Rodeo Beach Geology

Red and green chert pebbles, carnelia, sandstone and basalt.

Rodeo Beach formed about 5,000 years ago when sea level rise slowed after melting at the end of the last glacial period. At that time, this sandy barrier beach dammed an ancient stream valley to form Rodeo Lagoon. Strong waves and currents produce the unusually coarse and pebbly beach sand here today.

Watchful beachcombers can find a rainbow of pebbles dotting the shore. The vibrant colors are the result of the most common rock found in the Marin Headlands: bedded ribbon chert, which you can see exposed nearby at the south end of the beach. Traces of iron in the chert are oxidized in different amounts to produce a wide spectrum of colors, including red, green, yellow and black pebbles.


Admire these natural beauties all you like, but please don't take them from the beach.