Although rhyolitic rocks generally weather to soils that are considered to be nutrient poor, the Howell Mountain American Viticultural Area and other wine appellation districts located on the west slope claim that grape vines planted in these soils are stressed and as a consequence produce wine grapes that are superior in quality for wine making when compared to grapes grown on unstressed vines planted in the richer loams of the valley floor.
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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 Benecia area at the southern end of the range, and in the Lake Hennessey area to the east of St. Helena.
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