Archive for the ‘Oakland sandstone/shale’ Category

The Eocene mudstone, part 2: Shepherd and Thornhill Canyons

29 April 2019

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.

The Eocene mudstone, part 1: East Ridge

1 April 2019

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.

Redwood Ridge and the Parkridge land bridge

19 February 2018

Redwood Ridge is a name I made up to keep things straight. Let’s start with the part of the USGS topo map showing the south end of Oakland’s redwood country. Redwood Ridge is just east of Skyline Ridge (another name I made up), which starts where Joaquin Miller Road meets Skyline and extends to Lake Chabot.

Oakland was a redwood lumbering town before it was anything else, and the great redwood groves gave their name to features all over the hills. Redwood Peak sits at the top of the map, and east of it is Redwood Creek running down a straight valley that leads to Upper San Leandro Reservoir. That valley has no formal name, so I dub it Redwood Valley, the valley of Redwood Creek.

A major tributary of Redwood Creek flows out of a steep-walled valley named Redwood Canyon, clearly marked on the topo map starting with the 1947 edition. So, Redwood Canyon cuts through Redwood Ridge and ends in Redwood Valley at the point where Redwood Road meets Redwood Creek. Got all that? Good, because I won’t repeat it.

From here on out I’ll show maps that have been tilted for easier viewing. Here’s Redwood Ridge in the handy terrain view of Google Maps.

This post is about the south part of Redwood Ridge. It’s a pretty cool piece of land, just to look at on the map.

The top side is bounded by Redwood Valley and the left side is defined by the lower part of Redwood Canyon, a classic water gap. Now look at the bottom side. On the right is Grass Valley, with Grass Valley Creek flowing through it down to Lake Chabot. On the left is a smaller valley that lines up with the upper part of Redwood Canyon. It has an unnamed stream in it. I’ll call it MacDonald Creek, because that’s the name of the trail there.

The last thing to notice is that little land bridge leading from the end of Parkridge Drive, right where the valleys of MacDonald and Grass Valley Creeks meet. The two creeks have been eroding their way toward each other. They seem to be evenly matched, but I think Grass Valley Creek may have a slight edge. The photo portion of this post starts there.

But first, the bedrock map. It shows that those two creeks have been exploiting the softer rock of the Shephard Creek Formation (Ksc), sandwiched between the Oakland Conglomerate (Ko) holding up Skyline Ridge and the Redwood Canyon Formation (Kr) holding up Redwood Ridge. Rare are the places where Oakland’s bedrock is expressed so clearly on the landscape.

And here’s the park map with the details on the trails.

As you descend Parkridge Drive to the trailhead, Redwood Ridge appears as an island of forest.

In my three visits here, dog walkers made up the great majority of people using the park. (Be sure you or your walker supports the park by carrying a permit and following the rules.)

Starting out across the bridge feels magical.

And at the right time of day as if by magic, the bedding planes of the Shephard Creek Formation appear out of nowhere. The geologic map indicates that these beds are overturned.

The view from the bridge extends to the right down Grass Valley toward distant Mission Peak overlooking Fremont.

And to the left, the view from front to back encompasses MacDonald Creek valley, Redwood Canyon, the massif of Redwood Peak and Round Top beyond with its bare southern shoulder. Redwood Canyon still grows a few redwoods, but in the mid-1800s they must have filled the canyon to the brim.

The MacDonald Trail is excellent for all users, including horses and (since 2016) bikes. The woods are enchanting in any weather, but they photograph best on shady days.

So does the bedrock in the road. The Redwood Canyon Formation is primarily fine- to medium-grained sandstone that shows the marks acquired over 80 million years of geologic history. It’s soft enough to be graded without blasting. The ridge stands as high as it does not because the rock is especially hard, but because it absorbs water so well, inhibiting the surface runoff that so effectively erodes the stream valleys all around it.

Off the road, the sandstone occasionally crops out in bulbous boulders. When Jim Case mapped these rocks for his PhD thesis in the early 1960s, he described these as “cannonball concretions,” but from my observations so far I think he was mistaken, and the description of this unit on the geologic map (circa 2000) does not mention them either. I think this is ordinary weathering like you see in arid and semiarid country all over the West.

The previous three photos are from the north side of the trail. The south side offers wider views of Grass Valley and beyond to Loma Prieta and the Sierra Azul west of San Jose at far right.

And you must not miss the stub of Brittleleaf Trail, which leads to a sandstone spur overlooking lower Redwood Valley. Surrounded by blooming manzanita at this time of year, the tranquil spot hums with bees and invites a long sit. Naturally I inspected the sandstone and determined to my satisfaction that its beds are overturned and dip steeply at 75 degrees. Notice that the fractures in the sandstone have no relationship to the original bedding.

The view south from here looks over the reservoir and watershed lands, the bare green ridge known as The Knife west of San Ramon, and the Diablo Range mountains south of Livermore against the horizon.

The view north, from far to near, includes the Briones Hills, tower-topped Mulholland Hill in Orinda and Moraga, the south end of grassy Gudde Ridge with its water gap where Canyon Road cuts through, a bit of wooded Canyon Ridge, and chaparral-covered Pinehurst Ridge, the type area of the Pinehurst Shale. All are worthy destinations of their own.

This is the best time of year to see these lands. Among other reasons, the poison oak has begun to sprout, making it easily visible, but not yet spread over the woods and side trails, keeping you out.

The Pinehurst Shale

23 January 2017

Much of Oakland’s high hills consists of our local piece of the Great Valley Group, the colossal set of sedimentary rocks that runs the length of the Central Valley along its western wall. (How our piece got over here is, as they say, poorly constrained.) The group is well exposed in Shepherd Canyon and points south, and it continues on the far side of the hills as far as the Upper San Leandro Reservoir. There the Redwood Canyon Formation, the upper unit of the group, is overlain by a little rock unit called the Pinehurst Shale. On the geologic map, it’s the slices of darker green nestled amid the Redwood Canyon Formation (labeled Kr).

pinehurstshalemap

It’s named for Pinehurst Road, naturally, and its type locality is at the Pinehurst Staging Area on the far side of Redwood Regional Park. You know the place if you’ve ever driven out there. It’s right at the symbol labeled “45.”

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This exposure shows strong bedding, but it’s all shale — rock with little material coarser than clay size. Some of it’s soft claystone that crumbles and erodes easily. Here it is in a rain-carved rut in the fire road that leads into the park.

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Other parts of the Pinehurst Shale are very hard and resistant because they contain a goodly share of silica. Here’s what that siliceous shale looks like in outcrops along the trail . . .

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. . . and here it is in hand specimen. (For display only; I put it back.)

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Farther up the trail, the shale gives way to siltstone of the Redwood Canyon Formation. It’s quite a dramatic change.

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Both of these rock units contain microfossils from the Campanian Age, making them somewhere around 75 million years old. Whatever once lay above them has been removed by faulting, leaving a gap of maybe 60 million years between them and a little ribbon of Sobrante Formation across the reservoir on EBMUD land.

There’s another cool thing here. Here’s the rock just to the right of the top photo. See that big chunk of rock by the right edge, sticking out crosswise against the bedding?

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That is a sedimentary dike. It marks a place where once upon a time, before this rock was fully solidified, an imbalance in fluid pressures caused the one layer to force a crack upward through the overlying layers and squirt into it. Usually dikes are something you expect to see in lava beds, or producing “sand blows” on the ground after large earthquakes. This next shot is taken from the right side, looking at the top of the dike.

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An exposure like this, on a roadcut, is usually fair game for grooming by pulling out the shrubs and sweeping away some of the rubble. I do that here and there during my outings, but this spot looked a bit chancy.

I contented myself with pulling up some young French broom plants. Now’s the best time of year to do that, when the soil is saturated. If we all did a little of that, it would help fight the invasive problem. The land is worth the effort.

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Visiting our parklands can take you very far away.