This subsection is on the Point Reyes Peninsula. It is bounded by the San Andreas fault on the northeast and by the Pacific Ocean on all other sides. The climate is temperate and humid; it is greatly modified by marine air. MLRA 15c.
Lithology and Stratigraphy
This subsection contains predominantly Mesozoic granitic rocks and middle Miocene marine sedimentary rocks adjacent to the San Andreas fault and middle and lower Pliocene marine sedimentary rocks southwest of the Miocene rocks. Also, there are substantial areas of beach and dune deposits along the northwestern ocean shoreline.
This subsection is on steep hills and mountains adjacent to the San Andreas fault, and an elevated coastal plain from there southwest to Point Reyes. Slopes on the coastal plain are nearly level on remnants of the Pliocene marine surface, to moderately steep below that surface. The elevation ranges from sea-level up to 1407 feet on Mt. Wittenberg. Ocean currents, waves, and wind (coastal marine and eolian processes) are active along the outer edges of the coastal plain. Fluvial erosion is the main geomorphic processes on the coastal plain, although eolian erosion and deposition has been active in the past. Mass wasting is another process that is active on steep slopes in the hills and mountains adjacent to the San Andreas fault.
The soils of the granitic terrain are mostly Pachic Haplustolls and Ultic Haplustalfs. On the Miocene sedimentary rocks they are Typic Dystropepts and some Lithic Haplustolls. On the Pliocene sedimentary rocks they are Ultic Haplustalfs and Mollisols. And on the recent dunes they are Psamments and Ustic Dystropepts. The soil temperature regimes are mostly isomesic, with some mesic. Soil moisture regimes are mostly ustic, and possibly some udic.
The predominant natural plant communities of granitic terrain are mainly Douglas-fir - tanoak series, Bishop pine series, and Coast live oak series. Plant communities of the Pliocene sedimentary rock terrain are mainly Pacific reedgrass series and Coyote brush series. All of the plant communities of these two terrains may be present in the Miocene sedimentary rock terrain. The dunes support a succession of plant communities, from bare dune through European beachgrass series and Sand - verbena - beach bursage series to Dune lupine - goldenbush series or Yellow bush lupine series.
Characteristic series by lifeform include:
- Dune vegetation: Native dunegrass series, European beachgrass series, Sand - verbena - beach bursage series.
- Saltmarsh vegetation: Cordgrass series, Ditch-grass series, Pickleweed series, Saltgrass series.
- Grasslands: California annual grassland series, Introduced perennial grassland series, Pacific reedgrass series
- Shrublands: Dune lupine - goldenbush series.
- Forests and woodlands: Bishop pine series, Douglas-fir - tanoak series.
The mean annual precipitation is about 20 to 40 inches; there is considerable summer fog. Mean annual temperature is about 50° to 53°F, and there is less than 14°F difference between lowest and highest monthly means. The mean freeze-free period is about 275 to 300 days.
Water runs off rapidly, and most of the streams, which are small, are dry by the end of the summer. There are no natural ponds or lakes in the subsection.
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