Number 3 on the Top 100 Great Geosites of the UK, Siccar Point is a place all Geologists simple must make the pilgrimage to! Located just an hour East of Edinburgh, it really is worth a visit on a sunny day, just as I did this July. Be warned though – the trip is not for the faint hearted!
The outcrop is located off the A1, just East of Cockburnspath. The trailhead is marked by a small carpark, located along the access road to Drysdales Vegetable Plant. The path takes a short route along the cliff top through fields to give a fantastic overview of the site from above. If the weather is good, and you are feeling energetic it is a steep decent down to the outcrop via a grassy slope that conveniently has a rope for added stability. It is well worth the exertion!
Siccar Point
Siccar Point beautifully demonstrates the geological unconformity. That is where a surface separates two rocks of different ages. There are many different sub-divisions of unconformity types, but what makes Siccar Point so spectacular is the strong angular nature of the unconformity. Below, greywackes and shales were laid down on the seabed during the Silurian Period (400 million years old). They were then uplifted, folded and deformed by the Caledonian mountain building giving them a vertical appearance at Siccar Point. Over 55 Million Years, these rocks were eroded and formed the land's surface. Then, during the Devonian Period (345 million years ago) this palaeosurface was covered by multiple layers of sand, deposited by rivers.
The whole section has since been eroded by the forces of the sea so that both the underlying vertical grey Silurian and overlying sub horizontal red Devonian sands and conglomerates can be seen with complex infilling geometries. The diagram gives one such example, where multiple flood events have gradually infilled the palaeosurface topography. Floods ripped up and eroded the underlying rocks and clasts or pebbles of grey rock can be seen within the characteristically red sandstones deposited above.
The Foundations of Modern Geology
Unconformities such as the one at Siccar Point are commonly seen in rocks around the Globe. What makes this one so unique is its historical value. The outcrop was first described over 200 years ago by James Hutton. At that time, most people believed that the Earth was only a few thousand years old. He , however, believed that the Earth was “composed of the productions of the sea, and of other materials similar to those now found upon the shores” and thus must have been formed through countless repeated cycles of deposition over geological time. He set out to find evidence for such a theory. Many outcrops were described as evidence for his theories, but Siccar Point remains the most spectacular. John Playfair who sailed to Siccar Point with Hutton describes the outcrop with much wonder:
“On us who saw these phenomena for the first time, the impression made will not easily be forgotten. The palpable evidence presented to us, one of the most extraordinary and important facts in the natural history of the Earth, gave a reality and subsidence to those theoretical speculations…”
Siccar Point, and the now-named Hutton’s Unconformity were the first conclusive proof of deep geological time. This is despite the lack of accurate dating at the time (indeed the Silurian and Devonian Periods had not yet been defined!) But, the works of Hutton and Playfair went on to create and define the Science of Geology where, as Hutton explained:
“we find no vestige of a beginning, —no prospect of an end”
3D model of the main outcrop of the unconformity at Siccar Point. Dudek (2016) https://bit.ly/2tqPBCv
--- Happy Exploring! ---
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