Day #97 Deskcrop: Conglomerate with Slickensides

Today’s deskcrop comes from the Cedar Mountain area of the northern San Rafael Swell in Utah. This rock is from a thin unit that is found at the contact between the Jurassic Entrada and Summerville (Curtis) formations. The conglomerate contains a variety of pebble types including limeclasts, suggesting an arid depositional environment. This particular piece also has some nice slickensides developed on one side (below). I’ll follow up with some more slickensided deskcrops over the next couple of days.


… with Slickensides

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Day #96 Deskcrop: Granodiorite of Wagy Flat

During the course of my M.S. and Ph.D. research I collected a great many samples of the granitoids of the southern Sierra Nevada Batholith as reference samples of the plutons that may have been parental to the conglomerate clasts from the Gualala basin whose provenance I was attempting to ascertain. Today’s deskcrop is from one of those southern Sierran plutons. I’m guessing a bit at the exact locality of the sample, as my field notes are shamefully thin on detail. I really need to get back to this area and reacquaint myself with the details of its geology…

Granodiorite of Wagy Flat

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Day #95 Deskcrop: Charnockite

A couple of weeks back Callan kicked off his transect of the Virginia Appalachians with a great post on charnockite. Well, I did a little digging and I’ve unearthed a deskcrop of charnockite of my own. Mine is from the Adirondacks, most likely in the neighborhood of the Canada Lakes. My piece is pretty fresh so you see the green color of the orthopyroxene but not any of its dun weathering color. My deskcrop also shows a bit more strain than Callan’s. See my discussion in Callan’s comments for more geologic background on the Adirondack charnockites.


I’m not certain, but that may be a slightly larger crystal of andesine plagioclase showing a bit of labradorescence near the right side of this deskcrop.

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Day #93 Outcrop: Baked Contact(?), Green Sand Beach

I’ve been trying to find a nice example of a baked contact so that I could jump on the geomeme that Callan and Silver Fox started earlier this week. I thought about maybe using a skarn contact (I think I’ve got pictures of one somewhere), but that’s not exactly aligned with this meme. What I did find I’m not 100% certain qualifies as a baked contact, though I certainly think it might.

These shots are from a ledge above the famous Green Sand Beach near Ka Lae (South Point), Hawaii. My interpretation is that what we’re seeing here is a basaltic lava flow capping the littoral deposits that form the source of the eponymous green sand. This is one of those situations where I took the photos because there appeared to be something geologically interesting going on, even though I wasn’t exactly sure what it was. I’m not certain whether the rusty layer at the contact represents contact metamorphic baking from the lava flow, or later hydrothermal(?) alteration by groundwater, but in any case I think it’s the best I’m likely to do to keep the current geomeme going. Perhaps some more familiar with this locality or these sorts of processes can fill in the details or set the record straight.

Baked Contact(?), Green Sand Beach

A Closer Look at the Contact

And of course, the main attraction of this day’s hike was beckoning me to turn around and descend to collect a deskcrop. It’s not easy to linger on an undocumented curiosity when the siren below is singing her song…

The Green Sand Beach – Eroding Littoral Cone

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Day #92 Deskcrop: Favosites

Why don’t we close out the work week with a Fossil Friday, where I get to display my general ignorance of paleontology (which is why Fossil Fridays are likely to be few and far between). Today’s deskcrop is from the same locality as the deskcrop of Petoskey Stone I featured back in early February. As best I can fathom, this is a fossil of some flavor of Favosites. If a qualified fossil fancier (paleontologist) would care to proffer a refined identification, I’d find that useful.


This post is brought to you by the letter “F”.

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Day #91 Deskcrop: Centerlinite

For today’s deskcrop I present the type specimen of centerlinite. Closely related to asphaltite (and its derivative products), it is distinguished from other minerals by its distinctive taxicab yellow color on top (below), though remarkably similar to other minerals of the group on the bottom (2nd photo below). Usually found as a surface encrustation on Quaternary (Anthropocene) asphalt deposits, centerlinite can only be considered a mineral if you’re not especially picky about the “naturally occuring” part of the mineral definition, as all known samples appear to be anthropogenic. Further work will be necessary to establish the color causing mechanism of its distinctive top side.

Centerlinite (top)

Centerlinite (bottom)

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Day #90 Deskcrop: Gore Mountain Garnet

To celebrate the quarter pole of the year in deskcrops and outcrops (and the fact that I’m temporarily caught up with posting again), I’m breaking out a nice piece of Gore Mountain garnet for today’s deskcrop. Remarkably large garnets have historically been mined for use in abrasives at the Barton Garnet Mine, just west of Gore Mountain in New York’s Adirondack Mountains. This mine is world famous for its huge garnets (up to a meter in diameter). They occur in an amphibolite grade metagabbro and often the garnets have coronas of hornblende or plagioclase. Alas, most are fractured and not of gem quality. Nonetheless they make a colorful deskcrop that no geologist should be without.

Gore Mountain Garnet

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Day #89 Deskcrop: Herkimer Dolostone & Quartz Encrusted Vug

Near Middleville, New York the Herkimer Dolostone has developed a vugs which are commonly filled by doubly terminated quartz crystals known as “Herkimer Diamonds“. The one trip I made to the quarries at Middleville in search of Herkimer diamonds turned out to be less than completely successful. I lost a sweatshirt that day and found precious few “diamonds”. Probably my best find that day was from a low roadcut outside the quarries where I was able to recover this deskcrop of Herkimer dolostone with a large vug lined with lots of small singly terminated quartz crystals.

Not Exactly a Herkimer Diamond

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Day #88 Deskcrop: Bay of Islands Peridotite

Today’s deskcrop is only the second one so far this year that I didn’t collect myself (here’s the first). It’s a piece of the Bay of Islands Ophiolite from near Corner Brook, Newfoundland, Canada. For some reason or another (probably undergrad field camp) I missed my family’s one and only vacation to Newfoundland. This sample was a gift from my sisters, who know a good looking peridotite when they see one. There are some lovely serpentinized(?) fractures that give this rock its distinctive texture. I don’t know the exact locality of the sample, but I’ll locate it near the Blow Me Down Massif, because it’s got such a great name (and would be a reasonable source for a rock like this).

Bay of Islands Peridotite

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