The Rainbow Sands of Rodeo Beach
Size:  (not chosen)
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This is an earthcache and so there is no physical cache to find. Instead, you will get to discover and learn something interesting about the geology of an area. Please adhere to the earthcache guidelines of Leave No Trace – Outdoor Ethics and be respectful of your surroundings. The beach sands and pebbles at Rodeo Beach are protected since the area is in National Park Service parklands. Please only take photos and only leave footprints.
Rodeo Beach is unique among California's beaches, both in terms of its geology and history. This earthcache will educate you about the unusual texture and mineral composition of the beach and give you an opportunity to explore a beautiful area with an interesting past.
PhotoRodeo Beach and Fort Cronkhite (J5 Crew, 2010)
Rodeo Beach Geology
Rodeo Beach is both a barrier beach and a pocket beach. During most of the year, the beach forms a barrier between the Pacific Ocean and Rodeo Lagoon. Sometimes after heavy rains, the barrier is cut by a channel, allowing the lagoon to drain into the ocean. As a pocket beach, Rodeo Beach sands do not migrate up or down the coast. Instead, they are carried a short distance offshore in winter, tumble about in the surf, and then return to replenish the beach in the spring and summer. Thus, the sands of Rodeo Beach are native to the Marin Headlands and reflect the Franciscan geology of the closest hills and cliffs. Local Franciscan rock types primarily include chert, basalt, and graywacke sandstone.
A number of researchers examined Rodeo Beach in the early 1970s and confirmed that due to the similarity of the beach composition to the local rock character or lithology, significant input of sand through longshore transport or drift does not exist. Further, it also was concluded that the volume of sand from Franciscan rock origin has not been significantly increased in modern times and higher sea levels. Probably much of the unique sand present on the beach had its origin in the erosion of coastal bluffs due to wave energy and rising sea level after the last glacial period when the sea level was about 100 meters lower than present. A rapid sea level rise occurred between about 18,000 and 5,000 years ago, but it has been relatively stable for the past 5,000 years.
Rodeo Beach is comprised largely of dark, coarse, pebbly sands and its mineral composition sets it apart from every other beach in the State of California. Specifically, the pebbly sands are composed predominately of red and green chert (about 55%), mafic volcanic rock fragments of submarine origin, such as pillow basalts (about 30%), and lesser amounts of graywacke sandstone (about 10%) and finer mineral grain, such as feldspar and hornblende (about 5%). Although a very minor constituent of the beach sand, carnelians deserve special mention because they are responsible for much of the historic interest in the beach by pebble collectors. This earthcache focuses on the abundant radiolarian chert and rare carnelian pebbles.
Radiolarian Chert – Chert pebbles are the most abundant on the beach. Chert is a sedimentary rock and much of it is formed from microscopic silica skeletons of marine zooplankton called Radiolaria. Radiolarian chert forms where two conditions are met. First, a deep, open ocean setting is required where there is little continental mud or carbonate sediment to dilute the “rain” of dead radiolarian shells settling to the seafloor. Second, the upper ocean waters need to be relatively rich in nutrients in order for abundant Radiolaria to thrive. Most local chert is red or brown and less commonly green, but it may be a range of colors. The color reflects the amount of oxygen present in the sediment when it became rock. If oxygen is plentiful in the sediment, it oxidizes small amounts of iron present and the chert is red or brown. If oxygen is scarce, the iron is reduced and the chert is green or black. The green chert pebbles are often mistaken for jade.
Carnelian – Carnelians were once abundant on the beach but intensive collecting over the years has made them scarce. However, with a little patience and luck, you should be able to spot some because they “stick out like a sore thumb” against the sea of pebbles (Carnelian Pebble Closeup). Please take time to enjoy them and then leave them for future visitors. Carnelians, which are commonly used as semi-precious gemstones, are translucent and bright orange to reddish-orange in color. They formed in small spherical cavities or vesicles in volcanic pillow basalt rock produced by air or gas bubbles while the molten rock was solidifying. Carnelians are comprised of microcrystalline quartz, also called chalcedony (chal – SID – ny). Chalcedony is a cryptocrystalline variety of quartz where crystals are too small to be recognized with an ordinary microscope. Pillow basalts at other Marin beaches may contain quartz or red jasper, but only the pillow basalts at Rodeo Beach contain many carnelians. Red jasper is sometimes mistaken for carnelian; however, jasper is opaque, not translucent like carnelian, and can be red or yellow.
Rodeo Beach demonstrates that not all sand is alike. In addition to mineral composition, geologists classify sands based on their texture (appearance). There are three main textures to consider: size, roundness, and sorting.
Size generally tells you how long a particle has been eroding in a system. The grain size usually decreases the older the particle. “Sand-sized” means particles from 1/16 to 2 mm in diameter, “granule-sized” means particles from 2-4 mm, and “pebble-sized” means particles from 4-64 mm. Rodeo Beach sand contains many pebble-sized particles, making it overall unusually coarse.
Roundness is defined as the presence or absence of corners and sharp edges on the particles. Particles with many edges are “angular.” Particles lacking edges are “rounded”. Particles get rounder as they are transported; their corners get broken off as they bump against other particles. Although locally derived, many of the particles at Rodeo Beach are well rounded, indicating that they have been tumbled repeatedly by the surf as they move on and off shore seasonally.
Sorting refers to the range in size of particles. Poorly sorted sediments show a wide range of grain sizes, while well-sorted ones have similar sized grains. Like rounding, sorting increases with transportation. If sand is deposited in a turbulent area, the sand typically is not well sorted. Conversely, if there is a quiet setting, the sand typically is well sorted. Overall, the sand at Rodeo Beach is not well sorted, owing to the turbulent environment and particle size differences between rock types present of different hardness.
Please note that failure to meet these logging requirements may result in log deletion.
This earthcache is designed to not only teach geocachers about the local geology of a beautiful beach, but it provides an opportunity to hone your search skills. In order to log a find for this earthcache, you must send me a private e-mail that identifies the cache name and waypoint (GC2H5H8) and answers to the following questions:
Go ahead and log your find when you submit your answers to the above questions. Please do not include your answers in your cache log.
- What kind of weathering process do you think created the majority of the Rodeo Beach sand?
- Why do you think the southern or northern end of the beach close to the tidal zone or around the Rodeo Lagoon sand berm provide the best opportunity for pebble hunting?
- Find a sample patch of pebbly sands for investigation and mark its location on your GPS. Using the texture guides above, what are the size, roundness, and sorting of a typical sand sample from your area of investigation? What are the coordinates of your sample?
- And now for the fun and challenging part… Find and document via a photograph posted with your cache log sorted sample(s) of red chert, green chert, and carnelian pebbles to demonstrate that you have learned how to properly identify these colorful pebbles. Good luck on the hunt!
Additional Logging Request
Though not a requirement, it would be appreciated if you take a photograph of you and/or your GPS at Rodeo Beach and post it with your cache log to verify your visit. Please do not log an armchair find!
Golden Gate National Recreation Area – Marin Headlands Information
Rodeo Beach is located in the Fort Cronkhite area of the Marin Headlands, one of the most special places in the Golden Gate National Recreation Area with access to multiple trails, fantastic views, a dog-friendly beach, fascinating geology, and an interesting history. Fort Cronkhite is a former World War II military post, and many of the fort’s historic buildings are preserved and provide homes to park partners, including the Marine Mammal Center, The Headlands Institute, and the Golden Gate Raptor Observatory. Here is a link to the National Park Service brochure for the Marin Headlands area:
Geology and Natural History of the San Francisco Bay Area: A 2001 NAGT Field-Trip Guidebook, pp. 61-86: “Geology of the Golden Gate Headlands” by William P. Elder, National Park Service, Golden Gate National Recreation Area
Glossary of Franciscan Geology Terms, the National Park Service Golden Gate Recreational Area (Website)
Marin Headlands, the National Park Service Golden Gate National Recreation Area-Marin Headlands (Website)
The Rainbow Sands of Rodeo Beach, informational guide prepared by Roxi Farwell of the National Park Service located at the Marin Headlands Visitor Center
The Unique Beach Sand at Rodeo Cove, Mineral Information Service, December 1970: vol. 23, no. 12, pp. 238-241, by John R. Wakeley
Wetland Habitat Changes in the Rodeo Lagoon Watershed, Marin County, CA, October 2004, by Chuck Striplen, Robin Grossinger, and Josh Collins for the National Park Service, Golden Gate Recreation Area
(No hints available.)