Day #110 Deskcrop: Potsdam Sandstone

Today’s deskcrop is a piece of the Cambrian Potsdam Sandstone from near Alexandria Bay, NY. The Potsdam at this locality lies nonconformably over gneisses of Proterozoic age. One can see bedding laminations in this sample as well as alteration zones at the top and bottom of the sample marking where groundwater flowing along bedding planes has begun to weather the sample. I also GigaPanned the roadcut at this locality, but I’ll hold that view for one of this coming weekend’s “Outcrop” posts.


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Day #109 Deskcrop: Sillimanite Schist, Tracy Hill Road

Concluding the Barrovian metamorphic index minerals theme I began last week (as a series of clues to WoGE #198), today’s deskcrop is a sillimanite zone muscovite-rich schist from Dutchess County, New York. There is no sillimanite visible in hand sample (like most sillimanite +/- K-feldspar-zone metapelites I’ve known, but not all), though there are some bodacious garnets hidden in that sea of muscovite. The Barrovian-sequence metapelites from Dutchess County were the topic of a two-week metamorphic petrology lab in my undergrad days at Colgate University, and names like Tracy Hill and Still Road bring back memories of that lab with a fondness that undoubtedly grows with the passage of time.

Sillimanite Schist, Tracy Hill Road

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Day #108 Outcrop: Carlin Canyon Angular Unconformity

This is the second of two GigaPanned outcrops I’m posting this weekend accompanied by text from Annals of the Former World, by John McPhee, upon which I cannot improve…

Interstate 80, in its complete traverse of the North American continent, goes through much open space and three tunnels. As it happens, one tunnel passes through young rock, another through middle-aged rock, and the third through rock that is fairly old, at least with respect to the rock now on earth which has not long since been recycled. At Green River, Wyoming, the road goes under a remnant of the bed of a good-sized Cenozoic lake. The tunnel through Yerba Buena Island, in San Francisco Bay, is in sandstones and shales of the Mesozoic. And in Carlin Canyon, in Nevada, the road makes a neat pair of holes in Paleozoic rock. This all but leaves the false impression that an academic geologist chose the sites-and now, as we approached the tunnel at Carlin Canyon, Deffeyes became so evidently excited that on might have thought he had done so himself, “Yewee zink bogawa!” he said as the pickup rounded a curve and the tunnel appeared in view. I glanced at him, and then followed his gaze to the slope above the tunnel, and failed to see there in the junipers and the rubble what it was that could cause this professor to break out in such language. He did not slow up. He had been here before. He drove through the westbound tube, came out into daylight, and, pointing to the right, said, “Shazam!” He stopped on the shoulder, and we admired the scene. The Humboldt river, blue and full, was flowing toward us, with panes of white ice at its edges, sage and green meadow beside it, and dry russet uplands rising behind. I said I thought that was lovely. He said yes, it was lovely indeed, it was one of the loveliest angular unconformities I was ever likely to see.

Carlin Canyon Angular Unconformity

The river turned in our direction after bending by a wall of its canyon, and the wall had eroded so unevenly that a prominent remnant now stood on its own as a steep six-hundred-foot hill. It made a mammary silhouette against the sky. Mu mind worked its way through that image, but still I was not seeing what Deffeyes was seeing. Finally, I took it in. More junipers and rubble and minor creases of erosion had helped withhold the story from my eye. The hill, structurally, consisted of two distinct rock formation, awry to each other, awry to the gyroscope of the earth-just stuck together there like two artistic impulses in a pointedly haphazard collage. Both formations were of stratified rock, sedimentary rock, put down originally in and beside the sea, where they had lain, initially flat. But now the strata of the upper part of the hill were dipping more than sixty degrees, and the strata of the lower part of the hill were standing almost straight up on end. It was as if, through an error in demolition, one urban building had collapsed upon another. In order to account for that hillside, Deffeyes was saying, you had to build a mountain range, destroy it, and then build a second set of mountains in the same place, and for the most part destroy them. You would first have had the rock of the lower strata lying flat-a conglomerate with small bright pebbles like effervescent bubbles in a matrix red as wine. Then the forces that had compressed the region and produced mountains would have tilted the red conglomerate, not to the vertical, where it stood now, but something like forty-five degrees. That mountain range wore away-from peaks to hills to nubbins and on down to nothing much but a horizontal line, the beveled surface of slanting strata, eventually covered by a sea. In the water, the new sediment of the upper formation would have accumulated gradually upon that surface, and, later, the forces building a fresh mountain range would have shoved, lifted, and rotated the whole package to something close to its present position, with its lower strata nearly vertical and its upper strata aslant. Here in Carlin Canyon, basin-and-range faulting, when it eventually came along, had not much affected the local structure, further tilting the package only two or three degrees.

Clearly, if you were going to change a scene, and change it again and again, you would need adequate time. To make the rock of that lower formation and then tilt it up and wear it down and deposit sediment on it to form the rock above would require an immense quantity of time, and amount that was expressed in the clean, sharp line that divided the formations-the angular unconformity itself. The lower formation, called Tonka, formed in middle Mississippian time. The upper formation, called Strathearn, was deposited forty million years afterward, in the late Pennsylvanian time. — John McPhee


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Day #107 Outcrop: Eocene Auriferous Gravels

Two GigaPanned outcrops this weekend accompanied by text from Annals of the Former World, by John McPhee, upon which I cannot improve…

The dry bed of an Eocene river carries Interstate 80 past Gold Run. The roadside records the abrupt change. As if you were swinging off a riverbank and dropping into the water, you go out of the metavolcanic rock and into the auriferous gravels. We stopped, stood on the shoulder, and looked about a hundred feet up and escarpment that resembled an excavated roadcut but had not been excavated by highway engineers. It was capped by a mat of forest floor, raggedly overhanging. The forest, if you could call it that, was a narrow stand of ponderosas, above an understory of Manzanita with round fleshy leaves and dark-red bark. The auriferous gravels were russet, and were full of cobbles the size of tomatoes-large stones of long transport by a most impressive river.

I-80 Roadcut in Eocene Auriferous Gravels

To the south, across the highway, the scene dropped off into a deep mountain valley. The near end of the valley was three hundred feet below the trees above us. The far end of the valley was nearly twice as deep. A mile wide, this was a valley that had not been a valley when wagons first crossed the Sierra. All of it had been water-dug by high-pressure hoses. It was man-mad landscape on a Biblical scale. The stand of ponderosas at the northern rim was on the level of original ground.

The interstate was on a bench more than halfway up the gravel. Above, us behind the trees, were the tracks of the Southern Pacific. In the eighteen-sixties, when the railroad (then known as the Central Pacific) was about to work its way eastward across the mountains, it secured the rights to this ground before the nozzles reached it. Moores and I made our way up to the tracks, where the view to the north was over a hosed-out valley nearly as large as the one to the south, and bordered by white hydraulic cliffs. The railroad, with the interstate clinging to its hip, ran across a septum of the old terrain, an isthmus in the excavation, an unmined causeway hundreds of feet high made of gravel and gold. — John McPhee

I-80 Roadcut in Eocene Auriferous Gravels, near Gold Run, California
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Day #106 Deskcrop: Powell Kyanite Schist

Is it getting hot in here? The pressure is certainly rising.

Today’s deskcrop is a piece of the Powell kyanite schist from northern Wisconsin. This is an absolutely gorgeous rock, particularly in thin section. If you take a close look around the full scale version of the image below you’ll see both weathered (bottom) and fresh (top) surfaces of this rock. The keen eyed observer will be able to distinguish porphyroblasts of red garnet and pale blue kyanite among the mass of biotite, muscovite, plagioclase, and quartz grains that make up the bulk of this rock.

I think we’ll give the metamorphic theme a rest for the weekend, but when we come back on Monday you can expect to see a sillimanite zone rock in this space.

Powell Kyanite Schist

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Day #105 Deskcrop: More Black Hills Staurolite Schist

Mmmm, staurolite! One of the loveliest porphyroblasts – particularly when twinned (see the left end of today’s deskcrop, below), staurolite is the metamorphic index mineral above garnet in the progressive metamorphic sequence in metapelites. This particular sample has a more schistose texture than the staurolite schist I previously featured, and comes from a bit more remote locality.

When I collected this one back in 2003 web-based mapping tools were still in their infancy. Nonetheless, I have a distinct memory of planning the search for this locality with them and printing out a bunch of satellite images of this area of the Black Hills to aid my back-roads navigation. Today I can do all of that on the fly, in the field, and in real time on my Motorola/Verizon Droid cellphone. Oh, how times have changed!

The kyanite zone is up next, with a deskcrop from Wisconsin…

Staurolite Schist, Black Hills

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Day #104 Deskcrop: Garnet Schist, Black Hills

Got garnets? Oh, have we got garnets!

Look at all the lovely garnet porphyroblasts in this lovely piece of schist from the Black Hills of South Dakota. In fact this outcrop is just up the road from another one featured in this series back in March. This particular bed was just chucky-jam full of little 2-3mm diameter dodecahedrons of joy. Boy, would I like to see what this one looks like in thin section.

Having visited the garnet zone, the next scheduled stop is the staurolite zone. I have a feeling we’ll be revisiting the Black Hills on Tax Day…

Garnet Schist, Black Hills

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Day #103 Deskcrop: Biotite Phyllite, Walloomsac Fm.

We’ve arrived in the biotite zone (just barely). Today’s deskcrop comes from the Middle Ordovician Wolloomsac Formation in Dutchess County, New York. This deskcrop was metamorphosed during the Taconic and Acadian Orogenies and is just inside the mapped biotite isograd. There’s supposed to be macroscopically visible porphyroblasts of biotite, but I’m hard pressed to recognizing them, myself.

Tomorrow we’ll move on to the garnet zone…

Biotite Phyllite, Walloomsac Fm.

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Day #102 Deskcrop: Ellsworth Schist

Tax week has been a pretty busy one for me with grading and other meatspace commitments. As I catch up on Deskcrops for the week there’s a theme that geologists will recognize that relates to an earlier blog post. Nuf said. ;-)

I’m kicking the week off in the chlorite zone. Today’s deskcrop is Ellsworth Schist, a Cambro-Ordovician rock unit cropping out in coastal Maine near Mount Desert Island. This rock unit has experienced quite a bit of deformation but appears to have only been heated to a point such that chlorite is the highest grade metamorphic index mineral present.

Tomorrow we’ll visit the biotite zone…

Ellsworth Schist – Chlorite Zone

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Where on (Google) Earth #198?

I greatly enjoyed finding Péter’s lherzolite locality in the French Pyrenees, though it took me a week searching Switzerland before I read his clue carefully and considered the possibility that such gorgeous alpine geology was not hidden in some nook in the Gruyère.

I especially enjoyed the geological aspect of Péter’s challenge – so much so that it has inspired (indirectly) the locality I’ve chosen here. As always, the object of Where on (Google) Earth is to identify the locality of the image below (latitude and longitude will generally suffice), but also to explain the geological significance of the site. Quite often that is simply a matter of interpreting the landforms that can be readily identified in the GE image. However, the landforms seen here are merely the key that will help you unlock the deeper geological significance of the site. I would ask that you refrain from identifying the locality until such time as you are prepared to map out a sufficiently detailed geological explanation. This may be frustrating if you find the locality quickly (a distinct possibility, in this case), but I hope that you’ll find the challenge of unearthing the geological significance of the region a worthwhile quest in its own right.

I think WoGE #198 will be relatively easy to locate, so I’m choosing to invoke the Schott Rule – wait an hour for each WoGE win before answering, please. Post time: 3/13/2010, 23:37 Central Standard Time (USA) – 3/14/2010, 4:37 GMT.

Where on (Google) Earth #198.

Dig in!