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) Plate The 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

A Pickup Walk (Hiking Edgewood)

(co-published in http://EdgewoodSecrets.blogspot.com)

Invasive European grasses and Star Thistles
are one foe the Edgewood Warriors fight
tooth and nail

I know it's called a pickup game, when you run into other basketball players on the courts and have a competitive round. So I guess I had a pickup walk today.

I was hiking Clarkia and Lower Ridge trail, just appreciating the fact that my knees are back in service, when I found myself picking up a lone hiker, Diane. She hadn't found her hiking group, was pretty unfamiliar with the trails (she'd come in via Sunset Gate) and wanted company.

I ended up docenting  along Clarkia, up to Inspiration Heights, down along Lower Ridge trail to the fence that overlooks the Bluebird meadow and back to Sunset Gate, at which point we ran into her group

Discussed and seen along the way...

- Serpentine rock and soil discussion and challenge of nitrogen dump/non-native plant invasion. Also successes of Weed Warriors due to just plain hard work plus cunning and analysis 

- Why the erosion scars aren't a trail/the challenges of their trail-like appearance - And yes we ran into two erosion scar explorers that I had a chat with on Inspiration Heights. Hopefully they didn't go back that way, as I encouraged them to go on the trail. Much discussion with my new hiking pal, over how to discourage this behavior without being patronizing and actually getting desired behavior. 

- We met Steve and  Denora  out rangering and Diane had her birthday photo taken with them. Steve indicated perhaps more signs indicating erosion scar versus trail may be forthcoming?

- We enjoyed the beautiful summer colors of deerweed, tarweed and poison oak. We both think the seedheads we saw in with the tarweed is yarrow. I keep meaning to look up that pink dry headed looking flower that's in and around Ridge trail. I think it's a seed head not a bloom. It reminds me of the sea thrift I saw in Cornwall, just a little bit.

- Told her how she could find the plant database/photos lookup  on Friends of Edgewood web pages, as both of us were wondering about that pinky flower/seed head.

- Told her to come look for the brilliant green Hair Streak Butterflies during bloom time for the deerweed. Discussed the importance of the Bay Area Checkerspot and how it saved the preserve. Diane was glad we weren't hiking through the golf course this area was, at one point, destined to be.

- Diane wanted to know about animals we see in the area. Pointed out Western Fence Lizards, mentioned my few views of rattlers by me and others and where noted ...Much pointing to the area on Serpentine Loop Trail  from Ridge Trail looking down to discuss the scurry zone and habits of the cottontails. Also discussion of the jackrabbits when they go mad with testostorone in the springtime and their hare 'ness ( Here's a nice web link on their being hares and not rabbits http://animals.nationalgeographic.com/animals/mammals/jackrabbit/). 

- Pointed out the frog pond, looking down on it from Ridge Trail (would be easy to have a talk about water in the preserve at this point, wouldn't it?)

- We should have asked Denora about the bobcats when we met up with her and Steve later on, as I know she once said there is one living in the vicinity of the ranger's house. Durn

- Of course we chatted about cougars. Doesn't everybody like to know about cougars?

- We talked about the different types of oaks, and after some quick mental review. I remembered  (and I think properly id'd ) coast live oak (thanks to a hint Alf once gave me), contrasted them with a description of Valley Oaks, and mentioned the scrub oak. I think that's what grows on Upper Clarkia, not Leather Oak? Remembered to tell her about the naturally hybrid ones.

- We talked about the Western Blue Birds

Dianne was very pleased with her one-on-one docent walk! We found her group back at Sunset Gate and she introduced me all around and bragged about getting the goods on the preserve. I was lightly quizzed by a couple of folks in regards to seeing freshly blooming Farewell to Spring, and I agreed I had seen one too. Was able to respond "Clarkia, like this trail" when asked what is the real name. So I guess I passed the test. Good thing that was one I know.

Despite it not being a high bloom time, there's a lot to talk about out in the chaparral zone