Friday, January 31, 2014

[Geology2] Quake Hike Series: Point Reyes and the San Andreas Fault in Marin County

Quake Hike Series: Point Reyes and the San Andreas Fault in Marin County

marin-county-bear-valley-earthquake-trail-sign by Suzy Rosenburg on Flickr

Tectonic landscapes make for some lovely scenery. Also, as a resident of such a shaky state, it is a good idea to get to know the earthquake fault that makes California earthquake country.

The San Andreas Fault is at essence "plate boundary on land", the boundary between the Pacific & North American plates. It is a strike/slip or transform fault that runs over 700 miles long as the crow flies.

In Marin County, the San Andreas Fault runs the length of Tomales Bay in West Marin, passing through the communities of Stinson Beach, Bolinas, Inverness, Olema, and Point Reyes Station. This is a part of the world where you can see the effects of Pacific Plate as it slides northwest with respect to the North American Plate. The plates are moving past each other at a rate of about 2.5 inches per year, about the same rate that fingernails grow.

Tomales Bay by DBerry2006 on Flickr

Tomales Bay by DBerry2006 on Flickr

The geology of Point Reyes is visible in the rock and flora differences in the east and west sides of Tomales Bay, which is the fault trace in this area. The east side is variegated sandstone, shale, and clays overtopped with grasslands, chaparral, and occasional oak trees.

The west side is almost all granite, and covered with a thick forest of oak, pine, spruce, bay, and fir trees. The granite that makes up the Point Reyes Peninsula has traveled far north along the San Andreas Fault from its source in the Southern Sierra Nevada.

As much of the land on either side of Tomales Bay is park land, it's a great place to hike along the San Andreas Fault. The All Things Marin blog suggests the easy, ranger-guided "On Shaky Ground" hike along the fault trace within the Point Reyes National Seashore.

Point Reyes by Michael_Peck on Flickr

Point Reyes by Michael_Peck on Flickr

Knitting Nurse blog describes what she considers her favorite hike: a day-long trek out to the end of Point Reyes. The hike she describes is 9.5 miles to the point and back, but it takes in forests, beaches, and vistas of grazing tule elk.

The Marin Independent Journal describes a vigorous 8.5 mile hike that takes you to Bolinas Ridge capped with stunning vistas of the tectonic landscape. A day-long hike, the trail starts in the trees, then wanders through marshes, grasslands and rolling hills, all created by the action on the fault.

The USGS has published a great resource for exploring the San Andreas Fault in the San Francisco Bay Area that maps out hiking trails all over the fault zone. You can also get free trail guides online and at the ranger station at Point Reyes.

Slurp by dotpolka on Flickr

Slurp by dotpolka on Flickr

Before you head up on your hike, you can stock up on snacks in West Marin's gourmet shops, bakeries, and restaurants in Point Reyes Station, Bolinas, Olema, and Marshall. And while you're there visit some oyster farms along Tomales Bay.

So take advantage of the dry weather we are having, and take a day to explore the beauty of West Marin's tectonic landscape.



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