Science File -

An unconformity in geology is a boundary between two sets of rock where some of the 'history' is missing – the layers do not match up. In this case, the schists (hard metamorphic rocks) that make up the land near Lochranza are tilted, so much that they are not far off vertical. On top of these schists lies a layer of sandstone that is almost horizontal. I knew roughly what I was looking for, but it took me a while to be sure that what I was looking at really was the unconformity that helped James Hutton to formulate his theories about the Earth in the late 18th century. The rocks are there for all to see, but need to be looked at closely and with some knowledge to appreciate what they are telling us. 

There were problems with these ideas, however, even before Hutton and his unconformities. In Italy Steno was looking at sharks' teeth and other fossils found on mountain tops and deciding that if it looks like a shark's tooth, and still looks like a shark's tooth when you cut it up and examine its internal structure, then it probably was a shark's tooth and not some formation in a rock that happened to resemble a shark's tooth. Here was one of the first suggestions that the Earth might have a much longer history than the biblical account of creation suggested. 

Hutton's unconformity backed up this idea that the earth must be far, far older. Our explanation for unconformities like this are that the schists were metamorphosed from earlier rocks by heat and pressure far beneath the surface of the Earth. They were then subjected to earth movements that tilted them and they became exposed at the surface of the earth by the weathering and erosion of the rocks above them – or perhaps the exposure happened before the folding. Once at the surface, they must themselves have been eroded to provide the flattish surface on which the sandstone was subsequently formed. Some sandstones are formed on the surface from desert dunes, but others are formed on and beneath the sea bed when sediments are deposited. The rocks on Arran must therefore have sunk beneath the sea again (or the sea rose to cover them), which allowed sand to be deposited, buried and cemented to form rock. As we now see these rocks just above sea level, further earth movements must have taken place to uplift them and to erode the rocks or sediments that must once have been above them. The two unconformable layers of rock therefore suggest a lengthy history indeed. 

All was not quite so straightforward for Hutton, however. At the time it was not even known that rocks such as sandstone were formed from sediments. Many other observations made by Hutton helped to formulate our current ideas of rock formation, with granites and similar rocks being the frozen remains of molten bodies of rock, and sedimentary rocks being laid down usually beneath water. He developed the 'uniformitarian' idea, which held that the processes we see on the surface of the Earth today are the same ones that were responsible for shaping the rocks and the landscape – the unending cycles of erosion, consolidation and uplift. The opposing view was that changes to the Earth were all caused by 'catastrophes' such as earthquakes and volcanoes. Hutton had developed these ideas over some years, and the unconformity near Lochranza was one further piece of evidence that his ideas could explain how our current landscape was formed.

Hutton found other, similar, unconformities; one near Jedburgh in the Scottish borders, and the most famous one at Siccar Point, on the coast east of Edinburgh. He published books with his ideas and evidence, but his ideas were not immediately accepted. Nearly 30 years after his death another geologist, Charles Lyell, took up his ideas and built upon them with yet more evidence, and his views were the main influence on geology for some time. Lyell was a strict uniformitarian, denying any role at all for sudden events, but in fact both ideas have merit. You only have to look at a flash flood or the self-destruction of a volcano to realise that catastrophic events do have a part to play. Even glaciers, slow by our human standards, should probably count as a fast-moving 'catastrophic' mechanism for erosion when compared to the time taken for most geological processes. 

So Hutton's unconformity is a fine example of science as we know it – careful observation leading to an idea, the combining of this observation with many others to formulate a hypothesis, which can then be tested by further observation. Hutton even attempted experiments to test his ideas about the formation of igneous rocks, but at the time the apparatus was not available to approximate the temperatures and pressures required. 

Well, not quite. As far as can be gathered from records, Hutton's reasoning went something like this: The Earth must have a purpose, and what better purpose than to support life. But if the Earth was designed for this, it was not a very good design because bits of it keep wearing away and the soil necessary for life, although formed by weathering and erosion, is also washed away and destroyed by the same processes. If these processes continued the Earth would eventually become uninhabitable. Perhaps the design had included the capacity for repair? From the idea that the purpose of the Earth is to support life, Hutton deduced that there must be some means of repair and set out to gather evidence to support this view.  

The Seashell on the Mountaintop, by Alan Cutler 

 (The last two contain a lot more geological detail, for those interested.)