Sonoma County Transect

The Sonoma County Transect Explanation

Geologic History with ages in million years (my)

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 speckled 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 2009, Terry Wright