S., R. 3 and 4 \V., NLD.; San Mateo, Hay ward, 

 and Palo Alto 15-minute quadrangles. Ownership: 

 Multiple, but mainly Ideal (dement Company, 420 

 Ideal Cement Building, Denver, Colorado 80202, and 

 State of California (1967). 



.Accumulations of shells are found in many of the 

 shallow bays and inlets of C'alifornia. By far the largest 

 are the accumulations of native oysters {Ostrca lurida 

 Carpenter) deposited in Holocenc time in south San 

 Francisco Bay. The deposits of south San Francisco 

 Bav have served as the principal sources of shells 

 dredged in C-alifornia and, since 1962, have been the 

 sole source of shells dredged in the state. Prior to 1962, 

 oyster shells also were dredged commercially from 

 Newport Bay in Orange ('ounty. 



Although oyster shells were obtained from San 

 Francisco Ba\' as carlv as 1891 for garden walks and 

 other purposes (Skinner, 1962, p. 95), shells were first 

 dredged commercially in 1924 for livestock feed and 

 soil conditioning. Since 1925, when a cement plant 

 began production at Redwood ("ity, the shells have 

 served mainlv as cement ravv materials. From 19^1 to 



about 1950, shells also were calcined to make lime, 

 which was used for reacting with salt works bittern to 

 produce magnesium compounds. In recent years, 

 three companies have been active in shell dredging. 

 Ideal Cement (^oinpany was the major producer, util- 

 izing shells and associated mud for the production of 

 cement. * Pioneer Shell Company dredges and proc- 

 esses shells mainly for cattle and poultry feed. South 

 Bav Dredging Company also dredged and uashed 

 shells but sold all of their product to Bay Shell Com- 

 pany for further processing and eventual sale for live- 

 stock feed and soil conditioning. ** 



A summary of past and present operations, as of 

 1969, is given in table 6. More detailed data appear 

 below under individual company descriptions. 



Occurrence of Shells 



In spite of many years of commercial shell dredging, 

 numerous technical reports on the sediments of the 

 bay, and thousands of drill, core, and dredge samples 

 collected, surprisingly little detail is known about the 

 size, extent, and characteristics of the individual shell 

 accumulations. Most investigations of the bay sedi- 

 ments treat the shell occurrences incidentally or in 

 only general terms. The sediments of San F'rancisco 

 Bav are probably best summarized by Treasher 

 (1963) and Trask and Rolston (1951). Detailed data 

 on the Holocene sediments, in which most of the 

 shells occur, are presented in unpublished reports by 

 Conomos (1963) and Gram (1966). Investigations 

 specifically concerned with shell deposits are reported 

 by Hart ('l966a) and Story et al. (1966). Many other 

 pertinent references are listed by Trask (1953), Terry 

 (1955), and Goldman (1969), 



The principal shell accumulations consist of native 

 oysters deposited in the shallow parts of south San 

 Francisco Bay. Most of the deposits lie in the upper 

 part of a Holocene soft mud unit — referred to as 

 "younger bay mud" by Treasher (1963). Partly ve- 

 neering this unit are surficial inodern oyster shell ac- 

 cumulations. Both the mud-associated and the 

 surficial deposits have been dredged commercially, 

 but the former constitute by far the largest reserves. 

 Shells exposed on the bay bottom, excluding periph- 

 eral beaches and bars, indicate the general areas where 

 buried shell deposits exist (figure 2). 



Smaller deposits of oyster shells also occur uith 

 older bay mud but are too small and deeply buried to 

 be of economic value. Fragmental shells (mostly 

 clams) associated with sand in several parts of the bay 

 conceivably could be a source of shells but have not 

 been dredged for that purpose. 



I he princqial shell occurrences — younger bay mud 

 deposits, surficial deposits, and shells associated with 

 sand — are described below. 



• Shell-dredging and cement manufaciurc by Ideal Cement Company ceased 



al the end of 1970, (See fi«)tnolc tjclow under Ideal Cement Company), 



•• Shell-drcdging by South Bay and processing by Bay Shell ceased in IVftV 



