Archive for the ‘Oakland rocks’ Category

How useful is the Orinda Formation?

30 April 2018

Walking along the paved trail north of Inspiration Point, I was brought up short by a splendid outcrop of conglomerate.

It’s strongly reminiscent of the Orinda Formation conglomerate exposed to the south in Claremont Canyon, in Sibley Volcanic Preserve and along Route 24 east of the Caldecott Tunnel.

Naturally I fired up the geologic map (I keep USGS map MF-2342 on my tablet) to see how the locality is mapped. It’s the little hill northwest of Inspiration Point, right above the word “Nimitz” where a power line runs.

But instead of Orinda Formation (the orange unit labeled “Tor”), which underlies Inspiration Point, it’s mapped as “Tus,” or “unnamed sedimentary and volcanic rocks (late Miocene).” Turns out there’s a major fault that separates two big blocks of young East Bay rocks — that thick black line with the teeth that represent the upthrown side — and even though the rocks look the same, we can’t say for sure they are.

The area of “Tus” rocks is rather large; in fact it’s the largest single rock unit on the geologic map.

I poked around the literature and found that the Orinda Formation has drastically shrunk over the years. As one example, here’s part of a 1973 map of the Lafayette area (Calif. Div. Mines & Geology Map Sheet 16) that classified a bunch of rock as Orinda Formation, drawn with the exquisite attention that emanates authority.

But the details are quicksand. First, the map is not about bedrock per se, but landslide hazards. Second, the author’s citations are generally very old, not that there’s anything wrong with that, but still. Third, the author’s idea of the Orinda is a unit that mixes lava beds (lumped today in the Moraga Formation) with the coarse sediment that defines the Orinda today. A long footnote explaining his thinking shows that he basically made an arbitrary choice of stratigraphic nomenclature to match the informal usage of local engineering geologists, who tend to talk about “Orinda-type” materials (like I was thinking at my outcrop) without making sure the stuff actually matches all the criteria for the Orinda Formation.

I’ve read my share of geologic engineering reports; any large construction project has to have one prepared. They’re good for their purpose — ensuring that the work is suited for the ground — but they don’t critically assess all the details of the science. And they probably shouldn’t. Instead, they line up the existing literature, outmoded and current alike, and discuss or dismiss it on the way to reaching their conclusions and advising their client.

Maps like Sheet 16 propagate obsolete or informal nomenclature, and thus stratigraphic concepts that are outmoded or discredited persist in the geotechnical literature like zombies long after research scientists have moved on. But I don’t blame people. The old idea of the Orinda Formation, widespread and simple, was very handy. The current idea of the Orinda, constricted and specific, is less handy because it leaves a large area of bedrock with the mumbly label “unnamed sedimentary and volcanic rocks (late Miocene).”

Geologic maps aren’t written in stone. Only stone is, and we’re still learning to read it.

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Land and rocks of westernmost Piedmont

2 April 2018

The western end of Piedmont includes the headwaters of Pleasant Valley Creek, which is tucked under Grand Avenue. But the slopes and gullies of the valley can’t be hidden, and what may seem like a scramble of streets is a nice place to walk around. The topography I’m talking about is north of Oakland Avenue, between Grand and the former quarry of Dracena Park.

None of it’s bedrock; nothing wrong with that. It’s the uppermost part of lobe 3 of the Fan, that big arc of ancient alluvium hills across central Oakland. The bedrock part, farther uphill, is much steeper walking, and the rock (the blue field labeled “Kfn” on the geologic map) doesn’t show itself either.

I should make an exception to that statement — Dracena Park is a great place to see the bedrock. But for the purposes of this post it’s just a pretty place, either in the former main pit:

or in the valley on the north side, now full of redwoods.

This is the most dramatic bit of stream valley in the area. Elsewhere, it’s easy to trace the drainage lines; in fact during our recent batch of rain, you could see water coming out of the ground just like in the old days before people lived up here.

This slopy bit of suburbia is criss-crossed with hidden history. The Key System streetcars used to serve the neighborhood. Indeed, a railroad syndicate once planned a major route through here that was going to run through upper Fruitvale and on to San Jose. The right-of-way appears in this 1927 map (courtesy of the Oakland Library History Room), long after the plans were abandoned.

The part of it running as far as Oakland Avenue did end up with rails as part of the Key System’s C line. If you look closely at the area in Google Maps, the lot lines give the route away.

The transit routes, road and rail, took advantage of the saddle in the ridge of lobe 3, at this spot where Pleasant Valley Road eases over the hill and becomes Grand Avenue. The view is west across the saddle toward Rose Street.

OK, enough of that. There are indeed rocks in this neighborhood. See that clever stone wall in the foreground in the previous shot? Here’s a closeup.

Other rocks are more laid back, understated but cool.

Still others, the only word for them is homey.

They’re all mostly landscapers’ stone, purchased at a commercial yard. But here and there you’ll see genuine domestic Oakland rocks. This little wall is made of serpentine/blueschist from around here, probably the pit at Serpentine Prairie or nearby.

And the Franciscan red chert is very likely from Piedmont itself, which sits on a hill of melange, a marble-cake rock unit that mixes chert, basalt, sandstone and serpentine in a mudstone matrix.

Some of it can be downright psychedelic. Stuff like this was quarried in Piedmont before any buildings were here at all.

Even the sidewalks, the oldest ones anyway, incorporated aggregate from Piedmont’s original quarries.

The town has grown up since then. It puts on a good front — very nice homes, lovely grounds, a fine place for walking and taking in the views (especially when the leaves are down in winter). But it began as a rugged, dusty mining district with horses and dynamite.

Oakland geology ramble 5: Grass Valley

19 March 2018

Over the last few weeks I’ve been exploring the remote land just east of Skyline Boulevard, over the city line in Anthony Chabot Regional Park. Time to show you some of the charming features of Grass Valley, seen here from Redwood Ridge near the Parkridge land bridge.

In classic Geology Ramble style, this walk (a set of them actually) starts at one bus stop and ends at another. It starts at the top end of the 39 line and ends at the top of the 46L. The challenge is that you can only do this on weekdays because neither line runs on weekends; moreover both lines only run once an hour. Here are the two routes I’ve done so far, in the Google Maps terrain view just to give a feel for the territory (800 x 100 pixels). There are 12 photos in this post, locations given in a map at the end.

The route on the right runs down Redwood Ridge, and the first part is just like what I showed you last month only more so. Where it meets the other route, at the Bort Meadow staging area, you get this view of the valley floor. You can see it looks completely different in cloudy weather. Without the distant views across the Bay and south into the Diablo Range, it’s intimate and secluded. (The rest of my photos are from the western route.)

Amelia Sue Marshall, in her new book East Bay Hills, a Brief History, says that Grass Valley was never settled by whites or Ohlone because the stream was unreliable. Back in the day, redwood lumber was hauled through the valley to Castro Valley; only a few redwoods actually grew there. Later, cattle were driven through it between Oakland and Moraga. The Grass Valley Ranch raised cattle there for many years until the water companies moved in. The East Bay Water Company and the Contra Costa Water Company both planted huge eucalyptus forests there, and finally East Bay MUD took them all over. When they secured Sierra Nevada water, their East Bay land was transferred to the East Bay Regional Parks District, and that’s the story.

The ramble starts at Skyline High School. The scenic way is up the median path of Skyline Boulevard, charming in all seasons. It also exposes the Oakland Conglomerate, as I’ve posted previously.

The road up to the city stables is marked by a sign. Go past the gate and enter the Goldenrod Trail, an old dirt road popular with horse riders. It’s pretty, and you’ll see outcrops of Oakland Conglomerate along the way.

You knew I’d get to the rocks. The gross terrain is shaped by the rocks beneath it, but not much rock is actually exposed. These rocks began as sediment, and to sediment they quietly return, mostly covering themselves in a forested blanket of soil.

The important map units are Ko, Oakland Conglomerate; Ksc, Shephard Creek Formation (mostly shale); Kr, Redwood Canyon Formation (mostly sandstone). Grass Valley is strongly confined by the shale. The upper slope, though, is conglomerate. You can usually tell by the well-rounded stones embedded in it — samples of ancient mountain ranges. They get as big as this.

And there’s more than rocks to shake a camera at. For instance, banana slugs.

Take the Ranch Trail or the Buckeye Trail down into the valley. Near the bottom the woods open up nicely.

After a while you reach Bort Meadow. A hundred years ago the water company called it the Big Trees area, but the park took down most of the eucalyptus and it’s much better now.

The stream gradient is very gentle; this area looks like it could have been a lake at various times in the recent geologic past, especially if landslides dammed it.

Grass Valley is still a ranchy, horsy, countrified place. Though it’s gone from drivin’ dogies to walkin’ doggies.

The first time I came through the valley, unsure of my pace relative to the bus schedule, I was trotting. After that I knew I could amble instead. The whole hike is less than 6 miles, and the climbs aren’t that strenuous.

Farther down the woods rise up and close in, first oak and then eucalyptus.

By the time you reach the Stone Bridge the woods are thick. I haven’t yet gone farther toward Lake Chabot; the public transit logistics are pretty daunting. Take a minute to look at the streambed above the bridge; the Shephard Creek Formation is well exposed there, but look from afar because the creek bed is fenced off as sensitive habitat.

From the bridge turn up the Jackson Grade, where you’ll meet the bottom end of Skyline Boulevard. (There’s a water faucet at the top of the grade.) From there it’s a quick downhill into the Grass Valley neighborhood. The eucalyptus allee on Grass Valley Road is pretty to look at, harrowing to drive and inadvisable to walk.

Instead, cut over to Scotia and down Shetland and contemplate the classic postwar burbia around you as you head for the bus stop.

As promised, here’s the photo key.

You could take this route the other way, but it’s uphill, from about 500 to about 1100 feet.

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 Fremont Peak.

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.