Oakland Geology

Part of exploring Oakland’s geology (and writing the book on it) is digging deeper, ever deeper. Two posts ago I dug into the unsung body of Eocene-age mudstone in the high hills, doing a systematic survey of its mapped extent, and had to stop halfway. Since then I’ve surveyed the other half, and it still feels like I’ve just begun. But so be it.

The ideal is to learn all of the significant outcrops. That would take a trip down every road and byway, and I’ve done that once already just for reconnaissance, not to pinpoint outcrops. Because life is finite, this time I figured out a shortcut based on the geologic map, where significant outcrops are ready-mapped.

The outcrops in unit “Tes,” the Eocene mudstone, are marked by those little symbols: a line with a tick sticking from the midpoint, labeled with a number. Each symbol tells you the orientation of the rock beds at that spot. The long line shows their strike — the direction the beds would align if you shaved the ground level — and the tick signifies their dip direction — the downhill direction of the beds. The number is the angle, in degrees, at which the beds slope in the dip direction.

For my purposes, all I wanted was the location, which is right where the tick is. I plotted those locations on a street map and set off to visit each one.

Before we start, this is an interesting image. It shows that the terrain where unit Tes is mapped is stronger, more resistant to erosion, than the Redwood Canyon Formation (Kr) to its south and the Sobrante Formation (Tsm) to its north.

This survey will go from east to west, the same way I walked it. The first outcrop, on the rim of Shepherd Canyon at Skyline, labeled “53,” was too far to hike so I skipped it. So we’ll start down in the canyon at the one on Woodrow Drive. I’ve shown you this one before; it’s where I found that cool concretion back in 2008. Supposedly the beds there are vertical, with the original upper side facing south (the black ball on the symbol means that there are indications of the original top and bottom of the beds). You can’t tell that from the outcrop, because it’s pure shale and the rock is so degraded, but there are still concretions weathering out. According to the map, then, we would be looking at the top surface of that concretion.

Around the corner on Paso Robles Drive is this exposure. It matches the map symbol in displaying overturned beds with a 65-degree dip. If you flip it over in your mind, you can see that a layer of fine-grained sand spilled over a muddy seafloor, and the flat surface is its underside.

The next symbol, the one marked “70,” is on Saroni Drive just east of Sayre Drive, but there’s no rock visible there today. It appeared to me that a new house has been built on the spot, or maybe the outcrop is in a back yard and is inaccessible. But farther west on Saroni, right at the edge of the “Tes” belt, some of the rock is exposed: a clean siltstone with the typical blonde color.

Now we cross the crest of Colton Boulevard and enter Thornhill Canyon. The next outcrop, on the east side of Armour Drive, is a roadcut exposing shale that has degraded since it was mapped. But you can still see the bedding’s steep leftward (northward) tilt, along with some near-vertical jointing.

The outcrop just west of Aspinwall Road is on a large vacant lot that used to be accessible (I recall visiting it during a walk led by Dennis Evanosky a few years back), but is now fenced off. Too bad. On the uphill side of Aspinwall is an exposure of clean siltstone, but its orientation is unreliable — these might be loose boulders, not living bedrock. Typically a geologic mapper measures strike and dip at several spots using a special compass/clinometer, often called a Brunton after the most highly regarded manufacturer. I have one, but a smartphone app does almost as well.

Crossing to the north wall of Thornhill Canyon, a steep climb up Beauforest Drive gets you to Valley View Road. The roadcut where the symbol labeled “80” sits is all mossed over. The Eocene mudstone prefers to support vegetation rather than crop out, and until some maniac cleans off the overgrowth or the hillside collapses, whichever comes first, this exposure is retired.

Two more exposures to go. The first of these is farther down Valley View, right next to the uppermost leg of the Upper Merriewood Stairs. It’s a good one, displaying a dip of 56 degrees east just like the map says.

Once you make it up the stairs, the rest of the walk is real pleasant, up to Broadway Terrace and across to the end of little-traveled Virgo Road. Unfortunately, there’s no sign of an outcrop there — either it’s covered with grass at this time of year, or a new house has obliterated it, or I’m just blind. But if you poke around, the views are wonderful. So that’s some consolation.

Getting back home from here is left as an exercise for the reader.

With all that work, I managed to confirm just three of the nine outcrops in this part of the map. Should I, and future mappers, accept the rest of these measurements if they can’t be confirmed? Should we accept them now? One approach to this conundrum is to consider previous geologic maps. I have four of them, and none of them agree. Some of the outcrops on this map also appear, with the same numbers, in the county geologic map of 1996, but that’s because the same guy, Russ Graymer, prepared them both. He measured just two or three outcrops that also appeared in two maps from the 1960s, and his numbers didn’t match theirs. The earliest map, published in 1914, might as well show a different planet. (See how it showed Knowland Park in this post from 2015.)

So I guess the upshot is that every generation of geologists learns the landscape anew, and by extension, that includes me. The certainty of a geologic map is always provisional and subject to correction, or at least to change. It can be disconcerting to realize that geologic knowledge is not necessarily cumulative, authority may not be authoritative, and rocks are not that firm a foundation.

I’ve spent an inordinate amount of time lately in the obscure part of the Oakland Hills between Piedmont Pines and Canyon. Much of it leaves me puzzled, and some of it leaves me dazzled, but I did manage to slow down and get a decent look at the rock along the East Ridge Trail in Redwood Regional Park. You’ve walked on it if that trail is your favorite hike, starting from the Skyline Staging Area.

The first mile of the trail exposes this rock in the roadbed. Stay on the trail, because actual outcrops are hard to find, and soon the poison oak will seal the woods off for the year. It presents colors of greenish-brown, buff, tan, dun and gray but the overall impression is a light brown.

The Eocene mudstone doesn’t have a formal name. It’s just a strip of fine-grained, mostly silty rock with a bit of sandstone here and there, that’s mapped across the midsection of both Thornhill and Shepherd Canyons and peters out along East Ridge (which has no formal name either; elsewhere I’ve called it Pinehurst Ridge). Here it is as shown on the geologic map labeled “Tes”, a dagger of cyan-ish color with the blade pointing east. This post is about that east-pointing blade, where there are no homes to spoil the ground. (Part 2, when I get around to it, will gather notable outcrops in the residential neighborhoods of the handle.)

What do we know about it? James Case gave it a searching look for his 1963 PhD at UC Berkeley and assigned it an age, based on fossil shells and foraminifers — one-celled “animals” with carbonate skeletons — in the early to middle Eocene, somewhere around 50 to 40 million years old, maybe a bit older. The specimens came from “thin beds of fossiliferous limestone” that Case noted on East Ridge.

I found some in the trail. It fizzed very nicely in a drop of acid, as you’d expect. The shells were small and mostly fragmentary. Naturally I left it there, under the East Bay Regional Park District’s protection.

Dorothy Radbruch of the U.S. Geological Survey looked at this rock unit again in the late 1960s and called it “sandstone and shale,” primarily fine-grained sandstone. She noted that it was fairly strong, holding up 1:1 slopes, which is reassuring for homeowners in that part of the canyons.

Most of what I saw in the roadway was siltstone — usually massive, or featureless, but occasionally laminated like this.

The USGS’s Russ Graymer, in the 1990s, characterized it more simply as green and maroon mudstone with occasional sandstone. He stated confidently that it was faulted on the top and bottom — just another small card in the well-shuffled deck of Coast Range rocks in the greater San Andreas fault zone.

In brief, it’s an isolated body of pretty clean mixed fine sediment that must have formed off the seacoast, not too near. It got lost in the shuffle as California was sliced, diced and rearranged between the middle Eocene and now.

Here’s a detail of the geologic map, plus the equivalent area in Google Earth, in case you feel like poking around. But note that just north of the East Ridge Trail, it’s East Bay MUD watershed land.

Top to bottom: Tor, Orinda Formation; Tcc, Claremont chert; Tsm, Sobrante Formation; Tes; Kr, Redwood Canyon Formation. The line with the teeth is a thrust fault, south side up.

The woods are rapidly closing in from their winter openness, and the slopes are in that brief interval between slippery-wet and crumbly-dry. I’m itching to return while I can, and it’s not from the poison oak, yet. Already I’ve missed the manzanita blooming season, except for a rare straggler . . .

and the land beckons.