Point Reyes Peninsula, California
Posted by: 
DougK
N 37° 56.078 W 122° 41.843
10S E 526591 N 4198605
The San Andreas fault line runs through the Olema Valley, stretching from Bolinas Lagoon in the south to Tomales Bay in the north end. Point Reyes peninsula resides on the Pacific Plate to the west, which is moving north at the rate of 1 inch a year.
Waymark Code: WM9NCJ
Location: California, United States
Date Posted: 09/08/2010
Views: 8
Point Reyes Peninsula is a unique piece of land off the coast of California. The geology of this land mass is much different from the land across the Olema Valley to the east.
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The reason for this is plate tectonics. Point Reyes Peninsula is a trianglular section of crust and extends out into the Pacific Ocean. Point Reyes sits on the Pacific plate and mainland California sits on the North American plate. The boundary between these two land masses is known as the San Andreas faultline.
Satellite views of Point Reyes show a huge rift line running NW-SE, separating the Point Reyes Peninsula from the main body of California. At this point the fault line runs from Tomales Bay in the north to Bolinas Lagoon and Stinson Beach in the south. The Olema Valley parallels the San Andreas fault line.
The Pacific plate with Point Reyes has been sliding north along the coast of California for millions of years.
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Photo from Richard L. Gunther
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Dramatic movement along the San Andreas fault line can be seen on the Earthquake Trail at the Bear Valley Visitor Center. Another nice display at the visitor center is the 3D relief model of the fault line area and the terrain around Point Reyes.
From the Introduction to Point Reyes Peninsula: A Relentless Journey Northward by Richard L. Gunther:
Point Reyes Peninsula is a unique piece of continental crust attached to western Marin County in northern California. The peninsula is unique because it is vastly different geologically than its mainland neighbor. The reason for the geological differences is because Point Reyes is a long way from its source region. Point Reyes has been traveling for millions of years on the Pacific plate. At this time it is adjacent to Marin county that is part of the North American plate. The boundary between these two plates is the San Andreas fault.
The San Andreas fault is a strike-slip fault that separates Point Reyes Peninsula from Marin County. The fault zone travels through Bolinas Lagoon in the south through Olema Valley and out Tomales Bay in the north. In the famous earthquake of 1906 Point Reyes Peninsula moved approximately 5 meters north relative to Marin County.
The massive earthquake of 1906 and the unique geology of Point Reyes was not fully understood until the 1960's. During the 1960's the theory of plate tectonics became widely accepted. The processes of plate tectonics when applied to Point Reyes Peninsula and California brought a more comprehensive understanding to the processes that formed them.
With the acceptance of global tectonics in the 1960's geophysicists began to look closely at the west coast of North America. They realized that the west coast was actually made up of many large pieces of different sections of continental crust. The pieces of crust were named terranes. The terranes had traveled from various places around the Pacific theater. The first terrane added to the west coast is called the Sonomia terrane. Sonomia was added 375 million years ago. Next came Smartville about 225 million years ago. The Franciscan terrane began the accretion process 175 million years ago. The most current terrane is Point Reyes Peninsula. Point Reyes Peninsula had its origins many miles south of its current location.
Posted coordinates are at the head of Bolinas Bay, where the earthquake fault line comes out of the ocean and follows the Olema Valley north to Tomales Bay. Pictures anywhere along the fault line or on Point Reyes will satisfy a visit to this waymark.