Crucifix Site 1: Sediments

On the September 2009 GSA field forum in the Owens Valley, the final stop of our first day was to check out the so-called “Crucifix Site,” along Chalk Bluff Road (north of Bishop, California, at the southern margin of the Volcanic Tableland). It’s called the “Crucifix Site” because there is a metal cross erected there:chalk_bluff_stuff_13

This is the site of some pre-Bishop-Tuff volcaniclastic sediments. The place is interesting on several levels, including the sediments themselves, and the subsequent deformation they have experienced. Here’s a look at the outcrop:chalk_bluff_stuff_03

Some annotations help to call one’s attention to primary sedimentary structures and interpretations:chalk_bluff_stuff_03anno

Lots of the sediment itself was made of little beads of obsidian, usually surrounded with a “chalky” weathering rind:chalk_bluff_stuff_07

A cool little channel cross-section was visible, too:chalk_bluff_stuff_04

For some reason, this is what pops into my mind when I run into a well-exposed Cheshire channel cross-section:

Birds and wasps had tunneled into the softer layers, resulting in horizontal rows of holes. I tried to ignore these modern bioturbations so I could focus on the ancient tale in the rocks themselves. Some cool soft sediment deformation was visible, like these flame structures (upper part of the central gray layer):

Zoomed in on a pair of flame structures, and the down-sagged material between them:chalk_bluff_stuff_11


…And another set:chalk_bluff_stuff_12


This was odd: The lowermost layer (upon which my field notebook rests) is unperturbed, but the layers above it are all churned up in one small area (center), flanked by a couple of bird holes:

Zoomed-in on the area in question:chalk_bluff_stuff_09

Annotated, for your viewing pleasure; green is top of undisturbed layer; red shows boundaries of zone of disrupted sediment:chalk_bluff_stuff_09anno

I would be pleased to hear from anyone who knows more about sedimentology than me about the wavy bedding in the second (& third) photo, and this weird sediment disturbance in the lowermost photo. Also: with the flame structures, it looks like coarser material in the lower layer (gray) is the less viscous participant, while finer-grained (white) material is sinking downwards. Isn’t this the opposite of the way it usually works?


Virginia water well shows seismic waves

This site, from the USGS, shows depth to the water table for a well in Virginia:


You’ll notice the tidal influence on the water table (broad sine-curve-like up and down crests and troughs at ~12 hour intervals), and then a sudden perturbation which caused some wiggles almost two and a half feet of magnitude! This, presumably, is the seismic waves from the Chilean earthquake arriving — surface waves, I would guess, but I’ll happily be corrected if that’s not the case.

Major hat tip to Cian Dawson, who tweeted a link to the site around 6am D.C. time (which would be, what, 3am in California, Cian? Sheesh!). In the same tweet, a link to this hydrograph in Christiansburg, Virginia, showing its response to various historical earthquakes.

Frozen soil lifts off

When I was out poking around in the woods, confirming for local geophile Barbara that indeed her local geologic map wasn’t 100% accurate, I noticed this on the frozen ground:

We have seen this before, in a post back on NOVA Geoblog, almost exactly a year before I took this photo. Here’s another shot from the more recent excursion, taken a foot or so over from the first one:


What’s happening here is not that I am showing you particularly high-contrast photos of pebbles and cobbles in the mud. Instead, the reason for the dark line around the sedimentary clasts is that the mud is frozen. When water freezes into ice, it expands in volume by about 9%. This extra volume means that the ice can’t occupy the same space that the liquid water did. So it pooches upwards, as “up” is the direction in which it is least “hemmed in.” Down? No — the expanding ice is not capable of pushing the entire Earth out of its way. North/south? or East/west? Well, there’s already soil there, and it’s pushing back, so there’s no expanding out in those (horizontal) directions. So, “up” it is. That’s all we’re left with: “up” is σ3.

If I were to draw this as a cartoon, here’s the “before” picture:


As the sheet of frozen mud expands upwards, it detaches from the non-expanding (in fact, shrinking, but not by anywhere near 9%) cobbles and pebbles. As the mud ice lifts up higher and higher, the gap between it and the clasts gets more and more pronounced.


Shadows in those gaps make them appear dark to the camera lens.

Ice pulls all kinds of neat tricks like this in the winter. What’s a cool ice phenomenon you’ve observed lately?

Piedmont rocks exposed in a creek

One of the cool things about being the local geoblogger is that people get in touch with you about local geology. Sometimes this even leads to meeting up for field trips. Here’s two quick photos from a recent (January 2010) field trip to a creek near Springfield, Virginia.

My host was Barbara X, a local aficionada of Piedmont geology. She has lived in this particular neighborhood for many years, and is very familiar with the local woods and drainages through decades of dog-walking there.

Her main question for me was “Could the geologic map of this area be wrong?” She showed me the map, and then took me out to an outcrop which clearly was of a different rock type than the map indicated it “should” be.

The offending intruder, a meta-basalt with two prominent joint-sets:


A short distance downstream, a cut bank revealed some saprolitic rock that is more typical of the Piedmont province:


I think we’re seeing bodies of schist/ gneiss (highly foliated in cross-section), as well as coarse-grained, lighter-colored bodies of granite. All of them have been weathered to hell: you can scoop handfuls of this “rock” out of the outcrop if you want. If you’re a plant, you can plunge your apical meristem right into it, and let the roots follow.

This is typical “outcrop” around here: though the mid-Atlantic region has a fascinating story (including the Appalachian mountain belt, like these rocks), the wet climate has rotted most rock away. The only other thing that’s worth mentioning about this particular outcrop are the upper-left-to-lower-right brown lines: those are fracture traces decorated with rust. The fractures serve as plumbing to move fluids around in the subsurface, and their dissolved cargo of elements can then react with the rock on either side of the fracture.

Rockies course applications open

For those of you who are potential NOVA students (really, that’s pretty much anyone on the planet), I wanted to let you know that applications are now open for the July 2010 Regional Field Geology of the Northern Rockies course that I co-teach with Pete Berquist of Thomas Nelson Community College. A more detailed description is available on my website.

Contact me via e-mail if you want more information or download an application here.

To whet your appetite, here’s Rockies 2009 student Jason Von-Kundra mapping Mississippian-aged carbonates in the Bridger Range of Montana:


Lola → ammonite

Seeing my cat in this posture:
…made me think of this:

Where did those hind legs go?

Snowy décollement

Earlier in the month, during the big snowstorms, my window got plastered with snow. This snow formed a vertical layer which then deformed under the influence of gravity. Looking at it through the glass, I was struck by how it could serve as a miniature analogue for the deformation typical of a mountain belt.

Let’s start our discussion by taking a look at an iPhone photograph of the snow:

So here’s what I notice about this (vertically-oriented) photo:

The big sheet of snow is sliding downward over the face of the glass. This surface of slip is thus analogous to a low-angle thrust fault. Here, the maximum principal stress (known as σ1 to structural geologists) is gravity. The minimum principal stress (σ3) is perpendicular to the window, and the intermediate principal stress (σ2) is horizontal, parallel to the bottom edge of the window (i.e., left-to-right). As deformation proceeds, the snow slab folds up on itself and pooches outward in the area of least stress (σ3); away from the surface of the window.

As the snow layer moves downward, it creates a major fold which thickens the snow in a big line perpendicular to gravity, parallel to σ2. Along the vertical part of the window frame, the snow sheet has detached in a vertically-oriented fracture (i.e., parallel to σ1). Oblique to both σ1 and σ2 is a series of smaller folds with diagonal axes.

We can see a similar pattern in this map of the Himalayan mountain belt:

Note that the map* is oriented with north at the bottom, and south at the top, so as to be able to better compared it to my window. Note the broad arc of the Himalayan mountain front (~parallel to the Nepali border) which is perpendicular to the motion of India relative to Eurasia. The minimum principal stress direction (σ3) is vertical, which is why the mountains grow upwards (and the crust thickens downwards into the mantle, too, making the Himalayan mountain belt the site of the thickest crust on the planet). Along the edge of the impactor (analogous to our snow sheet), for instance in northern Burma, we see the same “splay” of folds with axes perpendicular to the the India-Eurasia convergence vector. The crust there is not as thickened.

Though a gooey slab of snow on my window isn’t a perfect analogue for Himalayan mountain-building, we can see some similarities in gross morphology — structural similarities that are fundamentally tied to the orientation of the principal stress directions.


* Modified by me from a Google Maps “terrain” view.