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.

March 7. March 7, 1785 – James Hutton

Today we take a break from earth history per se to mention an important anniversary. On this date, March 7, 1785, in some ways, the modern science of geology was born. On this day the first of two papers by James Hutton was read before the Royal Society of Edinburgh, Scotland. The pair of papers was entitled Concerning the Systems of the Earth, its Duration, and Stability. These papers, with little change, formed the core of Hutton’s Theory of the Earth, published in 1795. Many of the ideas first clearly expressed there serve as the basis for modern geology.

James Hutton

Hutton was born June 3, 1726, son of a Scottish merchant who was Treasurer of the City of Edinburgh. Hutton’s early interest in chemistry led him into medicine – he was a practicing physician – and ultimately into geology.

He studied unconformities – breaks in the rock record – cross-cutting relationships, and the tilting of formerly horizontal strata to come to his famous conclusion, expressed later by Charles Lyell as “the present is the key to the past,” the idea that processes operating today must have operated in the past, and that they produced the features we see now. Hutton actually said "from what has actually been, we have data for concluding with regard to that which is to happen thereafter."

An important aspect of his conclusion was that while there was much change in earth history, the processes were fundamentally the same, and operated continually. This led to his famous concluding statement, "The result, therefore, of our present enquiry is, that we find no vestige of a beginning,–no prospect of an end." Today we call this uniformitarianism, a big word that just says the processes affecting things on earth happened uniformly over geologic time.

At the time there was plenty of opposition to Hutton’s ideas. Prevailing thought included the catastrophists, who believed that short intense events created the things we see in the rocks, rather than the gradual effects of erosion, deposition, and multiple relatively small tectonic events such as earthquakes. In fact uniformitarianism was sometimes called gradualism.

Hutton was also opposed by the Neptunists, who were in their own fight against the plutonists. Neptunism suggested that everything was formed in the sea, and plutonists said everything came from molten, volcanic origins. Hutton’s ideas allowed for a nice compromise between those two fairly rigid and extreme views, and gave room for both processes and gave both considerable importance.

The ultimate adoption of the concept of gradual change, uniformitarianism, became so well entrenched in geological thought that even into the middle part of the 20th century, the idea of any kind of catastrophe was ridiculed. Consequently when Luis and Walter Alvarez suggested, in 1980, that a catastrophic impact caused the extinction at the end of the Cretaceous Period that wiped out the dinosaurs, there was considerable laughter at the idea. The good news is that the idea was tested scientifically, and the evidence mounted, culminating in the discovery of the “smoking gun” -- a huge crater in the subsurface of Yucatan that has been dated to just the right time.

Today, I would say that geologists firmly believe that uniformitarianism has been the dominant factor in the evolution of planet earth – but there have indeed been some changes in the processes affecting the planet over its 4.6-billion-year life, and there have indeed been some catastrophes too.

But James Hutton first clearly expressed the basic concepts that govern modern geologic thought, on this day back in 1785.

—Richard I. Gibson

Image from Wikipedia (public domain)