Siccar Point on the Berwickshire coast is one of the most famous geological sites in the world. It is the site of Hutton’s Unconformity, where almost vertical beds of marine Lower Silurian greywackes are overlain by gently sloping beds of non-marine Upper Devonian conglomerates. The unconformity was discovered during a boat journey made in 1788 by the founder of modern geology James Hutton, the scientist and mathematician John Playfair and the geologist James Hall. The Silurian beds are approximately 435 million years old, while the overlying Devonian beds are aproximately 375 million years old. The unconformity therefore bridges a time gap of around 60 million years.
The unconformity provided one of several pieces of evidence that Hutton produced in support of his theory that the Earth was much older than was widely believed in the eighteenth century. Although he did not know the ages of the Silurian and Devonian rocks at Siccar Point, he recognised that long periods of time would have been necessary for the Silurian rocks to have been tilted and eroded, and for the Devonian sediments to have been laid down on top of them.