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Deep past: Lower Antelope Canyon, Navajo Tribal Lands, Arizona (Image: Ralph Lee Hopkins/National Geographic Creative)

, National Academy of Sciences, Washington DC, until 15 January 2015

A new book and exhibition explain how geology has taken us from an unchanging Earth to the knowledge that the rocks are shifting under our feet

IN 1650, James Ussher, archbishop of Armagh, Ireland, published a book in which he stated that the Creation occurred on 23 October, 4004 BC. Other scholars disagreed, some dating the world to as early as 4103. Isaac Newton eventually weighed in with a later date, 3988.

All were some way off. The modern estimate is that the Earth is 4.5 billion years old, provoking scientifically educated audiences to scoff at the literalism of those chronologists. Historian Martin J. S. Rudwick, however, believes Ussher and the rest deserve respect.

In Earth鈥檚 Deep History, he argues compellingly that biblical chronologies mark the origin of geology and are important in understanding the subject today.Chronology was a sophisticated field in the 17th century, a multidisciplinary endeavour to produce a timeline of world history. As Rudwick writes, the Bible was one source among many, important because it was believed to be the only available textual record of 鈥渢he beginning鈥�.

One task facing chronologists was to reconcile scripture with other evidence, such as coins and monuments. An important consequence was that material evidence came to be seen as authoritative in its own right, a means of discovering aspects of history for which no textual records existed. Scholars realised that 鈥渘ature might, metaphorically, have its own antiquities鈥�. Fossil shells could supplement scriptural sources; more radically, they could reveal Earth鈥檚 own history.

That might not seem groundbreaking, but Rudwick is skilled at elucidating pre-modern ways of thinking. As he writes, 鈥渢he natural world was鈥� taken to have been a stable backdrop throughout human history. That nature might have had its own dramatic action began to seem plausible only when the ideas and methods of historians were transposed into the natural world, from culture into nature.鈥�

Much of Earth鈥檚 Deep History is concerned with the ramifications of this. Once the idea of terrestrial history was established, the age of the world could be investigated in ways Ussher never imagined. For example, strata were no longer seen as structural attributes of an immutable Earth. Instead, they were deposited over time, serving as a terrestrial 鈥渁rchive鈥�.

Of course, it was easier to conjure a metaphor than to act on it. As Rudwick explains, Earth鈥檚 archive could be read in multiple ways. Most 鈥渕en of science鈥� agreed the planet was millions of years old, yet the actual age could be inferred only by estimating the rate of stratification 鈥� and that raised more profound questions. Were strata steadily created, or was the process more capricious?

The former position was most powerfully argued by the 19th-century geologist Charles Lyell, who believed that the world was a steady-state system of deposition and erosion, and past geological processes were analogous to those observed in the present. The latter notion, that Earth鈥檚 past was erratic, emerged from fossil research by Georges Cuvier, revealing the world had gone through several mass extinctions.

Rudwick credits Lyell with giving geologists 鈥渁 better appreciation of the power of present processes, acting over vast spans of deep time鈥�, but is wary of Lyell鈥檚 theorising. He prefers Cuvier鈥檚 observations because the extinctions belong to Earth鈥檚 history, whereas Lyell鈥檚 steady-state model was posited as a universal law 鈥� and Rudwick insists that geology is a historical science. Like human history, Earth鈥檚 history is 鈥渉ighly contingent throughout, and therefore utterly unpredictable even in retrospect鈥�.

聯Earth鈥檚 history is highly contingent throughout, and thus unpredictable even in retrospect聰

Rudwick鈥檚 book is authoritative and riveting, and its historical breadth is bound to make geology exciting for readers from both sciences and humanities. As it happens, one of his previous books, , helped inspire an exhibition now on at the National Academy of Sciences in Washington DC.

Imagining Deep Time showcases contemporary artists seeking to embody the 鈥渄eep time鈥� of geology and cosmology through painting, sculpture and photography. It lacks coherence, but there are standout works such as Jonathon Wells鈥檚 Boston Basin, a composite photo showing a thin sliver of Boston skyline above millions of years of strata. The proportions alone capture the grandeur of geology, the depth of history Rudwick evokes.

In one respect only is Earth鈥檚 Deep History a little shallow. Ironically, Rudwick鈥檚 strength as a historian undermines his arguments about the nature of geology. Drawing a contrast between geology and physics, Rudwick insists on a 鈥渄istinction between establishing historical realities and finding causal explanations鈥�. His version of geology is unconcerned with causality because he deems causality hopelessly out of reach for sciences mired in contingency.

However, his fine analysis of geology鈥檚 roots shows how keenly historians work to find causal explanations, and how worthwhile such explanations can be. Whatever other similarities they have, history and geology at least share the virtue of being explanatory.

This article appeared in print under the headline 鈥淭he birth of geology鈥�