Glacial deposits new and old in the Scottish isles

A post by Chris RowanI’ve just spent the last few days travelling around western Scotland, with the furthest point I visited being the Scottish island of Islay.

On the ferry to Islay

Islay is probably most famous as a whisky powerhouse; it is currently home to eight active distilleries, the products of which have a peaty kick that can warm the coldest of palates (I’m quite a fan).

Like most of Scotland, Islay has been strongly shaped by the last ice age – indeed, the channel that separates it from the mainland was carved by a glacier. In many places the bedrock is covered by a thick layer of glacial till, a poorly sorted mixture of ‘rock flour’ produced by the grinding action of moving ice, and larger pieces of rock carried within the glacier itself, all deposited and jumbled together as the ice sheets melted 10-20,000 years ago.

Islay till

Glacial till, Islay, W Scotland

It’s quite interesting to look at these recent deposits and compare them to the much, much older glacial units that I’ve been studying in Oman (image below). At least at an outcrop scale, they do look pretty similar, even if the Omani diamictites were probably deposited in a marine environment rather than a terrestrial one. “The present is the key to the past”, indeed.

However, it turns out that Islay has an even more direct link with the glacial deposits of Oman. As the road rises up from the ferry terminal at Port Askaig, you drive past an impressive roadcut.

Port Askaig Formation roadcut

A roadcut through the Port Askaig Formation on the road up from the Ferry Terminal

The unit exposed here, and along the coast to north of the Ferry Terminal, is the Port Askaig Formation. This is admittedly not the most whimsical of names, but the formation itself is actually rather intriguing, because it is also a diamictite – a poorly sorted sediment with large clasts of granite and other rocks embedded in a fine-grained, silty matrix.

Large granite clast (and others) in Port Askaig Formation

Boulder in the Port Askaig Formation

Boulder in the Port Askaig Formation, with Chris for scale

The Port Askaig Formation is quite poorly dated, but dating of the granitic basement that it rests on, and some overlying volcanic formations, tell us that it is somewhere between 600 and 800 million years old. So, the late Neoproterozoic in Scotland was a time of cold and glaciers – just like in Oman. In fact, the Port Askaig and its correlative units in other parts of Scotland and Ireland were possibly the first evidence of Neoproterozoic glaciations to be recognised. If the Snowball Earth theory can be said to have had a birthplace, it could well be here.

Cool geology, with a whisky to finish: what more could you ask for from a holiday?

Categories: deep time, geology, outcrops, past worlds, photos, Proterozoic, rocks & minerals
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Comments (9)

  1. Bob O'H says:

    I’ve decided I’m going to be careful about agreeing to share a car with you. If it’s anywhere near outcrops, it coulsd be aslow (albeit interesting) journey.

  2. KJHaxton says:

    Oh see Bruichladdich would be my choice! Lets face it, that’s almost as hard to pronounce as Allochthonous :) Other than that, I agree with Bob, it would be hard to get anywhere fast on a road trip with you jumping out the car at every outcrop.

  3. It’s worse than you imagine. He picks his itineraries based on interesting rocks! :-)

  4. Andrew Alden says:

    I’ve only Bowmore in the house at the moment, but that’s good enough to toast your fine post!

  5. Lab Lemming says:

    “So, the late Neoproterozoic in Scotland was a time of cold and glaciers – just like in Oman.”

    Whoah there. Saying that a 200 million year interval contained a few thousand years of glacial deposition is hardly earth-shaking. You might as well equate the Devonian and Permian glaciations.

  6. Lab Lemming says:

    On the other hand, the Devonian and Permian could be considered part of the same ‘glacial event’ in that they were both casued by parts of Gondwana being close to or at the south pole (ditto carboniferous, of which I know almost nothing). But being able to make that statement requires good geochron and paleogeography.

  7. Passerby says:

    Some authors might not agree with your conclusion.

    >The thick succession of diamictite interbedded with current-deposited sandstone preserved within the Port Askaig Formation is not consistent with deep freeze conditions proposed by the snowball Earth hypothesis.

    Neoproterozoic environmental change recorded in the Port Askaig Formation, Scotland: Climatic vs tectonic controls. Arnaud and Eyles (2006). Sediment. Geol. 183: 99-124.

    A distillery tour and a few days spent looking over Islay geology would make for an excellent field trip to Scotland.

    Bruichladdich would be my preference, too. A tad less bite than the southern Isle brands.

  8. Lab Lemming says:

    If Arnaud and Eyles (2006) interpretation of a tectonics-dominated lower succession is correct, then it might be worth running the matrix detrital zirocns, on the off chance there was concurrent volcanism or unroofing of synorogenic batholiths…

    None of which were seen by Cawood et al. (2003). Oh well.

    Journal of the Geological Society; 2003; v. 160; issue.2; p. 231-246

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