Geology of the Southeastern Highlands of Redwood

Found some interesting rocks out in the hinterlands of the south end of Redwood today; remind me of slag or coal clinkers that I have sometimes seen along old railroad grades, but these chunks are in disparate locations in an area whose geology is mainly sandstone and siliceous shale.

These next two photos are of lower quality, as I had used up the battery in my camera and so took these with my phone.  Seem intriguing enough to be worth it, however.  The yellow veins are of something powdery that comes right off on your finger and is hard to remove.

Each of the areas in which I encountered these sorts of rocks had signs of human activity long, long ago, with mature trees growing in the center of old roadbeds and with similar regrowth of vegetation in level areas that look like old camp or work sites.

The predominant type of native rock I am encountering on these west-facing slopes, both as outcrops and parent material, is sandstone.  Often these outcrops are quite beautiful and graced by giant live oaks, and in some places even look wind-sculpted.  I wonder how many ages of time they have stood here, as the forest has come and gone.

Interspersed with these layers of sandstone are occasional layers or seams of a very different sort of rock, one that favors a different assemblage of plants and underlies most if not all of the sky prairies I have been describing here in the southeastern highlands.  The soil is often thin or non-existent, and the rocks can be quite beautiful.  And many flowers seem to favor them greatly. As always, clicking on these photos expands them and shows vastly more fine detail.

Ranger Pamela recently looked into the geology of these particular rocks, and, if memory serves, described them to be Pinehurst Shale, a siliceous shale that is composed largely of quartz.  According to the USGS “Preliminary Geologic Map Emphasizing Bedrock Formations in Alameda County,” which is available online, these shale deposits — as well as the sandstones mentioned earlier — hearken back to the Campanian Age of the Late Cretaceous Epoch, or roughly seventy to eighty million years ago.  The height of the Age of Dinosaurs!

This is inference on my part, but the siliceous nature of these Pinehurst Shales may go towards explaining the existence of the sky prairies.  There is not much in the way of nutrients in rocks that are largely silica, and such relatively harsh conditions often do seem to favor native species.

Some of the non-natives are pretty tough too, though:

Broom is tough, and supplies some of its own nutrients.  Within a few years, if not stopped, this plant will give rise to a thousand and this prairie will be gone.  I have been finding many such spots where five years ago, or ten, the bunchgrasses were as lush, the wildflowers as bright, and now all that remains is wispy, forlorn shreds of dead and half-dead purple needlegrass in the deep gloom beneath the broom thickets.

I have photos of these graveyards, but they are too mournful to post on a bright, cheery Easter morning, which is when I am writing this.  Shall post them soon, together with new action shots of weed wrenches at work bringing light and renewal to these once and future prairies.

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