Rusty Rocks, or Fallen Meteorites from Mars? ♂ EarthCache
Rusty Rocks, or Fallen Meteorites from Mars? ♂
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Coyote Hills are made up from various rocks from the Franciscan formation. These rocks are generally shattered, deeply weathered and are cut by numerous minor faults and shear zones. Although extreme distortion is present locally, the larger rock units lie uniformly Northwest generally parallel to the Hills, and dip vertically to the Southwest. These Hills were once an island in the bay, but are now surrounded by tidal mud flats.
The Franciscan formation, named for its type locality in San Francisco, is one of the major geologic units in California; it possesses great thickness and areal extent, complex structure, and a varied but distinctive variety of rock types. These many different rock types include hard dark-colored sandstones (graywackes), thin-bedded shales, ferruginous and manganiferous cherts, altered basalts (greenstones) and minor limestones. These rocks have undergone repeated episodes of deformation and are folded, faulted, and generally sheared. They are locally recrystallized, altered, and are occasionally metamorphosed and intruded by large serpentine bodies and minor basic igneous rocks. In short, the Franciscan formation is recognized for its diversity and complexity.
Typical Franciscan cherts are abundant and are the only rocks which form good exposures in the Coyote Hills. They are well-bedded, fine grained, siliceous rocks, predominately dark red in color, with local variation to white, green, yellow or black. The coloring, which may change rapidly both between and within the beds, is dependent on the amount and state of combination of the iron (which causes the red color) and manganese which are present as impurities. The individual chert beds range from one-half to six inches in thickness and often are separated by thin ribbons of siliceous shale. Although some of the chert bodies appear to be only slightly disturbed, more commonly they are extremely contorted and shattered. Veining and recrystallization of the chert is common and the boldest outcrops are formed by large masses of red or yellow chert (jasper) so thoroughly recrystallized that the original bedding is obscured. Thin chert horizons, too small to be differentiated, are often imbedded with other types of sedimentary rocks.
The massive variety of chert is a hard, brittle, abrasive rock which does not fracture easily. While the thinner bedded types do rupture easily and break into sharp, rectangular fragments. At the posted coordinates you will see examples of both large and thinner bedded chert in the same area. You will know which rocks you should be focusing on for the questions because they will stand out here.
To log this Earthcache, send me an email with the following information:
1. The name and GC# of this Earthcache.
2. What type of rock is this? (This is to see if you were paying attention 
) What type of rock/mineral makes up the lighter colored ribbons you see within the large rocks here?
3. Describe how the larger rocks surface generally feels. Is it smooth or rough? Strong or brittle?
4. Nearby, you should also see some of the thinner bedded chert as described above. How does the appearance of this smaller, thinner bedded chert compare to that of the larger boulders in the area?
Please do not post your answers in your log.
Congratulations:
FTF (First to log & visit) kittyshrink with CroakDesigns.
FTF (first to send their answers in) retiringyoung.
Additional Hints
(Decrypt)
Erq Juvgr naq abg Oyhr