IMPORTANT:
This is an Earthcache. There is no physical cache at the coordinates provided. In order to log a "Found it" for this cache, you must send the answers of the questions below to the cache owner via the message center or email function of the site.
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Materials needed for this Earthcache: Camera, GPS, Measuring Device
It is recommended to download this cache beforehand, cell signal can be spotty.
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Learning Objective: This Earthcache aims to teach you about the formation and geologic history of Elephant Rock and the Wilson Grove Formation.
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The Wilson Grove Formation
Elephant Rock, so called as it resembles an elephant laying on its back, is composed from the oldest sedimentary layers of the Wilson Grove Formation. This formation was deposited during the late Miocene (about 10 million years ago) to the early Pliocene (about three million years ago). It consists of fine and coarse sandstone, as well as gravel and other rocks that create conglomerate. In this formation, the sandstone erodes faster than some of the other rocks in the conglomerate, leaving eroded pockets with small boulders embedded inside.
Elephant Rock as seen from the viewing area near the road.
Elephant Rock
Elephant Rock itself, however, exhibits only some characteristics of the broader Wilson Grove formation due to its heavy erosion. To start, it consists of only more coarse sandstone and gravel, not the fine sandstone that can be found elsewhere in the Wilson Grove. When this formation was originally deposited, it was submerged in an area with much higher wave activity than other areas of the same formation. These waves carried away the fine sediment and only left the more coarse sandstone and gravel which ultimately became conglomerate. The rock’s close proximity to the San Andreas Fault also caused erosion in the form of jointing. The stress from the fault causes this jointing, which then erodes and creates the individual rock outcroppings. In addition, precipitation of calcium carbonate in the rock creates weaker sections called concretions, which erode into circular holes and protrusions. Keep an eye out for these formations as you visit.
From left to right: an erosion feature empty of the original concretion; an erosion feautre that still contains the original concreton, and a shell found embedded in the rock.
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Questions
In order to log this Earthcache as a "Found it" you must send the answers to the following questions to the cache owner via the message center or the email function of the website. You do not need to wait for a response from the cache owner to write your log.
Logs without answers may be deleted without notice.
#1: If you are sending in answers for a group, include the usernames you are sending answers in for. If it is just you, simply put your username at the top of the message.
#2: Find an erosion feature that appears to be empty of the original concretion (see photos above). Using a measuring tool or estimation, find the diameter of the hole. Noticing that almost all the holes are similar in size, why do you think the hole eroded to this size and not bigger?
#3: Find an erosion feature that appears to still have the concretion nodule in it (see photos above). Using a measuring tool or estimation, find the diameter of the concretion. Do you think the size of this concretion nodule has anything to do with the fully eroded hole you measured in question 2? Predict what will happed over time to this concretion nodule. Will it erode further?
#4: Look across at the elephant rock, and notice the erosion patterns. Describe what you see.
#5: Find the shell embedded in the rock at N38º 14.968’ W122 56.653’ (Waypoint 1). How do you think it got here?
#6: Take a picture of you or a personal item at the coordinates of the cache. For example, a picture of your water bottle with the formation in the background will suffice, but just a picture of the formation will not.
Further information can be found from Sonoma State University Geology from Above: Virtual Field Trips (https://geologyfromabove.cs.sonoma.edu/post/1/Geology_of_the_Sonoma_Coast).