James Hutton (1726-97) based his Theory of the Earth on field observations, and then used the Theory to predict what would be seen somewhere else. His procedure is like that of a laboratory scientist doing a crucial experiment. Specifically, Hutton concluded that younger sedimentary strata were deposited on the eroded edges of upturned older rocks, and he predicted that Siccar Point was the place to find the evidence. It had long been believed that the Earth was 6,000 years old, but at Siccar Point in 1788 Hutton proved that the Earth is vastly older than that. He was correct - we now know that its age is nearer to 6,000 million years old!
Hutton was a key figure in the Scottish Enlightenment. His intimate friends included Adam Smith 1723-1790 (Economist), Joseph Black 1728-1799 (discoverer of carbon dioxide and latent heat), James Watt 1736-1819 (whose name is our unit of Power), John Clerk of Eldin 1728-1812 (Naval Tactician), John Playfair 1748-1819 (Mathematician and Physicist), and Sir James Hall of Dunglass 1761-1832 (Founder of experimental geology). Playfair and Hall accompanied Hutton to Siccar Point in 1788.
Hutton was a prolific author on science and philosophy, and one of the first industrial chemists - producing sal ammoniac from the city soot. He also farmed in Berwickshire and improved Scottish agricultural practice. He described natural selection 15 years before Darwin was born, and is recognized as the Founder of Modern Geology. It is tragic that so few people know his name.
Weathering and Decay: Hutton saw that soil is washed away by rain; he also recognized that soil forms by the decay of rocks. If these processes were long continued, all land would be reduced to sea-level. Boulders, pebbles, sand, and mud are travelling from the mountains to the sea floor.
Distribution of Rocks: Scotland is divided into three parts: (a) the Highlands, bounded by the Highland Boundary Fault from Stonehaven to Rothesay; (b)the Midland Valley; and (c) the Southern Uplands (including the Lammermuirs), bounded by the Southern Uplands Fault roughly from Cockburnspath to Girvan. Though roads were primitive, Hutton was a prodigious traveller and he noticed how the various rocks were distributed. In the Midland Valley stratified (layered) rocks are common - sandstone, shale, limestone, and coal. He also correctly recognized volcanic rocks; e.g. at Arthur's Seat, Edinburgh Castle, Campsie Linn, Kinnoull Hill, the Ochils, and North Queensferry, and many other places. The rocks in the Highlands and Southern Uplands are also usually stratified, but they are hard and the layers often contorted and at high angles.
Hutton's "former land": Smeaton used Quarry Mill sandstone to build the Perth Bridge (1766-1771) e.g. Photo 1: coarse reddish sandstone with scattered, water-rounded pebbles. Most of the pebbles match Highland rocks. The sandstone has the characteristics resulting from flash floods in a desert bounded by hills to the north.
Photo 2: horizontal strata at Barns Ness (between Dunbar and Cockburnspath). From bottom to top the rocks tell a story: (a) coal (remains of a forest like the Florida Everglades); (b) shale (compressed mud) with marine fossils, (showing that the forest was killed when the land sank below the sea ; and (c) coral limestone - indicating that, with further subsidence, warm clear seas succeeded the muddy waters. Photo 3: horizontal strata of red sandstone near Jedburgh.
As Hutton put it: "From the present state of things, we have it in our power to reason from effect to cause, and read the annals of a former earth" [1788, p.287-288]
"The land on which we rest is not original. Before the present land was made, there had subsisted a world composed of sea and land, with such operations at the bottom of the sea as now take place. While the present land was forming at the bottom of the ocean, the sea was then inhabited by animals, as it is at present. Hence we conclude, that the greater part of our land had been produced by natural operations. To make this land a permanent body, two things were required: (1) the consolidation of loose material, and (2) the elevation of those consolidated masses from the bottom of the sea, where they were collected, to where they now remain above the level of the ocean; i.e. the present land had been first formed at the bottom of the ocean, and then raised above the surface of the sea " [1785, p.5-7 simplified. Italics added].
Folded strata: Though deposited in horizontal layers, strata are often tilted and folded when raised to form new land. Photo 4: folded strata at Calico, California. Photo 5: folded strata near St Abb's head, on the Berwickshire coast. [Contrast with the horizontal strata of Photo 3.] Hutton concluded that the highly folded strata of the Highlands and the Southern Uplands were older than the gently inclined strata of the Midland Valley. Hutton's idea about what we call an unconformity is explained in Photo 6 (diagram from Arthur Holmes) and printed on the last page.
Photo 7 Siccar Point. Horizontal red sandstone on vertical "schistus"
Photo 8 Siccar Point - the classic view
Photo 9 Sir James Hall's cross section (1788).
Photo 10 Photograph of Sir James Hall's exposure.
Photo 11 Close up of "schistus" fragments in overlying sandstone.
Photo 12 Breccia [angular fragments]overlying vertical strata.
"The Theory confirmed from observations made on purpose to elucidate the subject" [Hutton]: To confirm the age relations Hutton had to find the contact between the older and younger rocks. He searched the southern boundary of the Highlands and the northern boundary of the Southern Uplands. This was in vain because - unknown until much later - these are fault boundaries. Here are Hutton's words (Vol.1, 1795, p.453-456):
"Having got a distinct view of the primary and secondary strata of the globe, I was considering where we might most probably succeed in finding the junction of the low country and alpine schistus. I inquired of Mr Hall of Whitehall [Sir James Hall's uncle] ... Mr Hall having had occasion to examine the Pease and Tour burns, in planning and superintending the great improvement of the post road upon Sir James Hall's estate [Dunglass] while Sir James was abroad, he informed me that the junction of the schistus [older] and sandstone strata [younger] was to be found in the Tour burn [Hutton pronounced "Tower" as "Tour"].
"It was late in the spring 1788 when Sir James left town [Edinburgh], and Mr Playfair and I went to Dunglass about the beginning of June. We had exceeding favourable weather during the most part of our expedition; and I now propose to give an account of the result of our observations.
"St Abb's Head and Fast Castle are head lands projecting into the sea, and are the bulwarks of this shore, which is embayed to the westward, where the sea preys upon the horizontal strata. The solid strata are every where exposed either in the cliff or on the shore; we were therefore certain of meeting with the junction in going from Dunglass to Fast Castle, which is upon the schistus; and we first set out to examine the junction in the Tour and Pease burns, where we had been informed it was to be found. ...
"Having taken boat at Dunglass burn, we set out to explore the coast; and, we observed the horizontal sand-stone turn up near the Pease burn, rising towards the schistus. We found the junction of that schistus with the red sand-stone and marly strata on the shore and sea bank, at St Helens, corresponding in general with what we had observed in the burns to the westward. But, at Siccar Point, we found a beautiful picture of this junction washed bare by the sea. The sand-stone strata are partly washed away, and partly remaining upon the ends of the vertical schistus; and, in many places, points of the schistus are seen standing up through among the sandstone, the greatest part of which is worn away. Behind this again we have a natural section of those sand-stone strata, containing fragments of the schistus .... [emphasis added]
"The ridge of the Lammer-muir Hills, in the south of Scotland, consists of primary micaceous schistus, and extends from St Abb's-head westward, till it join the metalliferous mountains about the sources of the Clyde. The sea coast affords a transverse section of this alpine tract at its eastern extremity, and exhibits the change from the primary to the secondary strata, both on the south and on the north. Dr Hutton wished particularly to examine the latter of these, and on this occasion Sir James Hall and I had the pleasure to accompany him.
"We sailed in a boat from Dunglass, on a day when the fineness of the weather permitted us to keep close to the foot of the rocks which line the shore in that quarter, directing our course southwards, in search of the termination of the secondary strata. We made for a high rocky point or head-land, the Siccar, near which, from our observations on shore, we knew that the object we were in search of was likely to be discovered.
"On landing at this point, we found that we actually trode on the primeval rock, which forms alternately the base and the summit of the present land. It is here a micaceous schistus, in beds nearly vertical, highly indurated, and stretching from S.E. to N.W. The surface of this rock runs with a moderate ascent from the level of low-water, at which we landed, nearly to that of high-water, where the schistus has a thin covering of red horizontal sandstone laid over it; and this sandstone, at the distance of a few yards farther back, rises into a very high perpendicular cliff.
"Here, therefore, the immediate contact of the two rocks is not only visible, but is curiously dissected and laid open by the action of the waves. The rugged tops of the schistus are seen penetrating into the horizontal beds of sandstone, and the lowest of these last form a breccia containing fragments of schistus, some round and others angular, united by an arenaceous cement.
"On us who saw these phenomena for the first time, the impression made will not easily be forgotten. The palpable evidence presented to us, of one of the most extraordinary and important facts in the natural history of the earth, gave a reality and substance to those theoretical speculations, which, however probable, had never till now been directly authenticated by the testimony of the senses.
"We often said to ourselves, What clearer evidence could we have had of the different formation of these rocks, and of the long interval which separated their formation, had we actually seen them emerging from the bosom of the deep?
"We felt ourselves necessarily carried back to the time when the schistus 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 a superincumbent ocean.
"An epoch still more remote presented itself, when even the most ancient of these rocks, instead of standing upright in vertical beds, lay in horizontal planes at the bottom of the sea, and was not yet disturbed by that immeasurable force which has burst asunder the solid pavement of the globe. Revolutions still more remote appeared in the distance of this extraordinary perspective.
"The mind seemed to grow giddy by looking so far into the abyss of time; and while 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."
"Amid all the revolutions of the globe the economy of Nature has been uniform, and her laws are the only things that have resisted the general movement. The rivers and the rocks, the seas and the continents have been changed in all their parts; but the laws which direct those changes, and the rules to which they are subject, have remained invariably the same."
Playfair, Illustrations of the Huttonian Theory, Paragraph 374.
Monument to Sir James Hall of Dunglass in Dunglass Chapel:
(Probably composed by Sir Walter Scott)
Sacred to the Memory of
Sir James Hall of Dunglass Bart.
President of the Royal Society of Edinburgh
A philosopher distinguished amongst
the eminent men of an inquiring age
Not less by the originality, boldness,
and accuracy of his speculations
The ingenuity and resolute perseverance
With which he substantiated
Various important theoretical views in his
Favourite science of Geology
By a series of brilliant and convincing
Born 17th Jan. 1761. Died 23d June 1832
For further information about James Hutton and geology see: James Hutton: Founder of Modern Geology, Donald B. McIntyre & Alan McKirdy, National Museums of Scotland, with coloured plates. ISBN 1 901663 69 8.