Hutton's Unconformity

Natural History, June, 1999 by Margaret W. Carruthers

In his 1803 eulogy of Scottish geologist James Hutton (1726-97), mathematician John Playfair recalled the clear day in the summer of 1788 when he, Hutton, and Sir James Hall sailed south along the Berwickshire coast in southeastern Scotland in search of evidence concerning the age of Earth. They rounded Siccar Point and landed at the base of what was to become one of the most famous sites in the history of geology. Before them, nearly vertical layers of gray slate rose up from the sea in a sheer cliff face, only to terminate abruptly against gently dipping beds of red sandstone.

   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 epocha 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.... The mind seemed to grow
   giddy by looking so far into the abyss of time. (Transactions of the Royal
   Society of Edinburgh, vol. V, pt. III, 1805)

Now known as Hutton's Unconformity, the formation reveals a missing chapter in what Hutton referred to as "the annals of the Earth." Hutton recognized that constructive geological processes, such as volcanism and mountain building, created strata of rock and sediment and that breaks, or "unconformities," in the chronology were caused by erosion or an absence of deposition over long periods of time. The gray vertical beds were originally flat-lying marine sediments that over eons hardened and were turned on end. They were slowly planed away and overlain by a horizontal layer of sand deposited at the edge of a now-vanished sea. Geologists estimate that roughly 80 million years elapsed between the deposition of the Lower Silurian gray slate and the Upper Devonian Old Red Sandstone.

Independently wealthy, an enthusiastic chemist, an innovative farmer, and a skilled mineralogist, Hutton was a true man of the Enlightenment. He was trained as a doctor but chose to forego a career in medicine in order to pursue his own research. Gradually his interests shifted from the human circulatory system to geology and the "blood of the earth." According to Playfair, he began to look "with anxious curiosity into every pit, or ditch, or bed of a river." Hutton published on a wide array of topics, including artillery, botany, linguistics, meteorology, and--more than sixty years before Charles Darwin and Alfred Russel Wallace--on organic evolution. In 1785, before the Royal Society of Edinburgh, he presented his principle of uniformity: the idea that the processes we observe shaping Earth today are the same as those that shaped it in the past.

Theologians and natural philosophers in Hutton's time believed that Earth was less than six thousand years old, based on the Anglo-Irish prelate James Ussher's mid-seventeenth-century calculations. Geology was also influenced by the charismatic German mineralogist Abraham Werner, who contended that most rocks were sediments deposited during the biblical Flood, and who dismissed active processes like river erosion and volcanism as recent and insignificant phenomena. While Werner regarded Earth as remaining static until the next divine catastrophe, Hutton saw it as dynamic, preferring to consult "God's books"--the rocks themselves--rather than the Bible for insights into the history of the planet.

Hutton's theory of the earth was published as a paper by the Royal Society in 1788 and then in book form in 1795. Unfortunately, he was such a poor writer that few read his book. We owe Hutton's legacy to his friend John Playfair, whose 1802 Illustrations of the Huttonian Theory of the Earth made the ideas accessible and appealing. It was not until the 1830s, however, when Charles Lyell published Principles of Geology, that geologists were generally ready to accept and implement Hutton's approach and to move from biblical interpretations of Earth's physiography to those based on observation, experimentation, and reasoning.

After two hundred years, Hutton's principle of uniformity is still at the heart of geology. Scientists recently observed, for example, copper sulfide and zinc sulfide being deposited on the seafloor by scalding fluids jetting out of hydrothermal vents (see "Neptune's Furnace" page 42). This find led to a reinterpretation of the 2.8-billion-year-old copper and zinc ores of Timmins, Ontario, and linked them to ancient seafloor processes.

Siccar Point is now a Scottish National Heritage (SNH) site. With permission from the SNH and others, Museum geologists Graham Stewart and Heather Sloan, and a crew from Research Casting International in Ontario, Canada, spent sixteen days last summer making a roughly fifteen-by-twenty-foot mold of the unconformity. It involved surveying, constructing rigging, and painting a section of the north-facing cliff with latex. Now cast in fiberglass and painted and textured to simulate the outcrop, it is a philosophical centerpiece of the new Gottesman Hall of Planet Earth.