Hutton's Unconformity – Siccar Point
Siccar Point is a place of pilgrimage for geologists worldwide – here in 1788 James Hutton made one of the fundamental advances in geological understanding.
At that time geologists like Hutton were aware of the harder, folded rocks (which they called ‘schistus’) that formed the Lammermuirs and the cliffs of Berwickshire; they were also aware of the flatter lying sedimentary strata that formed the East Lothian coastal plain – but “where was the join?” they asked. Looking for an answer James Hutton with friends set sail from Dunglass and sailed east, studying the sea cliffs. At Siccar Point they found the answer at the locality known thereafter as ‘Hutton’s Unconformity’.
The junction between the two types of rock was laid bare by the action of the waves on the rock platform and cliffs. The ‘schistus’ beds were seen to stand vertically; they were cut off along an irregular surface; the red sandstones and conglomerates lay above them and were inclined gently seawards.
Here several chapters of the geological record could be seen in one place:
Chapter 1 The oldest rocks were laid down at the bottom of deep seas inhabited by surface floating animals such as graptolites. The sediments ere buried and became rocks.
Chapter 2 Continents moving closed the sea and squeezed the rocks till the strata were almost vertical, heated them gently so they became hard and pushed them up to form mountains.
Chapter 3 The mountains were worn down by rain, flood, river, wind, trimming off the top of the strata.
Chapter 4 Torrents and flash floods carried boulders, gravels and sands, and deposited them in valleys and plains.
Chapter 5 More, but gentler, continental movement affected the rocks, tilting the newer strata to the north-east.
Chapter 6 Erosion wore down the mountains again to the position we see today.
So remember, although the landscape seems unchanging, a human lifespan is but a moment in geological time. Every day somewhere in the world, lavas are erupting, sills and dykes being intruded, faults are causing earthquakes, and all types of rocks being eroded and redeposited as sediments. Over geological time the landscape of Scotland has changed dramatically and will continue to change. Hutton’s Unconformity gives us a glimpse of the forces that have been at work.
The same geological phenomenon had previously been noted by Hutton, but not fully understood, by the banks of the Jed Water at Allar’s Mill near Jedburgh. Although now degraded and overgrown the section was beautifully illustrated in the drawing by Hutton’s friend, the artist John Clerk of Eldin.