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.
Most of the sandstones in the park formed under muddy conditions that caused some to be so dark that they are almost black. More than 75 percent of the Franciscan rocks in Coe and the surrounding areas of the Diablo Range are sandstones. At Coe, almost all of the Franciscan sandstones were subducted to depths on the order of ten miles, where heat and pressure converted them to a low-grade metamorphic rock called metagraywacke. Included with the sandstones are some interbedded mudstones and conglomerates that represent finer and coarser-grained sedimentary rocks.
Cherts range in color from white to green, red, and brown and are composed almost entirely of silica. Most Coe cherts are a deep red-brown and are layered with thin lenses of shale, which used to be mud. The chert was highly deformed as it was carried down beneath the continent and then lifted during the building of the Diablo Range. Hydrothermal waters at great depth have leached the red-brown cherts in some areas, leaving behind the white and green cherts. As chert changes from the original red-brown color, it loses its glassy luster and becomes more earthy looking.
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.
A different kind of alteration occurred in some of the chert near the Rooster Comb, where manganese was introduced during a period of hydrothermal activity. The age of the Rooster Comb chert has been dated as approximately 90 to 120 million years old, while the cherts of Pine Ridge are approximately 150 to 180 million years old. The dating process is based on the presence of tiny fossils called radiolarians, which can be dissolved in hydrofluoric acid and examined under the microscope.
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.
Greenstone 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.
Description - Blueschist, also known as glaucophane schist, is greenstone or, less commonly, chert or sandstone, that has undergone intense metamorphism deep in the subduction zone. It is characterized by blue, needle-like crystals, often intergrown with garnets and white mica. Outcroppings of blueschist were favored by Native Americans as ceremonial sites. There are numerous large blocks or craggy outcrops (called knockers) of blueschist in Coe Park. Some of these make up the prominent peaks above the Kaiser-Aetna road north of Pacheco Creek, but the best place to see them is along the creek downstream from the big culvert.
Description - Serpentine is also a green rock, occasionally pale green but most often dark and mottled by patches of black. It is seamed by a crisscross pattern of closely spaced fractures with polished surfaces called slickensides.
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 are restricted to the depleted soils that form on these rocks.