Edgewood Rocks! (Geology)

Geology is key to the diversity of habitats and the success of Edgewood as a nature preserve. Because of the specialized geologic makeup of this 467 acre site, rarely seen native California wildflowers and other plants have maintained a toehold.

Because of Edgewood geology, animals dependent on plants that grow in this specialized area find a home.

Because of Edgewood geology's contributions to rare and endangered plant and animal communities, local citizens preserved this area as a natural space for future use by plants and all animals (including humans)


Key Components of Edgewood Geology

1. Tectonic Plate* Boundary
   
   • The North American Plate
   Edgewood lies along the western edge of this formidable slab of rock, that floats over the Earth's mantle in much of North America.
   • The Pacific Plate
   This plate is the largest of the tectonic plates. Though it's not beneath Edgewood, it does provide a foundation for the Santa Cruz Mountains, which can be viewed from the western side of the park.
   • The San Andreas Fault
   This fault marks the border between the North American and Pacific plates. Evidence of this junction can be seen in the valley covered by the Crystal Springs Reservoir, beyond the 280 freeway on the western side of Edgewood preserve. Many earthquakes in the area are due to movement of these two great plates along this boundary.
2. Rock Types
All the rocks at Edgewood started out on the ocean-floor. They arrived in the preserve via subduction**, the movement of one tectonic plate beneath another, and were later recycled as continental crust. Much of the rock within the park is part of the Franciscan Complex.
1. Serpentinite/Serpentine
A particularly significant rock in the Franciscan Complex when it comes to the success of Edgewood meadow and chaparral plants, and the survival of the Bay Area Checkerspot Butterfly. Serpentine rock, and the soils and plants it contributes to, are found on the Clarkia and Serpentine Loop Trails. Only 1% of CA rock is serpentine. It breaks down into soils that produce masses of flowers, and other plants adapted to it's unique composition.

2. Other Rocks in the Franciscan Complex
These rocks include greywacke, melange, and greenstone. They contribute to soils, and therefore plant and animal life, in the central ridge (Ridgeview Trails) area and upper oak woodlands of the popular Sylvan Trail.

3. Whiskey Hill Formation
This sandstone occurs in the woodland area of Old Stage picnic grounds. It extends into areas beyond the preserve, notably in Pulgas Ridge Open Space (Dogs not allowed at Edgewood are permitted at Pulgas Ridge!). Handley Rock , a popular local rock-climbing site and an intriguing view from Edgewood, is composed of Whiskey Hill 

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Web Resources

*What is a tectonic plate?
**Subduction
Geologic Timeline of Western North America
The North American (Tectonic) PlateThe Pacific (Tectonic) Plate
Friends Of Edgewood: This site is an excellent source for everything from docent hike schedules to Edgewood wildflower search (Use the Photos tab in the upper right hand corner, to figure out what kind of flower you saw).
Edgewood Quick Facts