No Vestige of a Beginning…

Not only does the wee Isle of Arran record a mighty geological history, its rocks are famous for its role in many important discoveries in the history of geology. In the late 18th century, the widely accepted theory for the origin and evolution of the Earth was that proposed by Abraham Gottlob Werner (1749-1817) of Freiburg, in Germany. According to Werner, all rocks had been deposited or crystallized in a few thousand years from an ancient, receding ocean that had originally covered the entire globe. This cleverly kept his geological theory onside with the Church and the Biblical idea of a Noah’s Flood. Werner hypothesized that all rocks had been laid down in a continuous and systematic order, with the oldest at the bottom and the youngest forming on top. Coal deposits burning underground were hypothesized as the source of heat in erupting volcanoes! Because of its reliance on a great ocean, Werner’s theory soon became christened ‘Neptunism‘, after the Roman god of the sea, Neptune.

Figure 1. Geological map of the Isle of Arran.
Figure 1. Geological map of the Isle of Arran.

But, by the end of the 18th century a rival hypothesis for the origin and evolution of the planet had been proposed. James Hutton (1726-1797) was an Edinburgh doctor and he also ran a farm in Berwickshire on the coast east of Edinburgh. When he looked at the landscape around him, Hutton realised for the first time that we live on a dynamic and ever-changing planet. Hutton saw that mountains were being actively weathered and eroded away. The resulting products were carried away to the sea where they would eventually be dumped as sheets or layers of sludge on the seafloor. Over time, younger layers would be deposited on top of the older ones underneath, gradually burying them deeper and deeper into the Earth’s crust. The sludge would harden into rock and then somehow this rock was pushed and squeezed back up to the surface again to form new mountains. If this ‘rock cycle’ didn’t happen, then Hutton hypothesised that there would be no mountains left as the whole surface of the Earth would have been worn away. This recycling of the rocks which form the Earth’s crust meant that, as Hutton famously wrote later, he could see, ‘no vestige of a beginning, and no prospect of an end.’

Hutton published his ‘Theory of the Earth’ as a brief abstract in 1785, but it attracted a lot of criticism from the Neptunists. It was, after all, just a nice idea and what Hutton needed was to find hard evidence to back-up his claims. In particular, he needed a mechanism by which the rocks buried beneath the seafloor could be uplifted to the surface again to form new mountains. Hutton had developed a hunch that the mechanism for uplift might be found in the nature of granite and how it was formed, often at the centre of mountains, and he set off around Scotland in search of field evidence to support his theory. He investigated granite outcrops in Glen Tilt in September 1785 and on the SW coast of Scotland during September 1786.  Finally, Hutton and his friend and artist, John Clerk, arrived at Ardrossan Harbour in the summer of 1787, and seeing Arran’s Northern Granite, realised that the island might provide the key evidence that he was looking for. He was to write later his expedition to Arran: ‘In setting out upon that expedition, I had but one object in view; this was the nature of the granite, and the connection of it with the contiguous strata. But upon examining the island, I have found it sufficiently interesting and comprehensive to make it the subject of a natural history…. Let us therefore, see how far the natural history of Arran shall be proper to try the Theory of the Earth.’

Figure 2. Hutton’s cross section through the North Arran granite and metasedimentary country rocks. Water colour by John Clerk of Eldin.
Figure 2. Hutton’s cross section through the North Arran granite and metasedimentary country rocks. Water colour by John Clerk of Eldin.

Hutton made two discoveries on the Isle of Arran of such fundamental geological importance that they would change forever our understanding of geological processes. In North Glen Sannox, the somewhat apocalyptic valley where many undergraduate students cut their teeth mapping, Hutton finally discovered a well-exposed contact between the granite and the Dalradian schists.  This outcrop was the key piece of evidence he was looking for, as he had finally found unequivocal field evidence to disprove the Neptunists’ theory that granite formed from precipitation out of water: “Nothing can be more evident than that here the schistus had been broken and invaded by the granite; as in this place the regular stratification of the vertical schistus is broken obliquely by the other rock …” (Theory of the Earth, V 3, p. 221).  His second major discovery was on the north shore of Arran by Lochranza. Here the Dalradian metamorphic rocks have been tilted until they were nearly vertical, and are overlain in a structure we now call an unconformity by Carboniferous sandstones. The unconformity near Lochranza is actually the first unconformity discovered by Hutton, even though the more photogenic example that he discovered subsequently at Siccar point is more famous.

Figure 3. Hutton’s Unconformity near Lochranza. The Dalradian schists dip steeply towards the left of the photograph, the overlying Carboniferous sandstones dip shallowly to the right.
Figure 3. Hutton’s Unconformity near Lochranza. The Dalradian schists dip steeply towards the left of the photograph, the overlying Carboniferous sandstones dip shallowly to the right.

From his Arran fieldwork, Hutton felt he had discovered the evidence needed to demonstrate that the Earth was hot inside, and hot enough to melt rocks if they were taken deep enough. He concluded that this internal heat in the planet must bend and twist the rocks back up to form new mountains. Whilst we would now invoke a different mechanism for mountain building, Hutton was right about the Earth being hot inside and this coupled with his ideas of a dynamic, recycling planet were enough to change the way people thought about Earth for ever. Since heat deep in the planet was so crucial to Hutton’s theory it became known as ‘Plutonism’, after the Roman god of the underworld, Pluto.

By Dr. Chris Nicholas, Trinity College Dublin

References

http://www.scran.ac.uk/packs/exhibitions/learning_materials/webs/43/index.htm

http://www.arranmuseum.co.uk/what-will-you-see/geology/the-geological-history/

Clark, N. D. L. and H. Corrance, H. (2009) New discoveries of Isochirotherium herculis (Egerton 1838) and a reassessment of chirotheriid footprints from the Triassic of the Isle of Arran, Scotland. Scottish Journal of Geology 45, (1), 69–82.

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