Posted 13 April 2016 by A.G. Sylvester ©
The Monterey Shale has several pockets of “burnt rock” resulting from the combustion of natural gas in very rich organic layers. The pockets are in a belt about 14 miles long along Oak Ridge south of Fillmore. The largest pocket is in Grimes Canyon where vesicular black, reddish-brown, yellow, and white rock is sold as “lava rock” and used for decorative stone. Low temperature oxidation of pyrite appears to play an important role as a catalyst for the spontaneous combustion of the hydrocarbon in the rock. Rock temperatures exceeded 3,000° F, melting the rocks and producing small intrusive masses.
To reach the Grimes Canyon quarries, proceed south from Fillmore on CA 23 past the Elkins Ranch oil field (PM 21.5) at the base of the grade. Shallow wells produce from the Sespe Formation at the crest of the Oak Ridge anticline. Outcrops on both sides of the canyon at PM 20.5 expose the Monterey burnt shale. The quarries are on privately owned and require trespass permission to visit them.
Continue uphill and south on CA 23. All the south-dipping beds are on the south flank of the Oak Ridge anticline. Quarries west and east of the large roadcuts in the switch backs at crest of the grade (PM 19.6) expose yellow sand and gravel of the lower Saugus Formation, which consists of interbedded shallow-marine to brackish water sandstone, siltstone, pebble-to-cobble conglomerate, and coquina beds that grade laterally and vertically into non-marine sandstone, siltstone, and conglomerate. The quarries produce bulk sand and gravel for road base and plaster. The lower Saugus Formation was deposited in a delta by west-flowing streams in Pleistocene time (about 2.5 to 0.7 million years ago).
Roadcut at the crest of CA 23 in yellow sand and gravel of the Saugus Formation (34° 20.57N, 118° 54.46W).
CA 23 continues downhill about three miles to Moorpark through light gray to brown pebbly soil derived from weathering of upper Saugus Formation gravel, sand and clay, which also dip south in the south flank of the Oak Ridge anticline.
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Roadside Geology of Southern California 