|Here we are, back at Lida Summit, looking west on S.R. 266.|
Offhand, I can think of a few places where a geologist might park or pull over other than roadcuts or viewpoints. For one, I would park off a paved road! And while down some dirt road away from traffic, I would park in a place that wouldn't allow my truck to roll forward or backward even without chocking the tires with a rock or chock blocks. That spot might be some random wash or ditch of some kind—if I thought the low area was out of the way of any potential flash flood—or it might be on a hill, provided the truck wouldn't roll when I let out the clutch: a site with a natural declivity or some in-place, roll-resistant rocks. Then I'd chock the truck anyway.
Speaking of ditches, that brings me to a second possible parking position: a hidden spot. While in the field I might not want to advertise my presence for a variety of reasons. For example, I might decide to indulge in some sort of exploration espionage and therefore wouldn't want my presence to be suspected. Mostly, though, I'd chose to conceal my vehicle, and thus my presence and general whereabouts as a means of personal protection: in order to hide myself from random strangers who might prove to be a threat.
|A view of the summit from the west side: summit ahead, roadcut right.|
|And here's the roadcut. My truck is alongside for scale.|
Is the rock in our roadcut igneous (granitic rock or rhyolite, perhaps), sedimentary (blocky-weathering siltstone, perhaps), or metamorphic (quartzite, maybe)?
|It's ash-flow tuff!|
Vertical jointing stands out clearly in these photos. I’m not sure from this one exposure if the jointing qualifies as poorly formed columnar jointing, but it looks quite possible. Columnar jointing is fairly common in some parts of ash-flow tuff sheets, especially in the strongly welded zone. The jointing is often rectangular to square (Ross and Smith, 1961), though it can also be five- or six-sided, as is common in basalts. Consequently, it's not always possible to tell the difference between basalts and dark-weathering, columnar-jointed ash-flow tuffs from a distance.
Hints of the strong eutaxitic texture present in this exposure can be seen cutting subhorizontally across the photos, with the planar feature dipping more or less to the right (that would be in an easterly direction).
|In this photo, the eutaxitic texture becomes, perhaps, slightly more visible.|
|Strongly or densely welded vitric ash-flow tuff.|
Back in the semi-dark ages, I learned, somewhat incorrectly, that the term "fiamme" refers to the tiny flame shapes at the end of flattened pumice lenses in rocks like these. I also learned to call the lenses "flattened (or collapsed) pumice," rather than "fiamme." The lenses are definitely fiamme, even though Ross and Smith (1961) used a definition that might seem more specific than the one commonly used today. They defined fiamme as
where "piperno" is a rock characterized by lenses of glass."the Italian name used to describe black glassy inclusions in piperno and which have a cross section shaped like the tongues of flame. These are often several centimeters in length, but may range from microscopic size to several feet in length,"
The definition and genetic interpretation of fiamme is addressed at length by Bull and McPhie (2007). They point out that the presence of fiamme is not diagnostic of welded ash-flow tuff (the same way that the absence of fiamme is not diagnostic that you are looking at something besides a welded ash-flow tuff).
|Moderately or strongly welded lithic-rich vitric ash-flow tuff.|
We can examine these rocks a little more closely.
|An enlargement from ash-flow tuff #1.|
|An enlargement from ash-flow tuff #2.|
Thesis: Finding an Area
Finding a Thesis: Battle Mountain to Austin to Gabbs
Finding a Thesis: Pole Line Road
Finding a Thesis: Pole Line to Belmont
Finding a Thesis: Klondyke District
Finding a Thesis: A Joshua Tree Aside
Finding a Thesis: Into the Palmetto Mountains
Finding a Thesis: Farther into the Palmetto Mountains
Finding a Thesis: A Bit O' Geology in the Palmetto Mountains
Finding a Thesis: Future Stories from the Palmetto Mountains
Post updated 2Dec2015 to add related posts.