The northern-most San Andreas rift valley at Alder Creek in coastal Mendocino County is a highly fractured tectonic zone, featuring at least two main stems of the San Andreas Fault itself, and riddled with multiple minor transform and thrust faults, both on-land and offshore.
The rural northern landward extent of the San Andreas fault forms most of the arrow-straight Gualala and Garcia River valleys east of Point Arena, and extends northwestward (from the western bend in the Garcia River) as a string of tectonic pressure ridges and creek beds, offsetting marine terraces as it goes, finally exiting the landscape back under the Pacific Ocean at the north end of Manchester Beach as the Alder Creek drainage valley.
