Columnar Basalts - The Palisades - Napa County EarthCache
Hidden : 1/1/2013
Difficulty:
1 out of 5
Terrain:
4 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:


This EC is located in Robert Lewis Stevenson State Park right on the Table Rock/Palisades Trail between Highway 29 and the historic Oat Hill Mine Trail.  To observe and log this cache your feet never need leave the trail.  Two items before you try to reach this EarthCahce:
 
  1. It is a long, arduous hike on a treacherous, primitive trail that at points would lead someone to think its abandon.  But yes, this is a public trail, but past Table Rock this trail is narrow and hazardous, and although an official trail it's condition is in places barely better than a deer track on a steep hill slope. Trail maps and more information are available at (http://napaoutdoors.org/parks-trails/oat-hill-mine-trail )
  2. Other than natural hazardous include high temperatures in summer, cold icy conditions in winter, and rattlesnakes.
  3. Be sure to bring lots of water, sturdy hiking boots (tennis shoes won't do), and other supplies for a long hike on rough terrain.
 
Sonoma Volcanics:  The Palisades include a series of volcanic rocks, including lava flows, ash fall tuffs, volcanic breccias, and other extrusive igneous rocks.  The Palisades are part of what is called the Sonoma Volcanics.  Beginning more than 25 million years ago (Ma) faulting changed from a subduction and off-shore spreading center to the strike-slip lateral movement along the San Andreas and other faults.  Volcanism began in southern California and migrated northward.  The current point of this propagation is off shore of Cape Mendocino at what is geologist call the Mendocino Triple junction (a whole complicated topic unto itself).
 
As this volcanism migrated northward it left a trail of extrusive igneous rocks.  Examples are the San Neenach (24 to 22 Ma) and San Emigdio (21 Ma) Formations (Fm) near Santa Barbara, Quien Sabe Fm (10.5 to 8 Ma) inland of Monterey Bay, Berkeley Hills Volcanics (10.2 to 8.5 Ma), Sonoma-Tolay Volcanics (13 to 2.7 Ma), and finally the Clear Lake Volcanics (2.1 Ma to 10,000 years before present).  As you see this volcanism started in the south and migrated northward. 
 
This EarthCache - Columnar Basalts:  This EC can be observed without your feet even leaving the trail.  What you are looking at are some columnar fracturing in a basaltic lava flow in the wall of the trail.  It appears this outcrop was created during construction of the trail.  Most likely as the basaltic lava flow was deposited horizontally or near horizontal.  The fractures form as the flow cools, perpendicular to the surface of the flow, so presumably these fractures were originally vertical.  Other famous examples of this kind of fracture are Devils Postpile National Monument (Eastern Sierra, CA.  http://en.wikipedia.org/wiki/Devils_Postpile_National_Monument ) and Giants' Causeway (Ireland http://en.wikipedia.org/wiki/Giant%27s_Causeway ).  This columns are always hexagonal when viewed from the top, which is covered by soil at this location. 
 
Logging Requirements: You will notice to some vugs (a.k.a., vesicles).  These voids are where gasses in the basalt expanded just like bubbles forming in your soda when you pop the cap and release the confining pressure.  These vesicles are frozen into place when the lava cools.  Sometimes the vesicles remain voids (like here) or sometimes are filled with minerals at a later time.
 
A picture is provided, but it has been rotated to the original formation position, vetical (so you cannot tell the current angle), and with nothing for scale so you cannot tell the size of the vesicles.  To log this cache send an email to the CO with the following info:
 
(1) Name and GC code of the cache.
 
(2) Deviation of the angle in degrees of the joints FROM THE VERTICAL. 
 
(3) Largest measurement you can find for the largest vesicle you can easily find.
 
References:

Harden, Deborah.  1997.  "California Geology"

Enderlin, Dean 1995.  "Trailside Geology of Oat Hill Trail.  Calistoga, Napa County California" 

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