Upper Arroyo Viejo: My first Oakland fossil

Two weeks ago I wrote about the upper part of Arroyo Viejo, in Knowland Park, and said that I hadn’t walked the whole section exposed along the stream. Soon afterward I returned there and did the deed. I had a special goal of locating fossils in the Knoxville Formation.

This time, instead of following deer trails along the hillside, I went up the streambed, starting at the conglomerate outcrop I showed in that post. This is a closeup.

KJk-traverse1

It was pretty easy going for the most part. There have been no recent landslides into the creek, and in general you can see that the creek has been cutting down leaving tree roots–and bedrock–exposed. Most Oakland creeks aren’t this vigorous.

KJk-traverse2

Most of the section is coarse-grained rocks, sandstone and conglomerate. Here’s an outcrop that exposes conglomerate on the left and sandstone on the right, all steeply tilted.

KJk-traverse3

Researchers from both the US Geological Survey and UC Berkeley have found and collected fossils near here. (In fact I think they were all the same person, Jim Case, who did his Ph.D. work here in the early 1960s and then expanded it for the USGS in Bulletin 1251-J.) Many rocks have microfossils in them, which are useful for experts but not very thrilling for the average amateur geologist. What caught my interest was that these were macrofossils, the remains of regular everyday-sized organisms.

To find macrofossils, you want shale. There were chunks of it in the streambed, but outcrops were scarce.

KJk-traverse4

The fossils were reported as being shellfish of the genus Buchia and unspecified belemnites. Buchia was a group of chunky, oblong bivalves much like clams that lived during Jurassic and Cretaceous time. Belemnites were squidlike creatures with internal shells that lived around the same time. You’ll see polished specimens in any rock shop.

I didn’t come prepared for fossil hunting, with the right hammers and chisels and so forth; I just wanted to see if I could spot some on the ground. Rocks like this, I gave a searching inspection.

KJktraverse-5

Then, there it was in the middle of the streambed–a belemnite mold.

KJk-belemnite

A mold is the space that once held a fossil. Here the stream managed to dissolve the calcite shell and pluck it out of the rock. So this isn’t much of a fossil, and belemnites aren’t useful for zeroing in on the age of a rock the way Buchia is, either. But it’s proof of concept. I was as thrilled as when I was a kid grubbing brachiopods out of the Devonian shales of upstate New York, once upon a time.

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