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<br />1 <br />east (70 to 80 degrees). Clay and silica are produced from <br />' the Laramie Formation and Fox Hills Sandstone, both of Upper <br />Cretaceous Age. <br />1 Generally, the Laramie Formation is divided into two <br />' parts. The Upper Laramie is described in USGS Map I-790-A <br />as being "light gray micaceous siltstone stained yellowish <br />orange, light, olive, and pinkish-gray silty claystone, <br />grayish-brown lignitic claystone, minor white and yellowish- <br />' orange friable ridgeforming sandstone, and near the top are <br />thin layers of conglomerate composed of pebbles of sedimentary <br />rocks. Sand grains are quartz and chert. Yellowish-orange <br />sandy ironstone concretions." <br />The lower part of the Laramie Formation is described <br />' as almost entirely yellowish-gray ironstained and white 'salt <br />' and pepper' friable sandstone composed of quartz, biotite <br />mica and kaolinized feldspar. Sandstone 110 feet thick at <br />' base is considered Fox Hills Sandstone by one authority. Gray <br />sandstone contains gray shale chips. Contains thin ironstone <br />layers and shale layers near base. Sub-bituminous coal beds <br />' as thick as 8 feet lie in the lower 200 feet above basal sandstone. <br />Abandoned coal mines are areas of potential subsidence, as <br />' in the valley south of Alameda Parkway. Gray or white claystone <br />in beds more than 10 feet thick is used for manufacture of <br />' brick and tile. Contains fossil leaves, wood and other plant <br />remains." <br />' 6 <br />1 <br />