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.

Fault Breccia on Slickenside Ridge

Up above our last hiking stop on Slickenside Ridge, MOH and I came across this wonderful exposure of a breccia (hiking stick for scale).

Here's a closer view, showing massive and drusy quartz cementing  the breccia and filling vugs left in the rock.

This is a type of fault breccia — though I was so into my admiration of the breccia that didn't think to look for the fault  — and you can see hints of slightly oblique slickenlines on the smooth upper surface in the first photo.

In fact, it just occurred to me to show you what I mean, so here are two photos of that upper surface, one without drawings, one with.

Planar fault surface from the 1st photo, enlarged.

Same photo with some lines drawn in.

The possible slickenlines in the upper left are the ones I noticed first; the second possible slickenline direction shown in the lower right is vaguer, might be my imagination. More field work required!

A zoomed view of the "best" part of the breccia.

Looking northwest toward Winnemucca Mountain (hidden from view by my choice of framing).

Now I've walked back over to the spot where I left you in the last post, looking down Water Canyon toward the northwest. Just above the bright reddish orange dike rock on the right side of the photo, on the hill across the canyon, you can see the lower part of a rock wall first mentioned in this early post about the dikes.

Looking back down the hill and across the canyon.

And so now, as we turn to head back down the hill, we might think about investigating the rock wall on the far hill, the one we sometimes call "Dike Hill."

Things You Find in the Field: Screwy Contacts?

While wandering up one of our hiking hills back in mid-March, I noticed some orangey brown rocks that at least in part seemed to be cutting across the general trend of beds in the dark, brownish gray quartzite. Are these dikes? Maybe just some slumping or sliding of large boulders? Or maybe it's just the angle of my view.

For the first photo, I'm standing between the first switchback and the first large outcrop of quartzite (next photo). The first switchback is at Location 1. Locations 1 through 8 are identified in the final Google Earth image at the bottom of the post.

View from higher on the hill, looking across the canyon.

Now, having rounded the bend of the first switchback and made it up a very rocky jeep trail, I'm standing in the shadow of a large, jutting outcrop of quartzite [Location 2]. A narrow bed of quartzite holding up a ridge across the canyon runs parallel to some vaguely orange-brown, scrabbly-looking or blocky-weathering material [at Location 8].

We wander past more quartzite, hoping to find our mysterious, unknown rock around the bend. Really. That's what we do on hikes.

Aha! A pale yellowish to light brown rock in contact with our darker, brownish gray quartzite! Right there in the road!!

The rock above the road on the skyline is made of the same stuff. What is it? [Contact is at Location 3.]

It's a dike rock!

What I hold in my hands is a white to pale grayish yellow to moderate yellowish orange and light brown rhyolite porphyry with conspicuous quartz phenocrysts.

Now I'm standing on the dike above the drill pad [Location 4] from a day or two ago, looking south across Water Canyon [Location 8]. A fuller explanation of what we're looking at can be read here, with contacts drawn in.

From that same perch just above Location 4, I look west across a field of orangey brown outcrops of more of the same rhyolite porphyry, toward the dark brown quartzite knobs [Location 5] near the west end of the road.

Here's the view from the quartzite knobs [Location 5] seen in the last photo, looking east up Water Canyon and across the same field of dikes and quartzite outcrops [toward Locations 3 and 4].

Most of the exposures in this photo, saving ones in the foreground and a few on the far left and right (and on hills in the far distance), are of rhyolite porphyry.

An unexpected rock wall [Location 6] cuts diagonally across the photo, running from the foreground outcrops of quartztite to a couple middle-ground outcrops of porphyry. In the previous photo, the wall had looked more like a dike. Trust me when I say it isn't! (More on that later.)

Having reached our hiking and geologizing goals, we'll now head back down the hill.

From the middle of the hill, we look down toward Water Canyon across a few bouldery outcrops of orange-brown porphyry and dark gray quartzite [at about Location 7], with more massive exposures of quartzite lower on the hill.

Now we've arrived back at the first switchback [Location 1], and I realize the outcrops below it are made of porphyry.

What we see directly across the canyon at Location 8 is mostly porphyry, with a couple thin beds of quartzite running straight up the ridge.

Looking back to the north from just across the canyon.
(This photo is from a hike on a more recent day).

The first switchback [Location 1] is just right and uphill of the bold porphyry outcrops on the slope just above the trees. These outcrops of mostly porphyry seem to contain a few blocks or beds of quartzite, although I haven't walked the area to know for sure. The orange-brown outcrops on the far skyline left of center include the exposure at the drill pad [Location 4] that we saw in the earlier post. A couple irregular dikes cut across the middle hill, connecting the switchback exposures to the drill pad exposures.

The contacts don't seem so screwy when you know what's going on!

Google Earth image of our area, with north to the right.

Numbers of locations referenced in the post.

Dumortierite at the Champion Mine, Nevada

Here we are, back at the lower workings of the Champion Mine, standing on the second wall, looking at a funny, corrugated outcrop. If I look at this outcrop with a tilted head, I might imagine a sideways, not-quite-upside-down Nevada with corrugations of the Basin and Range forming large-ish mountain ranges (or I might imagine caterpillars crawling).

Closeup of the corrugated outcrop.

Most of the outer edges of the corrugations are terminated by planar, thin rinds of fibrous, purplish-red dumortierite. I'm not really sure what created the shape of the corrugations, but I'm thinking that they might be structural in origin, combined with some incipient cavernous or spheroidal weathering.

Thin rinds of dumortierite on schistose, Triassic rhyolite.

Fibrous to bladed dumortierite up close.

The orientation of the fibers or crystals reminded me of the slickenlines one might see on a slickensided fault surface, but I didn't have time to really check out this possibility by taking a lot of orientations with my Brunton.

Dumortierite in irregular veins cutting the Rochester Rhyolite.

Veins and a vein breccia.

Nice vein of fibrous dumortierite.

I'll have some more photos of dumortierite and dumortierite-sericite schist later, and soon, we'll head up towards the upper Champion Mine workings.

To be continued...

Rock Walls at the Champion Mine Lower Workings

We had just started up the hill to the lower workings of the Champion Mine in the Rochester mining district, Nevada.

Three rock walls.

The lower workings are a mess of mine dumps, scattered junk, and locally heavy brush amidst three well-defined rock walls below a cliff of Rochester Rhyolite.

Corner of the lower of the three walls.

The rock walls are built of dumortierite-sericite schist from outcrops of the Triassic Rochester Rhyolite, part of the larger Koipato Group of Triassic volcanic and volcaniclastic rocks. I'm guessing that the walls were possibly used to provide flat places for mining activities near the workings on the very steep slopes, rather than as foundations for shelters, buildings, or mills: we found no signs of such structures.

View from the lower wall, looking WSW down Rolands Canyon.

The second or middle wall as seen from below.

The third or upper wall, medium brown in the upper left of the photo.

Standing on the second wall, the third, or upper wall becomes apparent, high on the slope, nearly blending into the cliffy outcrops of Rochester Rhyolite. The funny, corrugated, white to beige or orange-brown outcrop at the far end of the flat platform contains dumortierite in fibrous masses, which we'll see in a later post.

View from the upper or third wall, with the first and second walls below.
My jeep is no longer visible.

Now we've gone back to the bottom of the hill, and are looking straight upward at all three walls.

From just above the jeep parking location, only the first and second walls can be seen.

But, before we had made it all the way back to the parking area to drive away toward continued adventures, MOH and I had turned and walked up the very old, impassable road to briefly investigate the upper workings.

To be continued...