Oakland Geology

Surprisingly, the town of Piedmont has its share of woodland trails — well they’re paved sidewalks, but they’re unused, covered with duff and overhung with untended shrubbery. On a weekday afternoon, you can walk quietly on miles of these soft paths and encounter only a handful of property workers. It’s in that spirit that I recommend a geologizing stroll around the highest part of Trestle Glen Creek’s eastern watershed.

The shaded-relief map below shows the creek valley in eastern Piedmont, with Dimond Canyon on the right edge. The area I’m featuring is the triangle just right of center bounded by Crest Road, Pershing and Estates Drives and Hampton Road.

And here’s a closeup just to display the street names. During weekdays, the 33 bus stops at Lexford and Hampton, where the two valleys in this little watershed join.

The geologic map shows that this area is solidly within the block of Franciscan sandstone (Kfn) that underlies most of Piedmont. The hilltop above it consists of Franciscan melange (KJfm) that includes bodies of chert (fc), notably the one on Pershing that I’ve called the best bedrock in Oakland.

The neighborhood is gracious. This view looks up Huntleigh Road, which runs on the valley floor. As I traversed the streets, I used sidewalks that almost never feel a human foot. At times it was easy to imagine being in a Tolkien novel.

Lexford Road, in its own valley, is more secluded and more whimsical architecturally.

For the geologist, these streets are valuable because they aren’t as tightly landscaped as in most of Piedmont, and the Franciscan bedrock can be seen and studied at leisure in several places where the road builders exposed it. That’s unusual for this town.

Plenty of hand specimens are available too, if that rings your chimes.

There are even a few empty lots here. Unlike the existing homesites, these are especially challenging due to the steepness of the terrain and the strength of the rock — not just on the surface, where the weathered sandstone has fractured into rubble, but also deeper down where foundations would need to be dug into the hard, unweathered bedrock. When this lot was cleared recently, it had shed enough rubble to nearly cover the sidewalk.

A large house has been proposed here for many years, and the record of the intricate wrangling needed to invite and address everyone’s concerns is mind-numbing. However, the record does include geotechnical reports that give us a glimpse underground.

What’s on the site now is this set of what are called story poles, which serve to outline the planned building.

Geologists acquire a certain ability to see the ground through everything growing or sitting on it. It’s an ability to visualize the landscape as if it were covered with story poles instead of vegetation and structures. This bit of watershed is a good field site to practice.

This entry was posted on 13 November 2017 at 8:00 am and is filed under Franciscan rocks. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.