1978 



Limestone in the Coast Ranges 



29 



l)c used commerciall\, although they may have been in variations of individual shell lenses and groups of 



the past. lenses are not available. However, some idea of the 



chemical composition of a high-grade mud-associated 

 Shells associated with sand. Fragmental shells, ^h^n deposit is indicated by sample 1 in table •/ I'hc 

 perhaps composed mostly of clams, are associated sample is a product of hydraulic dredging and un- 

 with sand in shoal areas north of San Francisco where doubtedly is higher in lime (CaO) than the in-place 

 currents ar.- strong. Sand-shell deposits are shown to sample because of the inherent washing effect of the 

 be exposed northwest and southwest ot Angel Island dredging process. Sample 2 (table 7), which is the 

 by Trask ( 19.V?, figure {}-.?). Samples ot dredgings just same as sample 1 exxept that it has been wa.shed, con- 

 west of Alcatraz Island are reported to contain at lea.st fai^j. 53 j^^^ linie. Samples 3 to 7 are washed shells 

 50% shells. A l.vfoot-thick layer of sand containing an f^om surficial deposits and contain .vV07% to .^4.3 1 % 

 undetermined percentage of broken shells also is ij^^g Unwashed sample 8 from a shell beach contains 

 known at San Bruno Shoal, v\ here the sand is buried 52 9% lime, 

 under 1 .^ to 20 feet of mud and shells. This shelly sand 

 has been used extensively for hydraulic fill at Foster Development and Reserves 



City. Whether the shells are distributed through the , . , 1 -,- ,^ n- r . n . 



% I II ^ ,A- \ „ i„ ^; It IS estimated that 2.'< to 30 million tons of shell have 



sands or are locally concentrated is unknown. In ei- , , , , r . o i- • n • 



., ■■ '. ., ,, , ,u„ ,u^ii j.,u, , „,u;^u been dredged from south San brancisco Bay since 



ther case, it IS conceivable that the shell debris, which ° l n r u- j ■ u ' r 



J , .u ,u ■ , i A \A u„ 1924. I he great bu k of this was used in the manutac- 



tends to be coarser than the associated sand, could be ^ ° ., r , , „ , , , r 



■ 11 r- ■ . J L cu 11 1 ^.,, ,,r ^,^A ture of cement. Most of the shells dredged came from 



easily beneficiated by screening. Shell-bearing sand , ... . , c- n r>j ri 



J •. re zr '■ D u u I )„ j^.u the vicinity of the San Mateo Bridge east ot the main 



depositsof San Francisco Bay have been dredged sole- ,. , • , , ^. ,.0 j j 1 1 



ly for fill purposes so far ' "'^'P channel (see figure 2). Some dredging has been 



•^ * ^ reported south of Dumbarton Bridge, and bottom 



Composition of Shell Deposits depth changes recorded in nautical charts of different 



years indicate that additional dredging has been done 

 Samples indicate that more than 95% of the shells ^^^j. ^^^^ Qakianj Airport, south of Alameda. Recent 

 dredged commercially are derived from the very pro- dredging by Ideal Cement Company has been con- 

 litic and widespread native oyster Ostrea lunda Car- j^^,,^^ just north of the San .Mateo Bridge causeway. 

 penter. I his oyster produces a rather flat and very -y^^ company operates* a hydraulic dredge with a 

 thin shell that is a maximum of 2 '/, inches long. Minor ^^^^^^ j^^^d to develop a bench-cut that is worked east- 

 amounts of debris from other mollusks, plus barnacles ^.^rd. Pioneer Shell Company and South Bay Dredg- 

 and bryozoa, also are present. The presence of the Company" dredge hydraulicallv in water no 

 Japanese littleneck clam, introduced into the bay in ^^ ^j^^n 10 or 11 feet,' reportedly near the San 

 1930, readily differentiates the modern shells from ^^^teo Bridge causeway. Surficial deposits of loose 

 those dredged from older buried deposits. Imported ^j^^„^ ^^^^ ^.^^^^^ ^^^ ^^^y bottom or accumulate local- 

 oysters once raised corjimercially, may also constitute ,^, ^^^ believed to be the main sources of shells used by 

 some of the surficial shell debris locally. ^^^^^ companies. Both Pioneer and South Bay wash 

 As indicated above, the most important shell depos- ^^^ ^j^^„^ ^^^^^^^ ^^^-^^ j^^j j^,^ j,, 1^^^^ process- 

 Its or horizons consist of multiple shell lenses in mud. 



The discrete shell lenses also contain variable amounts • Operat.on.s ceased at end of W70. 



of interstitial mud. Specific data on the compositional •• South Bay ceased dredging in iw* 



Table 7. Chemicol analyses of oyster shell samples from south San Francisco Bay. 



Oxides (dry weight basis) 



in percent I - ^ ■>_ i_ ^ ^ ^_ 



Ignition loss 35.94% 43.47% 43.14% 4J.(t6% 43.60% 43.00% 43.50% 43.60% 



CaO 45.14 " 53.80 53.07 54 31 53.2 53.1 53.1 52.9 



MgO 1.58 0.34 0.51 0.51 0.44 0.74 0.64 0.62 



SiOt 9.70 1.40 2.15 0.98 0.72 1.5 0.74 0.83 



AijOs 1-84 0.15 0.34 0.11 0.43 0.84 0.37 0.47 



FejOj 1.65 0.11 0.24 0.09 0.11 0.16 0.09 0.09 



p,Oj 0.17 0.05 0.05 0.05 0.04 0.05 0.05 0.06 



Narf) 1.86 NO NO ND ND ND ND ND 



K,0 043 ND ND ND ND ND ND ND 



Total 99.31% 99.32% 99.50% 99.91% 98.54% 99.39% 98.59% 98.57% 



ND = Not done. 



Samples 1^1 — analy/.ed by I- l-ofgrcn, I9fi2. 



Samples 5-K — analyzed by C Smith and M. 'I avcia, IV67. 



Sample 1— from Ideal Omeni Cji barge. Rcdwmx) City plant, unwashed sample dredged east of channel near San Mateo Bridge. 



Sample 2 — same as sample i but washed by hand <ivcr a 28-mesh screen. 



Sample J — from primary stockpile of Pioneer Shell Co . Peialuma. sample dredged, washed and screened near San Mateo Bridge. 



Samples 4 and !— from primary stiKkpile at Bay Shell C:o . Alviso; shells dredged, washed and screened near San Mateo Bridge by South Bay Dredging i.-n 



Samples 6 and 7— dried, crushed and screened shell from Bay Shell Co plant. .Mviso Sample ft is "medium shell", sample 7 is "flour shell" 



Sample 8 — unwashed shell from beach at Brewer Island. San Mateo County. 



