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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.
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.
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.
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?
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!
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, …
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).
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.
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.
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.
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.
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 …
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 …
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.
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:
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.
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 , …
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.
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, …
25 May 2017
Chert-slab conglomerate from the Fig Tree Group, Barberton
Deep in the Archean, things suddenly got violent in the deep water of the Mapepe Formation’s oceanic setting. What was a quiet precipitation of chert suddenly was torn apart and tumbled downslope. Was an earthquake to blame? A bolide impact? This is the result.
