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Earth's Deep History:

How it was discovered and why it matters

Martin J. S. Rudwick

Usher was only one of many trying to sort out the precise date of Creation. Problem was that other data was available. They couldn't decipher Egyptian hieroglyphs, but they did have ancient Greek accounts of what the Egyptian timelines recorded, and they indicated many centuries of geneology prior to 4000 BC. And the Greeks told of Babylonian civilizations even older then the Egyptian ones. When Jesuits arrived in China, they translated early Chinese records which indicated a much longer human history than the Bible chronology allowed.

There was only one way to deal with these conflicting accounts - ignore them. They were obviously just legends, or spin invented by leaders to establish their authority. The Bible was the only valid authority.

First explanation of fossils was that they were just mineral equivalents, corresponding to real animals or plants. Seems strange to us now, but fitted available facts - no idea of how organic material could become mineral, or could be buried so deep in mines etc. And many of them bore no resemblance to living animals.

Even when things such as shark's teeth were recognized as such and not 'tongue-stones', and that since they were embedded in limestone used in Etruscan buildings they must pre-date the Etruscans, Steno had trouble convincing people that such things could have survived since Noah's Flood.

In C18 fossils with very delicately preserved features were found in several quarries around Europe. They were preserved in very thin layers of rock, which had obviously been deposited by very fine-grained mud, very slowly and in very calm water. These could not have been laid down by Noah's Flood - a brief and violent event.

From mid-C18 on, idea that a single Flood event, whether a short-lived rise in ocean levels or a violent cataclysm, was dropped by scholars. There was simply too many and too varied examples of fossil deposition that clearly happened at different times and for different periods.

Recognition of many extinct volcanoes in central France, yet no historical records or even local folklore to suggest that they had erupted in human times. Found that a lake which a Roman poet had recorded fishing on was in fact formed by a lava flow that had blocked the valley, so must have happened well before Roman times. Finally recognized that the ridge tops were also old lava flows, which meant that the lava had once flowed down prehistoric valleys, the surrounding hills of which had subsequently eroded away, leaving just the solidified lava to record where the valleys had once been. All implying unimaginable time to occur.

1808 map of Paris geology by Cuvier showed layers of Coarse Limestone, full of fossil shells similar to marine molluscs today, which meant that a vanished sea had once extended across much of Paris. But there were also finer grain sandstones full of fossil shells of species that only grew in fresh water. Taken together, these meant that Paris had seen repeated but irregular alternation of marine and freshwater environments. And later, rivers had carved out valleys in these rocks, and filled them with alluvial gravels.

At the same time, studying sedimentary rock in foothills of Italian alps, geologists realised that while some of the shells were of species that still existed today, at least half were of species that had gone extinct.

By mid-1820's study of stratigraphy showed the sequence of development - the oldest plants - huge ferns - were in coal seams, and seemed to make up the coal itself. In the younger, Secondary rocks, cycads and conifers were abundant. And only in the tertiary rocks did flowering plants appear.

Many of the fossils being found seemed 'out of place' - tropical coral reefs and palm trees in chilly England. At first this was interpreted as backing up the idea of a cooling Earth.

1870's studies of glaciated landscapes found remains of forests sandwiched between layers of glacial till or boulder clay. So there must have been successive episodes of glaciation, separated by interglacial periods. And these, going by the plants found, must have been warmer than present day Europe.

Scientists couldn't figure out why these episodes happened, but they could see from the evidence that they had.

(New Scientist)

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's 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 "the beginning".

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 "nature might, metaphorically, have its own antiquities". Fossil shells could supplement scriptural sources; more radically, they could reveal Earth's own history.

That might not seem groundbreaking, but Rudwick is skilled at elucidating pre-modern ways of thinking. As he writes, "the natural world was... 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."

Much of Earth's 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 "archive".

Of course, it was easier to conjure a metaphor than to act on it. As Rudwick explains, Earth's archive could be read in multiple ways. Most "men of science" agreed the planet was millions of years old, yet the actual age could be inferred only by estimating the rate of stratification - 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's 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 "a better appreciation of the power of present processes, acting over vast spans of deep time", but is wary of Lyell's theorising. He prefers Cuvier's observations because the extinctions belong to Earth's history, whereas Lyell's steady-state model was posited as a universal law - and Rudwick insists that geology is a historical science. Like human history, Earth's history is "highly contingent throughout, and therefore utterly unpredictable even in retrospect".

Rudwick's 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, Bursting the Limits of Time, 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 "deep time" of geology and cosmology through painting, sculpture and photography. It lacks coherence, but there are standout works such as Jonathon Wells's 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's Deep History a little shallow. Ironically, Rudwick's strength as a historian undermines his arguments about the nature of geology. Drawing a contrast between geology and physics, Rudwick insists on a "distinction between establishing historical realities and finding causal explanations". 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's 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.

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