Discussion Topic |
|
This thread has been locked |
ydpl8s
Trad climber
Santa Monica, California
|
|
Send in these guys.
|
|
Bob Harrington
climber
Bishop, California
|
|
Here's my explanation of why the hole wins:
Cycles of precipitation, evaporation, and capillary rise result in the most soluble common salt, NaCl, on the surface of the playa. Dig the hole, then it rains enough to saturate the upper several feet of the playa with an Na-rich solution, which disperses the clays. Soil water flows into the hole, percolating through the clay or (more likely) through preferential flow paths along desiccation cracks, transporting the dispersed clay into the hole, filling it up. Alternatively, the precip picks up Na from the playa surface, runs off the playa surface, and ponds in the hole, and the Na-rich solution works on the walls of the hole, dispersing the clays, and the walls of the hole slough into the hole. Either way, the tendency toward clay dispersion works on filling up the hole.
The effect of precip on the pile is different. The rain is dilute, the clay remains flocculated, so the pile remains relatively intact.
So, how long did it take the hole and pile to disappear? There are a lot of circumstances that could change the outcome -- wind would (I think maybe) affect the pile more than the hole, the hydrology and chemistry of the playa would affect the process speculated about above, flooding of the playa might favor destruction of the pile through wave action, etc. ...
Here are a couple for more shots of the Inyo monster:
|
|
Minerals
Social climber
The Deli
|
|
Wow, you guys are good!!! That’s way more than I would have come up with for an answer on why the hole wins the race. And I didn’t expect the photo to generate so much interest. I was looking through some photo files the other day and happen to run across that slide scan and had been meaning to post it here in the past because I like the photo and the contrast between the very small scale and the very large scale (hole and pile versus playa and mountains).
The hole was gone within a year or so but I think it took a couple or a few years for the pile to disappear. I’m not sure as I didn’t do much of a thorough follow-up and only checked on it a few times when I was passing through the area. So, it wasn’t really a true “experiment” – it was more of a spontaneous thought of… “hey, I’ll dig a hole for the heck of it and see what happens!” This was several years ago and I also spaced on taking a photo of the fresh hole.
Anyways, yeah, it seems that weather conditions would be a major factor and it would be interesting to see what would happen without any precipitation. After the first rain, it appears that a solid crust formed on the pile as it dried out (just like the playa surface), which made it much more resistant to wind erosion, whereas if it stayed dry, the finer silt and clay particles would be carried away by wind and destabilize the slopes of the pile. The hole would obviously remain longer without the presence of water.
I’m guessing that the white “ring” around the hole is a concentration of salt(s). Yes? The playa doesn’t flood too often and it looks like it wasn’t flooded between the time that the hole was dug and the photo was taken.
Here’s a photo of the playa in October of 2010, which was a very wet month, even in the desert. The edges of the playa had a “bathtub ring” of sorts and it looked like the entire playa surface had been covered in water not too many days before this photo was taken. As the playa began to dry out, mud cracks (or desiccation cracks, as mentioned above) formed. On this day, a slight wind was pushing the water across the playa and the fresh cracks were becoming submerged. Notice how the edges of the polygons curve upward and are exposed above the water level.
Oh!... speaking of playas… this reminds me… You know the famous “sliding rocks” of The Racetrack Playa in Death Valley, right? Anyone see or hear of this phenomenon occurring on other plays…?
“Do you dig many pits way out in the desert?”
OT… but… you mean like this one? :)
Back to really old dead things…
Bob, did you know that there are trilobite fossils in the area or did you just happen to stumble across that nice find while hiking? It looks like the rock was already split in two, judging by the little bit of sand on the surface. And I presume that it’s now proudly displayed on a bookshelf or something of the sort in your house? What’s the geologic rock unit?
BASE104 - email sent.
|
|
Bob Harrington
climber
Bishop, California
|
|
The trilobite is from the Poleta Formation, annually trodden by many UC field camp students out in the Poleta Folds of Deep Springs Valley. We were just out hiking, my wife found it, then I found the other half after poking around for another while. Quite a find. Check out all the little bitty trilobites around the big one. We knew we were in the Cambrian, so such finds were a possibility.
Sliding rocks -- my "Racetrack Playa Rocks" file has a paper (Schumm, J. Sed. Pet., 26(3) 284-286, 1956) which discusses "playa stone tracks in California and Nevada..." The reference to Nevada is Clements, Wind-blown rocks and trails on Little Bonnie Claire Playa, Nye County, Nevada, J. Sed. Pet. 22, 182-186, 1952.
...OK, a little googling turned up a table in Paula Messina's disseration listing playas with reported sliding rocks, all in the Mojave.
Here's something to ponder
It's a road cut north of Mono Lake. Brush in upper left for scale.
What's with the pit? Is that a grave?!?
|
|
Reilly
Mountain climber
The Other Monrovia- CA
|
|
Bob, what a find!
Near the base of Fitzroy. A diamond in the rough.
|
|
BASE104
climber
An Oil Field
|
|
In sedimentary basin analysis, we call a hole "accomodation space."
It has a huge impact on how and why you get certain depositional environments of different sedimentary rocks.
Trust me. It goes far deeper than shales, sandstones, and limestones.
I do a lot of basinwide correlations of different rock sequences. Every day I yank up cross sections that are 50 or more miles long and correlate them. After a while, I have a total spiderweb of cross sections and can create a 3D model (easy on computers these days) of the basin. That, along with lithology and looking at the actual rocks for depositional environments, enables you to unravel 500 million years of history.
It is fun and not that hard. For me, anyway. I have been doing it for 25 years. I specialize in deltaic environments but have been working carbonates for the last 4 years.
Geology is so fun. You don't just look at the surface. You have to understand the 3D picture throughout the time that the rock records. Things also happen to rocks after they are deposited, so diagenisis is also an important part.
The general public has no appreciation for sedimentary rocks. They are super cool.
|
|
Bob Harrington
climber
Bishop, California
|
|
I want to be in a band called Accommodation Space.
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
Here's another photo taken near Mono Lake of a similar feature to the one Bob Harrington posted above...
... soft sediment deformation triggered by a seismic event or deformation resulting from a glacier advancing into the lake??
|
|
Dr.Sprock
Boulder climber
I'm James Brown, Bi-atch!
|
|
have you guys heard of the lisping geologist?
|
|
Minerals
Social climber
The Deli
|
|
|
Feb 13, 2012 - 12:12am PT
|
Ah, Poleta. Jfailing posted a Google Earth photo of those folds in post #61 of this thread. Looks like an interesting area – I’ve driven by on 168 but never stopped to take a close look. Have you ever checked out the Papoose Flat Pluton and its contacts with the surrounding host rock? That’s been on my list for years.
Thanks for the references on sliding playa rocks. I found evidence on a playa here in Northern Nevada last spring, which is the first time that I’d seen this outside of the Racetrack Playa. Funny that I haven’t seen more evidence, given that I have been on a lot of playas here in Nevada. But maybe that’s because there are no rocks on ‘em in the first place! I mentioned what I had seen last spring to a professor friend and he thought it was interesting. Paula Messina and the Mojave… will keep that reference in mind and have a look.
Cool photo of the soft sed deformation. There is a photo in the new book, Geology Underfoot in Yosemite National Park, by Allen Glazner and Greg Stock, 2010, that is pretty much the same as your photo.
From the photo caption on page 251:
“Intensely contorted beds of basaltic Black Point tephra atop planar beds of the same material exposed at stop 1. The folds developed as the upper layers slumped while underwater – to the right in the photo. The boundary between the planar and folded layers is the slippery surface upon which the sliding took place. The white layer at the base of the tephra (foot level) is composed of a few inches of fine lake sediment (deposited at the bottom of Lake Russell) and is underlain by pinkish ash from the Mono Craters.”
And from the text on page 250:
“The amazing pear-shaped layers are folds that formed in the upper sandy layers when they slumped downhill, toward the center of the lake, during earthquakes. How did this happen? The cross-bedded sedimentary layers deposited on the lakebed were saturated and, being parallel to the lakebed, had a slight tilt toward the center of the lake. Earthquake shaking was enough of a jolt to cause some of the layers to slide downhill. The beds were folded and crumpled as they slid across the slippery surface of the beds below. Similar intensely folded layers of sediment occur in sand layers derived from other volcanoes around the lake, indicating that this process was common.”
Tradster, is that the same road cut? Looks very similar. Nice.
“Geology is so fun.”
Agreed! And it’s tough to ignore when we spend so much time outside having fun.
Oh… a grave? I suppose it’s a grave of sorts, for scrap metal from an abandoned ore mill. There is an old concrete foundation close to where the photo was taken, and a skarn mine a ways to the west. I thought I had found some more target material one morning, but after a little excavation exercise, it turned out to be more metal than I could manage. Darn. So I filled the hole in. But that’s not geology… Back to geology!
|
|
Minerals
Social climber
The Deli
|
|
|
Feb 13, 2012 - 12:17am PT
|
And…… a BUMP to bring back Weschrist and FortMental!!! They have certainly made positive contributions to threads like this one.
|
|
TGT
Social climber
So Cal
|
|
|
Feb 26, 2012 - 01:25am PT
|
Tucker has this in his collection, Don't know where it came from, but anyone have an idea of what the composition of the black inclusion in the quartz is?
It obviously grew as a crystal. Looks like it's octagonal
|
|
BASE104
Social climber
An Oil Field
|
|
|
Feb 26, 2012 - 04:07am PT
|
hmmm. Tourmaline often looks pretty black and crappy like that.
Hard to do geology from pictures, though.
|
|
Jaybro
Social climber
Wolf City, Wyoming
|
|
|
Feb 26, 2012 - 12:23pm PT
|
What minerals said.
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
|
Mar 18, 2012 - 11:48am PT
|
Outcrop of late Miocene Neroly Fm, andesite cobble-rich conglomerate, Altamont Hills, CA
This outcrop contains a long, 1 to 2 ft diameter, cylindrical cavity. At first I thought it was man made, but upon further inspection it appears natural.
Here is a photo of the feature looking back toward the face of the outcrop
Another view of this feature in the opposite direction taken with a flash light for illumination. The feature appears to extend 10s of feet.
Nearby feature, claystone-filled cavity...possible clue?
I'm puzzled as to how this long cylindrical feature formed...any ideas?
|
|
Grampa
Trad climber
OC in So Cal
|
|
|
Mar 18, 2012 - 12:19pm PT
|
If a block of soft material was covered during deposition of the conglomerate, then that soft material would erode faster than the harder conglomerate and leave a cavity.
Or aliens dug it out.
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
|
Mar 18, 2012 - 12:55pm PT
|
A long cylinder-shaped claystone rip up...the feature in the last photo is a remnant??
|
|
BASE104
Social climber
An Oil Field
|
|
|
Mar 18, 2012 - 02:11pm PT
|
I would say that it isn't a claystone. It is hard to tell from one little picture what the size of the clasts are in the conglomerate, but it looks to me like a boulder sized chunk is missing.
PM me whenever this thread wanders into sedimentary rocks.
edit: looked at the pictures and retract the above statement. I would love to look at it, but you have to follow the outcrop all around to really understand it.
The clasts are very well rounded, so they traveled a fair distance before being deposited in a high energy environment. That is conglomerate 101.
|
|
tuolumne_tradster
Trad climber
Leading Edge of North American Plate
|
|
|
Mar 18, 2012 - 06:45pm PT
|
BASE104: interesting to think about the California paleogeography at the time these sediments were deposited. Very different than today. Probably like the Cascades.
The light grey lithology in the last photo of my post above is definitely claystone or silty claystone. The conglomerate contains predominately andesitic volcanic cobbles derived from the Sierra foothills. The Neroly Fm is roughly correlated to the Mehrten Fm in the Sierra foothills and also consists of andesite-rich fluvial and lahar deposits.
from a USGS open file report...
Tn Neroly Sandstone (Miocene) -- Blue, volcanic - rich, cross-
bedded sandstone and conglomerate; mainly nonmarine. The
Neroly Sandstone in this assemblage contains tuffs with K/Ar
ages of about 9.7 Ma (Black Diamond Park Tuff ) and 10-12 Ma
(Alves Tuff) (Sarna-Wojcicki, written communication, 1990).
Includes minor siltstone, locally mapped (Tns).
Some info on the Mehrten.
http://search.datapages.com/data/doi/10.1306/03B5AB06-16D1-11D7-8645000102C1865D
Here are a few more photos of the Neroly
|
|
Darwin
Trad climber
Seattle, WA
|
|
Y'all think there would be interest in a Caves and Karst thread? I know cavers have deep seated issues w.r.t. cave locations, and those issues make surfers look like open and welcoming come-share-my-break peaceniks. Nonetheless, the BC Limestone thread and others makes me think we could pull it off. I think I'm too timid to start the thread, though.
My real question that I asked some posts ago.
How often do large bodies of calciferous rocks exist without caves?
I'm thinking of the large body of calciferous rock in the White Mountains NE of Bishop, CA? I don't know if there are caves there, but I found no evidence of them in my brief camp out there. Is the explanation for no-caves as simple as lack of water to erode/dissolve the rock? nfw.
Darwin
|
|
|
SuperTopo on the Web
|
Let us know!