Archive for the ‘sausal creek watershed’ Category

Serpentinite of Visionary ridge, Joaquin Miller Park

6 May 2010

The ridge behind Woodminster Theater, in Joaquin Miller Park, is mapped as serpentinite. You start to notice this around the Moses pyramid, but as you explore you’ll find a lot more of it plus various associated metamorphic rocks.

joaquin miller serpentinite

For instance, on the Siwalik Loop Trail I spotted a fine chunk of actinolite schist, detailed below.


The ridge doesn’t have a name on the map, so I’ll give it one. It has the Moses pyramid, the Elizabeth Barrett Browning monument, the fireplace that Joaquin Miller built for his own cremation, and Lookout Point where Miller loved to contemplate Oakland and the bay. So by my decree, this is Visionary ridge.

At Lookout Point, the large boulders surrounding the viewing patio caught my eye. High-grade blocks!


A closer look showed me the garnet-muscovite-glaucophane association that marks blueschist. There was also some green (chromian) mica. Bring your magnifier, not your hammer, if you come this way.


Redwood Canyon Formation

8 April 2010

The Redwood Canyon Formation sits above the Shephard Creek Formation in what’s called a conformable relationship, indicating that the two units represent a continuous span of time. The difference between the two formations is that where the Shephard Creek is fine-grained, the Redwood Canyon Formation is coarse, consisting mostly of wacke (dirty sandstone) and some siltstone. Neither of these rock types is rich in clay, whereas the mudstone and shale of the Shephard Creek Formation are pretty much defined by it.

redwood canyon formation

Again, this unit dates from the Campanian Age of the Late Cretaceous Epoch, which extends from 83 to 71 million years ago. And again, it’s steeply tilted by activity along the nearby Hayward fault, although it was originally laid down in flat-lying beds. It holds up steeper slopes in Shepherd Canyon, being a stronger rock than the underlying unit. Elsewhere, the Redwood Canyon Formation turns into the Pinehurst Shale going upsection, but in Shepherd Canyon that unit does not appear, having been removed by faulting.

There’s another photo from this formation here.

Shephard Creek Formation

29 March 2010

Moving up the stratigraphic section along Shepherd Canyon Drive, as seen in the Railroad Walk, we go from the Joaquin Miller Formation to the Oakland Conglomerate. (Because I’ve already posted three pages on the Oakland Conglomerate, I’ve put links to them instead on the Joaquin Miller Formation post.) Then comes the Shephard Creek Formation, separated from the underlying Oakland Conglomerate by a fault. It appears in this sloppy outcrop at the very beginning of the Railroad Walk under a tree.

shephard creek formation

Because the walk arcs into and then out of the underlying Oakland Conglomerate, you see the Shephard Creek Formation again at the far end of the big cut in this better outcrop. But beyond that the rock is hard to find.

shephard creek formation

It’s a unit of mostly soft, fine-grained rock, mudstone and shale. Shale is more or less pure clay, and mudstone is shale with some fine grit (silt) in it. The unit also has some siltstone and a few thin beds of coarser wacke. What stands out about it is that, as the mapper says, it’s “distinctly bedded.” The whole thing is of Campanian age, meaning the stretch of the Late Cretaceous Epoch between 83 and 71 million years ago, presumably early in that time. (The age is named for the well-studied rock sequence around Naples, Italy, in the Campania region.) It’s not really as blue as this hand specimen—blame the light from the sky for that—but it does get dark where thin coatings of iron minerals accumulate.

shephard creek formation

I would say that this rock does not support the steepest slopes. The canyon is a little wider here, along Shepherd Canyon Park, than it is upstream where the rocks change to the Redwood Canyon Formation and the hills grow higher.


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