We can see a side view of these rocks in the figure above.
Different symbols and patterns show the relationships among rock
units and faults. This diagram is reference point for virtual field trips accessed by links below. These pages are a distillation of information from my various classes and field trips on Sonoma County Geology.
West of the San Andreas Fault. Bedrock west of the fault is
beautifully exposed on the ocean side of Bodega Head at Windmill
Beach. The salt and pepper texture Bodega Head Diorite
(100 million years) intruded as molten rock. It contains dark
patches which are the remains of ancient layered rocks ripped off
by the diorite as it pushed its way toward the surface, called the Sur Series These
rocks are laced by pink stripes of coarse grained granite
pegmatite and aplite dikes. Faults offset the stripes, relatives
of the San Andreas Fault.. Orange layered sands overlie the
diorite, and modern quartz-rich sand eroded from the granite
forms the beach. We call this sequence "Salinia" because similar
rocks form the Santa Cruz and northern Big Sur mountains near
Salinas.
East of the San Andreas Fault. Bedrock east of the San
Andreas Fault is mixed up bunch of rocks called the Franciscan
Complex (140-42 million years). Its parts formed on the ocean
floor far from here and accumulated in an oceanic trench . These
rocks then mixed together by faults as the ocean floor slid under
the continent. Some rocks were faulted deep into the earth where
they were effected by high pressure, but fairly low temperatures,
giving rise to blueschist. The California state rock, serpentine
is also typical. Magnificent examples of these rocks are
exposed at Shell Beach on the Sonoma County coast south of Jenner.
Our landscape is created by opposing forces: constant uplift from pressure caused by the grinding of tectonic plates along the San Andreas fault: and continuous erosion by rivers, landslides and humans.
West of the San Andreas fault, the Sur series formed as ancient sedimentary layers which were changed by heat and pressure (250 my). Molten rock then cooled deep below to form speckeled Diorite and white bands of coarse granite seen at Bodega Head (100 my) . The San Andreas Fault moved the Sur series and underlying Diorite at least 320 miles north relative to the mainland during the last 29 million years.
After uplift and erosion of 5 miles of overlying rocks, sand and gravel accumulated on a beach and became Marine Terrace deposits (40,000 years). These ancient beaches are being uplifted and eroded today along the coast. Look for Native American white shell middens in dark organic soil above the sands at Bodega Head. Alluvium is sand and gravel forming today in river valleys and on beaches.
East of the San Andreas Fault, the Franciscan Complex is a faulted mixture of ocean crust rocks (140-42 my). Its contents formed on the ocean floor, some many thousands of miles southwest from here. They mixed along faults and stuck to the continental edge as the ocean floor slid down under western North America, visible at Shell Beach on the coast. After 30 my of uplift and erosion, the Petaluma Formation formed from sedimentation in a shallow sea (10my). After more uplift, tilting and erosion, the Sonoma Volcanics resulted from erupting lava and ash (3-8 my). The Wilson Grove Formation of sands and fossil shells formed in a shallow sea to the west of the volcanos (3-5 my). With yet more uplift and erosion, the Russian River cut its path across rising mountains to the sea. Glen Ellen gravel, sand and clay formed in local valleys (1my).
Alluvium is the eroded sand and gravel that washes into the river valleys and up onto the beaches of Sonoma County , part of the ongoing dynamics of geologic change.
Copyright 1998, Terry Wright
11/28/98 TW