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.

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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.

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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).

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Bay of Islands Peridotite

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Day #87 Outcrop: Glacially Polished Potsdam Sandstone

I’ve recently been uploading field photos I took early last summer and one of those outcrops is just crying out to be blogged about. This outcrop is located in the St. Lawrence River Valley in upstate New York, northeast of Alexandria Bay in the Thousand Islands region. The bedrock here is Cambrian Potsdam Sandstone and the outcrop in question is a great lesson in glacial processes. The top of the outcrop is beveled off and polished by glacial scouring. The surface was actually quite slippery with a thin film of water from a passing rain shower.

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Glacially Polished Potsdam Sandstone

Looking at the surface in detail, you can see both glacial striations (right) and chatter marks (left) side-by-side in the image below. Not much question which way the ice sheet was moving here…

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Glacial Chatter Marks (left) and Striations (right)

Finally, there are some remarkable joint patterns exposed on the polished surface of this outcrop, just so the structural geologists don’t feel left out.

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Remarkable Jointing Pattern

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Day #86 Outcrop: Half Dome Granodiorite Erratic

We’ve visited the Half Dome Granodiorite before, both as a crushed “sand” and as a deskcrop. Well, it’s time to visit this beautiful rock once again, this time in the form of a glacial erratic, in its natural habitat.

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Half Dome Granodiorite Erratic

This particular erratic is located on the shores of Tenaya Lake in Yosemite National Park. It’s an example of the equigranular phase of the Half Dome Granodiorite (there’s also a porphyritic phase). If we take a closer look you can see some of the minerals that make up this rock: plagioclase feldspar is most abundant, followed by quartz, K-feldspar, hornblende and biotite. There’s also accessory sphene (titanite) but I doubt you’ll spot it at this resolution.

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Take a Closer Look at its Minerals

And I’d be remiss if I didn’t let you in on the even more spectacular view from this vantage point.

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Tenaya Lake and Polly Dome

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Rudderless

I’m struggling with the topic of this month’s Accretionary Wedge.

This AW is to share your latest discovery with all of us. Please let us in on your thoughts about your current work. What you are finding, what you are looking for. Any problems? Anything working out well?

For the better part of the last few years I feel like I’ve been trying to share my discoveries via this blog and at geology meetings. Most of this has focused on using GigaPan photography in geology and creating virtual field experiences with GigaPan and Google Earth. I’ve also put a good deal of effort into building up the Geoblogosphere and aggregating and sharing geology news on the web. Most recently I’ve been sharing the deskcrops and outcrops of my life. And of course, I teach geology to Fort Hays State University geology students (and anyone else who cares to listen), which I enjoy, but more importantly pays the bills and makes the rest of this possible.

For better or worse, that status quo is not sustainable. Changes are coming, though not immediately. Over the next year and a half I need to either find a way to make the things I do here valuable to my peers and/or the world at large, or find something else to do. As much as I enjoy being on the bleeding edge of the intersection of geology and the web, sometimes it’s lonely and unrewarding.

“Who doth not answer to the rudder shall answer to the rock.”

Day #85 Deskcrop: Rand Schist

We’ll close out our westward journey this work week with a final stop in California. Today’s deskcrop is a piece of epidote-bearing blueschist, collected from the Rand Schist west of Randsburg, California. The sample was collected during a Penrose Conference in the Tehachapi Mountains, and if memory serves me, this particular leg/locality was led by Carl Jacobson. Though this may not be the most representative sample from this locality, I recall collecting it for is large epidote crystals – best seen on quartz in the second photo below.

The locality of this sample is very fuzzy in my memory, so take its placemark with a big grain of salt.

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Quartz – Epidote – Blueschist

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Quartz – Epidote – Blueschist

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Day #84 Deskcrop: Sillimanite (Fibrolite) Gneiss

Following Horace Greeley’s advice we continue our journey westward. Today’s deskcrop is a sillimanite (var. fibrolite) gneiss collected from a roadcut along the Beartooth Highway, south of Red Lodge, Montana. I collected this sample during the NAGT Teaching Petrology Workshop field trip in July 2003. I take special pride in this sample because it marks the only sillimanite bearing sample I’ve collected to date that I felt confident of my field identification of that mineral (in this case, the variety fibrolite – best seen in the upper left quadrant of this image). What’s more, I believe it was the only macroscopically visible sillimanite-bearing sample collected by the entire group of petrologists that day. I would not pretend my field skills exceeded that of my many eminent colleagues that day – I’m quite content to chalk it up to good luck.

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Sillimanite (Fibrolite) Gneiss

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Day #83 Deskcrop: Staurolite Schist

Continuing westward, today’s deskcrop comes from the Black Hills of South Dakota, just north of the town of Keystone. You can make out some lovely porphyroblasts of staurolite in this deskcrop, some of which have the distinctive “fairy cross” twinning. Although garnet is not apparent as a phenocryst in this sample it does occur in nearby samples. The metapelites of this region of the Black Hills have mapped isograds suggesting that the heating event responsible for the metamorphism was coincident with the intrusion of the Harney Peak Granite and its related pegmatites.

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Staurolite Schist

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Day #82 Deskcrop: Wausau Syenite

We’ll stay in Wisconsin for today’s deskcrop – the ~1520 Ma Wausau syenite. This deskcrop comes from an outcrop in the back of the parking lot of the Wausau Insurance Company, and it’s as nice and pristine a syenite as you could hope to collect. This one’s probably quartz normative but there’s no macroscopic quartz to be seen.

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Wausau Syenite (Weathered)

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Wausau Syenite (Fresh)

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