FRIENDS OF EDGEWOOD NATURAL PRESERVE

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By Bill Korbholz

Typical of much of the land in this geologically active corridor along the San Andreas Fault, Edgewood Natural Preserve consists of a hodgepodge of rock and soil types. Within the Preserve are found rocks from volcanic lava, sand from beneath the sea, and a distinguished form of rock that, because of its slippery nature, oozed up from deep within earthquake fault traces.

The central ridge, the Preserve’s most striking topographic feature, consists primarily of Franciscan greenstone. Red in color due to oxidation, this type of rock has volcanic origins and consists chiefly of basalt. If you follow the Ridgeview Loop, you will be trekking through deep red soil which has eroded from Franciscan greenstone.

Surrounding the central ridge’s greenstone, you’ll find a lot of rock which is greenish-gray to bluish-green in color, and often very slick. This of course is serpentinite (serpentine rock) and Edgewood has the largest exposed formation of serpentine rock on the peninsula. Because this type of rock is relatively soft and slippery, it tends to squeeze up through rock fissures caused by earthquake faults, and that is the reason it is so prominent in this area. You can enjoy the beauty of serpentinite, our state rock, along the Serpentine Loop, which circumnavigates the central ridge. For an in-depth description of serpentine rock, see Carolyn Curtis’ article "What is Serpentine?" beginning on the next page of this newsletter.

In the northeast quarter of the Preserve you’ll find concentrations of Franciscan and Butano sandstone. This soft rock consists of graywacke sandstone with imbedded mudstone, siltstone, and shale. The Sylvan Trail, perhaps the most beautiful trail in the Preserve, takes you through these sandstone concentrations and past the Preserve’s only year-round running spring.

Also in the Preserve you will find alluvial deposits that have drained down the western slope of the central ridge, and Franciscan sheared rock along the northwestern boundary.

Send mail to wm at this domain with questions or comments about this web site. Copyright © 1999 - 2008 Friends of Edgewood Natural Preserve. Last modified: January 20, 2008.