The Sands of Time EarthCache
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Ocean Beach is the widest and longest expanse of sand on San Francisco's shores, extending from the Cliff House to Fort Funston along the Pacific Ocean. Sand that has blown east from the area underlies more than third of the San Francisco. Here, you can learn more about the sand's origins and the geology it reveals.
Unlike other forms of geocaching, there is no container here. To complete this earthcache and answer the logging questions below, you will need to bring a magnet, a magnifying glass or hand lens, and your gps receiver or smartphone. Be sure to submit your answers before logging this earthcache as a find.
Ocean Beach is part of Golden Gate National Recreation Area. As a National Park Service parkland, it is a protected area. Please only take photos and only leave footprints.
The Geology
During the last ice age, around 20,000 years ago, the ocean was about 400 feet lower than it is today. Mountain glaciers developed during that period, and ground the granitic rocks of the Sierra Nevada down to sand and silt, which was carried off by great rivers. Larger rocks were deposited in the Sierra, but small sand grains were carried through the Carquinez Straight, down to the valley that is now known as San Francisco Bay. The sand was carried across the then exposed continental shelf to the coast, near the present day Farrallon Islands. As the sea began to rise after the glaciers started melting, the sand along the coast was picked up by the westerly winds and blown back across the land to cover much of the northern part of the San Francisco Peninsula
Today, sand still accumulates along Ocean Beach and blows landward towards the City. Cement walls hold some of it into place at the shore, but crews regularly clear the Great Highway to keep the sand in check.
The sand here comes from a variety of sources, including that which was brought from the Sierras during ice ages. Some of it also came here as spoils carried to the bay as a result of hydraulic mining in the Gold Country during the 1800s
In part, this sand also originated from the Colma and Merced formations south of Ocean Beach. The sandstone cliffs at Fort Funston are easily eroded by ocean waves and rainfall. Although sand in California generally moves like a river along the coast to the south, counter-current eddies in the area bring sand northward along Ocean Beach.
These forces have formed San Francisco Bay ebb tidal delta offshore of Ocean Beach. This delta consists of a vast submarine crescent of the sand that lies on the ocean floor outside the Golden Gate, arcing from Point Bonita and Potatopatch Shoal to the north, down to Sloat Boulevard. The sand you find here may have once been part of this delta.
Logging Requirements
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The mixture of sand from the sources described above form an unusual combination of grains. To complete this earthcache start at the listed coordinates and walk toward the graywacke sandstone rocks found below the Cliff House, stopping at two locations along the way. Mark the coordinates of the places you stop and email them to the earthcache developer along with the following information:
1. As you walk closer to the beach, stop and look at the sand with your magnifying lens. You will see that it consists of a variety of grains. You may find pink garnet crystals, greenish and orange hornblende, as well as quartz, feldspar, or other minerals from the Sierras. You could even spot a rare grain of gold from the Sierras. Describe what you find - note the color of the grains and whether they are fine or coarse. Do the grains in the area you are looking appear similar to each other? Did your magnet pick up any of the grains?
2. Black magnetite, with strong magnetic qualities, can often be found at Ocean Beach. Because it is denser and heavier than other minerals it tends to get concentrated on delicate drifts forming a black sand that is sometimes mistaken for oil or tar. Magnetite comes from igneous rocks of the Sierra Nevada and is formed here when gentle waves have just enough energy to carry away the lighter grains but not enough to move the heavier grains. Look for a location that you think might have magnetite. Mark your coordinates and include them with your answer.
Use the magnet you have brought to see if you can find it. Look at the grains through the magnifying lens. Describe what you observe at this location. Did the magnet pick up any more grains here than it did at your first stop; are the grains here larger or smaller than at your first stop?
3. Is there a difference at the two locations in the coarseness or size of the sand -- whether the sand is sharp and angular or rounded and smooth or if the grains appear to be smaller at one location or the other? From what you observe, do you think the sand was recently deposited or came a long way away?
4. A photo of you, your gpsr, your magnet, or a personal item that identifies yourself should be included showing the "black sand" location where you tested for magnetite. Changes in the earthcache guidelines (June 2019)no longer require photos to be optional.
Please submit your answers before logging this as a "find." Logs that have not fulfilled the logging requirements will be deleted.
Park Information
You can take public transit to Ocean Beach or park at the designated locations along the Great Highway. Note that the weather varies dramatically and is often cold or foggy. The ocean here also has strong rip tides and feeder currents resulting from tidal movement, westerly winds, and the steepness of the beach offshore. During very low tides you may be able to see some of the channels that the movement of the water has created. But under any conditions, use caution and do not go swimming or wading.
Further Information
Doris Sloan, The Geology of the San Francisco Bay Region, University of California Press, 2006
The Geology of the San Francisco Bay Region: 2007 video presentation by Doris Sloan.
The Geology of the Golden Gate Headlands: Will Elder, National Parks Service.
Geological Field Trips: Ted Konigsmark's Guide to SF Geology, including Ocean Beach sands.
The Science Behind the Black Sands of Ocean Beach: A brief summary.
Additional Hints
(No hints available.)