AW #58: Beware of the Signs!

Evelyn Mervine at Georneys is holding another Accretionary Wedge carnival (two in a row!), this time it's AW #58: Signs! I was first thinking of some geographic signs along the lines of her original signposts meme, and realized the ones I was thinking of consist of license plates rather than mileage signposts, and that all the ones I have are probably non-digital and currently a bit hard to reach (nevertheless will try to post some when I'm not quite so busy). Instead, I dug up a sign from a trip to Crater Lake a few years back.

Hard to say whether this sign on a very steep part of the west rim of Crater Lake is to warn one away from the danger of the steep slope, or a warning that the mountain (cinder cone or entire caldera?) might blow!

Glacial grooves and striations, field sandal for scale.

Either way, if one persists past the sign in true geological fashion, one will find a great exposure of glacially striated and grooved volcanic rock. Read a little more about the glacial history of Crater Lake here.

Things You Find in the Field: Another Rock Wall

Is that a dike leading across the far field to the brownish outcrops?

As I mentioned near the end of my last post about "Slickenside Ridge" , MOH and I had seen a rock wall the first day we hiked up the north side of the canyon to investigate some dikes of rhyolite porphyry. The photo above shows our first view of that rock wall, looking from east to west, before we knew for sure it was a wall and not a dike.

We eventually meandered over to the brown quartzite outcrops and found for sure that we were indeed looking at a rock wall.

We determined to walk back along the wall to discover what we could about who or why it was built.

Aha! Barbed wire.

A former fence post?

Barbed wire and flowers with boulders of dike rock.

More wire, strung along more or less parallel to the length of the wall.

A rusty, old can of unknown age and purpose.

Looking back toward the quartzite knob from a wired exposure of dike rock.

By now we've made it to the middle of the expanse between the location of our first view on the east and what we thought would be the end of the wall at a couple ribs of dike rock in the middle of the area — but the wall, and its associated wire and posts, continues on farther than we expected.

The wall continues on toward a fairly large rib of dike rock.

Fence post with barbed wire attached.

The wall here is composed mostly of yellowish-brown dike rock
and barbed wire.

Post sticking out of a pile of rocks.

Here we've walked to the far eastern side of the large rhyolite porphyry rib, to find a post or two stuck in a cairn made of the same rock.

And the wall continues on toward the dirt road we hiked up.

The truth is, I don't really know who built the rock wall or why, though it was obviously used at some point as a base for a barbed wire fence, presumably a cattle fence. The wall is made mostly of fairly large boulders of both rhyolite porphyry and quartzite, not particularly neatly stacked (or since fallen into disarray), and the wall uses large outcrops and ribs where possible, saving some of the effort that would be required to build wall its entire length were the outcrops not present.

A view of the lower part of the rock wall as seen from across the canyon.

It was after climbing Slickenside Ridge that we discovered that the wall extends at least all the way to the floor of the canyon, where it's built below a rocky section of hill with a lot of outcrops. I suspect that the wall winds up through the rocks as it did near the top of the hill, but we haven't walked that steeper section to check it out.

Textures from a Tufa Dome III

These are the last photos from this short mini-series showing textures of tufa from a tufa mound near Trinity, Nevada (read more about it and get directions to the spot here).

These photos show what appear to be a piece from the outer edge of a formerly spherical part of the tufa mound, like one I described here earlier, and ones seen in Figures 18 and 19 from Larry Benson's 2004 circular, The Tufas of Pyramid Lake.

Enlargement of the same piece.

I like the first photo for showing the bright orange lichen in the background. The second photo, besides being an enlargement, shows some pale to medium yellow lichen on the piece I'm holding.

Textures from a Tufa Dome II

Here's one more quick photo in this short-lived (and widely spaced!) series showing some textures found around a tufa dome near Trinity, NV, which is described more fully here. These are fan-shaped plates, probably originally composed of thinolitic tufa, possibly now calcium-carbonate cemented as in this photo from Pyramid Lake (page-up slightly to see the photo).

Textures from a Tufa Dome I

Just a quick post in a new, short-lived series of photos of tufa from a tufa dome. The dome is discussed in more detail here, a post which includes directions to the dome from the junction of I-80 and U.S. 95 at Trinity.