Sea Rock up Here? EarthCache
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An educational Earthcache located on Hawk Hill within view of the Golden Gate bridge. EC location is easily accessible and the knowledge learned at the coordinates will help you complete the task.
The headlands on both sides of the Golden Gate are composed of ancient oceanic rock pushed to the earth's surface--chert, serpentinite, basalt, and graywacke sandstone. The rocks you are standing on once resided on the bottom of the Pacific Ocean.
The sedimentary rock consists largely of the skeletal remains of minute marine organisms, radiolaria. Sedimentary rocks are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment is commonly formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers.
Scientists believe the radiolarian chert, as the rock is known, and the entire headlands complex, was forced to its present position over millions of years by a collision of the continental and Pacific plates. Since chert is mostly composed of radiolaria fossils, its layers tell us the origin of these rocks. It takes about 20,000 years for one inch of chert to develop. The radiolaria in these rocks are tropical species from the equatorial Pacific which lived millions of years ago."
In order to claim credit for this Earthcache you will need to email the answers to the three questions below to the cache owner. Failure to send in the answers may result in your log being deleted.
Question 1: What makes up the hard, weather resistant chert?
Question 2: How long might it have taken for the chert layers you see in this exposure to develop on the ocean floor?
Question 3: Estimate the elevation above sea level at this location (Can you believe this location used to be underwater?)
Resources: Google, Wikipedia, Geology books found at the Marin Headlands Visitor Center. Placed with permission of the Golden Gate NPR.
Additional Hints
(Decrypt)
Cyndhr & TCF