A couple of hand samples from the Patrick Creek site. On the lower right is some nice sheared serpentine (Coating less sheared, serpentinized ultramafic rock, though that's not apparent from the photo alone). On the upper left, a rock that I was quite uncertain about- it was from a loose block on the creek bank, and may have been transported there to armor that bank. It's much finer-grained than I expect the unaltered harzburgite to look. The scribble below the "?" says "Pat Ck," for Patrick Creek, and illustrates why I nearly always have a black Sharpie in my pocket. A couple little flakes of serpentine can be seen to the right of the piece I collected, mashed into the road surface.
As you can see, most of the cliff face is lined with a talus pile, from debris falling from the slick, crumbly serpentinite face. I'm always a little nervous about getting up close and personal with this sort of outcrop, though less so in dry weather. However, those piles at the base are an explicit warning that one should be alert to what's over your head. For me, it's less about keeping a sharp eye out- I'm often caught up in looking for nice seams of serpentine- but paying close attention to what I'm hearing.
From this distance, the outcrop looks quite chaotic. The fact is, up close, it seems even more so. There is, however, a distinct fabric to the rocks; the shearing seems to more or less parallel the road.
You know Americans aren't afraid to speak up, right? And speak up we did! Following are the tweets from my stream alone, and they've been coming in here at hundreds per hour- it's impossible to keep up! (unattributed are mine; others' are preceded by (RT @User).
Someone, somewhere, may be having fun in a manner of which I disapprove. RT @GOP: Tell us your top issues
Nasty, filthy, horrible tacksesses. RT @GOP: Tell us your top issues
Over-regulation of drinking water quality in West Virginia RT @GOP: Tell us your top issues
Florida looks too much like the nasty part of male anatomy. RT @GOP: Tell us your top issues
The rock here was first described to me as "tectonized harzburgite," which at the time was utterly opaque. My understanding of the classification of ultramafic rocks, other than dunite, is still pretty muddy, but I grasped pretty quickly that "tectonized" is simply an adjective meaning that the rock has been sheared and crushed beyond recognition. So the fact that it was once a harzburgite is kind of beside the point; it mostly isn't anymore. Due to hydration/alteration of the Mg-rich minerals- olivine in particular- the rock as a whole can now be described as serpentinite. Finding samples in which the harzburgite is apparent is possible, but it requires purposeful effort. I have posted on this outcrop before, in a broader discussion of serpentine.
"Grassy Flat," at this point, is basically mis-named; it has been cleared and driven on to the point that it's mostly bare ground, dirt and rock. But it is another good example of a fluvial terrace, much like the area upon which Gasquet was built. We're looking across the Smith River here, hidden behind the brush, to a large hillside of unconsolidated debris. I can't say with any certainty what its origin is, but I think we can narrow down potential sources. Some that spring to mind are an older, higher terrace, glacial moraine or outwash, an older, filled and abandoned river channel (these are quite apparent in other locations along this drainage, which is why this possibility occurs to me), or deposits from debris flows- essentially, an alluvial fan.
Looking carefully, the sorting of the clasts is poor to the point of being non-existent. I see very little evidence of bedding, with the exception of an orangy band that starts just below the tree line near the middle, and dips off to the right. The lack of sorting and bedding seems to weigh against a fluvial origin, so I'll withdraw the terrace and channel options, as well as the outwash conjecture. We're much too low in elevation for glaciers to have reached this spot, so I don't think a moraine is a viable guess, either. (Backing out in the FlashEarth view supports this: the drainages are clearly "V-shaped.") That leaves debris flow deposits/alluvial fan as the remaining option. As I said, I can't be certain, but given the situation and topography, that seems to be the best conjecture: an alluvial fan that's been carved out by subsequent meandering and erosion by the Smith River.
The Smith River National Recreation Area stretches along the river from Jedediah Smith Redwoods NP, nearly to the OR-CA border, and there's a nice little visitor center in Gasquet. There are a number of taxidermy birds and mammals, and interpretive displays on the geology, biology, climate and ecology of the region. The thing that most impressed me was the knowledge of the staff- or more accurately, their ability to put their hands on this or that pamphlet with details they didn't carry around in their heads. The center is no big deal, but it was a fun, quick stop. And I was pleased that they highlighted the unusual geology of the area.
Even if you choose to skip this stop, a feature to notice as you drive through town is the well-developed river terrace upon which the town is built. This is a nice, flat, well-drained surface for construction, and the Smith River has incised down into it far enough that I can't imagine flooding is ever an issue. I would guess the development of this particular terrace dates to the Pleistocene. Flying over the region to the Bay area, the higher peaks show lots of evidence of glaciation, which would have resulted in sedimentation in the lower reaches of stream valleys.
The thing that caught my attention with this benchmark, on the southwest abutment of the Mary Adams Peacock Memorial Bridge, was the "Reset 1984" note. During the visit, I was puzzled, but my guess was that it had been re-surveyed to check for any tectonic offset- vertical or horizontal, who knows? Turns out, the bridge was "remodeled in 1985," and this benchmark's "reset" is probably associated with that construction.
This was another random stop, at the bridge just west of Gasquet (GAS-key), California. A nice little pull-out with a bathroom, but unfortunately, it didn't look like there were any good bedrock exposures. We didn't clamber down to the river. This was, at the time it was built (1932), the only bridge in California named after a woman. Some other biographical notes on this woman can be found here, in the middle of the article.
Incidentally, I've forgotten to explicitly remark on this up to now, but remember how I kept complaining about how the coast was so windy, cloudy and cold? We're pretty well out of the redwood zone here (they end at about Hiouchi), though still on the edge of coastal climatic influence, but the rest of this day was gloriously sunny and warm. It was a much appreciated change. If you glance over the climatic data for Gasquet at the first link, you can see that average temperatures are moderate year-round, with fairly small differences between average highs and average lows each month. This tells you that the climate here still has a very strong marine influence, but it becomes markedly less as you drive inland.
The sheeted dike component of the Josephine Ophiolite is pretty much restricted to the segment of the river between this bridge and downstream to the bridge just above the confluence of the north and south forks.