View of Siccar Point, Scotland. Photo by Craig Chesek, © American Museum of Natural History.
Dacite columns that formed tens of thousands of years ago when a lava flow cooled rapidly against a glacier. Photo by Jackie Beckett, © American Museum of Natural History.
James Hutton (1726–1797), a Scottish farmer and naturalist, is known as the founder of modern geology. He was a great observer of the world around him. More importantly, he made carefully reasoned geological arguments. Hutton came to believe that the Earth was perpetually being formed; for example, molten material is forced up into mountains, eroded, and then eroded sediments are washed away. He recognized that the history of the Earth could be determined by understanding how processes such as erosion and sedimentation work in the present day. His ideas and approach to studying the Earth established geology as a proper science.
In the late eighteenth century, when Hutton was carefully examining the rocks, it was generally believed that Earth had come into creation only around six thousand years earlier (on October 22, 4004 B.C., to be precise, according to the seventeenth century scholarly analysis of the Bible by Archbishop James Ussher of Ireland), and that fossils were the remains of animals that had perished during the Biblical flood. As for the structure of the Earth, “natural philosophers” agreed that much bedrock consisted of long, parallel layers which occurred at various angles, and that sediments deposited by water were compressed to form stone. Hutton perceived that this sedimentation takes place so slowly that even the oldest rocks are made up of, in his words, “materials furnished from the ruins of former continents.” The reverse process occurs when rock exposed to the atmosphere erodes and decays. He called this coupling of destruction and renewal the “great geological cycle,” and realized that it had been completed innumerable times.
Hutton came to his chosen field by quite a roundabout route. Born in Edinburgh in 1726, he studied medicine and chemistry at the Universities of Edinburgh, Paris, and Leiden, in the Netherlands, and then spent fourteen years running two small family farms. It was farming that gave rise to Hutton’s obsession with how the land could hold its own against the destructive forces of wind and weather he saw at work around him. Hutton began to devote his scientific knowledge, his philosophical turn of mind, and his extraordinary powers of observation to a subject that had only recently acquired a name: geology.
Around 1768 he moved to Edinburgh, where a visitor a few years later described his study as “so full of fossils and chemical apparatus that there is hardly room to sit down.” In a paper presented in 1788 before the Royal Society of Edinburgh, a newly-founded scientific organization, Hutton described a universe very different from the Biblical cosmos: one formed by a continuous cycle in which rocks and soil are washed into the sea, compacted into bedrock, forced up to the surface by volcanic processes, and eventually worn away into sediment once again. “The result, therefore, of this physical enquiry,” Hutton concluded, “is that we find no vestige of a beginning, no prospect of an end.” Relying on the same methods as do modern field geologists, Hutton cited as evidence a cliff at nearby Siccar Point, where the juxtaposition of vertical layers of gray shale and overlying horizontal layers of red sandstone could only be explained by the action of stupendous forces over vast periods of time. There Hutton realized that the sediments now represented by the gray shale had, after deposition, been uplifted, tilted, eroded away, and then covered by an ocean, from which the red sandstone was then deposited. The boundary between the two rock types at Siccar Point is now called the Hutton Unconformity.
The fundamental force, theorized Hutton, was subterranean heat, as evidenced by the existence of hot springs and volcanoes. From his detailed observations of rock formations in Scotland and elsewhere in the British Isles, Hutton shrewdly inferred that high pressures and temperatures deep within the Earth would cause the chemical reactions that created formations of basalt, granite, and mineral veins. He also proposed that internal heat causes the crust to warm and expand, resulting in the upheavals that form mountains. The same process causes rock stratifications to tilt, fold and deform, as exemplified by the Siccar Point rocks.
Another of Hutton’s key concepts was the Theory of Uniformitarianism. This was the belief that geological forces at work in the present day—barely noticeable to the human eye, yet immense in their impact—are the same as those that operated in the past. This means that the rates at which processes such as erosion or sedimentation occur today are similar to past rates, making it possible to estimate the times it took to deposit a sandstone, for example, of a given thickness. It became evident from such analysis that enormous lengths of time were required to account for the thicknesses of exposed rock layers. Uniformitarianism is one of the fundamental principles of earth science. Hutton’s theories amounted to a frontal attack on a popular contemporary school of thought called catastrophism: the belief that only natural catastrophes, such as the Great Flood, could account for the form and nature of a 6,000-year-old Earth. The great age of Earth was the first revolutionary concept to emerge from the new science of geology.
The effect that this portrait of an ancient, dynamic planet had on the thinkers who followed in the next century was profound. Charles Darwin, for example, was well acquainted with Hutton’s ideas, which provided a framework for the eons required by the biological evolution he observed in the fossil record. English geologist Sir Charles Lyell, who was born the year Hutton died and whose influential book Principles of Geology won wide acceptance for the Theory of Uniformitarianism, wrote, “The imagination was first fatigued and overpowered by endeavouring to conceive the immensity of time required for the annihilation of whole continents by so insensible a process.” The “ideas of sublimity” awakened by this “plan of such infinite extent,” as Lyell referred to it, inspired not only Hutton’s contemporaries, but generations of geologists to come.