Archive for the ‘Oakland sandstone/shale’ Category

McAdam’s quarry

11 April 2022

One of my first outings during the pandemic era was a hike up Shepherd Canyon in search of Alexander McAdam’s sandstone quarry. But I was missing a telltale clue. Now I think I’ve found it.

I wanted to locate the quarry because it produced the stone used in the historic First Unitarian Church in downtown Oakland, the only example I’ve found of a truly local rock used as dimension stone in a building instead of crushed stone in an anonymous construction.

The clues I had pointed to a location “at the head of Thirteenth Avenue” somewhere “in Medos Cañon, back of Piedmont.” I thought this meant some place in present-day Montclair, but all the old maps I checked didn’t show any land there belonging to McAdam. Then I tracked down the 1894 Wagner map of Alameda and Contra Costa Counties, and here’s where his ranch was: over the ridge above Shepherd Canyon, in the valley of Redwood Creek.


Shepherd Canyon in the center; the old Thorn Road runs around its north edge. The Heyland property at lower left is in Dimond Canyon and the Hays School is in present-day Montclair. The McAdam property is at lower right past the county line, shown by stippling.

The land is within Redwood Regional Park, so I surveyed the territory last week. Here’s a closeup of the map, showing McAdam’s 131.24 acre holding.

And here’s where it sits in the geologic map.

This is very steep country, in an area where the redwood groves had been logged out forty years before. McAdam used it as ranchland, like others in this remote district, but he also operated a successful quarry here somewhere. It wasn’t in the coarse, crumbly Oakland Conglomerate (mint green) or the shaly stuff of the Shephard Creek Formation (pale green), but somewhere in the thick-bedded, fine-grained sandstone of the Redwood Canyon Formation (Kr on the map, olive green).

Fortunately, Redwood Regional Park maintains the old trails and logging roads, so I superposed McAdam’s land on the park’s trail map and gave it all a good look. Note: the poison oak is very healthy this year.

The only place a quarry would make sense is at the very top of the property, but if you’re up for it, the Tres Sendas trail takes you down to some nice woods.

The rock here is appropriate, just like the stone in the church.

And some of it fractures nicely enough to be dressed into ashlar blocks.

But it would be nuts to operate a quarry down in the back forty. Up on the ridgetop in the northwest corner of the land is where I think McAdam had his pit. It’s the backdrop of this view over the spot where McAdam’s ranchhouse was, at the westernmost corner where the Waterloo Staging Area is today.

It was accessible via the Castle Canyon road, as seen in the 1897 topographic map. The McAdam place is in the center, at the end of the road.

The hilltop is just off high-lonesome Wilton Drive, where I last took you three years ago for the Shepherd Canyon circumambulation. The views from there are still wonderful.

The slope immediately below, too steep to think about descending, is where I think the quarrying went on. The outcrops of Redwood Canyon Formation display good rock.

I’m glad to put this little mystery to bed. And if the Unitarian Church must replace its stone with authentic materials, they know where to look.

Load casts in the Shephard Creek Formation

11 October 2021

I see that I haven’t given my photos of this interesting feature their own page. They show some dramatic load casts on the south side of Shepherd Canyon, near the east end of Escher Drive. A load cast is made when heavy material, like a mudflow, crosses soft sediment and sinks its feet into it.

My most memorable lesson about features of this kind happened in 2008 during a visit to Point Reyes. The path leading to the lighthouse passes a chaotic scene that most visitors ignore.

The scene was an offshore basin, probably like the offshore Monterey Canyon today, where every now and then an undersea debris flow, full of gravel and sand, fell rudely upon nice quiet beds of deep-sea clay or soft mud. The results included scour marks . . .

and rip-up clasts, hunks of (easily eroded) clay swept into the flow . . .

and downward-pushing load casts accompanied by upward-pointing flame structures.

It was a great pleasure to come upon high-quality load casts in the Oakland Hills, in the mudstones of the Shephard Creek Formation, and lead group walks past them. Here’s the overall scene, photographed in February 2016. A thick layer of massive (i.e., unbedded) sandstone overlies thin-bedded shale and mudstone.

Near the base of the sandstone, on the lower right side, are these well-exposed load casts.

The previous June, I took a closeup of the underside.

Unfortunately, a Google Maps image from January 2021 appears to show that this feature has crumbled off the roadcut. That’s how geology goes in the Oakland Hills, and that’s one reason I constantly take photos. I also tell myself, in consolation, that new examples could appear on any given day.

The classic 1995 text Sedimentographica has good photos of these and many more features of sedimentary rocks. I treasure my hard copy, but maybe the publisher’s online version will outlast it. That one, YOU can enjoy.

Upper Castle Canyon

30 August 2021

Sausal Creek is formed by the junction of three streams, two of which are well known: Shephard Creek, which drains Shepherd Canyon, and Palo Seco Creek, which drains the bulk of Joaquin Miller Park. In between them is Cobbledick Creek and its steep-walled watershed, hidden green heart of the Piedmont Pines neighborhood.


Source: Alameda County Flood Control and Water Conservation District

The creek has two branches: the northern one, which I’ll call Beaconsfield Creek after Beaconsfield Canyon at its head, and the eastern one, Castle Creek, which drains Castle Canyon. The easternmost portion of Castle Canyon, a gorge running from the hairpin turn on Mastlands Road, is an 8-acre preserve that’s formally part of Joaquin Miller Park. Over the weekend, that parcel was renamed Dick Spees Canyon, with a bench and plaque, to honor the longtime politician and activist who helped keep the land undeveloped.

The interesting thing about Dick Spees Canyon, and the valley of Cobbledick Creek below it, is that it coincides with the inactive Chabot fault, a deep feature of the East Bay that runs roughly parallel to the Hayward fault. It runs diagonally across this digital elevation model of the area; Dick Spees Canyon is right in the center.


From nationalmap.gov

And the bedrock map of the same area is here. The Chabot fault is the dashed line with the pairs of tick marks on the right-hand (downthrown) side.

The Chabot fault juxtaposes two very different rock units directly across the canyon from each other — the serpentine rock of the Coast Range ophiolite (sp) on the west and the Joaquin Miller Formation (Kjm) of the Great Valley Sequence on the east. Dick Spees Canyon aligns very nicely with the upper part of Palo Seco Creek, forming an unusually good topographic expression of this obscure fault. Leona Canyon is another place it stands out; also in upper Knowland Park. The fault has been traced past Hayward. While it appears to have a long history, it’s very much inactive.

The fault is why I made a point of visiting here in 2019 and returned last Thursday. The parcel is almost completely undeveloped; only a rough footpath runs up the narrow valley floor from a short formerly graded stretch, then zigs up to Castle Drive. (I must advise all visitors not to try walking it downhill until the rainy season firms up the soil there.) The following photos are from both visits.

Here’s the lower end of the property. It’s a steady climb.

Soon the rocks make themselves evident, serpentinite on the right . . .

. . . and mudstone on the left.

Whoever built this fire ring used stones from both sides of the fault.

The valley floor is littered with dead cedar and eucalyptus trunks that need clearing out. And all sides of the canyon are very steep. I don’t expect anyone to cut any trails up them for a long time.

But if they do, visitors might glimpse the views enjoyed by the ridgetop residents who surround this neglected gulch with its interesting geology.