Woodson Mountain Spheroidal Weathering

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N/S ? ??.??? W/E ??? ??.???  [] In California, United States

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This open space area called Woodson Mountain (or Mount Woodson), once called by natives as the Mountain of Moonlit Rocks and by early settlers as Cobblestone Peak, now appears on maps as Woodson Mountain in honor of Dr. Woodson who homesteaded some nearby property in 1875. Woodson was a surgeon in the Confederate Army during the Civil War and after achieving local prominence, the mountain was named in his honor. It currently is part of the City of Poway trails system.

Rocks from this mountain were used to construct the famous Ramona Castle or the Mt. Woodson Castle. Designed by John Vawter and Emmor B. Weaver, the Mt. Woodson Castle combines English cottage and French castle influences. It is now part of the Mt. Woodson Golf Club.

A California Conservation Corps (CCC) camp was built on 40 acres of land at the base of Woodson Mountain in 1933. In 1934 some 300 men were stationed at the camp and were in charge of building the road to the top of the mountain as well as lookout towers around the county and a forestry station at Woodson Mountain.

EDUCATIONAL INFORMATION

This prominent peak with its distinctive boulder strewn appearance stands at 2894’ and appears much different from surrounding peaks in the county. You may have wondered where did all these boulders came from? Well, they were actually always there! These boulders formed in place by spheroidal weathering, which is a form of chemical weathering in which concentric shells of decayed rock (ranging from a few millimeters to a couple meters, WP1 & WP2 ) are successively loosened and separated from a block of rock, transforming angular blocks into round boulders as you see today.

These boulders are remnants of rock that cooled below the earth's surface. As cooling progressed the rock contracted and cracked (WP5 ). Millions of years later, after uplift brought the rock closer to the surface, warmer temperatures, elevated atmospheric carbon dioxide, and decaying vegetation combined with rainfall to chemically weather the surfaces of the rocks (WP4 ). The degree of weathering was controlled by the ability of these solutions to penetrate rock. As the decomposed material was stripped away, large round boulders were left behind to form boulder strewn hills like Woodson Mountain.

The light colored bedrock of Woodson Mountain is a type geologist’s call Woodson Mountain Granodiorite (WP3 & WP3 ). It has speckles of mafic minerals (dark-colored minerals rich in iron and magnesium), such as biotite (black mica) and hornblende. Granodiorite is classified by the unique amounts of the framework minerals know as quartz (30%), plagioclase (50%), and alkali feldspar (20%) that compose it. This type of rock weathers into boulder strewn slopes rather than smooth slopes, perfect for the local rock climbers.

At these views of Highway 67 (WP6 & WP6 ) a road cut shows that weathering along joints forms large, mostly unweathered rock surrounded by more weathered, softer, browner material which is easily eroded, leaving the least weathered rounded rocks in place.

To LOG this CACHE:
1. Measure the "pealing" thickness of this example of a weathering boulder located at WP2.

2. Submit with your log a photo of yourself and GPS at WP5, micro-fractures.

To log this cache e-mail me the answer  HERE .

TECHNICAL

Spheroidal weathering: (“onioning”) is a form of chemical weathering, in which concentric shells completely surround a corestone. Causes of spheroidal weathering include the following hypotheses; expansion (several types), unloading, constant volume alteration, Liesegang phenomena, micro-cracks, and possibly several other mechanisms. The following sources were used to generate this cache.

Here's a list of references and additional reading material:

• Beck, C.D. 2004. On Memory’s Back Trail: A Story History of Ramona and the Backcountry of San Diego County. Backcountry Press. 297p.
• Clifford, H.J., F.W. Bergen, S.G. Spear, & D.M. Burns (ed.) 1996. Geology of San Diego County: Legacy of the Land. Sunbelt Publications. 175p.
• Kennedy, D. 1999. San Diego County Climbing Guide. Deadpoint Press. 177p.
• Ollier, C.D. 1971. Causes of spheroidal weathering. Earth-Science Reviews. Vol. 7, Issue 3, p. 127-141.
• Ruth, L. 1982. The Ramona Castle: Irene Amy Strong’s Home and the Craftsman Movement. The Journal of San Diego History. Vol. 28, No. 3.
• Shad, J. 1992. Afoot and Afield in San Diego County. 2nd Edition. Wilderness Press. 304p.
• Walawender, M.J. 2000. The Peninsular Ranges: A Geological Guide to San Diego’s Back Country. Kendall/Hunt Publishing. 114p.

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Woodson Mountain

WP1

Large outer shell peeling
WP2

Boulder in the process of peeling
WP3

Decomposed Woodson Mountain Granodiorite
WP3

Unweathered Woodson Mountain Granodiorite
WP4

Large boulder undergoing spheroidal weathering
WP5

Fracturing and peeling in process
WP6

Road cut on Hwy 67 revealing the "peeling onion".
WP6

Another road cut on Hwy 67. Notice the "core" stone.

Logged Visits ( 46 total. Visit the Gallery (76 images) )

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Cache Logs

September 5 by BadSquirrel (1510 found) Very interesting cache...I always enjoy learning something new! We stopped at all the waypoints and got schooled, then captured the pic at the final. Thanks for the work you put into this! [view this log] Monkeys at the final Measuring at WP2

December 26, 2008 by Mocha! (1957 found) If it were not for the Splashes, I could not log my first earthcache 'cause my waterboy forgot the camera. Since I'm a canine, I'm hopping you'll waive the first requirement. If not, well, it's not about the #s and will understand a log deletion. You don't get any candy if you don't ask, right?! Thanks for the cache. [view this log]

December 26, 2008 by splashman (12513 found) Hey Jason! We hiked up the Mtn. today with Chuy!, Mrs.Chuy!,Ginger!, Mocha! and 3cd's. Wow is this ever cool up here! Enjoyed checking out all of the examples of the weathering, etc. on the way. Took a pic of some weathering, but Splashman said I had as much as the rock! (let's see how much he misses dinner for the next year) Thanks for the fun, The Splashes [view this log] Weathering comparison...................hmmmm Chuy!, Ginger! & Mocha!

November 25, 2008 by The Speis (649 found) 21st of 23 finds on our all-time best day ever! We enjoyed all the different shaped rocks we encountered on our hike today. TFTC [view this log] Todd, Sadie, and Luna at micro-fractures

November 15, 2008 by Live and Let Live (357 found) Really enjoyed learning about Mt. Woodson. Thanks for all the great history. We and the pups really had fun. [view this log] Pups and us on Mt. Woodson

Current time: 11/21/2009 10:22:26 AM
Last Updated: 9/6/2009 2:14:58 AM
Rendered: From Database
Coordinates are in the WGS84 datum