I paid a visit yesterday to part of the Hayward fault in Oakland, but while there I felt the pull of a neighborhood treasure I call Rettig canyon. The name is from Rettig Avenue, which traverses it. Here’s the topography, from Google Maps. This is just south of the LDS temple.
The fault runs from top middle down Jordan Road and exits where Victor Avenue leaves the map. The hills to the west are the southernmost part of the Piedmont block of Franciscan rocks. Rettig canyon cuts right through the hills thanks to Peralta Creek, which comes here from Butters Canyon on its way to the bay through Peralta Hacienda and Foothill Meadows Park.
Normally when you see a stream cutting through a bedrock ridge, you explain it as either stream capture or a water gap. That is, either the stream eroded its way headward through the ridge or was running that way already when the ridge rose underneath it. Given the intense tectonic activity here, I’m inclined to call it a water gap, as I do Dimond Canyon (with the addition of tectonic stream capture).
I saw some possible evidence of this in the streambed. But first, a look at the scene.
Rettig Road is a single lane through the canyon and is coned off as a landslide zone. It’s been that way for at least six years; I hope a local will say more in the comments. The canyon is steep, dark and thickly wooded. You can scarcely see the stream, but you can hear the water everywhere.
But there is a place to scramble down to the streambed. It’s well populated with rocks that appear to be local Franciscan melange, pretty jagged and hence not transported far.
I was looking for bedrock and found some candidates like this scaly schist. I didn’t have my hammer and was reluctant to disturb the scene anyway, so I can’t say much about it. It might be serpentinite.
This is the outcrop that excited me, showing what looks like a thrust contact.
Ignore the green patches; that’s just algae. The rock on the left is fairly soft and foliated parallel to the contact. I picked out a small piece and can’t say much about it, but in the hand lens it looks like a highly altered talcy kind of stone. At the base is a good centimeter-think layer of nice gray clay, then we hit clean tightly packed sediment with highly tilted bedding; indeed it’s tilted steeper than the contact above it. So my best guess is that it may be the contact between the Franciscan and much younger Pleistocene sediments. Due to squeezing along the Hayward fault, the older rock has been thrusted up and over the sediment. This isn’t unheard-of, but I haven’t seen it documented around here so I could easily be wrong. But that would explain the rising ridge, the topography of Jordan Road (which sits in a long trough here that may well be a sag basin) and the course of the stream.
I couldn’t resist bringing home a pocket-sized cobble of beautiful actinolite schist.