Archive for the ‘Oakland rocks’ Category

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.

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Care and grooming of rocks and outcrops

5 February 2018

Photographers know that everything about a great shot depends on how you set it up. As I capture images of geological subjects, part of the setup involves prepping the model. Often it’s just a matter of removing a few stray twigs to get a decent picture, like these serpentine stream cobbles in Joaquin Miller Park.

Only rarely does nature present rocks in an untouchably satisfying way — and to tell the truth I might have brushed off a cobweb before I took this portrait of laminated siltstone in Shepherd Canyon.

But around here, as opposed to the Grand Canyon, rock exposures are generally kinda shabby. What are you going to do? Rocks are made underground, and once exposed to surface conditions they start to break down. And vegetation doesn’t care where it sheds things.

In many of my outings, I find myself doing some housecleaning. Whenever I lead a walk, it’s mandatory for me to go over the route beforehand and groom the features I plan to show off, like the splendid exposures of the Oakland Conglomerate in Montclair.

This textbook example of load casts always requires a thorough weeding session, followed by a good sweep with my umpire’s brush. Because shale wants nothing more than to return to clay, and weeds like nothing more than decayed shale.

There’s an outcrop in Dimond Canyon, along the trail, that I always take a minute to groom. The last time I did this I took these before-and-after shots.

This is something that any of you can do too, if the spirit moves you. I know people who routinely carry tools to prune snaggy or undesirable vegetation along the trail in a discreet way. Sometimes I do that, but more often I stick to grooming the outcrops. Because who else will? While one of my principles is to “leave the stone alone,” the stone can use a bit of care here and there. That’s my contribution to the geoparks movement in a country that badly needs it.

The St. James Drive roadcut

11 December 2017

Recent work in far east Piedmont has exposed some excellent bedrock worth a close inspection. Because the town government won’t put an interpretive sign there, this post will have to do.

To my knowledge, there are only two sites of powerline towers in Piedmont, one at the mouth of Estates Drive and the other at 298 St. James Drive, near Park Boulevard. Last year the power company replaced the latter pair with shiny new towers, and as part of the work it cleared the roadcut of its cover of acacia trees (Google Maps shows the site as a thicket going back to 2007). Shortly afterward I discovered it and had high hopes, though it wasn’t much to look at in late October.

By January, the exposure had been stripped of loose rock. Already it was clear that it would be a showcase of slickensides.

By August, a strong concrete wall had been put in place and landscape plantings made.

The slickensides turned out to be fabulous. These are the polished marks made as movement along faults grinds rocks against each other.

And here’s a closeup.

Also visible is evidence of brecciation, the geologist’s word for shattering rocks and cementing the pieces together.

The rock here is sandstone of the Franciscan Complex, specifically part of the Novato Quarry nappe. This is a thick slice of fine-grained sandstone that was laid down off the ancient coast of California, then shoved against the continent’s edge and pulled apart into lumps by the San Andreas fault system. A bunch of it underlies Marin County, and more makes up Point Richmond and El Cerrito as well as Piedmont and points south. This tectonic history probably accounts for the wear and tear visible in the roadcut.

When I visited the roadcut again last week, I annotated and recorded the site in the ROCKD smartphone app and announced it on Twitter. I’m trying out the app just for fun as a way to make some of my observations public. I’m looking at other apps for more rigorous mapping purposes.

The Dunn-Spring Quarry, north Berkeley

27 November 2017

Glendale-La Loma Park, a little ballfield/playground complex in the north Berkeley hills on La Loma Road, is a repurposed quarry that’s had a true Berkeley history. The original quarry, operated by J. J. Dunn, appears to date from 1892. John J. Dunn, a Canadian immigrant born in 1839, was a major contractor in California building roads and sewers starting in the 1870s. This is Dunn portrayed in 1896 in the Oakland Tribune.

In 1900 Dunn advertised the quarry for sale “on account of sickness,” and died of Bright’s disease (kidney failure) in St. Helena that June.

In 1904 the Dunn quarry was reopened by Louis Titus, former head of People’s Water Company, as part of the Spring Construction Company. Its dawn-to-dusk blasting operations infuriated local residents, who obliged it to shut down in 1909 by threatening the business-friendly city council with a recall campaign. The company made the gaslighting claim that they were not operating a quarry, even though the rock from the pit was being used for streets and homes in the Thousand Oaks tract, but were in fact building a reservoir for the People’s Water Company.

After its abandonment the Spring quarry became an attractive nuisance, drawing generations of youngsters to its steep sides and deep swimming hole. Even those who didn’t swim in the cold, murky groundwater must have enjoyed the view.

In 1950 an 11-year-old boy drowned, and the city fenced off the quarry. The city acquired more property around the site in 1957, and eventually it became what you see today.

Unfortunately I have found no details of the site’s geology, although Titus called it a basalt quarry. The geologic map shows the downhill side as basalt of the Moraga Formation and the uphill side as the conglomeratic Orinda Formation.

This is the basalt. The brownish streaks are slickensides — friction marks from faulted fractures.

Other volcanic rocks here include rhyolitic tuff, a minor component of the Moraga Formation. It’s distinguished by its light color and broken (brecciated) texture.

Superficially, it resembles the Northbrae Rhyolite that makes up Grotto Rock Park and its sister parks, but unlike that heroically strong stuff it degrades quickly when exposed at the surface.

At the foot of the cliff you’ll find pieces of this excellent conglomerate from the Orinda Formation.

But the fact that it has crumbled down the cliff and will fall apart in your hands means that the rock face would be treacherous climbing. The city should be more forthright in discouraging climbing here; instead they just say “check out the other Berkeley parks with rock features…” Stay off it, and if an earthquake strikes while you’re standing there, jump back fast.