Archive for March, 2008

Montgomery Ridge

16 March 2008

bowlofchert.jpg

St. Mary’s Cemetery, north of Mountain View Cemetery, is on a ridge that runs toward the bay and peters out at the Kaiser hospital on Macarthur and Broadway. The ridge is on bedrock at the high end and changes to old alluvial fan sediment just past Pleasant Valley Boulevard. I call it Montgomery Ridge because Montgomery Street runs approximately up its crest. My yard lies on the edge of this ridge down near its end. I find these Franciscan chert cobbles scattered thinly in the dirt, and I’ve been putting them aside. They are rough, but not jagged, so I take them to be natural, in-situ alluvium rather than fill or crushed rock. That’s where things stood until the other week, when I found a cutbank on upper Howe Street dug into the ridge, and the same chert was tumbling out of the hillside from a layer just beneath the topsoil. Walking down Montgomery, I saw more chert chunks in the soil by the road at the corner of John Street. My favorite pieces are the greenish ones, like this one by the side of upper Howe Street.

howegreenchert.jpg

This chert comes from the Piedmont block, but the geography is different today. Today, streams have incised the old fan and they’re too feeble to carry this kind of material. I picture much drier conditions, and flash floods strewing the chert across the surface of the ancient fan. The next thing is to see where else it occurs. Let me know if you find it in your neighborhood.

Rockridge Shopping Center quarry

10 March 2008

51stquarry.jpg

Halfway up Broadway, where it crosses 51st Street/Pleasant Valley Avenue, there used to be the big Bilger quarry. The Oakland Paving Company mined a body of traprock (mapped as Franciscan quartz diorite, a near-basalt), crushed it, and shipped it out via a rail spur that crossed Broadway at 42nd Street and curved toward the former depot at 41st and Shafter streets (you can see the trace in Google Maps). This is what’s left. I’m standing at the very edge of the Catholic cemetery overlooking the Rockridge Shopping Center. Beyond it is a remnant of the original hill where California College of the Arts sits. It was a family estate before it became a school; perhaps they couldn’t stand the quarry operations and decided to give the place away. Still, the views from there are very nice, and the old quarry walls beautifully display the sandstone that adjoins the traprock as well as the faulted contact between them. Click the photo for a bigger version.

The old pit holds water from the Rockridge Branch of Glen Echo Creek after it traverses the Claremont Country Club golf course. A glory hole at the other end carries the overflow into a culvert that runs down the west side of Broadway, daylighted in two small places (by the new Kaiser parking structure and along Brook Street) before it joins Glen Echo Creek proper just north of 30th Street. The photo below shows where the creek joins the pit. I think the little waterfall on the left must be drainage from the country club grounds. It’s still pretty, and both are seasonal hidden treasures. The creek is barely visible from the other side—you have to stand by that little promontory at the front corner of the Longs store.

51stquarryfalls.jpg

The fall of gravity

6 March 2008

brookwoodpath.jpgOakland is full of historical reminders, some of them enshrined in official registers, others neglected. A century ago, neighborhoods were planned with abundant footpaths and stairways, especially in the old streetcar suburbs within a mile or so of the town of Piedmont. There are roughly a hundred of them there, ranging from small flights of steps to alleyways with their own street signs. That was a time when gravity was respected, if only as a vestigial memory. For our entire previous history as a species, the default method of travel was on foot. The Key System streetcar lines first made the Oakland hills inhabitable by the masses. They ran up the gentle grade of stream valleys, and commuters would walk the rest of the way home to their modest castles. Within a generation, the automobile made this system temporarily obsolete. For the moment, no one walks in the hills, and the city is slowly giving up on the paths, which were once essential infrastructure. They are still useful, though unrecognized, as alternative routes in case of fire or quake.

As I explore Oakland’s geology, I walk by habit and by resolution and as a sign of respect to gravity. Gravity is the key to landscape, and until the rise of mechanical transportation gravity was the prime influence on the layout of cities. Walking helps me view my city with the eye of a geologist at a field site, and I enjoy experiencing Oakland as its 19th-century inhabitants did. And, of course, if I didn’t walk I would be hopelessly out of shape.

Conglomerate in Claremont Canyon

1 March 2008

orindafm.jpgOn my same hike up Claremont Canyon I passed excellent outcrops of the Orinda Formation. This is a sequence of coarse-grained, fluvial gravel that dates from about 12 million years ago, the late Miocene Epoch. The rocks in the conglomerate come largely from Franciscan sources to the west, places that today are in Marin and Sonoma counties, on the other side of the Hayward fault. There must have been high, rocky hills of the stuff, and vigorous streams that tumbled and rounded this gravelly sediment as they carried it into a freshwater lake. Shortly afterward (shortly in geologic times, that is), volcanoes burst up as faulting broke the crust and covered all this with flows of basalt lava, the Moraga Formation. Only a few microfossils occur in this unit.

See more photos of the Orinda and Moraga formations from the spectacular Route 24 exposure here.


Follow

Get every new post delivered to your Inbox.

Join 4,029 other followers