Archive for June, 2009

Dunsmuir Ridge and the Irvingtonian gravels

26 June 2009

Just northwest of Lake Chabot are some tiny areas mapped as “Irvington Gravels,” high above the Sheffield Village neighborhood in the Dunsmuir Ridge Open Space. They caught my eye because Irvington (part of present-day Fremont) is the site of a famous set of Ice Age fossils, from which the Irvingtonian age of North American land mammals was established. Yesterday I checked the area out, in case there were some sabertooth-cat fangs lying around. This entry has a lot of photos.

You get there starting at the access at the end of Covington Road, a dirt fire road that goes straight up a steep hillside. The Hayward fault crosses the road partway up, at a little level spot at the edge of the woods. To the west of the fault, the rocks are mapped as San Leandro gabbro (Jurassic rocks of the Coast Range Ophiolite), but it’s really hard to tell:


Across the fault the rocks change to Late Jurassic volcanic rocks of the Coast Range Ophiolite, the same stuff exposed in the big Leona quarry:


Higher up are three small terraces where the gravel is mapped. This is looking south from the northernmost one:


It looks like a hopeless task to find rocks here. Luckily for me, the fire roads have recently been graded, so there was a window into the substrate. As I approached the terraces, the roadbed started to display river cobbles, quite unexpected in this setting:


I made a point of crossing the grassy slope to the other two terraces, looking for stones the whole way. Nada. From the southernmost terrace, here’s the view north. Click on the picture for a stereopair:


There’s a house on a knoll at the same height as the terraces. The upper part of the Knowland Park Zoo land also lines up with the terraces. No gravel is mapped at either place, but there might be some.

Now the cobbles in the roadbed start to look interesting:


Above is another, higher terrace. It’s over 500 feet above the starting point and a bit of a trudge.

Just below it are scattered outcrops of the volcanic bedrock:


The roadbeds on the upper terrace also have interesting cobbles. I took a few home to clean up and photograph. Remind me to bring them back on my next visit.


Russ Graymer, who prepared the Oakland geologic map, describes the suite of cobbles thus: “Cobbles . . . consist of about 60 percent micaceous sandstone, 35 percent metamorphic and volcanic rocks and chert probably derived from the Franciscan complex, and 5 percent black laminated chert and cherty shale derived from the Claremont Formation.” He holds that these little terraces started out near Fremont and were carried here by the Hayward fault. They started out at a much lower elevation too, I would think; just a sign that fault movements are not straightforward.


Unexpected crystals

19 June 2009


You don’t see a lot of crystals in Oakland rocks. But stopping to examine the boulders in the walls at 166 Tunnel Road, just over the Berkeley line along the Hayward fault, I was arrested by this perfect quartz crystal in a coarse-grained marble block. It’s maybe a centimeter long.

Suiseki time again

12 June 2009

I brought up the topic of suiseki last summer, and now it’s that time again. This weekend, Saturday and Sunday, is the annual suiseki show at the Lakeside Garden Center from 10 to 5, no charge.


This was one of the stones on display at the 2005 show. It’s part of my gallery of Earth art on, too.

Unfortunately I must miss this show. Fortunately, I will be touring the gold mines of the Sierra foothills instead.


9 June 2009

I visited Sibley Volcanic Reserve on Sunday and was transfixed by this:


The light-colored blobs are amygdules, or fossil bubbles. Many of the lava flows that issued from a small volcano here about 10 million years ago were full of gas bubbles. Later those filled with minerals, and today the amygdules are weathering out. The minerals involved include quartz, its noncrystalline variety chalcedony, and various zeolite minerals. Read a little more about them on my site. Amygdules were named by a Berkeley geology professor, Joseph Le Conte, in 1878. He surely saw them in these very hills.

I also sought out the labyrinths and found three localities. The first one is on the north side of Round Top, hidden in a sweet spot surrounded by vegetation and butterflies.