Farm Hill No. 2 Mine (Emerald Lake Mine; Upper Emerald Lake deposit; Farm Hill Cinnabar property), Redwood City, San Mateo Co., California, USA
|Latitude & Longitude (WGS84):||37° 27' 12'' North , 122° 15' 1'' West|
|Latitude & Longitude (decimal):||37.45361,-122.25028|
|Köppen climate type:||Csb : Warm-summer Mediterranean climate|
A former Hg occurrence/mine located in the W½ sec. 36, T5S, R4W, MDM, 3.8 km (2.3 miles) SSW of Redwood City center, just inside the city boundary in Stulsaft Park, on private land (?). MRDS database stated accuracy for this location is .
Local rocks include ultramafic rocks, chiefly Mesozoic, unit 3 (Coast Ranges and Western Klamath Mountains).
The existence of mercury mineralization at the site has been known since 1863, but only after being rediscovered in 1953 during site work for a housing development, was it mined.
Fragments of cinnabar and metacinnabar were found in top soil. The area is in the central part of an irregular mass of serpentine, which at the surface has weathered to an ocherous clay. Below this clay, cinnabar coats fracture surfaces in silica-carbonate rock and the surrounding tuff and shale of the Franciscan formation. Cinnabar, metacinnabar, and native mercury occur in vugs and along fracture surfaces in the rock.
Eglestonite, terlinguaite, calomel and native mercury are also present in the ore. Very rare wattersite has been identified in some samples (R. Erd, 1990). Montroydite rarely occurs with the native mercury. The area is now largely covered by streets and houses, but some cinnabar-bearing exposures are in an adjoining city park.
Commodity ListThis is a list of exploitable or exploited mineral commodities recorded at this locality.
8 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Oligocene - Paleocene|
23.03 - 66 Ma
|Eocene marine rocks|
Age: Paleogene (23.03 - 66 Ma)
Stratigraphic Name: Anita Shale; Avenal Sandstone; Butano Sandstone; Canoas Siltstone; Capay Formation; Coldwater Sandstone; Cozy Dell Shale; Delmar Formation; Domengine Formation; Gredel Formation; Indart Sandstone; Jameson Shale Member of Markley Formation; Juncal Formation; Junipero Sandstone; Kreyenhagen Shale; Llajas Formation; Los Muertos Creek Formation; Lucia Shale; Mabury Formation; Maniobra Formation; Markley Formation; Matilija Sandstone; Meganos Formation; Nortonville Shale; Point of Rocks Sandstone; Rose Canyon Shale; Sacate Formation; Santa Susana Formation; Santiago Formation; Sierra Blanca Limestone; Tejon Formation; Tesla Formation; The Rocks Sandstone; Tolman Formation; Torrey Sandstone; Tres Pinos Sandstone; Welcome Formation; Yokut Sandstone; Reliz Canyon Formation; Mount Soledad Formation; Ardath Shale; Scripps Formation; Friars Formation; La Jolla Group; German Rancho Formation
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. 
|Middle Eocene - Early Eocene|
38 - 56 Ma
|Whiskey Hill Formation|
Age: Eocene (38 - 56 Ma)
Description: Light-gray to buff coarse-grained arkosic sandstone, with light-gray to buff silty claystone, glauconitic sandstone, and tuffaceous siltstone. Sandstone beds constitute about 30 percent of map unit. Tuffaceous and silty claystone beds are expansive. Locally, sandstone beds are well cemented with calcite. At apparent base of section on north side of Jasper Ridge, just east of Searsville Lake, a thin greenstone-pebble conglomerate is present. In places within this map unit, sandstone and claystone beds are chaotically disturbed. This formation is as much as 900 m thick
Reference: Brabb, E.E., R.W. Graymer, D.L. Jones. Geology of the onshore part of San Mateo County, California. U.S. Geological Survey Open-File Report 98-137.