Oakland Geology

There aren’t many fossils to be found in Oakland. Maybe microfossils — there must be lots of those in the young hilltop rock units. But on the geologic map is a little body of rock up in Montclair that is supposed to have “well-preserved coral fossils” of Paleocene age (that’s 65 to 55 million years old). Recently I checked it out.

The rock unit has no formal name; it’s mapped simply as “unnamed glauconitic sandstone” and extends in a belt from Shepherd Canyon north over the ridge to Snake Road. This outcrop is at the intersection of Paso Robles Road and Shepherd Canyon Road. I examined it carefully and found nothing but massive fine sandstone, with almost no bedding and no sign of fossils of any kind.

I traversed all the roads in the area where this rock unit was mapped, and the same sandstone was everywhere I looked. At this outcrop on Paso Robles Road, weathering and lighting combined to bring out some subtle bedding planes, but again no sign of fossils.

At the north end of the belt, across Snake Road, is Armour Drive. Here is a large landslide scar that has broken Armour Drive in two, and there was plenty of loose rock for me to apply my rock hammer and take home a hand sample. This kind of rock is all I could find, period. But the unit is said to be “coarse-grained, green, glauconite-rich, lithic sandstone” interbedded with “hard, fine-grained, mica-bearing quartz sandstone.” That’s this stuff.

Somewhere in this belt is hiding a bunch of green, gravelly rock with coral fossils in it, and I haven’t looked hard enough to see it. On the other hand, geologic mapping is an imperfect art, and much of the hills has been mapped on the basis of aerial photos with limited work on the ground. Because I work exclusively on the ground, I’m in a position to do better. Maybe the fossils are only along one edge; maybe they belong to a separate subunit that needs to be mapped more precisely. Maybe someone made a mistake. So far, it’s a puzzle.

This entry was posted on 25 September 2008 at 4:44 pm and is filed under Oakland fossils/features, Oakland geology puzzles, Sausal Creek watershed. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.