Rocky Ridge
Map SatelliteRocky Ridge has a number of features of interest to the natural scientist. Paramount, perhaps, is the geological structure of the ridge. It is a hog-back ridge formed by a series of parallel ridges of marine sandstones of the San Pablo group, Neroly, Cierbo and Briones, all dating to the Upper Miocene. This group is faulted against Pliocene rocks of the non-marine Orinda-Mulholland complex. Overall, the ridge presents a stepped profile, with steep, rocky outcrops separated by almost level terraces. The hog-back ridge results from the rock layers eroding at different rates. The most resistant rock is the Cierbo sandstone, a well-cemented graywacke, which forms the outcrop near the summit and occurs as almost vertical rock slabs.
There are numerous fossils in certain of the beds, primarily invertebrates such as pelecypods and pectens, but there are some vertebrates, including an early horse, and leaf impressions and tree-trunk fossils.
Soils are of the Hugo loam series and are generally shallow.
Five plant communities are represented: grassland, oak woodland, riparian, chaparral, and north coastal scrub. Along the ridges the chamise, Adenostoma fasciculatum, dominates the chaparral and occurs as short, stubby, lichen-encrusted, widely spaced shrubs, as opposed to the more common occurrence elsewhere in thickets. In the oak woodland, three species of oak are found, valley, coast live, and maul, Quercus lobata, Quercus agrifolia and Quercus chrysolepis, the latter occurring in an almost pure stand. There is also a dense woodland dominated by the maul oak and California bay, Umbellularia californica. California huckleberry, Vaccinium ovatum, is present in the understory.
Rocky Ridge is near the meeting point of the cooler and moister western climate and the warmer and drier interior climate, and supports a varied fauna.
Integrity: Virtually undisturbed; there are dirt roads and military buildings near the summit. The area is fragile. It has been designated a natural preserve, with portions as environmental education areas.
Use: Educational, research, observational.
Ref: Jackson, Rodney, 1973. Educational Land Use Survey, East Bay Nun. Utility Dist., Oakland, Calif., 68 pp.
March 1976
Inventory of California Natural Areas
Revision © 2009 Steven Louis Hartman