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You are browsing the archive for primary structures Archives - AGU Blogosphere.

28 November 2022

Getting ripped

Patterns in the sand on a beach in Costa Rica…

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30 July 2022

Well-preserved mudcracks in Belt argillite, Glacier National Park, Montana

Fresh from the field, Callan shares a quintet of beautifully preserved desiccation cracks in Mesoproterozoic Belt Supergroups sediments, exposed in Montana’s geological gem, Glacier National Park.

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13 December 2019

Friday fold: blueschist & eclogite at Jenner, California

A pre-Fall Meeting field trip to the coast of northern California yields rare sights of garnet-bearing blueschist, plus eclogite, some pillow basalts, birds, waves, wind, and a lot of rain.

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30 August 2019

Friday fold: Calafia State Beach

The Friday fold is a guest contribution to “Mountain Beltway” from the manager of the AGU Blogosphere, Larry O’Hanlon. It shows apparent crumpling of a few sedimentary layers at the toe of a soft sediment slump at Calafia State Beach in southern California.

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20 May 2019

Digital manipulation as a teaching aid

Photoshop is a powerful image editing program. Its “cloning” tool allows the removal of “distracting” data from geological imagery. Examine these four examples and consider the ethical limits of the technique. Is it okay to remove fractures and lichens from an outcrop photo in order to allow novices to focus on the geological content you want them to learn from?

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18 March 2019

New discoveries in the Martinsburg Formation

A virtual field trip to examine some deepwater clastic sediments shed off the first phase of Appalachian mountain building, and deformed in the third phase. It’s a lovely day for a field trip to the late Ordovician!

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6 February 2019

Dalradian schist at Glen Roy

We saw last week how glaciation carved out a valley in Scotland called Glen Roy. As the glacier ground into the landscape, it liberated tremendous numbers of sedimentary particles from the bedrock, which is composed of Dalradian metamorphic rocks (mainly porphyroblastic schist in my observation). Then once that now-U-shaped valley had been deglaciated, a new glacier dammed it, making a lake that rose and filled the declivity of Glen Roy, …

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8 January 2019

Geology of the Galapagos Islands

Dozens of delectable geological images from the Galapagos Islands, showing fluid basalt flows and violent pyroclastic deposits along with many primary volcanic features (and a bit of sedimentology thrown in for good measure).

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15 July 2018

Chickie’s Rock, Pennsylvania

A virtual field trip to the deformed quartzites and metaconglomerates of Chickie’s Rock and Sam Lewis State Park in Lancaster County, Pennsylvania.

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5 June 2018

Cross-bedding in Archean komatiitic ash deposits

Cross-bedding isn’t just for modern sedimentary deposits; you can find it in truly ancient ash deposits too! Let’s head to South Africa and take a look.

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7 May 2018

2/3 pass rate on determining geopetal “up”

A visual challenge via Twitter to determine stratigraphic younging direction ends with equivocal results. So let’s use GIGAmacro imagery to school your sedimentological students on how three primary sedimentary structures look different right-side-up versus up-side-down.

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10 April 2018

An outcrop showcasing a strand of the Kentucky River Fault System

Roadcuts in Kentucky show Ordovician limestones of two distinct types, replete with fossils and primary sedimentary structures, and juxtaposed by a fault, one strand in the Kentucky River Fault System.

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14 March 2018

New media to show off exemplary features of the Devonian-aged Hampshire Formation along Corridor H, West Virginia

Last week, I was in Morgantown, West Virginia, to deliver a colloquium talk to the geology department at West Virginia University of geological visualization. The next day, I took some time on the way home to geologize a bit on the road called Corridor H, a gorgeous transect through the eastern Allegheny Plateau and western Valley & Ridge provinces. I focused that day on the Hampshire Formation, Foreknobs Formation, and …

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12 January 2018

Friday folds: Hayden Butte (“A Mountain”), Tempe

In keeping with the Arizonarific theme of this week’s posts (thanks to my participation in the 2018 Structural Geology and Tectonics Forum), I thought I would wrap up my ‘geology of the Phoenix area‘ posts with a walk I took on my last day there. This was to what Google Maps calls “Hayden Butte,” but the locals call “A Mountain.” Not “a mountain,” but “the mountain called ‘A‘.” It has …

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29 September 2017

Friday folds: soft sediment deformation in thin sections of MTD sandstone

The Friday folds are small soft-sediment deformational features within a dismembered, folded sandstone (a “ploudin”) from a mass transport deposit from the latest Devonian of West Virginia.

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22 September 2017

Friday fold: bend in a vesicular lava flow, Etna

It’s the First Friday of Fall! Here’s a sort of fold to help you celebrate: a section through a ∧ shaped bend in a vesicular basalt flow from the eastern flanks of Mount Etna in Sicily. It’s due to volcanic lava flowing rather than ductile deformation of a pre-existing solid rock (our usual habit with this feature), but I think we can appreciate it regardless:

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9 August 2017

A suite of new 3D models

A showcase of five new 3D digital models of awesome rock samples and outcrops, produced using Agisoft Photoscan.

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15 July 2017

Flow-banded rhyolite from Vulcano, Italy

I collected only a single rock on my summer travels in France and Italy. (Those of you who know me will realize how extraordinary this low number is!) It’s a flow-banded rhyolite from Vulcano, in the Aeolian Islands north of Sicily a few weeks ago. It contains porphyritic vesicular basalt xenoliths. I featured a similar sample on Twitter yesterday on the occasion (supposedly) of “International Rock Day”: For #InternationalRockDay , …

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6 June 2017

Loading sags in homogeneous lithologies?

Can soft sediment deformation “loading structures” (ball & pillow) occur when the two strata are identical in composition? Grok on these field photos and chime in with your best hypothesis.

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5 June 2017

3D models of sedimentary structures

I’ve been busy making 3D models lately. Here are three ones united by a theme of being sand that was deposited relative to mud. In one case we have scouring to make flutes, in another case we have have localized sagging to make “ball & pillow” structures, and in the third case we have an extraordinary submarine landslide deposit. For two of them, the shale has been preferentially etched away, …

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