Oakland geology ramble 2, Rockridge to Orinda

15 August 2016

The second geology ramble — my name for a long walk that starts in one place and ends in another — is a long and rugged one, just to show you I’m not kidding about these. From the Rockridge BART station to the Orinda BART station is a walk of more than 9 miles with a thousand-foot climb in the middle.

There are several ways to do this. This summer I’ve pioneered what I’ll call the middle route on two separate outings. Here they are, first on Google Maps and then on the geologic map (both images are 1000 pixels). The photos are a mixture from both traverses.



From the Rockridge station, the route along the south side of Route 24 is more direct while the alternative, up Chabot Road to Roanoke Road to The Uplands to Tunnel Road (dashing across Tunnel to the uphill side), takes you through more shade and past more rocks, starting on Roanoke.


The mixed lithologies of Franciscan melange (KJfm) give way to rugged outcrops of the Leona rhyolite (pink color) as you cross Vicente Creek on Tunnel Road. Admire them at the century-old estate called The Rocks. Beyond the Fire Garden is a short stretch without sidewalks that passes a long, excellent exposure of Leona Rhyolite. This was quarried in the 1930s and again in the 1950s during construction of upper Broadway, the Caldecott Tunnel and Route 24.


The rocks change to mudstone of the Great Valley Sequence (Ku), then the much younger Sobrante Formation (tan color), as you ascend Tunnel Road’s steady, gentle grade.


The change to the Claremont chert is dramatic as you near the ridgetop and enter Sibley Preserve.


Cross the park on the Round Top Loop trail, which goes through the coarse-grained sedimentary rocks of the Orinda Formation (Tor), although you won’t see much of them. Take the Volcanic Trail left, which leads into the structurally overlying basalt of the Moraga Formation (Tmb). The quarry that carved up these golden hilltops extracted that basalt. In 0.2 miles, at the right edge of this photo, is a road with a cattle gate that exits the park.


The views change dramatically on the east side of the hills, whether you’re looking to the left up Siesta Valley . . .


. . . or to the right toward Mount Diablo.


Straight ahead lies the unbuilt Wilder Ranch subdivision of Orinda. The valley it sits in is the continuation of Siesta Valley, and it’s underlain by nonmarine sedimentary rocks of the Siesta Formation.


Both valleys owe their shape to the large fold, or syncline (“sloping together”), in the Siesta Formation that’s noted on the geologic map. The Moraga Formation basalt is also downfolded by this syncline, and it crops out again in the hill with the quarry scar.

This subdivision looks bleak, but the developers are doing the job right. The lots are gray because they’re sealed with some tough, pliant substance that prevents all dust and weeds. And as you cross, the route takes the dirt road running from the intersection of Wilder and Bigleaf Roads to the big bend in Rabble Road. You’ll pass several vegetated catch basins designed to hold the extra runoff from the new properties.


This is another example of the flood-control practices I mentioned last week.

The route goes from Rabble Road to Boeger Ranch Road, but take the straight spur between them and follow it to the end, where a footpath connects with the end of Oak Road. All of this area is mapped as mudstone of the Mulholland Formation, of which I know nothing beyond its (young) age, Miocene and Pliocene. From Oak, take Stein Way down to busy-busy Moraga Way and from there head to the BART station. Sidestep as much of Moraga Way as possible by taking Camino Encinas.

If time permits, stop for a beer at The Fourth Bore in Theatre Square. If you take this ramble the other way, stop for a beer at Ben & Nick’s. Either way, you’ve earned it.

The first time I made this trek, many years ago, I took the northern route: up Claremont Canyon, north on the Skyline Trail, then down through the Lomas Cantadas maze to Camino Pablo. That was work. I’ve hiked up the canyon on Claremont Avenue several times, but the traffic is nerve-racking. The alternatives, through the Hiller Highlands or Grandview neighborhoods, are steep, sunny trudges. On the Orinda side it would be more fun to descend through the East Bay MUD land from the Skyline Trail (for which you need a hiking permit). I plan to attempt the northern route again when the weather cools. I have a vague scheme for a southern route, too.

See ramble 1 here.

Tiny steps toward flood control

8 August 2016

More than once on this blog, including last week, I’ve described streams as sleeping creatures that wake up in floods. Kittens that turn into tigers, nebbishes that become the Incredible Hulk, pick your own metaphor — streams do most of their geological work in spasms. The downpour that happens once in a century, filling a creek and scouring its streambed a bit deeper and wider, is one hour in a million hours.

Last week one such rainfall made a shambles of Ellicott City, Maryland. Hydrologist Anne Jefferson looked at this flood with a geologist’s perspective. Her insights apply here as well as there.

Oakland’s Lake Merritt is prone to floods as well as high tides. The tides are slowly growing as sea level rises, of course, but the greater threat of flood comes from the land side. Oakland’s soil absorbs much less rainwater than it used to. As Oakland grew, its dusty streets were sealed under asphalt, its grassy lots occupied by homes with guttered roofs, its footpaths paved over in concrete. Today these impervious surfaces shed the rain, and the runoff drains swiftly away to the nearest body of water.

In October 1962 the Columbus Day Storm dumped over 4 inches of rain in a single day. Lake Merritt rose more than 7 feet and left the surrounding roads waist-deep in stormwater. A few years later a large flood-control station was installed at 7th Street that regulates the tidal lake.


We can do better on the land side. Greener building practices, sustained from now on, will gradually offset our disruption of the local hydrology. This is one of those — panels of pervious pavement flanking the street trees near the lake on East 18th Street. They’re made of a porous concrete that lets water through, like a super mulch. They let the tree roots breathe, too.


Another green practice you’ll see near the lake and elsewhere around town is rain gardens, shallow basins filled with vegetation that catch and absorb rain runoff before it can reach the lake.

Oakland alluvium

1 August 2016


When someone opens up the ground in Oakland, no matter where, I think it’s interesting. This construction site on Telegraph Avenue between 29th and 30th Streets exposes alluvium, the stream-laid sediment that once supported productive farmland throughout Oakland’s flats. Mapped as “alluvial fan and fluvial deposits (Holocene)” or unit Qhaf on the geologic map, it covers more area than any other geologic unit.

The uppermost part, the brown stuff that the excavators have turned over in curls, is rich in organic matter and clay. A little deeper it turns tan as the organic matter thins out. It’s dense and firm, good ground for building.

The nearer you get to the bay, the finer grained this material gets — more clay, less sand and gravel. Streams have carried it down from their canyons in the hills over the last few hundred thousand years, pushing back the sea. And by “streams” I mean floods. The clear trickling streams we know are actually asleep. Floods are the one day in a thousand when streams awaken, picking up and carrying alluvium from place to place.

Occasionally the streams themselves jump their tracks. If you visualize the land in super-fast motion over geologically recent time, our streams whip back and forth over the coastal plain like firehoses out of control, winnowing the alluvium again and again. From the hills outward they build up low, cone-shaped piles of sediment called alluvial fans. Downstream, these coalesce into an alluvial plain.

The “h” in “Qhaf” refers to the Holocene time period. The Holocene (“fully new” in scientific Greek) began when the latest pulse of the ice ages ended, about 12,000 years ago. It’s been a mostly pleasant time. Many geologists argue, with good reason, that the Holocene has given way to a new permanent state of wrenching climatic changes. Because the natural balance of climate is strongly influenced by human activities, they argue, the climate system today is a writhing firehose we may be able to control. They propose to call our new era Anthropocene time.