Outcrops of burnt rock are found in a road cut and cliffs on the east side of Grimes Canyon a little south of Fillmore. There is a pullout with ample parking. The features can be seen from the car, but the best examples do require walking over uneven ground and weedwacking (terrain 2.5). Watch for snakes, they love the rocky slopes.
The Monterey Formation is common near the coast of California. In fact, similar rocks were deposited throughout the Pacific Rim during the Miocene (17.5 to 6 million years ago). All of these rocks share similar characteristics. They are
- Diatomaceous (made up of the shells of microscopic plankton called diatoms)
- Phosphatic (containing phosphate)
- Dolomitic (containing dolomine); and
- Rich in organic matter (oil and gas).
This last characteristic, being rich in organic matter, makes these rocks economically important because they often are reservoirs for oil and gas. Most of the off-shore oil rigs along the California coast are pulling oil from this formation. Across the Pacific, Japan has similar rocks and oil reserves.
Whether from a lightning strike, spontaneous combustion, or some other natural ignition source, the organic material in this area of the Monterey Formation somehow began to burn and change the surrounding rock in a process called combustion metamorphism. Exhaust and steam moved upward from the fire altering rock far above the subsurface fire. This movement of air sucked in oxygen from the atmosphere inward through fractures in the rock. References estimate that the temperature exceeded 1650 deg C, enough to melt the rock.
This changed the surrounding rock into ash, slag, and glass-like material similar to obsidian that is sometimes called fused shale. Thin ribbons of purple, red, yellow, and black fused shale can be seen in the road cut and cliffs. Additionally, during the recrystallization, the rock acquired a magnetism that is stronger than the surrounding rocks.
In the road cut the fused shale is in an approximate 3-inch thick ribbon near the top of the cut (see the picture with coordinates). In the cliffs, there are a series of layers with fused shale, but you should find some large examples in the rocks that lie at the base of the cliff at the coordinates. Look for the purple and black glassy rocks.
Fused shale was used by the local Native American Villages to create sharp tools and trade with other villages. Its use may have begun 5000 years ago. Currently the material is mined and used as a decorative stone.
Send me a note with :
- The text "GCZJMV Grimes Canyon Burnt Shale" on the first line
- The number of people in your group.
- The powdery material at the base of the road cut and compare it to ash you would find in a campfire;
- The glassy material and compare it to obsidian (or glass if you haven’t seen obsidian)
- If you have a magnetic compass, see if it is affected by the rocks in the area (post that in your log)
The above information was compiled from the following sources:
- Geology and Mineral Resources Study of Southern Ventura County, California, 1973, California Division of Mines and Geology Preliminary Report 14
- Richard J. Behl California State University, Long Beach, Monterey Formation, http://seis.natsci.csulb.edu/rbehl/Mont.htm
- Santa Barbara and Ventura Basins, Tectonics, Structure, Sedimentation, Oilfields Along an East-West Transect, A. G. Sylvester and G.C. Brown Editors, Coast Geological Society Field Guide No. 64
- Burnt Oil Shale Deposits of Ventura County, CA. The Ventura Gem & Mineral Society, Inc. (VGMS) Rockhound Rambling February 2004. http://www.vgms.org/bt/vgms0402.htm