1978 



LiMESTONF IN HIE COAS 1 RANGES 



19 



Limestone of the Franciscan Formation is shown as 

 four discontinuous masses extending more than 2 

 miles northwesterly from SVV'XSW'/, sec. 2 by Walker 

 (1950b, plate 1). The deposits, which lie on the steep, 

 vegetated slopes of (kahili Ridge, are of difficult access 

 except at the southeast end. There, typicall\- light- 

 gray, fine-grained, dense, high-calcium limestone, as- 

 sociated with thin-bedded chert, is exposed as occa- 

 sional low outcrops; and there is abundant angular 

 float over an area 400 feet by at least 800 feet. The 

 northwest extent of this mass is obscured by dense 

 vegetation, and limestone reserves could be large. Re- 

 serves, estimated solely for the exposed area of the 

 limestone mass, are substantial, amounting to about 

 20,000 tons of limestone and associated chert per foot 

 of depth. 



Because the deposits on Cahill Ridge lie within the 

 drainage area of potable water supplies of San Fran- 

 cisco, development possibilities probably are limited. 

 Nonetheless, the Cahill Ridge deposits may represent 

 important limestone reserves and warrant additional 

 investigation. This view is also supported by Walker 

 (19.>0b, p. 7-S). The dep)osits are undeveloped and 

 probably unexplored. 



California Aggregates (Royee) deposits. Loca- 

 tion: Sec. 1 1,1. 4S., R. 6 W. (proj.), M.D., in Pacifica, 

 '/; mile south-southeast of Rockaway Beach settle- 

 ment; San Mateo l.^-minute quadrangle. Ounership: 

 Ken Royce, Inc., ZO.*! S. Linden Avenue, South San 

 Francisco (1955). 



Franciscan limestone and interbedded chert are ex- 

 posed in two quarry areas at the crest of the ridge 

 between Rockaway Beach and Pedro \'alley. Accord- 

 ing to Harrow ( 196.^, p. 12 and plate 2), the limestone 

 occurs as two northwest-trending bodies — each about 

 700 feet long — which are part of a synclinal sequence 

 of basalt, chert, and other Franciscan rocks. However, 

 the bodies are broken bv faulting and sho\A' considera- 

 ble variations in bedding attitudes. At the lower 

 (northwest) quarry, the limestone is mainly dark blue 

 gray, fine crystalline, dense, thinly bedded, and in- 

 terbedded \Aith some chert. Some northeast dips can 

 be seen in the north face of the quarry, but most of the 

 beds are crushed and disoriented. At the upper (south- 

 east) quarry, the limestone is light and dark gray and 

 occurs in thin sequences with chert and basalt. 'Fhe 

 light-gray to dove-gray limestone is dense, fine 

 grained, and foraminiferal. The deposit apparently is 

 cut by a northwest-trending fault, as beds on either 

 side differ significantly in attitude. 



Only two chemical analyses are available for these 

 deposits. An analysis presented by Walker (1950b, p. 

 7, table 2) shows 32.23% silica for the quarry-run 

 material and reflects the large amount of chert present 

 with the limestone. The other analysis (Logan, 1947, 

 p. 308) shows 91.7% CaCO, and 5.1% SiOj for the 

 blue limestone. 



The deposits were worked as a source of crushed 

 rock used for road base and concrete aggregate from 

 1944 to 1952 (Davis, 1955, p. 435-436; Logan, 1947, p. 

 308). The lower quarry is developed as a trench 

 roughly 1,000 feet by 40 feet with a maximum depth 

 of 60 feet. The upper quarry is 400 to 500 feet in 

 diameter and 65 feet in maximum relief. The west end 

 of the quarry is now used to support a large steel water 

 tank. According to Davis (p. 46), both pits apparently 

 were "mined to their economic limits". Based on quar- 

 ry sizes, it is estimated that roughly half a million tons 

 of rock were excavated. Limestone reserves arc not 

 determinable but probably are small. As chert is so 

 prevalent in the limestone, future development of the 

 deposits appears to be limited to crushed rock uses. 



Carnegie deposit. Location; SE'X sec. 33 and SW'/^ 

 sec. 34, T. 3 S., R. 4 E., M.D., '/z mile south of Corral 

 Hollow Road at Carnegie (site) and 9 miles southwest 

 of Tracy; Altamont 7',4-minute quadrangle. Owner- 

 ship: Not determined (1962). 



The Carnegie deposit consists of several small vein 

 deposits formed during (^^aternary time along the 

 Tesla fault zone. The fault separates Franciscan rocks 

 on the south from the Panoche Formation (L'pper 

 Cretaceous) on the north (Huey, 1948, p. 62 and plate 

 2). According to Aubury (1906, p. 79), the main 

 deposit consists of a "series of aragonite veins" 3 

 inches to 3 feet wide which dip 65° NE. The hanging 

 wall consists of crushed shales although "considerable 

 serpentine inclusions" occur with the travertine. This 

 deposit is developed by a quarry face 225 feet long by 

 40 feet high and by four prospect tunnels. 



A quarter of a mile to the east in SW'/jSW'X sec. 34, 

 small, irregular veins and masses of travertine are ex- 

 posed over an area 300 feet by 200 feet. The travertine 

 is white to honey brown, banded, fine to very coarse 

 crystalline, locally vuggy, and somewhat brecciated. 

 These deposits are developed by small prospect cuts 

 and pits in which the veins range in thickness from a 

 few inches to a few feet. Exposures on the ridge to the 

 east may also be travertine. The deposits appear to be 

 too small to be of economic interest. 



Ihe only record of production is by the California 

 Lime and Cement Company from 1900 to about 191 1. 

 Most of the travertine was used to make lime at the 

 Carnegie Brick and Pottery Company plant. Some 

 limestone also was shipped to Stockton for use in glass 

 making. Minor prospecting east of the main deposit 

 was done more recently, but details are lacking. 



Other reference: Clark, 1955, p. 39. 



Collins deposit. Location: Sec. 30, T. 2 S., R. 3 E., 

 M.D., 1 or 2 miles southwest of Altamont; Altamont 

 7'/2-minute quadrangle. Ownership: CJraham Nissen, 

 Livermore ( 1966). 



"A small outcrop of crystallized limestone" is re- 

 ported by Aubury (1906, p. 64) on the summit of a hill 

 in sec. 30. As shown by Huey (1948, plate I), sec. 30 



