
Major Rock Types |
Today at Coe Park we can see much evidence of the ancient sea floor sediments called the Franciscan Complex. The Franciscan Complex is a jumbled mass of sandstone and shale, mixed with chert and volcanic rocks plus minor serpentine and scarce metamorphic rocks called blueschist. Sandstone
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There are two theories about the formation of chert. One theory proposes that it is formed from the skeletons of microscopic animals welded together by recrystallization. Another theory is based on the fact that almost all beds of chert are found above bodies of greenstone, a metamorphosed volcanic rock formed under water. In areas of deep oceanic volcanic eruptions, such as along the mid-oceanic ridges, hot lava as it emerges superheats the adjoining sea water, causing silica to be precipitated out in a gelatinous mass, which is slowly deposited on top of the lava. In time, the gelatinous mass becomes chert, according to this second theory.
Native Americans seem to have favored the red or green chert when they crafted arrowheads, spear points, and scrapers. Chert from Coe and surrounding parts of the Diablo Range have been found as far away as eastern Nevada-attesting to the esteem in which this type of chert was held by Native Americans.
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GreenstoneGreenstone is a submarine basaltic lava, hydrothermally altered to green minerals such as chlorite and pumpellyite. The greenstone probably formed at the mid-ocean ridges and were the basement rocks on which the sedimentary rocks were deposited. Thin layers of greenstone can be seen along the south side of the Rooster Comb and east of Red Creek, above Hat Spring. .Blueschist
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Serpentine
Serpentine particularly puzzled geologists before the 1960s because its composition strongly suggests that it originated somewhere in the earth's mantle. Bodies of serpentine have cut boldly through surrounding rocks as though they had been intruded as molten rock; yet the surrounding rocks are never cooked (metamorphosed) as they would be if the serpentine had been molten. Today, most geologists believe that serpentine forms by the addition of water to a rock called peridotite. The serpentine has later penetrated zones of weakness, especially along faults, where it is usually found today. Good outcrops of serpentine can be found along the Madrone Springs fault and in the extreme northeast corner of the park along the South Fork of Orestimba Creek. The serpentine outcrops are of great interest to botanists because many rare plants restricted to the depleted soils that form on these rocks.
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