24 



California Division of Mines and Geology 



Bull i97 



ly, the coquina approaches pure hmestone in composi- 

 tion; but it is generally quite sandy, grading into 

 fossiliferous (calcareous) sandstone. A sample of typi- 

 cal sandy coquina from the small quarry was analyzed 

 by Matti Tavela of the Division of Mines and Geology 

 in August 1963 and showed the following content: 

 36.5% CaO, 0.78% MgO, 23.0% SiO^, 3.7% Al^O,, 

 0.78% Fe,0„ 0.10% P^O,. Although total reserves of 

 impure coquina may aggregate several million tons, 

 good qualitv limestone is practically nonexistent. 



The Pleasanton deposit was worked around 1917 by 

 Pleasanton Lime and Cement Company as a source of 

 agricultural lime. Sandy coquina was obtained in 

 NWy; NW'X sec. 4 from a narrow hillside quarry 

 where a face 60 feet long by a maximum of 30 feet high 

 was worked. The limestone was hauled via narrow- 

 gauge rail to a nearby plant and burned in two oil- 

 fired kilns. At the quarry, an adit (now caved) report- 

 edly was driven 50 feet northeast to explore the depos- 

 it. Not more than a few thousand tons of limestone 

 was produced. 



References: Huguenin and Costeilo, 1920, p. 22; Laizure, 1929, p. 434; 

 Logan, 1947, p. 206; Hall, 1958, map. 



Rockaway (Calera Hill) deposit. Location: Ap- 

 prox. NVVy, SW'X sec. 2 (proj.), T. 4 S., R. 6 W., M.D., 

 in Pacifica just north of Rockaway Beach; San Mateo 

 15-minute quadrangle. Ownership: Ideal Cement 

 Company, 420 Ideal Cement Building, Denver, Colo- 

 rado 80202; leased by Rhodes-Jamieson, P. O. Box 1 18, 

 Oakland (1970). 



The Rockaway deposit is a large mass of Franciscan 

 limestone situated west of State Highway 1, where it 

 forms a resistant prominence known as "Calera Hill". 

 It has yielded large amounts of limestone and associat- 

 ed chert for use as aggregate and other crushed rock 

 products (Davis, 1955, p. 436; Logan, 1947, p. 308; 

 Huguenin and Costeilo, 1920, p. 178). The limestone 

 also may have been used as a source of lime, because 

 the remains of some kilns are reported (Eckel, 1933, p. 

 354). The earliest record of production is for E. B. and 

 A. L. Stone Company, who operated a quarry from 

 1910 to April 1919. Apparently, the deposit then lay 

 idle until 1942 but has been active ever since. The 

 other operators include Rockaway Quarry, Inc., 1942- 

 1953; Marks Materials, 1953-1967; and Rhodes-Jamie- 

 son since 1967. Just when Ideal Cement Company ac- 

 quired ownership of the property is not known, but 

 the limestone has not been used to manufacture ce- 

 ment. 



This deposit has been described and mapped as a 

 200-foot-thick sequence of limestone and associated 

 chert exposed over a triangular area 1,400 feet by 900 

 feet (Darrow, 1963, p. 12-13, plate 1). It originally was 

 exposed through more than 300 feet of relief from sea 

 level to the top of the hill. The beds form a syncline 

 that plunges 45° NW, and they are truncated on the 

 north by a fault. Minor faults and folds also seem to 

 complicate the structure locally. The mass consists of 



a well-bedded, highly fractured sequence of light and 

 dark limestone with thin interbeds and nodules of 

 chert. The percentage of chert varies from place to 

 place but constitutes a substantial portion of the 

 deposit. Interbeds of greenstone and other altered 

 rocks are also present to some extent. The light lime- 

 stone is distinct, being light dove-gray, fine grained, 

 dense, and foraminiferal. Limestone that is free from 

 visible impurities is represented by the chemical anal- 

 ysis of sample 2 in table 5. An analysis of a composite 

 sample representing a 35-foot section of light lime- 

 stone, as given by Walker (1950b, table 2), shows 

 48.73% CaO and 10.85% SiO;. 



Contrasting with the above is the dark limestone 

 which is generally dark blue-gray, fine crystalline, 

 dense, bituminous, and commonly well laminated and 

 platy. Although not as obviously associated with chert 

 beds as the light limestone, the dark limestone is often 

 quite siliceous. Variations in chemical composition 

 are indicated by samples no. 1 and 3 (table 5), neither 

 of which contained visible chert beds. A composite 

 sample representing a 40-foot sequence of dark lime- 

 stone is shown by Walker (1950b, table 2) to contain 

 44.81% CaO and 16.73% SiOj. Other analyses are 

 given in Logan (1947, p. 308) and Eckel (1933, table 

 2). 



Table 5. Chemical analyses of limestone from the Rockaway 

 deposit, San Mateo County. 



Riickawav Rockuwjy Roclamjv 



(hide #/ * ' #-' #-'• 



CaO 25.0% 54.50% 53.50% 



MgO 0.5 0.51 0.51 



SiO; 50.0 1.67 1.57 



.MiO, 0.38 0.19 0.35 



Fe,0, 0.30 0.10 0.17 



P,0, 0.08 0.07 0.16 



TiO, 0.08 ND ND 



Organic C 2.7 ND ND 



Ignition loss (less C) _22^ 42.96 43.39 



Total 101.04 100.00 99.65 



ND = Not done 



• ,\djusted anaUsis. 



Sample I — typical dark blue-gray finely-crystalline, bituminous limestone 



uith no visible impurities. 

 Sample 2 — typical light dove-gray, fine-grained, foraminiteral limestone with 



no visible impurities. 

 Sample i — similar to .sample 1 but partly weathered buff and platy 



Analyzed by Lydia l.ofgren, California Division of Mines and Geology, 



1952. 



When examined in May 1962, the deposit was devel- 

 oped from the southeast side by a quarry about 500 by 

 700 feet with a maximum face of about 150 feet. This 

 is bordered to the north and west by a series of 

 benches, the highest of which may be 200 feet above 

 the quarry floor. A tunnel adit on the east side of the 

 deposit is all that remains to indicate the former glory- 

 hole operation of E. B. and A. L. Stone Company. 

 From the extent of the workings in 1962, it is estimat- 

 ed that roughly 3 million tons of materials have been 

 quarried at this deposit. 



