Report of the State Mineralogist . orma-tions composed of radiolarian chert. The exceptional resistance whichthese offer to erosion causes the areas they occupy to present a marked 1Geology of the San Mateo Quadrangle, U. S. G. S. Folio No. 193. 168 MINES AND MINERAL RESOURCES. contrast with adjoining areas composed of sandstone. * * * The southern block is marked by notable linearity in its crests andvalleys and its geomorphy is in general much less mature than that ofthe northern block. The most remarkable linear feature is the valleyin which lie San Andreas and Crystal Springs lakes, a shor
Report of the State Mineralogist . orma-tions composed of radiolarian chert. The exceptional resistance whichthese offer to erosion causes the areas they occupy to present a marked 1Geology of the San Mateo Quadrangle, U. S. G. S. Folio No. 193. 168 MINES AND MINERAL RESOURCES. contrast with adjoining areas composed of sandstone. * * * The southern block is marked by notable linearity in its crests andvalleys and its geomorphy is in general much less mature than that ofthe northern block. The most remarkable linear feature is the valleyin which lie San Andreas and Crystal Springs lakes, a short segment ofthe San Andreas rift valley, a feature due to repeated faulting in recentgeological times. The trace of the fault of 1906 follows this rift forabout 300 miles. The valley is due in part directly to earth movement,but in large measure also to the mashing of the rock in the fault zoneand its consequent easy erosion. Northeast of the San Andreas rift liesthe equally linear Buriburi Ridge which slopes down to Merced Valley. Photo No. 10. San Pedro Point, San Mateo County, on line of Ocean Shore steeply-dipping sedimentary strata. Photo by Walter W. Bradley. and to the Bay of San Francisco. The slope of the Montara orographicblock in the area southwest of the rift valley is dissected by a series ofsubparallel steep-sided canons and intervening ridges, which are inlarge part remnants of the original tilted surface. Viewed in a largeway, however, this tilted slope, taken as a whole, from the crest ofMontara Mountain to the Bay of San Francisco, presents a broadlyterraced aspect. It comprises two terrace levels or steps, one the flat-topped ridge that is elsewhere called the Buriburi Plateau, the generalelevation of which is about 700 feet, and the other the Sawyer Plateau,comprising a number of flat-topped ridges that stand at elevations of1,100 and 1,200 feet. The crest of the tilted block known as Montara Mountain is com-posed of quartz diorite and is so
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectminesandmineralresou
