Although bedrock in most parts of the Howell Mountains is a geologic formation of volcanic origin known as the Sonoma Volcanics, older sedimentary rocks of the Cretaceous age Great Valley Sequence are found in the Benicia area at the southern end of the range, and in the Lake Hennessey area to the east of St. Helena. The Sonoma volcanics, which make up bedrock in all of the grape-growing areas of the Napa Valley side, are late Miocene to early Pleistocene in age, and made up largely of silica-rich rhyolite tuffs and breccias interbedded with silica-poor andesite and basalt lava flows, with some volcanic gravels that are related to fluvial processes. Serpentine is found in a few places where older sedimentary rocks are faulted against younger volcanics, and in some of these same areas, particularly around Sulphur Springs Mountain at the southern terminous, cinnabar deposits were mined in the late 1800s for quicksilver.