Cartography by Gary Newman Design
Sonoma County Geologic History
120 million years ago
The continental margin forms as the ocean floor slides under the edge of the continental plate. A broken-up mixture of ocean floor material accumulates along the subduction zone, forming the Franciscan Complex of schist and serpentine.
Sonoma County Geologic History
5 million years ago
The San Andreas Fault forms along the interface between the Pacific and North American plates. Magma works its way up the fault system, causing lava and ash flows from volcanos that form Mt. St. Helena and Sonoma Mountain. The shallow Wilson Grove Sea covers the area from present-day Forestville to Sebastopol and Bodega.
Sonoma County Geologic History
Today
Coastal hills have continued to uplift, cut by the strong erosional forces of the Russian River. Continued erosion has filled the shallow inland sea with sand and gravel, forming the Santa Rosa Plain. The San Andreas Fault remains active.
Software: Adobe Illustrator, Photoshop