The Faults of Fissure-ing in Sag ponds
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In geology, a Fault is a planar fracture or "Fissure" in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement.
Large fissures within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes. A fault line is the surface trace of a fault, the line of intersection between the fault plane and the Earth's surface. Since faults do not usually consist of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane.
Strike-Slip Faults are faults where the fault surface is usually near vertical and the footwall moves either left or right or laterally with very little vertical motion. Strike-slip faults with left-lateral motion are also known as sinistral faults. Those with right-lateral motion are also known as dextral faults. Each is defined by the direction of movement of the ground on the opposite side of the fault from an observer. A special class of strike-slip faults is the transform fault, where such faults form a plate boundary. These are found related to offsets in spreading centers, such as mid-ocean ridges, and less commonly within continental lithosphere, such as the San Andreas Fault in California. The massive 1906 San Francisco earthquake was the result of a right-lateral strike-slip fault.
A Sag Pond is a body of water collected in the lowest parts of a depression formed either near the head scarp of rotational landslides or between two strands of an active strike-slip fault. Sag ponds may be useful identification features in aerial photographs for mapping faults and landslides.
San Andreas Lake (from which the fault takes its name) is a "sag pond'' that naturally formed in the valley of the San Andreas fault. Strike slip faults are good places for lakes; the fault both creates a low spot to collect the water, and grinds up the rock underneath making an impermeable layer to hold the water in. Man has enlarged San Andreas lake with a dam, and created two additional lakes in the same valley, Upper and Lower Crystal Springs Reservoirs. The dam holding the reservoirs in place is located North of GZ under a bridge on highway 280.
These lakes hold the water supply for the City of San Francisco. The water first comes via aqueduct from Hetch-Hetchy reservoir in the Sierra Nevada mountains (next to Yosemite, more than a hundred miles away to the East), then in a tunnel under the Southern part of San Francisco bay, in a tunnel up the San Francisco peninsula (passing along the base of the hills just West of Stanford University), and finally enters the reservoirs at the ``Water Temple'' on the West side of Canada road South of GZ.
The posted coordinates should take you within close proximity of a long natural drainage fissure that runs from Cañada road and flows into a sag pond known as "Upper Crystal Springs Reservoir". There is no need to step off the trail, just make your best estimate when noting the logging requirements, which can be seen from the trail near the posted coordinates.
Send me an email with the following information to log this Earthcache:
1. Was there any water flowing through the channel on either side of the trail at the time of your visit? If so how strong was the water flow?
2. How many feet wide across is the large fissure to the North/West of the trail?
3. How many feet deep would you estimate the fissure to be where the fence intersects across it's gap?
PLEASE DO NOT TRY TO STAND OR CLIMB INSIDE THE LARGER FISSURE IN ORDER TO GET A MORE ACCURATE MEASURMENT. I will be lenient with your best guess from the trail.
Please, just be safe here, and watch out for poison oak!
Congratulations KCSearcher on FTF!
Special thanks to ByeTheWay for helping me gather more accurate geological info on the area, and finalizing this Earthcache!
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Coordinates are in the WGS84 datum