Comments on: Sheffield soil

By: Michael Carey

Michael Carey — Thu, 16 Oct 2008 05:38:06 +0000

Hello Andrew;
Yes, we hit the main fault trace; just where Leinkaemper (and the topography) said it would be. The gabbro was very fractured and had LOTS of carbonate in the fractures. Then, about 250 feet to the east, there was another minor fault, along with the Pleistocene creek channel. The creek channel, however, is the interesting thing there.

The report has not been published, I am not sure what the fate of the development is, especially these dark days.

By: Andrew

Andrew — Sun, 05 Oct 2008 00:45:51 +0000

Welcome, Michael. Did you trench the fault trace, or was that off the property?

By: Michael Carey

Michael Carey — Sat, 04 Oct 2008 00:18:36 +0000

By the way, I just found your web site- Love it.

By: Michael Carey

Michael Carey — Sat, 04 Oct 2008 00:16:58 +0000

Actually, I do know something about these rounded rocks-

I recently did an Alquist-Priolo fault hazard Investigation on the large property above the Dunsmuir house (Not officially published yet). We dug several long trenches- One of them encountered an old (probably early Pleistocene) creek channel that contained unconsolidated sands and rounded pebbles to cobbles. This channel was cut into the local KJF bedrock (greenstone and “Leona”), and probably had a minor offset fault along one edge. This early Pleistocene alluvium (and another area somewhat upslope of this one) are shown on the recent USGS geologic map of Alameda County compiled by Graymer. [Also on the Google-Earth version of Graymer, where the location is pretty accurate].

It must be said that the actual source of the gravels is unknown to me, and we were not really able to do proper sedimentological work (flow directions, etc) on the trench exposure. MUCH scrutiny would be required to determine the actual source. However, some of the clasts are definitely composed of high-grade metamorphic rock.

The only local source of high-grade metamorphic rock that I know about is the hard knockers in the El Cerrito hills, several miles northwest of the stream exposure. However, It is certainly possible that they may have come from elsewhere – possibly from upslope (east) of their present location. (Andrew- is there also some in Rockridge or Piedmont??).

This stream channel does NOT appear to be related to the current drainage system; if so, it may well pre-date the major uplift of the hills. It is possible that the channel is analogous to the Eocene gravels of the Sierra. I am not aware of other stream gravels up on the east bay hills, although there certainly may be other occurrences.

Imagine- ancient southwest flowing stream channels may have existed prior to the major uplift of the east bay hills. I know that this is based on inadequate data, and I am certainly no expert on the dating of the uplift; still—.

This is why I love working in Geology.

M.Carey mccadobe@aol.com

By: Todd

Todd — Thu, 04 Sep 2008 03:32:04 +0000

Andrew,
I go up this trail quite a bit! I have noticed some interesting rocks along the trail (if you keep going up higher). There are what appear to be river or ocean rounded stones in one area. Then in another there are rocks that are bluish green. I wonder if they might have some copper in them? I also think this because the pieces that are newly exposed are yellow/white whereas the longer-exposed faces have the green/blue in them.
If instead of going up high you go to the left and back into the canyon and its streambed, I have seen some very rounded rocks that are greenish, perhaps olivine?

[Todd, I haven't seen these rocks and can't comment, but the area is mapped as ophiolite and dark blue-green rocks would be not be unusual. Copper and olivine would not be relevant.]