Arran (geological time)


Thank you to Dave for sending in a card from one of Geography Cat’s most favourite places, the Isle of Arran, off the south-west coast of Scotland.

Arran is sometimes called “Scotland in miniature” because it has both highland and lowland areas within its 167 square miles. This happens here because Arran is cut through by the Highland Boundary Fault which is essentially a large crack across the surface of the crust which stretches right across Scotland, east to west.

The north of Arran is hard igneous rock, while the southern half is sedimentary layers which have been shot through with igneous intrusions. That means that the sedimentary rock layers were already there when there was some tectonic activity below them which forced magma between the layers, like squirting jam into a doughnut. The photograph below shows an excellent example of this; Drumadoon Sill, this is around 30m high and was formed by magma being forced between layers of sedimentary rock. The top layer has since been eroded away exposing the vertical columns of tough igneous rock:


Drumadoon Sill Photo: A Ball


The island has also been glaciated (covered in glaciers and eroded by them) several times since then, most recently in the last Ice Age which ended around 10 000 years ago. The glaciation changed the shape of the land by eroding less resistant rocks more (such as those sedimentary ones) and leaving more of the harder igneous ones. The glaciers also dumped loads of material as they melted when the climate warmed up, such as the Cat Stone, shown in the photo below. The Cat Stone is a granite boulder which was torn from its original location by a glacier and dragged along until the ice melted and the rock was dumped here:


Credit: Paul Birrell / Clach a’ Chash (Cat Stone) Arran


So, with all this variety in such a small space, it is easy to see why some people consider Arran to be a “geologist’s paradise”; there’s certainly a lot to study here, and it’s no coincidence that hundreds of geology and geography students visit the island every year.

There’s one place on Arran that Geography Cat particularly adores, and that is the site known as Hutton’s Unconformity:


Hutton’s Unconformity Photo: A Ball


James Hutton (1726-1797) recognised something in this scene that seemed not to conform to what was known at the time about the formation of rocks and the age of the Earth. You can see it for yourself in the following version of the same photograph:



The green lines show the layers of sedimentary rock which were formed around 520 million years ago. At the time those rock layers were horizontal. Tectonic movements twisted the layers up into the angle they lie at now, and then something eroded them before the rock layers with the pink lines were lain on top, approximately 360 million years ago. The intervening 160 million years must have been the time it took for the original layers to be crumpled up and eroded back to a level surface for the deposition of river sediments on top that were to become the upper layers of rock.

This all seems perfectly reasonable now, but when James Hutton saw this in 1787 most estimates of the age of the planet were in their thousands or a few million. Only Buffon de Maillet came anywhere close with his theory that the Earth was around 2 billion years old in the 1720s, but his work was written under a pseudonym and he struggled to get it published as it seemed so incredible.


james hutton
James Hutton, the father of modern geology Source:USGS



In the late 1800s the age of the Earth was still one of the most hotly debated topics amongst scientists and others. The “knowledge” we have today (that the Earth is 4.5 billion years old) has been pieced together like a jigsaw, from the work, experiments and observations of many. James Hutton’s realisation that the rocks on the Newton Shore of Arran must have taken much longer to form than a million years or two didn’t come to him in a lightbulb moment, it was by looking and thinking and examining, and more thinking, and discussing, and looking again that drew him to his conclusions.

And that’s why Geography Cat loves to see Hutton’s Unconformity. It is evidence, not only of the immensity of geological time but also of the way in which scientific knowledge evolves, with the patient work of many parties who are open to possibilities but careful in judgment. The “knowledge” we have is a collection of fact, theory and perception that is constantly being challenged and refined, just as the rocks on the shore are continuously battered by the sea and ground down into sediment again.



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