This site, near the easternmost point in Piedmont, overlooks one of Oakland’s most interesting geological features.
You are standing on a long ridge of bedrock, a block of the earth’s crust about eight kilometers from north to south, that underlies the city of Piedmont and surrounding parts of Oakland. There’s a gap in the houses where you can see into Dimond Canyon, which bisects this ridge. Sausal Creek runs down the canyon floor to the right, toward San Francisco Bay. (Dimond Canyon is a city park with hiking trails in the woods and along the stream.) The hill across the canyon is the Oakmore neighborhood. Both it and the hill on this side reach an elevation of about 250 meters, and Sausal Creek is about 100 meters below, so Dimond Canyon represents a unique and quite substantial feature in the ridge. It’s what geographers describe as a water gap—a place where a stream appears to have cut straight through a ridge of solid rock.
Geologists, unlike geographers, don’t just describe landforms but interpret them, using observations and evidence to explain how they were made and what they might mean. That is, they come up with stories based on the clues on the ground. And that’s what you’ll do here!
The rock here, on both sides of the canyon and in the canyon itself, is medium-grained gray sandstone, part of the Franciscan Complex. There are no earthquake faults running through it, so the canyon was not made by the ground pulling apart or opening up. Somehow, running water cut the canyon.
Geologists know two ways that water gaps are made. The first way starts with a stream doing its thing, then the ground slowly rises underneath it and the stream erodes into the growing ridge as fast as it rises. The canyon grows as the stream stands still while the rocks move up. The second way starts with a thick body of rock that a stream slowly removes by erosion, exposing the ridge as it works its way downward. The canyon grows as the stream cuts down while the rocks stand still. The first way happens in tectonically active regions, the second in quiet regions.
Look up Dimond Canyon toward the high hills. At the head of the canyon, a valley leads to the right past the shoulder of the Oakmore hill. (It also extends to the left, toward Montclair.) The Hayward fault runs along this valley, and you can visit several EarthCaches nearby for lessons about the fault.
Every major earthquake on the Hayward fault shifts the ground by as much as two meters. (The last one was in 1868.) It always shifts the same way, with the far side of the fault moving to the right. Geologists thus refer to it as a right-lateral or dextral fault. Over the long term, the fault shifts by about ten millimeters per year.
The headwaters of Sausal Creek are on the other side of the Hayward fault.
Although nearly all the motion on the fault is sideways, the Hayward fault has affected the East Bay landscape vertically as well. Faults in the East Bay Hills as far away as San Ramon and Walnut Creek have pushed the land westward against the Hayward fault, raising the Oakland Hills. And on this side of the fault, the Piedmont bedrock block was uplifted as it moved past the hills of San Leandro, which consist of a large body of hard, resistant gabbro.
Immediately below this spot is a church and its parking lot, surrounded by steep, rocky walls. A hundred years ago, this was the Heyland Quarry, which mined the sandstone here and sold it as crushed rock. Park Boulevard, running past the church, started out as a logging road during the redwood rush of the 1840s and 1850s. Loggers took advantage of this canyon to haul timber down from the redwood forest in the valley of Palo Seco Creek, in today’s Joaquin Miller Park just beyond the shoulder of the Oakmore hill.
To log this EarthCache, send me your answers to these questions.
1. What kind of water gap do you think this is, the first or the second?
2. If you were traveling back through time, which direction would you see the high hills move?
3. If you were traveling back through time, what would happen to Sausal Creek?
4. Did Sausal Creek create this canyon?
5. What signs do you see that the quarry presents a hazard?