The wonderful Sibley Regional Volcanic Preserve has a southern extension, still being developed, that opened to the public recently. It’s at the head of San Leandro Creek upstream from the village of Canyon on the west side of Gudde Ridge, a part of the hills that’s always intrigued me. The land is in the foreground of this photo of Mount Diablo I took from Huckleberry Botanical Preserve in April 2009.
The property was homesteaded in the 1860s by Patrick and Catherine McCosker and remained in the family for the rest of that century and all of the next. It was a cattle ranch for most of that time, with a small kitchen orchard. The Oakland, Antioch & Eastern railroad exited the old Redwood Peak Tunnel eastbound near the ranch entrance and stopped there at a station named Eastport, still a locality on the map.
About the time when plans were floated to punch a new state highway up Shepherd Canyon and through the hills to the Lamorinda area, the McCoskers started a rock quarry in hopes of profit during the highway construction. (Transport distance is a very important factor in the crushed-stone business.) It operated from 1958 to 1971, then continued as a rock crushing plant serving various construction projects in the area.
After that business petered out and the land languished for a while, the McCosker descendants sold a 250-acre chunk known as the Texas parcel to the developers of Orinda’s Wilder Ranch community, and as part of a deal the land was donated to the East Bay Regional Parks District. The “McCosker Sub-Area” opened to the public in December 2016.
The parcel occupies most of the valley north of Eastport, as seen here on the 1959 topo map. I’ll show the bedrock map of the same area later.
It’s precious property for native plants and a good wildlife corridor, tucked between the Huckleberry preserve and steep, remote grazing land in the Sibley preserve. The District classifies it as a Natural Unit, managed for its “unique or fragile habitat values with public access primarily limited to trails.” Its permanent stream, a major tributary of upper San Leandro Creek given the name Alder Creek, will have some 1300 feet of decaying culvert removed and be restored to good riparian habitat suitable for native rainbow trout.
To protect this wild country, access for dogs and horses will be tightly controlled. (At present, dogs are not allowed at all.) Plans include a small campground suitable for backpackers on the major Bay trails.
On a misty weekday afternoon, it feels very far from the rest of the Bay area.
On sunny days, though, the views are good. I went back yesterday to capture some.
The rocks here are mainly from three map units: the Claremont Shale (the gold strip along the valley floor), the Orinda Formation (Tor, conglomerate) and Moraga Formation (Tmb, lava flows, and Tms, associated sediments).
The Claremont peeks out of the woods near the entrance with its characteristic, steeply dipping stripes.
The Orinda pokes out of the hillside with its characteristic beds of coarse gravel and cobbles — but only in roadcuts.
Elsewhere, out on the bare hillsides, the ground is nearly pure clay soil with only rare pebbles exposed, tokens of a very different terrain dating from some 10 million years ago.
The Moraga Formation lava is harder stuff, and there are a few actual outcrops in the high hills.
Near the entrance is a flat platform with a large shed, evidently built from quarry waste. It will become a parking lot and recreation area named Fiddleneck Field. For now it’s a parking lot for boulders from other parts of Sibley. Check them out. They represent the Orinda and Moraga formations, plus a few white limestone chunks from the Moraga sediment unit or the slightly younger Siesta Formation.
Some of the lava features the distinctive amygdales, former voids filled with hydrothermal minerals, found at Sibley.
Another thing to notice here and there is small landslides, like this slump at the edge of the Fiddleneck Field platform. Others occur along the ranch roads, which were built long ago by ranchers without the advice of geotechnical consultants.
As far as I can tell, the quarrying took place in scattered pits of no great size. That area is screened by oak/bay forest and cordoned off with barbed-wire fence, so it’s hard to tell. I look forward to seeing this land opened up as the District does more work.
In the meantime, there are splendid views to be had.
Give it a visit some time.
Details of the plans for this area are in the Sibley Land Use Plan, available if you search for it from the East Bay Regional Parks District website.
Oakland Geology 
25 November 2019 at 10:36 am
I’d love to send you a photo of some of my hillside outcroppings in Canyon to identify for me, the formations. My old geology teacher told me there was a little fault running along the creek through our village.
25 November 2019 at 9:00 am
“After that business petered out. . . .” I see what you did there! Or was that an unintentional pun?