May 4. The Great Unconformity

At Siccar Point on the east coast of Scotland, there’s a dramatic angular unconformity, an erosional gap representing a break between different packages of rock. Relatively flat-lying rocks of the Devonian Old Red Sandstone lie over tilted rocks of Early Silurian age that stand nearly vertical beneath the sandstone. The Old Red Sandstone, which we discussed yesterday, was deposited in a terrestrial environment, in basins within the vast Caledonian Mountain Ranges. The Silurian rocks at Siccar Point contain graptolites and were deposited in the deep sea. So the unconformity represents not only a 70-million-year break in the rock record, but also a dramatic change in the depositional environment for the rocks, from marine to terrestrial. Not to mention the tectonic activity that tilted the Silurian beds and eroded them off before the Old Red Sandstone was deposited.

Hutton's Unconformity at Siccar Point (photo by Dave Souza)

James Hutton, the geologist who first clearly expressed the concept of uniformitarianism, the idea that processes active today acted in the past over vast spans of time, used his observations at Siccar Point to develop his ideas of deep time, the idea that the earth was very old, and that processes like erosion operated in the past.

This outcrop is often called “Hutton’s Unconformity” because of its role in leading Hutton to his conclusions that form some of the most fundamental concepts of geology.

Hutton visited many locations where the unconformity is exposed, but Siccar Point is probably the most famous. He traveled there in 1788 with his friend John Playfair, who later recalled

On us who saw these phenomenon for the first time the impression will not easily be forgotten...We felt necessarily carried back to a time when the schistus [by which he means the Silurian rocks] on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited, in the shape of sand or mud, from the waters of the supercontinent ocean... The mind seemed to grow giddy by looking so far back into the abyss of time; and whilst we listened with earnestness and admiration to the philosopher who was now unfolding to us the order and series of these wonderful events, we became sensible how much further reason may sometimes go than imagination may venture to follow.

—John Playfair (1805) Transactions of the Royal Society of Edinburgh, vol. V, pt. III.

Hutton presented his Theory of the Earth to the Royal Society seven years later, in 1795.

The angular unconformity at Siccar Point and throughout Scotland represents the Caledonian Orogeny. The collision between Laurentia, North America, which included northern Scotland, and Baltica or Europe, which included southern Scotland, squeezed the preexisting rocks so that they were tilted from horizontal to nearly vertical, and then lifted up above sea level into the Caledonian Mountains where erosion began. The erosion produced the sediments that were deposited millions of years later as the Old Red Sandstone, on the erosion surface represented by Hutton’s unconformity.

Today, Siccar Point is something of a holy grail among geologists, since it represents the place that Hutton saw as the final vindication of his theory. And that helped change significantly the way we think about the earth.

—Richard I. Gibson

Further reading
Making of an angular unconformity
Hutton’s Unconformity – Arran


Photo by dave souza at Wikipedia, used under Creative Commons license.