Geoscience Reference
In-Depth Information
On 22 June, just over a week into the field trip, the convoy reached a spectacular outcrop in the
northwest of the country. As with many of the best outcrops, Paul had stumbled across this one almost
by chance. Two years earlier he had been exploring a dried-out gully, trying to trace the uppermost
part of the glacial rocks. The gully was moderately hard going, choked with hefty pale grey boulders.
As Paul made his way laboriously up the main channel, a graduate student named Pippa Halverson
ducked off down an innocuous and apparently uninteresting side channel. A few minutes later Pippa
reappeared. “You might want to come and look at this,” he said.
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“This” turned out to be an outcrop that took Paul's breath away. To reach it, he and Pippa
scrambled down the side channel and then turned the corner that was hiding it from view. The rocky
ground rose steeply up ahead, but the surface was so pitted by the action of wind and weather that their
boots stuck to it like glue. There was little vegetation, just a few scrubby bushes and arthritic trees,
one with a gleaming bark like thickly smeared cream. And rising up on the left was a sheer cliff face
that was the embodiment of the Snowball story.
The base of the outcrop, later called “Pip's rock” in honour of its discoverer, was crammed with
ice-borne rocks of every shape and colour. These were the so-called dropstones that had been delivered
to the seafloor by ancient icebergs. White and pink and tan and orange, they stood out spectacularly
against the dull mudstone. They were the unmistakable sign of ice. But that wasn't all. Around halfway
up the cliff, the scene abruptly changed. Suddenly the mudstone transformed into a pinkish carbonate
that contained no interlopers, no boulders, no signs of ice at all. Below this knife-sharp edge between
rock types, the Snowball was in full force. A fleet of icebergs floated on an ancient sea, discharging
the rocks they carried into the soft mud on the seafloor. Above this edge, everything had changed. The
ice had melted, the sea had boiled with carbon dioxide gas and a milky carbonate rain. In spectacular
fashion, these rocks had captured the transition between icehouse and hothouse. This one outcrop en-
capsulated everything about Paul and Dan's story.
Paul is immensely proud of Pip's rock. When he takes you there, he can't resist building up a sense
of drama. He climbs on ahead over the giant boulders of the dried-out gully that leads you to the rock,
and as you round the corner to see the cliff face, he is already there, ready to gesture towards the out-
crop with a triumphant flourish. And Dan then calls for all present to doff their caps in mock respect
for the rocks. Nobody is allowed to use their geological hammers to pry rock samples from the sur-
face. Paul has decreed that this particular outcrop must remain pristine.
Geologists often have their sacred places, the ones that hold the key to their ideas. I've seen re-
searchers take off their shoes when walking on rock surfaces. Though they say it's because they don't
want to risk damaging the outcrop, it is a strangely reverent gesture.
Some of the most famous outcrops are sites of pilgrimage, for which geologists compile “lifetime
lists”. One place on every geologist's list is Siccar Point in southern Scotland, where the father of
all geology—an eighteenth-century gentleman farmer named James Hutton—first learned about the
Earth's great age. Before Hutton, the prevailing theory of how rocks appeared on the surface of the
Earth was called neptunism. This theory held that the Earth was once covered completely with a single
vast ocean. Each layer of rocks was formed in the ocean, the most primitive ones first, and the more
recent ones last. Eventually the ocean dried up and the rocks have remained the same ever since. This
idea fitted beautifully with the biblical notions of creation, Noah's flood, and an Earth that had exis-
ted—according to the most literal interpreters of the Bible—for just a few thousand years.
After Hutton, this biblical interpretation was swept aside in favour of a new, more rational ap-
proach. Hutton realized that rocks had been created at different times and in different ways. Some were
laid down on the floors of ancient oceans, others created by volcanic eruptions, and others still by the
