CENTRAL COAST RANGES REGION (B) 



This region covers the central part of the Coast 

 Ranges and an adjacent portion of the Sacramento 

 \'alley (plate IB). It includes all of Alameda, Marin, 

 Napa, San Francisco, Santa Clara, Santa Cruz, and 

 Sonoma Counties, and parts of Contra Costa, Merced, 

 Sacramento, San Joaquin, Solano, and Stanislaus 

 Counties. Although San Francisco Bay and its related 

 waterways physically subdivide the region, they also 

 serve as important harbors around which a large 

 population and attendant industries have become con- 

 centrated. 



Topographically, San Francisco Bay is a drowned, 

 northwestern extension of the Santa Clara Valley. 

 The valley and bay are bordered on the west by the 

 Santa Cruz Mountains and on the east by the exten- 

 sive Diablo Range. The mountain and valley features 

 north of San Francisco Bay are less distinctive, having 

 many local names, but essentially constitute the south- 

 ern end of the northern ("oast Ranges. 



Limestone and shell deposits are abundant in the 

 San Francisco Bay area, and many of them have been 

 developed to serve regional and local markets. The 

 earliest commercial developments were for lime, be- 

 ginning about 1850 at the Olema deposit in Marin 

 County and in 1851 at theCowell Home Ranch depos- 

 its in Santa Cruz County. Some lime also may have 

 been produced in 1851 from the Lime Ridge deposits. 

 Contra Costa County. The first cement (hydraulic) 

 was produced in 1860 from limestone near Benicia, 

 Solano County. Portland cement was first manufac- 

 tured in 1902 at Cement, Solano County, and In \')()} 

 at Napa Junction, Napa County. The strong demand 

 for lime coupled with excessive land transportation 

 costs were responsible for the opening of many small, 

 and often impure, deposits of limestone in the late 

 nineteenth and early twentieth centuries. A number 

 of deposits also uere opened in the IVZOs and early 

 19.?()s to meet the sudden expanded demand for agri- 

 cultural limestone. For additional historical data see 

 Bowen, 1951. More recently, a number of quarries 

 were established in cherty limestone to supply struc- 

 tural needs such as aggregate and road material. It is 

 emphasized here that many of the deposits, origmalK 

 operated as local sources of lime and structural and 



agricultural materials, are no longer of economic in- 

 terest. Limestone of potential industrial value appears 

 to be limited to the crystalline limestone of Santa Cruz 

 County and the shells of San Francisco Bay. However, 

 with selective quarrying and beneficiation, the lime- 

 stone of the Franciscan Formation may also have in- 

 dustrial potential. 



Geologically, the oldest limestone in the San Fran- 

 cisco Bay area is part of a metamorphic rock unit 

 similar to the Sur Series of the Santa Lucia Range to 

 the south. Most of the limestone is coarse crystalline 

 and high in calcium, although some is notably sili- 

 ceous and some is locally magnesian. Substantial but 

 undetermined limestone reserves exist in Santa Cruz 

 County, and small occurrences are known in Marin 

 and San Mateo Counties. Most of the lime manufac- 

 tured in the Bay area was made from the crystalline 

 limestone of Santa Cruz County. 



Even more widespread is the Cretaceous limestone 

 of the Franciscan Formation. Limestone of the Fran- 

 ciscan has been informally designated the C^aiera and 

 Laytonville types — the Calera-type being light to dark 

 gray and the Lavtonville-tvpe being pink to red (Bai- 

 ley era/., 1964, p. 68-77; Garrison and Bailey, 1967, p. 

 B94-B100). The former is much more common than 

 the latter. Both types of limestone are fine grained, 

 thin bedded and highly fossiliferous (Foraminifera, 

 nannoplankton). Large reserves of the Franciscan 

 limestone are available in San Mateo and Santa Clara 

 (>ounties and minor deposits are known in Marin, 

 Sonoma, and Alameda Counties. However, because 

 abundant thin interbcds and lenses of chert are almost 

 always present, most of the limestone deposits are of 

 value solely as sources of crushed stone. The one great 

 exception is the Permanente deposit which, as a result 

 of selective quarrying and recent beneficiation, has 

 provided rau materials for a major cement plant since 

 19.?9. A few other limestone deposits may be large 

 enough to be amenable to beneficiation and conse- 

 quently may be of potential industrial value. 



(.^laternary carbonate deposits of two types have 

 also been of considerable importance. Deposits as- 

 sociated with springs have yielded large quantities of 

 travertine, calcareous tufa, and caliche for cement 



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