Topanga Formation Features - El Moro Canyon EarthCache

Safety Warning: There was an active bee hive about 25 feet south, however, I wasn't bothered at all. There is also quite a bit of poison oak off the trails, so stay on the trail.

At this location you can observe 3 different features: cavernous weathering, grain size differences, and cross bedding.

The rock formation at this location is called the Topanga Formations. The Topanga Formation in Orange County is a sedimentary unit made up of conglomerates and sandstones that were deposited in a shallow ocean during the middle Miocene, approximately 10 to 20 million years ago.

In Orange County, the Topanga Formation has further been differentiated into three Members (from youngest to oldest): the Paulerino, the Los Trancos, and Bommer members. The outcrop at this location is the Bommer member.

Cavernous Weathering
At this location and many other locations throughout the park, you can see the results of cavernous weathering. Cavernous weathering creates cavities in a rock through chemical and/or mechanical weathering. There are two related processes, case hardening and core softening, along with salt crystallization that may have contributed to the cavernous weathering.

In the case of hardening and core softening, slightly acidic water, either rain or groundwater, percolates into the rock. The water then dissolves the cement that holds the individual grains together. The water then moves out to the surface of the rock and is evaporated, depositing the cement on the surface of the rock. The result is a weak interior and hard exterior. Once the surface of the rock is breached, the interior of the rock erodes much more quickly than the surface creating the cavernous appearance.

Salt crystallization begins with sea spray percolating into the rock. As the water evaporates off the salt crystallizes inside the rock. Salt crystals increase in size between 1 to 5 percent from their dissolved state. The result is the weakening and erosion of the rock from the inside.

Grain Size Differences
Scattered throughout the cliff side are area with larger rounded cobbles. Since the size of the grains that water can transport increases with the speed of the current, these areas represent times and locations where the current in the shallow sea was strong enough to move these larger cobbles. Since these cobbles are relatively rare, it appears that the currents were generally gentle.

Cross Bedding
Cross beds can be seen at the back of the weathered outcrop just below where the rock changes color from red to white. Cross beds are characteristic of sand dunes. Based on other evidence elsewhere in the formation, these dunes were formed under water. Because of the way dunes are formed, the direction of the current can be determined from the shape of the cross beds.

Water pushes sand grains up the long front side of a dune. As the grains are pushed over the top of the dune they settle onto a steeper back side of the dune forming the angled layering pattern.

Source: USGS

Periodically the velocity of the current will change. This will either deposit a layer of finer grained material on the top of the dunes or erode off the top. Then the process begins again, forming another layer of angled layers.

The direction of the current that formed the cross beds can be determined from measuring angle of the cross beds. Usually measurements from many different outcrops are averaged together.

Logging requirements:
Send me a note with :

1. The text "GCQ4ZR Topanga Formation Features - El Moro Canyon" on the first line
2. The number of people in your group.
3. Given just this one 2-dimentional example of crossbedding in this rock, figure out which way the current was flowing when this rock was deposited

• Geology Trail Guide for El Moro Backcountry, Crystal Cove State Park, date unknown
• Orange County Archaeology and Paleontology Guidelines, Procedures, and Policies(http://www.ocparks.com/uploadgraphics/OverviewFinalReport2.pdf)
• Preliminary Digital Geological Map of the 30'X60' Santa Ana Quadrangle, southern California, Version 2.0 Compiled by D.M. Morton, Open File Report 99-172, Southern California Aerial Mapping Project
• North American Geologic-map Data Model Science Language Technical Team, 2004, Sedimentary materials: science language for their classification, description, and interpretation in digital geologic-map databases, Version 1.0 (12/18/2004): Draft report posted on the North American Data Model website , 595 p.
• Discovering the Beauty of Charmlee Park's Geological Formations, GDEP Symposium, Long Beach, 8 August 2003, Geoscience Diversity Enhancement Project, A collaboration of the departments of: Geological Sciences, Geography, and Anthropology California State University Long Beach, CA 90840 (http://www.csulb.edu/depts/geography/gdep/posters03/charmleegeology/)
• Dorn, R.I. 2004. Case hardening, in Encyclopedia of Geomorphology ,ed. A.S. Goudie, Routledge: London, 118-119.
• PhysicalGeography.net | FUNDAMENTALS OF PHYSICAL GEOGRAPHY, CHAPTER 10: Introduction to the Lithosphere, (r). Weathering (http://www.physicalgeography.net/fundamentals/10r.html)
• USGS bedform sedimentology site: "Cross-Bedding, Bedforms, and Paleocurrents" RELATIONS BETWEEN CROSS-BEDDING, BEDFORMS, AND FLOW (http://walrus.wr.usgs.gov/seds/relations.html)

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