Archive for the ‘Oakland geology walks’ Category

Oakland geology ramble 6: Chabot to Leimert via the Oakland Conglomerate reference locality

11 June 2018

In 1914, UC Berkeley professor Andrew Lawson published the first decent geologic map of our area, the San Francisco Folio of the Geologic Atlas of the United States. That’s where the Oakland Conglomerate got its name. But while that fine rock unit lives on in name, the concept it represents has changed. Let’s look at a hundred years of progress in geologic mapping the area just west of Redwood Peak, the heart of the Oakland Conglomerate.

Here’s Lawson’s map from 1914. Orient yourself by finding Redwood Peak. The belt of rocks labeled “Ko” is the Oakland Conglomerate, the road winding along it is called Skyline Boulevard today, and the road heading leftward from it is today’s Castle Drive.

Lawson called Ko the “Oakland conglomerate member of the Chico formation,” a name and classification by which he meant that these rocks were notable but not important enough in the big scheme of things to single out. To be fair, he was dealing with a very large, detailed and cryptic field area at the time.

Fifty years later, along came James Case, who picked this area for his Ph.D. research at UC Berkeley. Lawson, who died in 1952, was no longer around to intimidate his graduate students, so Case was free to argue that Lawson had erred, having lumped too many different rocks in Ko. Some of them were really KJk, the Knoxville Formation, and others Case put in his brand-new Kjm, the Joaquin Miller Formation. (This is similar to what I was saying the other week about the Orinda Formation.) “It is therefore proposed,” he wrote, “that conglomeratic beds exposed along Skyline Boulevard west and northwest of Redwood Peak be considered the reference locality of the restricted Oakland Conglomerate.”

Case’s thesis was approved in 1963. His map, published a few years later in USGS Bulletin 1251-J, is so badly reproduced it’s embarrassing. Instead I’ll show Dorothy Radbruch-Hall’s elegant map of 1969, which incorporated Case’s work.

And for completeness’ sake here’s the current standard map, by Russ Graymer from 2000.

It’s a cool area. I recommend visiting it the way I did: take the 339 bus up to Chabot Observatory and hike down to Montclair or the Leimert Bridge to catch the 33 bus back downtown.

First, go behind the observatory buildings and find the fire road, which is part of the West Ridge Trail in Redwood Regional Park.

The trail exposes the conglomerate beautifully. The cobbles are exceptionally well rounded, a sign that they once tumbled a long way down a steep river into the sea.

You could meander your way to Moon Gate, on Skyline, where you’ll take the trail going left, or you could follow the steep and tempting path up to the water tank. From there the views look north past a wooded hill, with Round Top peeking up behind it, to the Briones Hills . . .

or northwest toward Grizzly and Vollmer Peaks.

A tiny trail on the other side will get you to some proper outcrops of massive sandstone, also part of the Oakland Conglomerate.

Either way, you’ll then be on the Scout Trail heading south, then angling east down to Skyline. Along the way you’ll pass a young and vigorous redwood stand, planted in 1978 thanks to good old Jerry Brown.

Cross Skyline and take the Castle Park Trail west. It’s a lot safer than walking on Skyline, and a nicer walk. When you hit Castle Drive, take the pavement down to the secret fire road called the West Trail.

This is the original century-old road shown on Lawson’s map. Go on, you’ll thank me later.

There’s a particular kind of peace to be found on abandoned roads.

Once the trail ends, back at Castle Drive, you can pick your own best way down, an exercise left to the reader. This was my way — 3.5 miles long, 1100 feet downhill. . .

through the redwoods and across Leimert Bridge.

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.

Geologizing on the 33 bus line

4 December 2017

My geologizing habits are unusual: being self-employed and semiretired, I go out on weekdays, when everyone else is busy and I have the landscape to myself. Moreover, most of my outings around Oakland are on foot, with the help of the Citymapper phone app and all manner of public transit, which I’ve praised before. In fact, I’ve written a whole compendium of places to see in Oakland this way.

So I’ve gotten to know the bus system pretty well. And for accessing lots of interesting, walkable terrain, the 33 line ranks right up there. Assembled from parts of the old 11 and 18 lines, the 33 runs from the bedrock hills and uplands of Piedmont, past Lake Merritt, to the bedrock hills and uplands of Montclair, taking in lobes 3 and 4 of the Fan as well.

The northern leg takes you to the well-appointed sidewalks of Piedmont, saving you a tedious climb. But there’s scenery on the way. First you pass the north arm of Lake Merritt, then cross lobe 3 of the Fan and reach bedrock right after crossing upper Grand Avenue.

Destinations in Piedmont include the former quarries of Dracena Park and Davie Tennis Stadium, the canyon and creek of Piedmont Park, and the headwaters of Trestle Glen Creek, plus smaller attractions like the slickensided roadcut of St. James Drive.

The southern leg of the 33 takes you to central Montclair, a jumping-off point for hikes in all directions. But first you pass the mouth of Lake Merritt, then enjoy a scenic ride up the valley of Park Avenue Creek and past the views over the glens (Trestle Glen and Dimond Canyon) in Glenview.

Above Glenview you could get off at Leimert Boulevard and trek through Oakmore, or go the other direction into easternmost Piedmont; your call. But then you’d miss the passage through Dimond Canyon.

From Montclair you can head west back downtown through Piedmont; north to Thornhill Canyon or past Lake Temescal to the Rockridge BART station; south to the trails of Joaquin Miller Park; or east to the high hills through Shepherd Canyon, up the Colton spine, and even all the way through Sibley Preserve, shown below, to Orinda.

None of those possibilities appear on AC Transit’s map of the 33 line, just a lot of human destinations and transit connections.

So climb aboard the 33 some time. Maybe I’ll see you there.

A stroll up Indian Gulch, or Trestle Glen

28 August 2017

Once upon a time there was a thriving native encampment near the head of San Antonio Slough, tucked under bountiful oak trees in a valley with a permanent stream. Then the padres of New Spain put the natives behind walls to earn their bread with the sweat of their brows, and a generation later the Mexican rancheros converted them to secular laborers. The valley, which took on the name Indian Gulch after the Indians were gone, has remained significant. The property line between the lands of the Peralta sons, Antonio on the south and Vicente on the north, ran directly up its streambed.

On the map, the valley with its tributaries looks like a long feather, arcing across the bottom of this map from Lake Merritt (the former slough) into the hills of the Piedmont bedrock block.

Today the valley is an Eden of the suburban sort, well worth a walk for its natural and human sights. These sights do not include the stream, now called Trestle Glen Creek and mostly culverted or hidden in back yards.

The lowermost part of the valley, going up Trestle Glen Road, has a very gentle grade, taking well over a mile to climb 100 feet. The sides of the valley rise a good hundred feet on either side.

The stream here is at grade, meaning it has cut down about as far as it can. The ground it has eroded is the sediment of the Fan, not bedrock. See it here on the geologic map.

This stretch of the valley ends just past Norwood Street as we enter bedrock country.

The grade steepens slightly, and the valley walls close in a little. The power line towers in the back of this view sit on bedrock.

Two very unobtrusive footpaths lead from here to either side of the valley where you can encounter the bedrock. The one on the south side is particularly discreet; you might have better luck coming down from Park Boulevard via Elbert Street to see this exposure.

It’s your standard Franciscan metasandstone, the same stuff that was quarried in Piedmont before it incorporated in 1907. There’s also a nice exposure on Trestle Glen Road a little farther up.

By now the valley has gotten distinctly narrower and steeper. Right at the city line at tiny Valant Place, Trestle Glen Road leaves the streambed and climbs up to Park Boulevard. Seen from Valant Place, the valley is a real ravine now.

You can’t walk up the valley any farther; it’s all on private land. But from the Piedmont streets that flank the valley — Indian Road, La Salle Avenue, St. James Drive — you can catch glimpses of the living stream.

The Uptown to Montclair ramble I posted a few weeks ago goes through higher parts of Indian Gulch. But the longest stretch of the unspoiled stream, the western branch, is totally secluded in private hands. That branch is where the rancho boundary went. You can spot it from the 33 bus, on Hampton Street, if you know where to look.

So is this a glen, or is it a gulch? Both terms refer to small, steep-sided valleys with running streams in them. However, a glen is typically wooded — the word comes from the Gaelic — and implies a green, secluded place. A gulch not only has steep sides, but also a steep slope with a rushing mountain stream, and the word is widely used in the Southwest. A gulch is forbidding, but, especially in California, it’s well suited for gold panning. This valley offers both wealth and seclusion today, so I call it a toss-up.