Shell Ridge EarthCache
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The ground you are walking on was laid down as sediments in a shallow sea about 12,000,000 years ago, or in the late Miocene epoch (Neroly Formation). Mount Diablo plays a role here as it was thrust upwards and folded the surrounding lands some 5,000,000 years ago. Because of this, the entire area around Shell Ridge was split apart and, when this happened, large blocks of these sediments seemed to rise and the edges became loose debris that was eventually washed away by small streams. You might be able to imagine what this terrain looked like several thousands of years ago if you stand on top of Shell Ridge and look down. These tilted blocks are lined up in parallel rows. The tilts range from 60 to almost 80 degrees from the horizontal. The ditch along the road leading to the Muir water tank reveals several rock strata with very-steep tilts. Mentally, move these blocks back down again and you have the level plateau of an uplifted sea floor. The highest point on Shell Ridge, (north of Ginder Gap) is 829 feet above the present level of the sea and still contains several varieties of seashells, once in the shallow sea.
Stage 1 (posted coords) is the the old quarry. This quarry was opened in May 1947 to provide crushed rock for driveways and fill in a nearby subdivision. No rock has been mined since 1947 since the abrasive resistance of rock produced here is too low to pass state specifications for use in buildings or highways. The rock is poorly consolidated sandstone and conglomerate of tertiary age, containing as much as 50% seashells. Stage 2 is up on the ridgeline.
Carefully examine the boulder you find at the posted coordinates. Please email me with the length of the largest shell you see and try to approximate the number of shells contained (in that rock) per cubic foot. Then head up the narrow trail that rings the quarry and estimate the length of the exposed rock ridge at the posted coords for Stage 2. On this ridgeline rock, you can clearly discern the upshift of the landscape. You must also post a picture of yourself (or your group) at this spot (hopefully with Mt. Diablo in the background).
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