Rock outcrops cover approximately 1,700 acres or 0.5% of the County. Over 50% of the County's rock outcrops are located in the Eastern Mountains and are generally located on the steeper rigdelines of the Sonoma Volcanics. There are three types of rock outcrop that are recognized in Napa County:
- Volcanic rock outcrop
- Sandstone rock outcrop
- Serpentine barren
Although rock outcrops are not treated as a biological community becuase species composition in these sites varies a great deal depending on the surrounding biological community, they do provide important habitat for special-status plants and wildlife.
Vegetation is sparse on rock outcrop areas. Lichens are found on the most exposed areas, while ferns and clubmosses ay be found in more sheltered portions of the outcrop.
Small trees such as leather oak and foothill pine, shrubs such as chamise, toyon and broom snakeweed (Gutierezia sarothrae) are associated with sandstone rock outcrops. Chamise and manzanita are common schrubs associated with volcanic outcrops, with common herbs include rock lettuce (Dudleya cymosa) and naked buckwheat (Eriogonum nudum). Leather oak, white-flowered musk brush, and white leaf manzanita are associated with serentine barrens, while common herbs include jewelflowers (Streptanthus spp.), globe gilia (Gilia capitata), and lace fern (Aspidotis densa).
Rock outcrops provide a key habitat feature to a variety of species, which may use these areas for nesting, foraging, or other purposes. Rock outcrops absorb heat during the day and radiate it during the night, providing a means for cold-blooded animals like the western fence lizard to maintain their body heat. Bats such as the little brown myotis (Myotis lucifugus) and Pale Townsend's big-eared bat (Piecotus townsendii pallescens) may roost in rock crevices. Rock outcrops also provide a vantage point by raptors to search for prey.
Reptiles and Amphibians
Western fence lizard
Use the map below to explore the various biotic communities throughout Napa County.