Oakland Geology

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.

This entry was posted on 11 April 2022 at 10:03 am and is filed under Oakland sandstone/shale, Oakland stone. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.