Monday, April 29, 2013

Things You Find in the Field: Pyramidal Objects #2

"Danger. Explosives."
I'm presuming from the sign that this metal, pyramidal object once contained explosives. It's location seemed random: it sat on uneven ground amongst some rabbit brush and sagebrush, near one small juniper tree and some scattered, irregular mine dumps.
Set of rungs or handholds going up one side.
It was sturdily built: notice the rivets or bolts lining the edges of the metal sheets. This type of structure is atypical compared to the usual sorts of powder magazines or powder houses I've run across in the field before, which are more often dug into the side of a hill, sometimes consisting of a short adit with a sturdy metal or wooden door, and sometimes being a small, dug-in room or building like this one found in Tonopah or this one from the Matchless Mine in Colorado.
No rungs on the opposite side.
The inside.
Inside, the metal can be seen to be riveted or bolted to sturdy, wooden crossbeams. The wooden floor was a bit worn, and pack rats had been using the area.

Monday, April 22, 2013

Things You Find in the Field: Pyramidal Objects #1

Quartzite on top of "Dike Hill."
I came across this small, pyramidal rock on top of the hill seen first in this earlier post about dikes and drill holes. I didn't know what its full shape would be until I walked over to it, having gotten a glimpse of it from below. The first view made it look like a nearly perfect, small Pyramid of Giza.
View showing all sides.
Yes! It's in the shape of a pyramid, in this case a right, rectangular (not perfectly square) pyramid, a little eroded and lichen-covered on its more northerly sides. Pretty well-formed for a rock, eh?

Friday, April 19, 2013

Crude Map of Some Porphyry Dikes in Water Canyon

A couple days ago, we were looking at some rhyolite porphyry dike exposures, mostly on the north ridge of Water Canyon, an area MOH and I hike every now and then.
Google Earth image of the area (north to the right).
Crude map of the dikes.
On this second Google Earth image, I've added the approximate extent of the rhyolite porphyry dikes in bright turquoise. Contacts should be considered approximate. The south end near the canyon appears to be complicated by a fault as shown in dark blue, and that slope, although mostly of porphyry, appears to contain either beds or blocks of quartzite, possibly caught up within a couple merging or anastomosing dikes.

Across the canyon farther to the south (left edge of the image), a thin bed of quartzite is shown in blue running along one edge of a dike. I'm not really sure that the bed correlates directly with what looks like another thin quartzite bed directly across the canyon from it to the north (right). Also along that southern edge, a single turquoise line marks the east extent of another dike area. I'm not really sure what is going on between that line and the blue quartzite line, but those rocks look more like sediments or quartzite than dike rock in Google Earth.

What might not be clear from the image is that at least one, and maybe two or more, intermediate to mafic dikes of unknown age cut across the switchbacked road east of the rhyolite porphyry dikes of presumed Tertiary age.
What appear to be pieces of intermediate to mafic dike rock, with associated greenish outcrop in the road just behind my hand.
The intermediate or mafic dikes or sills look slightly reddish brown on enlarged images.
Google Earth image with approximate trace of one intermediate or mafic dike added in bright green.
Google Earth image showing approximate areas of rhyolite porphyry dikes hachured in bright turquoise. 
In the last image, sans intermediate or mafic dikes, I've added hachures to make it more clear where the rhyolite porphyry occurs. Again, the areas just north and south of the canyon are rough approximations of reality, and all contacts as drawn represent my best estimation, having been followed in part in the field and filled in using Google Earth.

Tuesday, April 16, 2013

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.

Saturday, April 13, 2013

Accretionary Wedge #56: Landscapes as seen by a Geologist

Accretionary Wedge #56 is all about geologists and photographs, as explained by Andrew Alden, our host:
Once upon a time, you took a picture of something that lots of people photograph. However, because you are a geologist, it didn’t turn out the way it does for most people. Show us that picture, tell us what you see in it, and tell us about the way you take pictures.
I take a lot of photos. I can't begin to tell you how many rolls of film I used to carry with me for simple 3-day field trips back in the day of Kodachrome, but I can easily shoot a hundred or more digital frames in a day, the number being limited only by the number of gigs on my SD card and the percent charge on the two proprietary rechargeable batteries I carry with me: one in the camera, one in my pocket (except for yesterday, when I forgot the extra).

So, I take a lot of photos. I think about how I'm framing them when I'm shooting, about what will look good as a photograph. I think about what I'm seeing, about what the photographs might show myself and others. Mostly I just shoot, as many photos as possible.

I see a lot of landscapes and roadcuts when driving here and there (which is something I do a lot of). Sometimes the landscapes are like nice, wide, almost blank spaces to me, allowing my mind to roam or think non-stressful thoughts not generally related to life or work; other times I really contemplate the landscape and notice the shape of the hills, pediments, basins, and mountains, and I perceive or try to imagine or understand the way the landforms formed and evolved, how the rock formations are guiding the shapes in the land. I see contacts and faults, real and hypothetical, wherever I go.

Consequently, when I take a photograph — almost anywhere unless it is indoors or of people (and sometimes even then) — I am seeing something that isn't necessarily visible to non-geologists, and I see that something in the photo, even when it isn't obvious. The photographs I shoot often already have contacts and fault lines drawn and visible in my mind, and the erosional or depositional remnants of geologic time are jumping out of the scenery and into the frame. The landscapes and rocks are telling me their stories, even from a distance — sometimes they nearly shout their stories, sometimes they almost whisper. Either way, their stories are embedded into my photographs.
A fairly bland landscape photo looking across Rye Patch Reservoir.
In this photo, taken partly for the nice clouds and the inscrutable but interesting shapes of the hills in the lowlands across the reservoir, Majuba Mountain (AKA Majuba Hill) clearly stands out as a resistant, rather roundish knob.

I took no less than 12 photos of this hill as we drove by on our way to catch a plane. The place fascinates me, partly because I've never been there, and in large part because I know it's supposed to be a great mineral collecting site, with ores of copper, tin, uranium, gold, and silver and various minerals including torbernite, cassiterite, and tourmaline (read more here). There is also supposed to be at least five varieties of breccia. The knob is resistant and in the shape it's in because its a complex rhyolitic or granitic plug, and because of the attendant alteration that includes silicification. I see all that when I drive by, and all that shows up in my photo, at least when I look at it. But really, I've never been there, though I've had it on my list of places to visit for a very long time.

My last post shows an example of the contacts I see when I take a photo, and how the contacts can be drawn in at a later time for others to see.

Also, while walking or driving around, I notice landscapes and outcrops, roadcuts and exposures that tell stories of days gone by, days populated with geologists, pickup trucks, drill rigs, beer, dozers and backhoes, ice chests, map cases, and packs full of this and that, including hand samples, white bags filled with rocks or soils for assay, a water bottle or two, a small lunch, waterproof matches in a ziplock bag, an extra jacket or flannel shirt, a small 1st aid kit, and nowadays a SPOT. These things or places and their stories are harder to photograph, but I nevertheless see them almost everywhere I go.

Examples could be endless.

Friday, April 12, 2013

Things You Find in the Field: Dikes and Drill Holes

After hiking up the north ridge, I wandered over to a flat area that looked like an old drill pad, and found an iron-stained felsic dike at the end of the pad.
Looking south down the dike toward the canyon.
You can follow the dike to a linear outcrop on the middle ridge, and then across the canyon to a rocky patch just left of the darker, more prominent quartzite bed.
Same view, with the dike marked in turquoise.
The casing for the drill hole was still in the ground, verifying that the flat area was indeed a former drill site. I dropped a rock into the hole; it took about 2.5 seconds to hit bottom, indicating a depth of about 30 m (about 100 feet). The estimating method doesn't allow for rocks getting slowed down by hitting the sides of a narrow drill hole, so depths should be considered a maximum. Also, drill holes can be caved short of their original T.D., and a dropped rock might hit the water table prior to reaching the bottom. A splash or funny hollow sound can usually be heard when this happens; nothing can be heard from the rock after it hits water. The sound of the rock hitting bottom is hard to discern in deep drill holes.

It's been quite a while since I tested drill holes of known depth (this can help calibrate how much a rock might slow down by hitting the walls); drill holes now are plugged with mud and cement as the rods are being pulled out, so very few recent holes are left open the way this old hole was.

Thursday, April 4, 2013

Things You Find in the Field: Roads You Shouldn't Take

The road we'll be taking a look at begins at a circular parking area (above and below), and heads up the ridgeline to the right of the drainage centered in the "Bad Road" Google Earth image (above). I haven't attempted this particular road myself, and don't plan on it without walking the mid-slope part of it carefully. Actually, I don't plan on attempting it at all: It looks like a road for "high-markers."
The Prius parked in the circular parking area below the hill climb, which begins just left of the photo.
Having no photos of the beginning of the road, here's a view from Google Earth.
This first part of the road really looks just fine, and there is an almost flat area just over the first rise for turning around.
View of the upper part of the road, which forks.
The main part of the hill climb continues uphill on the right-hand fork, into or through one patch of rocks, and beyond into another. The left-hand fork looks like it's been used to access vehicles that have rolled or slid downhill into the gully.
Closer view of the bad area below, through, and above the rocks.
This part, and maybe the curve just below the fork, looks rather sidehilly from below.
If you happen to make it up this far (into the first set of rocks), don't look back! Instead concentrate on going forward!
Otherwise you might end up in the gully. Vehicle parts are scattered all over.
Interestingly, it appears that Google Earth and Maps captured a vehicle in the process of being dug out. (Click to enlarge, or view here.)
Uphill, the road appears to end in rocks and a series of individual tracks, some of which go back down into the gully.

Instead of this questionable road, I'd recommend taking the switchbacked road that takes off just left of the circular parking area (first photo, with Prius). This road is very rocky in a couple places and requires a high-clearance 4WD vehicle. (We saw a low-clearance vehicle turn around at the first switchback just the other day.) The switchbacked road will take you up to the main ridgeline road, an adventure for another day.

Photos taken in middle and late March during two hikes up the switchbacked road.

Monday, April 1, 2013

Things You Find in the Field: Melted Claim Posts

Once upon a time in Nevada, 4-inch diameter, perforated or unperforated PVC pipe could be used as claim posts. Prior to some time in the late 1970s or early 1980s, claim posts in Nevada consisted of 4" by 4" by 4' or 5' wooden posts, which even when wormy with holes, were quite heavy to carry on the long claim traverses we sometimes made when staking claims.

By 1993 (possibly as early as 1991) all such PVC posts had to be unperforated and capped to prevent birds from trying to nest in the pipes, then dying when they couldn't get out. In late 2011, removal of PVC posts was mandated in Nevada. Apparently, anyone can pull the old PVC posts out of the ground, but you are required to leave them lying on the ground "immediately adjacent to to the location from which it is removed" (it is otherwise a crime to remove or disturb claim markers and filing papers on public lands).

Most claims in Nevada are now staked with a wooden post "at least 1 1/2 inches by 1 1/2 inches by 4 feet" set 1 foot into the ground. Small posts like these were not allowed in the 1970s and 1980s.
A closer, side view of the melted pipe, showing its originally perforated character.
A brush fire went through this area in July, 2007 (a few photos here). I've seen other melted claim posts from other fires here and there, but this one shows the most extreme melting I've seen so far; the fire must have been relatively hot or slow-moving.