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The rocks at above this historic lighthouse represent deep-water landslides that have been transported a long way from their origin by faulting.
Adjacent to the Point Reyes Lighthouse Visitor Center is an excellent outcrop of the Point Reyes Conglomerate. A conglomerate is a sedimentary rock with fine-grained sand surrounding large rounded rocks. These rocks are thought to have been deposited in the early Eocene (about 50 million years ago) in deep-water landslides called turbidity currents or turbidites.
Turbidites form in the deep ocean near the continental shelf. Classic turbidite sequences are made up of fining upward beds of sediment. This means that the grains at the bottom of the sequence are larger than the grains at the top of the sequence. In a complete sequence the bottom sediments are pebble sized conglomerates, followed by coarse then fine-grained cross-bedded sandstone, then shale, and finally silt. This series is called the Bouma cycle.
A turbidite is formed by a density flow. In this type of flow, the fine grained sediments suspended in the water increases the density of the water. The denser water is then able to suspend larger particles than it normally would be able to. For example a rock will sink quickly in pure water, but sink much more slowly in thick mud.
Typically, the entire sequence is not seen because each successive flow will erode off some of the top of the last sequence, or if the deposit is at the end of the flow, only the finer layers will be seen.
Usually turbidites form off of a convergent plate margin (where two plates are colliding). A convergent margin provides the mountains that are good sources of sediment, sufficiently steep off-shore slopes and periodic earthquakes to trigger the undersea landslides. As with all the rocks on the Point Reyes Peninsula, these rocks have been moved a great distance northward as the Pacific Plate has slid past the North American Plate. These rocks are thought to be the same as the Carmello Formation found near Point Lobos south of Monterey (about 180 km south of this location).
One of the key pieces of evidence for this correlation are the pink to purple pebbles with pink to white crystals that are common in both rocks. These pebbles are a relatively distinctive volcanic rock called porphyry tuff and the crystals are feldspar. Both Formations also have distinctive quantities of zircon and titanomagnetite.
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- The text "GC16KZJ Point Reyes Conglomerate" on the first line
- The number of people in your group.
- Examine layers of rock in the outcrop and determine which part(s) of the Bouma cycle is missing if any.
- How many sequences can be seen in the outcrop
- Without climbing or removing any rocks, see if you can find on of the Porphyry tuff pebbles. If possible post a picture of it, otherwise describe it in your log
Access to the Point may be restricted during whale watching season. During this period, the Park runs a weekend shuttle from Drake's Beach to take visitors to the Point. Contact the National Park Service for the current status of roads and shuttles..
- Sloan, Doris, Geology of the San Francisco Bay Region, California Natural History Guide Series No. 79, University of California Press, Berkeley 2006
- Geology at Point Reyes National Seashore and Vicinity, California: A Guide to San Andreas Fault Zone and the Point Reyes Peninsula
- NPS Informational Sign.
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Point Reyes National Seashore
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Coordinates are in the WGS84 datum