The Geology Rocks at Rodeo Beach South

Mill Valley, Ca – Need a secluded spot to relax? Check out Rodeo Beach South, a short hike through a beautiful landscape to a beautiful beach. Plus, it’s right near Point Bonita, making this an easy hiking double feature.

About the Park

Mill Valley, Ca – Region of the Golden Gate National Recreational Area

Science Spotlight: Rodeo Beach Geology

When I walk along California’s beaches, I tend to be so captivated by the landscapes around me that I find it easy to ignore the beauty beneath my feet. While every beach has some beautiful geological history to offer, Rodeo Beach stands unique amongst other beaches. Its beautiful, coarse, and pebble-dotted sands reflect a rich geological history.

Remember learning about the 3 major rock families when in elementary school science? As a refresher, rocks can be categorized based on the way in which they are formed. The three rock families are igneous, sedimentary, and metamorphic rock – and it just so happens that you can find all three on Rodeo Beach, making it a gem in terms of California geology.

Looking at the beach geology more granularly, one source reports that Rodeo Beach sands are composed of the following: red and green chert (about 55%), volcanic rock fragments such as pillow basalts (about 30%), lesser amounts of graywacke sandstone (about 10%) and finer mineral grain, such as feldspar and hornblende (about 5%).

Carnelian, although representing a small percentage of the rock composition, is another notable find on Rodeo Beach. These semi-precious gemstones are bright to reddish-orange in color. These beach gems are formed when small pockets, or vesicles, of silica appear within cooling lava. Years of collecting have negatively impacted the amount of carnelian found on Rodeo Beach, so be sure to refrain from collecting.

© David Abercrombie, 2014, Flickr Album, some rights reserved.

Visit the Park

Please note that there is no entrance fee to enter the park. Dogs are allowed on leashes.

We recommend visiting the southern tip of Rodeo Beach since it’s a skip, hop, and a jump away from Point Bonita. Regardless of the area that you visit, be sure to keep your eyes peeled for shorebirds dotting the beaches, mats of invasive ice plants, and the mosaic of different rocks that make up the sand.

Here are some helpful resources to help plan your visit:


Author: Taylor Crisologo

Taylor studied biology at Cornell University, where she worked with the Cornell Lab of Ornithology on projects ranging from breeding herring gulls off the coast of Maine to dancing lyrebirds in Australia’s Blue Mountains. When she’s not researching great places to experience Bay Area nature, you can find her birding or reading a book at home with her husband and their two indoor cats (Max and Penelope).

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