Some of the best outcrops for these rocks are on the west side of the southern Sacramento Valley at Putah Creek and Cache Creek canyons.
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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.