Hutton’s unconformity and the birth of ‘Deep Time’

I sometimes ask a question to students in an introductory class about geology: “What is the most famous geological site in the world?” For students from the western hemisphere, the Grand Canyon in the USA is a popular choice. However, if you were to ask the same question to a group of geologists, you might get a different answer, and one option is Siccar Point on the coast some 65km southeast of Edinburgh in Scotland. Although the site itself is relatively modest, a gently sloping platform of rock partly washed by the sea at high tide, and it lacks the spectacular grandeur of the Grand Canyon, the historical significance easily outweighs the lack of scenic drama. I’ve taken several groups of visiting geologists to the site, and so far only one of them has knelt and kissed the ground, but the site could be considered to be one of the ‘holy’ sites of our science.

Fig. 1. The steep slope that leads to the exposure seen from below, with a large group of students from the University of Edinburgh ascending. It doesn’t look too bad from this angle.

It is difficult for most modern geologists to imagine the world when any interpretation of the geological record had to be constrained by the literal interpretation of the Bible. A particular problem is the short timescale of the account of the creation of the Earth in Genesis, and the age of the Earth as calculated by Bishop Ussher, who allowed only some 6,000 years for the whole of geological time. The person who is frequently credited with expanding geological time to the ‘deep time’ we know of today is James Hutton. Hutton lived in or close to Edinburgh during the late 1700s, and he had realised that the geological record required vastly longer time periods than allowed for in the Bible. Convincing yourself is one thing, convincing other people is often trickier. Hutton wrote a book describing his ideas. Unfortunately, although he was apparently a good speaker, he was not a good writer and his book was pretty much universally ignored.

A boat trip in 1788 with two friends secured Hutton’s place in scientific history. Accompanied by James Hall, an Earl who carried out melting experiments on rocks, and John Playfair, who might today be described as an ‘opinion former’, a man that learned people listened to, they sailed along the coast deliberately looking for a particular geological feature, which we now term an unconformity. Hutton had previously visited the coast and knew that there were two main rock types – what we now call turbidites, which are deformed into sometimes spectacular folds, and a reddish sandstone or breccia, which is unfolded, although it is sometimes slightly tilted from the horizontal. What Hutton was searching for was the contact between the two – where the younger, undeformed sandstone or breccia rests on the eroded turbidites. Hutton explained this as evidence for cycles of sedimentation, mountain building and erosion in the evolution of the Earth’s crust – the folded turbidites are sediments from an earlier cycle caught up in a mountain building episode, with the sandstones and conglomerates being later sediments that were deposited on top of the roots of the eroded mountain belt. Playfair was so convinced by Hutton’s arguments that he later described how “the mind seemed to grow giddy by looking so far into the abyss of time”. Playfair later popularised Hutton’s theory by writing Illustrations of the Huttonian Theory of the Earth in 1802, which, as a well-written text, sold many copies and effectively promoted Hutton’s theories.

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