6 April 2014

Edinburgh: Arthur’s Seat, Salisbury Crags and Hutton’s Section

Posted by Jessica Ball

Arthur's Seat from Holyrood Park

The Arthur’s Seat volcano in Holyrood Park

On the last day of my visit to Scotland, my advisor and her husband (both former UB volcanology folks) took me on a hike to Holyrood Park to visit Arthur’s Seat, Edinburgh’s volcano. The volcano we see now is tilted about 25 degrees to the east, but originally formed in a tensional stress regime on a continental margin about 350 million years ago. What’s left of the volcano is about 13 different lava flows, agglomerates from the crater, some intrusive sills and dikes of basalt.

But there’s more in the park than just the volcano. As I mentioned in the previous post, there’s a locality that’s truly worthy of a geologic pilgrimage: Hutton’s Section.

Hutton's drawing on this informational plaque is just a LITTLE off scale...

Hutton’s drawing on this informational plaque is just a LITTLE off scale…

In 1788, James Hutton, an Edinburgh scientist and philosopher, published his Theory of the Earth, which suggests that present-day geologic processes operated the same way in the past and can be used to interpret what we see in the rock record. He based this and his other conclusions partly on observations he made at the small quarry above, located at the base of the Salisbury Crags (a dolerite sill). The sill that forms the crags intruded about 25 million years after the Arthur’s Seat volcano ceased erupting, disrupting the sandstone layers that underlie the area. At Hutton’s section, you can not only see where the sandstone layers were ripped up by the intrusion, you can also see some contact metamorphism where they were ‘baked’ by the hot basalt.

In discussing this section, Hutton said that

“We know that the land is raised by a power which has for its principle subterraneous heat, but how that land is preserved in its elevated station, is a subject which we have not even the means to form a conjecture.”

He suggested that molten rock under pressure could intrude across or between sedimentary layers and sometimes reach the surface as lava flows, a conjecture that’s played out in both the Salisbury Crags and in the Arthur’s Seat volcano, whose feeder dikes intrude the same sandstones.

Dr. Caco explaining Hutton's reasoning

Dr. Caco explaining Hutton’s reasoning

The ripped-up bit is very dramatic for a geologist, although considerably smaller in scale than the illustration found on the informational plaque, which is an etching by John Clerk of Eldin for the book:

Scale is very important!

Scale is very important! Don’t let your professors catch you messing up a sketch like this.

And the real-life version:

The bent-up sandstone in question

The bent-up sandstone in question

The contact between ___ and the sandstone

The contact between the dolerite and the sandstone

In addition to the amazing geology, the park is a wonderful place for views of Edinburgh and the surrounding area. One of the tourist guides bills it as a ‘little slice of the Highlands’, although I was assured I hadn’t had the true Highland experience until I’d been out hiking in the wind and rain. It was just windy that day, so I suppose I’ll have to go back to Scotland and get properly rained on.

Looking down the ___ glen at the Firth of Forth

Looking north down the Hunter’s Bog at the Firth of Forth

The Salisbury Crags themselves are very impressive, although not the best choice for hiking when it’s especially windy.

From the top of the Salisbury Crags

From the top of the Salisbury Crags overlooking a quarry (a panorama, so while there’s a bit of an alcove here, it’s a lot straighter than it appears.

The edge of the crags, a sill of ____

The edge of the crags

Despite being covered in vegetation (including gorse, a deceptively evil bush that you should never, ever try to walk through), Arthur’s Seat has a lot of features that are easy to pick out from a distance. The lava flows that would have made up the early flanks of the eroded volcanic edifice are very clear, for example.

Tilted lava flows on the flank of the volcano

Tilted lava flows on the north flank of the volcano

The Edinburgh Geological Society has a great brochure with geologic and schematic maps of the area, which I’ve used to annotate this panorama and point out some of the lava flows and intrusions. (The British Geological Survey also has a 3D cut-out model of the volcano . I think I need this among the ‘deskcrops’ in my office, it’s so cute.)

Here's what the volcano might have looked like (with a few features labeled, courtesy of the Edinburgh Geological Society's "Discovering Edinburgh's Volcano" brochure (link).

Here’s what the volcano might have looked like (with a few features labeled, courtesy of the Edinburgh Geological Society’s “Discovering Edinburgh’s Volcano” brochure (link). Click to make everything readable!

It’s not a geologic field trip if you don’t stop in places with impressive views to do some arm-waving! My advisor and her husband have some plans in the works to take advantage of the fact that there’s a volcano practically in their backyard that hasn’t had a great deal of physical volcanology study done on it.

Looking out over Edinburgh (with appropriate weather)

Looking out over Edinburgh (with appropriate weather)

And being that Edinburgh is the home of many of geology’s pioneers, it kind of makes a nice circle that I had a chance to visit them there.

The wind got so bad I had to sit for this...

The wind got so bad I had to sit for this…

Three doctors!

Three doctors!

After getting our fill of the volcanic features, we made a special stop at the Camstone Quarry to look at the other rock type in abundance, and check out some beautiful sedimentary features.

My advisor amid the sedimentary features

My advisor amid the sandstone

This is a great location to remind you that the Arthur’s Seat volcano formed on a continental margin where coastal processes were still going on. There are lovely ripple marks and some fantastic ‘mud’ cracks in the sandstones here.

Who would have expected such great mud cracks at a volcano?

Who would have expected such great mud cracks at a volcano?

On the non-geologic side of things, my advisor showed me the gorse bushes growing on the volcano’s slopes. Gorse looks very lush and pretty and even has beautiful yellow flowers that smell like coconut, but it is the nastiest, prickliest bush you could possibly imagine. I couldn’t even hold onto a branch to get close to the flowers, it was so spiky.

Arthur's Seat from the 'back'

Looking south at the volcano, with lots of gorse.

Holyrood Park also has features for the archaeologist; this is St. Anthony’s Chapel, built in 1450 as a hermitage (a kind of isolated religious retreat). It’s supposed to be mostly made of basalt from the surrounding environs, and stands on one of the older lava flows in the sequence. It makes a lovely brooding ruin for photos.

____ chapel

St. Anthony’s chapel

On the hike back, we got to see a couple of the native birds having a battle over something. The pheasant won, and the crow eventually gave up and flew off. (We may have helped it on its way a bit, since we all expressed our indignance over seeing the harassment of the national bird of Scotland [apparently not the national bird officially].)

The national bird of Scotland fending off a crow

The national bird of Scotland A feisty pheasant fending off a crow. (“Feisty Pheasants” would be an awesome name for a rock band…)

On the way out of the park, we stopped at the southern end of the volcano to look at Samson’s Ribs, a beautiful exposure of columnar-jointed basalt. The guide says that this was an intrusion on the edge of the crater-fill agglomerate in the Lion’s Haunch Crater; if so, this marks one of the crater margins.

Columnar jointing on the flank of Arthur's seat

Columnar jointing on the flank of Arthur’s seat, with cars for scale.

A closer view

A closer view

From the base of the cliff

From the base of the cliff

More jointing

More jointing, with gorse (the yellow-flowering bushes).

Columnar jointing is always a good way to end a field trip, in my opinion!

Next in the Edinburg series will be bits and bobs from my walks around town, and the field trip Dr. Caco and I took to Rosslyn Chapel (yes, that Rosslyn!)