Archive for the ‘Oakland sandstone/shale’ Category

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.”

pinehurstshale1

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.

pinehurstshale2

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 . . .

pinehurstshale3

. . . 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.

pinehurstshale5

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?

pinehurstshale6

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.

pinehurstshale7

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.

pinehurstshale8

Visiting our parklands can take you very far away.

Watershed wilderness

5 October 2015

The lower end of Skyline Boulevard offers a tantalizing glimpse of the wilderness right next door. This post has large images, so I encourage you to click on them.

When you look east from most of Oakland’s highest hills, the center of attention is Mount Diablo. You can drive there and drive up and it’s a wonderful place. From the southernmost end of the hills, though, Diablo is hidden by Rocky Ridge.

rockyridge

Rocky Ridge reaches just over 2000 feet elevation and forms the west side of Bollinger Canyon, in Las Trampas Regional Wilderness. Between here, at the city stables just north of Keller Road, and Rocky Ridge lies Grass Valley, with its patches of grass and a power line running up it; then a darker, more distant ridge on the other side of Upper San Leandro Reservoir. From there to Rocky Ridge is an untrammeled area of mixed woods and fields and chapparal that’s East Bay MUD watershed land.

The ridge is about 5 miles away in a straight line but more like 8 miles on foot. With a permit, you can walk there but you can’t bike and you can’t camp. In a word, reaching that land from here is a pretty extreme challenge. You could reach it from the other side if you’re up for an 800-foot-plus climb out of Bollinger Canyon.

It’s so near, yet so remote.

Here’s the geology: fairly young sedimentary rocks, of late Miocene age (roughly 10 to 5 million years old), deeply folded to create dramatic exposures on Rocky Ridge’s flank. Don’t worry about all the symbols and labels, they’re significant only to a few specialists.

rockyridgegeomap

The photo is taken from the lower left corner where it says “Ko” (for the Oakland Conglomerate) and points toward the upper right corner. In the upper right quadrant, that set of stripes with the heavy line on its right edge represents the package of rocks making up the ridge, and the heavy line marks a thrust fault along which those rocks have been uplifted.

To help you visualize what the map is showing, the map includes a cross section of these rocks, drawn along that straight diagonal line near the top left. The point labeled B’ corresponds to the same point on the cross section, below.

rockyridgeprofile

Is your brain stretched to breaking yet? No? You may have the makings of a geologist.

For a much easier experience, hike in Grass Valley instead. That’s not watershed land, because it drains into Chabot Reservoir; instead it’s in the northern part of Anthony Chabot Regional Park, where only the locals go, and is as peaceful as can be. Some day I’ll post about it.