22 July 2016

Friday fold: The walls of Scalloway Castle

When in Shetland, one of my first stops was the museum in Scalloway, and one of the ancillary benefits of visiting there is the castle next door:

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Scalloway Castle includes building stones derived from the local limestone – a Neoproterozoic unit that has recently been chemostratigraphically correlated with Snowball Earth cap carbonates elsewhere in the world. But that need not concern us today. Today we are here for the folds!

Check out these fine examples:

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There are also some adjoining examples of boudinage:

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(Zooming  in on a boudin neck from the previous example…)

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Hope you’re having a happy Friday!

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11 July 2016

Kilometer to millimeter: 4 GigaPans to zoom in on Lewisian gneiss

I’m in the North-West Highlands of Scotland, enjoying spectacular geology and less-than-spectacular weather.

I’ve been fairly productive on the GigaPan front, regardless, nipping outdoors when the weather permits to shoot outcrops and landscapes.

One set I’m particularly pleased with is this suite of four images. They show the Archean-aged Lewisian gneiss, the oldest rock unit in the North-West Highlands, as exposed on a beachside outcrop east of Durness, Sutherland. The largest feature in the first GigaPan is about a kilometer across (entire field of view). Then the second GigaPan is a zoomed-in subset of the first, and the third is a subset of the second, and the fourth is the most zoomed-in of all, a detailed look at a region of the third. The smallest features to be discerned in the last one (mineral grains) are a fraction of millimeter across. Thus, the whole “nested” suite visualizes this place over six orders of magnitude.

In the first, you can see the beach, nearby glacial erratics, as well as some features of coastal geomorphology:
Link GigaPan by Callan Bentley

Now we’re down on the beach, checking out the wall-like sweep of outcrop. Can you find three examples of boudinage?
Link GigaPan by Callan Bentley

Here’s my favorite example of boudinage from the site, located about in the middle of the previous image. Here, you can see it’s a “double” layer of amphibolite that’s been stretched. How would you characterize the boudin ends?
Link GigaPan by Callan Bentley

Finally, we can examine very small scale features associated with the “stern” of the boudin, and the boudin neck in its “wake”. In particular, you can see the “fish mouth” shape to the boudin’s end, and see that it’s “mouth” is full of pegmatite – crystallized evidence of the fluid rich low-pressure zone the formed here when the amphibolite layer separated into chunks.
Link GigaPan by Callan Bentley

Whether you’re more into the geomorphology or the petrology (or the structural geology!), there’s something in this quartet to hold your attention. I hope you enjoy exploring them.

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8 July 2016

Friday folds: Kinkell Braes, Scotland

When I took you on a virtual field trip to Kinkell Braes earlier this week, I didn’t mention that the sandstones are folded there, now did I?

Let me remedy that omission now:

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That is a plunging anticline that you could actually take a plunge into:

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And here’s a syncline to match.

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Happy Friday. Hope your week was a good one, and that your weekend is even better.

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7 July 2016

Tombolo at St. Ninian’s Isle, Shetland

I came to Shetland for the rocks – but I’ve been surprised and delighted by the huge range of interesting coastal geomorphology to be seen here too. I’ve never seen so many sea stacks, wave-cut cliffs, and bayhead bars in my life.

One that is so “classic,” so “textbook” that I couldn’t resist it, is the tombolo that connects St. Ninian’s Isle to mainland Shetland.

In Google maps, it couldn’t be more plain:

I shot two GigaPans there, one from each side of the structure:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

I also made a Theta 360 spherical photo from the middle of the tombolo:

Tombolo @ St. Ninian’s Isle, Shetland #theta360 – Spherical Image – RICOH THETA

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4 July 2016

Virtual field trip to Kinkell Braes, Scotland

Walking along the shore east from St. Andrews, Scotland, along the seaside sandstones of Kinkell Braes, you encounter several extraordinary examples of geology. It’s a great place for the next stop on our Grand Tour of the geology of the British Isles.

Here’s the scene:

The first stop is a giant eurypterid trackway, potentially the largest invertebrate trackway in the world (Whyte, 2005), on the underside of an overhanging sandstone slab that’s only accessible at low tide. Even then, you can slip on the many slimy seaside rocks, something I can personally attest to having done twice during my visit. The trackway features multiple sets of sea scorpion footprints, three on each side of the critter’s body, plus a long central groove made as its telson dragged through the sand:

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I made two GigaPans of the trackway:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

And here are two Theta 360° spherical photos of the outcrop:

Giant eurypterid trackway, Scotland – Spherical Image – RICOH THETA

 

Giant eurypterid trackway,  Scotland – Spherical Image – RICOH THETA

 

Those should convey a sense of how difficult it was to image this thing – but once again, the 3D model comes to the rescue. Using a photo set I shot on the site, lying on my back on the slimy rocks, my student Marissa used Agisoft Photoscan to make this:

Photoscan model by Marissa Dudek

That’s the next best thing to being there!

Now, that trackway alone is probably enough for one day, but if you were to continue your stroll around the next major headland, Kinkell Ness, you’d see something else that’s quite remarkable:

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Let’s make this the second stop on our virtual field trip. Here’s a Theta 360° spherical photo of the scene:

The Rock and spindle: Permian volcanic vent complex intruding Mississippian sandstone, Kinkell Ness, Scotland #theta360 – Spherical Image – RICOH THETA

The prominent sea stack is a feature called the Rock and Spindle – it’s a resistant bit of a volcanic vent complex that intruded through the sandstones in the late Carboniferous or early Permian. In the photo above, you can see the light-colored stratified sandstones (with some shale) in the foreground, and the relatively dark volcanic rocks (basalt and agglomerate) starting about halfway along the bay.

The Rock and Spindle is a glorious, albeit phallic, thing:

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The most striking thing about it is the completely radial columnar jointing seen at the base:

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That’s just amazing – a beautiful set of columns, indicating a circular cooling front – perhaps originally a cylinder shaped intrusion or a dome-like hemispherical bulge.

Here’s a GigaPan of the Rock and Spindle:

Link GigaPan by Callan Bentley

The beach east of Kinkell Ness is a great place to look at intrusive relationships. In 1867, Archibald Geikie brought Charles Lyell and Roderick Murchison here to show them these great plutonic relationships. There’s plenty of agglomerate to be seen immediately adjacent to the Rock and Spindle, as these images show:

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Hand sample of the agglomerate:

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Some contains xenoliths of sandstone – the same sort of sandstone that preserved the eurypterid trackway:

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Here’s a Theta 360° spherical photo of this outcrop:

Sandstone xenolith in volcanic rock, Kinkell Ness, Scotland #theta360 – Spherical Image – RICOH THETA

 

…And (why not?) a 3D model of it, too… scooting in underneath will allow you to see the bedding plane of the sandstone.

Photoscan model by Marissa Dudek

But a short distance away from all this volcanic mayhem are outcrops of the staid sandstone (and shale) with which we started the trip:

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There’s a low cliff showing the contact between Permian volcanic agglomerate (top right) and Mississippian sedimentary rocks (lower left):

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I made a GigaPan of this instructive outcrop, of course:

Link GigaPan by Callan Bentley

One thing that’s clear in that outcrop is that the sandstones and shales were deformed a bit by the intrusion. I found some sweet examples nearby of small faults offsetting sedimentary layering, which I provisionally interpret as resulting from the stresses of the magma pushing its way through, kind of like the situation we observed last week in Dunbar with a different package of sandstone.

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I found similar small conjugate sets of wee faults in another nubbin of sandstone, but here they were offsetting a small volcanic dike – probably synchronous with the intrusion – the crack opening up along the faults like a set of interlocking teeth in a monstrous maw:

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The same sandstone outcrop showed a beefier dike cutting though it (left to right through the middle of the image). It’s a bit hard to see because it’s been preferentially eroded away…

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Here’s a good use of the Theta 360° spherical photo medium – get your perspective into that crack and see how it’s eroded out along the dike:

Permian volcanic dike intruding Mississippian sandstone, Kinkell Ness, Scotland #theta360 – Spherical Image – RICOH THETA

All told, you could do worse than to lace up a sturdy pair of virtual boots and follow in the footsteps of Geike, Lyell, and Murchison to explore this exemplary place. Thanks for joining me on another virtual field trip!

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1 July 2016

Friday fold: Clarely, an anticline

Eric Pyle sent in today’s Friday fold –

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Eric reports that you can find this fold seaside, near Poulatedaun, Co. Clare, Ireland.

Thanks for sharing, Eric!

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30 June 2016

Virtual field trip to Siccar Point, Scotland

Time for another virtual field trip on the Geologist’s Grand Tour of the United Kingdom: the most famous outcrop in the world. Today, we visit Siccar Point, Scotland.

You’ve probably already seen photos of this place – they usually look something like this:

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To those who aren’t familiar, here’s what going on:

There are two sets of strata here – and the contact between them is an ancient erosional surface. It’s an angular unconformity, the very first one identified — anywhere, ever.

When James Hutton brought his friend John Playfair here in 1788, he explained the significance of such a structure: It implied that (1) Silurian graywackes and shales were deposited horizontally, (2) they were subsequently folded and tilted to more or less vertical orientations, (3) a period of erosion commenced, chewing away at their upper ends, (4) erosion ceased and deposition resumed, but now the environment of deposition had totally changed – instead of deep marine strata, oxidized fluvial redbeds were laid down (the Old Red Sandstone) — again in an original horizontal orientation, then (5) these too were tilted to their present angle gently dipping off to the left of the frame, and (6) modern erosion went to work on the whole mess, carving it up into the three-dimensionally complex exposure we find today.

Hutton pointed out that this whole sequence would probably take a little while. Playfair recounted the sense of temporal vertigo Hutton had induced in him: “The mind seemed to grow giddy by looking so far into the abyss of time.”

Siccar Point, then, was the place where the immense depth of geologic time was first realized. It was the first time it was felt!

I got into the spirit of the place by putting myself in the place of (A) the turbidites, (B)  the ancient erosional surface, and (C) the red beds:

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But there’s much more to Siccar Point than just this one oft-photographed scene.

Just walking in to the site is a treat – looking both ways along the coast, you can see the relevant units – with Old Red Sandstone dominating to the northwest, and Silurian turbidites to the southeast:

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The first view of Siccar Point itself comes from a great height:

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I paused there to capture the view on GigaPan (see the resulting image later in this blog post):

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Then, we descended. As you can see, we had awesome weather – but this slope was so steep, next to a knee/chest level strand of rusty barbed wire, that I shudder to imagine descending it on a rainy day:

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Because the modern waves have sculpted Siccar Point into a complex shape, there are a dozen ways to view the unconformity, dipping down below it in places (entering the upended Silurian) and popping back up again, like a time-traveling gopher (daylighting into the Devonian).

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The unconformity surface is laced with boulders and cobbles – the rubble of the eroded Silurian graywackes that became a basal conglomerate for the overlying red beds.

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There is some tafoni to be seen, too – always a treat!

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Another thing you can see in the previous image is orangey-yellow lichen, of which there is quite a lot at Siccar Point. I was surprised at this – even though the two rock units here are very distinct in color, both actually appear similar because of all the lichen encrusting them!

In fact, there is very little “clean” rock exposed here – if it’s too high up, it’s covered with lichen, and if it’s too low in elevation, the tides bring nutritious waters up twice daily, meaning it’s prime for colonization by barnacles. In between, you might find some decent views into fresh rock. But don’t hammer! This is as close to sacred ground as geologists get! Here’s a view showing the vertical zonation:

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In the thin zone where the rocks are clean, you can see features such as graded bedding. Here, a single turbidite is imaged, with the coarse base on the left, and then fining (younging) toward the right, where the lens cap lies atop some thin laminations:

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Here’s another, with the opposite geopetal implication:

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It gave me a little frisson of satisfaction to walk out to the hinge of the plunging syncline I featured here as a Friday fold back in February:

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Here’s a view of the same fold, from the other side of that little bight — You can see the nested “>>” shape of the barnacle-encrusted turbidite layers, in this case plunging to the lower-left:

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Fractures in the upturned Silurian graywackes give the clean rocks a tesselated appearance:

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And not all the fractures are joints – some are faults!

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So there’s a lot of fun stuff to contemplate here. To provide a user-directed facsimile of a real-life visit to Siccar Point, I gathered some visuals for you. Here are four GigaPans of the site:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Next up, here are four Theta 360 spherical photos:

Planet Hutton! Siccar Point unconformity, Scotland #theta360 – Spherical Image – RICOH THETA

 

Siccar Point unconformity, Scotland #theta360 – Spherical Image – RICOH THETA

 

Siccar Point unconformity, Scotland #theta360 – Spherical Image – RICOH THETA

 

And then —our coup de grâce— we have two awesome 3D models of the unconformity surface – one zoomed out a bit:

Photoscan model by Marissa Dudek

…And closer in on a key area:

Photoscan model by Marissa Dudek

Honestly, I think these two 3D models (photo sets by me, model construction in Agisoft Photoscan by my student Marissa), are one of the most important things I have accomplished so far on this trip. Because Siccar Point is a wave-blasted headland, the outcrop surface is a complicated shape, and that makes photographing it complex. Essentially, there’s one view that easily captures the relationship between the two rock units, and that’s the one that everyone takes – the view that started off this post. But Siccar Point is bigger than that one view, and I think it takes something like these 3D models to convey a real understanding of the complicated geometry of the place – an important step for inducing the “giddyness” that John Playfair and James Hutton enjoyed while visiting the site.

I feel extraordinarily fortunate to have been able to visit Siccar Point with my family on such a beautiful day. I hope that you are able to access some of the giddy joy I felt there with this virtual field trip!

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29 June 2016

Trace fossils in sandstone from Barns Ness

Check out this sandstone cobble I saw at Barns Ness – it comes bearing gorgeous trace fossils. Can you spot them?

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Lens cap for scale in all these photos. The next three are close ups of the burrows from the previous image:

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Plus two more, from other cobbles I encountered::

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28 June 2016

“Dunbar marble” at Barns Ness, Scotland

Thanks to the website ScottishGeology.com, run by Angus Miller, I learned of Barns Ness, a Mississippian-aged limestone fossil site on the shore not far from where we are staying at Dunbar. We ventured out there on Saturday afternoon, in search of fossils.

The presence of the Dunbar Cemenet Works nearby is an indication that this is the most extensive limestone outcrop in central Scotland.

I set my field assistant loose on the beachy outcrops to find fossils:

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The first one we spotted was a familiar sight from the shales back homeZoophycos, a feeding trace fossil:

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Then, as we worked our way down-section, the crinoids started showing up:

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Other, meatier traces also appeared:

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We spied a few large cross-sectioned shells – though I’m not sure quite what they would be. Maybe massive brachiopods?

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Also, there were corals. Very nice corals.

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A few more examples:

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Finally, we found what, for me, was the object of the trip – the “Dunbar Marble,” a limestone (not a marble) that was quarried as a building stone (in particular, for fireplaces) because of its lovely super-rich content of rugose corals, which are the white bits shaped like little cornucopias:

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Here is the cleanest, clearest face I could find showing a cross-section of this extraordinary bed:

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All of the preceding examples of the “Dunbar Marble” have been battered a bit – exposed more through the action of physical weathering (think regular poundings by pebbles and beach sand) than chemical weathering (slow dissolution of the carbonate, a bit faster in some spots and a bit slower in other spots). But this example that my wife found bucks that trend. Click on it to see the fine details of the segmentation and septae in the corals. Bonus: there’s a modern limpet shell to examine, too:

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Here are three GigaPans of the outcrop, which as you can see is half buried in beach sediment, if you want to explore them a bit:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

While the GigaPans were shooting, I went for a stroll across the tidal flats. Heading for a prominent outcrop in the middle distance, I hopscotched my way accross a lumpy surface that looked stromatolitic:

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I couldn’t detect any convex laminations on that surface, but since the tide was coming in, I also didn’t spend much time on the task. Another possibility is that these lumps and hollows represent original features in the bedding – perhaps they are associated with tree roots, according to the pamphlet linked to above. However, I did spy a boulder that had some dome-ish looking internal layering:

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If I’m right that these are stromatolites, then the sample is “upside down” in these photos – the dome would bulge upward toward the sun in life position. Stromatolites are thus geopetal (“way-up”) indicators.

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Elsewhere, I found very crinkly little CaCO3 features, which didn’t quite appear regular enough to qualify as stromatolites, but I wasn’t really sure what else to classify them as. They didn’t appear to have any kind of regular anatomy like I would expect from an invertebrate animal:

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On a few of those, I see a shape and internal anatomy that suggests articulate brachiopods, as well as slightly-lighter colored internal sediment, suggesting a hollow chamber. But elsewhere (like the lowermost left), it looks like a layered repetitive crinkly mass.

I guess it could be more than one type of fossil. Anyhow: the tide’s coming in! Keep moving!

I arrived at my target outcrop, expecting it to be the weathering face of a tough bed, but instead I found a wall-like tabular projection. It’s a fault – weathering out in positive relief!

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The fact that the fault weathers out in as a “wall” is plain if you look at Barns Ness from the perspective of Google Maps. It’s the feature that strikes northeast-southwest across the middle part of this view. Zoom in and you can see it casting its shadow:

Close-up, it shows lots of brecciation:

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Fluids percolating along the fault cemented these fragments back together, and indurated the fault trace, making it more resistant to weathering than the neighboring host rock.

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The rising tide cut off my retreat along the path I had just trod, so I headed inland, and found that beneath the fossiliferous strata was a recessively-weathering shale (aren’t they all?) and at its base a thin coal seam:

 

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This site, then, is a nice exemplar of the many small transgressions of sea level during the Carboniferous that buried coastal swamps (bayous) under offshore marine sediments, condemning the plants to become coal. Groundwater flow seems to percolate through the limestone and sandstone well enough, but finds the shale an aquitard, resulting in some small effusive springs above the beach:

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All told, Barns Ness had much to hold the geologist’s interest – and I’d spend more time here if I had it.

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27 June 2016

Small faults in upper Old Red Sandstone, Dunbar, Scotland

Dunbar, Scotland, is a nice little seaside town that also happens to be the birthplace of the conservationist John Muir. My family and I have been based out of here this week on our European geological GigaPan expedition. But on our first morning, upon visiting Siccar Point (which is nearby), I threw out my back, and spent most of the next two days recuperating. I did manage a short walk to the beach in a cove on the northwest side of town.

My son instantly keyed in on the puddles, and took his pet centipede there to play in the water:

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The clean exposures of the bright red sandstone there caught my attention, and I then did my own ‘keying in’ – this time to the many small offsets in the primary sedimentary layering. They are exquisite!

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According to the local geological map, these rocks are the Devonian- Mississippian upper portion of the Old Red Sandstone – the same stuff that caps the unconformity at Siccar Point.

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It was apparently quite disturbed by the intrusion of a collection of igneous masses at ~345 Ma. Many of these small faults looked to me like soft-sediment deformation, so I don’t know that I would have invoked volcanic molestation if I hadn’t read about it, but regardless, they are a delight to behold. I think the primary reason I fell in love with structural geology is that structures like these are just plain beautiful.

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