Archive for the ‘Oakland geology walks’ Category

Oakland geology ramble 4: Uptown to Montclair

24 July 2017

This five-mile urban hike is more of a terrain-and-streams ramble than a bedrock ramble. It climbs over 700 feet, winding through the watersheds that freshen Lake Merritt and traversing some of Piedmont’s wildest land. Although I’ve walked the route both ways, I’ll present it here from west to east.

The route goes from the 19th Street BART station to the heart of Montclair on La Salle Avenue, where every 20 minutes the 33 bus will take you straight back to the starting point. Or vice versa.

The terrain map shows the watersheds and drainage divides along the way.

This route is just one of several good possibilities. I have a thing about views, so I favor ridge roads that thread the divides between stream valleys. There are three streams here: Pleasant Valley/Bushy Dell Creeks (Grand Avenue), Wildwood Creek (Lakeshore Avenue), and Indian Gulch Creek (Trestle Glen Road). My route follows Warfield Ridge, the divide between the first two streams. The eastern alternative would go up Longridge Road, the divide between the last two streams. And, of course, one could take the low roads that follow the three streams instead. I leave those as exercises for the reader.

For completeness’ sake, here’s the geologic map. The blue areas are bedrock and the other colors represent various bodies of sediment.

The interaction of geology and terrain adds interest to the hike, but truth be told, the elegant and extravagant residences along the way are just as attractive as the geology.

On Broadway and Grand Avenue, you tread the level ground of the late Pleistocene marine terrace (Qmt) along the foot of the Fan (Qpaf), the large body of Pleistocene alluvium that’s one of Oakland’s most distinctive geomorphic features.

To the right of the Grand Lake Theater, the tall red-and-white building stands on Warfield Ridge. Behind it is Round Top. Cut to the right of the theater and make your way to, then up Warfield Avenue. That building houses the Grandview Apartments, and it’s well named.

Warfield, as I said, is a ridge road. It goes up and down a bit, but mostly up.

If you’re like me you’ll want to catch your breath every now and then. There are views on all sides. Pause and enjoy them. To the left is Pleasant Valley.

To the right is Wildwood Valley.

And behind is where you came.

Stay on Warfield all the way to its upper end, at split-level Wildwood Avenue. Go on up Wildwood. Right after you skirt Witter Field, in the valley of Bushy Dell Creek, you’ll enter the Wildwood Creek watershed for real, and also finally encounter bedrock.

It’s humble stuff, Franciscan sandstone. But it makes good crushed stone, and it supported several quarries before Piedmont took on its current identity. Take a closer look at the terrain here.

South of the blue line, you’ll see four small gulches eroded into this rocky hillside. The third one was a rock quarry that was later made into Oakland’s Davie Tennis Stadium. The others remained unexploited and are now thickly wooded. All four have running water still, thanks to our wet winter.

I picked the route that goes through the rocky headlands between these gulches. Turn right off Wildwood to Wildwood Gardens, and wend your way through this elegant neighborhood, across the top of the Wildwood Creek watershed, to the start of La Salle Avenue. Stay on La Salle all the way to Montclair.

La Salle crosses a fairly flat part of Piedmont, then leads along the rim of Indian Gulch, the village’s greenest and most secluded district. The road then goes up the floor of the stream’s middle branch. At its intersection with Hampton Road is a newly refurbished sports park where you can (and should) refill your water bottle. Higher up this little valley is a former reservoir named Tyson Lake. You can’t get to it.

La Salle becomes pretty steep here as you climb out of the valley and crest the highest ridge at about 725 feet elevation. The change in grade is the clearest signal that the bedrock is changing, according to the map, from Franciscan sandstone to Franciscan melange. But you can’t see that, either — there are no exposures of the melange along this road.

Instead you can see the high valley of Montclair, home of the Hayward fault, as you finally start down. A walkway painted on the road helps keep the locals’ cars back. In that respect Oakland is more walk-friendly than Piedmont.

And here’s the view back from the end of the line.

The Hayward fault is mapped right on the corner of La Salle and Moraga. You’ll see the offset curbs much better on Medau Place, though, one block north.

Siesta Valley and the De Laveaga Trail

2 January 2017

Last week I attempted the Rockridge-to-Orinda ramble by a northern route. Was strenuous, but it got me into Siesta Valley, a place I’ve had my eye on for years, for the first time. At this time of year the sun is so low that the light is terrific.

First came the climb up Tunnel and Caldecott Roads, then through Hiller Highlands up to Grizzly Peak Boulevard, a good 1200 feet higher, then down across the Fish Ranch/Claremont saddle, where it looked like this.

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That’s Gudde Ridge, in Sibley Volcanic Preserve, with Round Top on the horizon. The stairsteps in the rock are the huge roadcut for Route 24. Siesta Valley is to my left, over the ridge.

Siesta Valley is watershed land under the control of East Bay MUD, so I made sure to have my hiking permit with me. Not a soul was around, but it’s a good habit. The De Laveaga Trail runs from the top of the valley, at the Scotts Peak Trailhead, along the valley’s north flank. (It’s named for one of Orinda’s founding families — see the comments.) I took the wrong way, going straight down the valley floor on the dirt extension of Wilder Road, which leads to the back side of the Cal Shakespeare center. That meant a steep descent and subsequent climb of a few hundred feet on a pretty wet road, but I saw some bedrock of the Siesta Formation I’d otherwise have missed.

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That included this fine chunk of freshwater limestone, all of which fizzed nicely in acid, just like it should.

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Once I gained the De Laveaga Trail, the views opened up. The basalt flows of the Moraga Formation crop out well here.

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This view looks down the Siesta Valley to its continuation as Wilder Valley, across the freeway in Orinda.

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Siesta Valley is very quiet. City noise and freeway noise are kept out by the contours of the land, and on a lazy weekday afternoon I could see myself having a nice nap.

The trail leads up and over the east wall of the valley at about 1500 feet elevation, then it’s a steep 1000-foot plunge down to Orinda. This is the view north. The cattle pond is there because cattle graze these slopes.

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The sun sets so early here that it gets cold at night. Shaded puddles stayed ice-covered all that day.

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The winter haze obscured the Sierra, but not Mount Diablo and the Lamorinda hills.

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As I raced the sunset, downtown Orinda looked inviting. This is a harsh downhill, though. Uphill would be worse, I think. Nothing to do but try it sometime.

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The verdict: Siesta Valley is a challenge to reach on foot, but that’s your only choice as vehicles of all kinds are forbidden on EBMUD land. It’s an obscure piece of land that requires a permit, but it’s a gorgeous place. With rocks!

A hunt for silica-carbonate

12 December 2016

The geologic map of the northern East Bay that I rely on has a few rock units that are very small and hard to notice. One of them is the ultra-purple unit designated “silica-carbonate rock.” The map shows only three small exposures — one in Oakland and two in Berkeley — but they’re close enough to each other to visit in an afternoon.

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So that’s what I did back in August, hiking from the lower-right corner to the upper-left through all three areas.

“Silica-carbonate rock” is what happens when serpentine rock is invaded by superhot carbonated fluids, which replace the serpentine minerals with quartz and magnesium carbonates (dolomite and magnesite). The spectacular mercury deposits of the New Almaden and McLaughlin mines are of this type.

I wasn’t too sure what to look for, except that a rock made of hydrothermal quartz and Mg carbonates would be white and messy. Fortunately, the U.S. Geological Survey library, in Menlo Park, has a boulder-size specimen of mercury ore sitting around.

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Unfortunately, that’s not a very informative specimen; moreover it’s labeled “calc-silicate rock,” which would be quite different (it’s what happens when lime rocks are invaded by silica-rich fluids). So who knows.

To traverse the first locality, I started at the end of Chabot Road.

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It’s a highly disturbed place where railroads, streets, culverts and freeways have come through over the years, and it’s hard upon the Hayward fault. There is little promise of bedrock here, but I kept a close eye out anyway. There was some float, or loose rock, that was likely local: some brecciated stuff from the Leona “rhyolite,” tumbled down from its exposures above Tunnel Road.

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Some more of the “rhyolite” plus gray sandstone from the Franciscan melange mapped to the west.

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And at the north end, during a strenuous climb, some Franciscan chert from the melange.

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The next locality is at the top of the UC Berkeley campus. That was hopeless, given all the buildings and landscaping. Except for Founders Rock.

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This excellent knocker has a plaque on the back that reads, “Founders Rock / College of California / April 16, 1860 / Inscribed May 9, 1896” but there are no geological notes. Close up, the rock is enigmatic.

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Not much to do here but scratch your head, and feel sorry for any geology students assigned to write a report on this rock.

Onward through Berkeley’s steep hills to Keith Street, the third locality. That’s a residential street with all of its bedrock hidden, but I scrutinized the stone walls, in case the builders had used local rocks. You never know.

Some of those were could-bes. (A reminder: all of my photos click through to a 600- or 800-pixel image.)

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What I see in these is a uniform light color, suggesting pervasive alteration to siliceous material; brecciation and deformation, typical of an active hydrothermal environment; hints of channels and fractures such as you’d expect from hydrothermal replacement; and bits of iron staining from weathering sulfides. Without chemical tests and petrographic thin sections to examine, none of that is definitive. I did drop acid on them, but there was no reaction, nor would you expect one.

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That’s OK, I still had fun. And North Berkeley neighborhoods are famous for their integration of stone with stylish dwellings of all vintages.

Because it was a one-way walk, from the Rockridge BART station to the 67 bus line, this qualifies as a ramble.