Marin Headlands: Chert, sandstone, and pillow basalt

Draft: Should be completed by early February 2025.

About our hike

Rocks we will see in this area include sandstone, radiolarian chert, greenstone, and pillow basalt.

  • group of people looking at red layered rocks along the side of a road
  • photo showing a footbridge leading out to a lighthouse on a cliff over the ocean
  • photo showing a tower of rocks in the ocean
  • Photo showing a racoon peering out from bushes
  • Photo showing a group of people looking at a bluff of sandstone above a beach.

This hike is adapted on Ted Konigsmark’s Geological Trips: San Francisco and the Bay Area, Trip 2. We start at Rodeo Beach, where this is a nice outcropping of sandstone, then head across the beach to see some pillow basalts. Then the uphill begins: We walk a couple miles and 820 feet up to Battery 129, where we can see hawks soaring (Hawk Hill) and millions of year old radiolarian chert. After a brief lunch break, we start back down and swing by the pillow basalts at Point Bonita lighthouse. Then a short hike back to the beach.

The hike is 7-8 miles, with about 1,000 feet of elevation gain.

Geology

Sandstone at north end of Rodeo Beach

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Pillow basalts at south end of Rodeo Beach

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Radiolarian chert at Battery 129

black and white images of microscopic radiolarian, all kinds of spiky shapes.

Radiolaria are microscopic, silica based skeletons. Radiolaria live in deep oceans, and their skeletons float down to the ocean deeps over millions of years.

Links for more information

Selected videos

Quick introduction to the layering we observe in the area.

Pillow Basalt

Nice description of the rocks we will see

Hydrothermal vent through the chert

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