Geoscience Reference
In-Depth Information
Two decades later, in the 1980s, Joe Kirschvink knew all about Budyko's ice catastrophe. Every-
body did. And he also knew its corollary: the Earth can't freeze. If modellers came up with a white
Earth in their experiments, they simply threw those results away.
So what about this new evidence from Williams and Embleton? Perhaps, Joe felt, he should probe
a little more. An Australian geologist whom Joe knew happened to be travelling to Williams and Emb-
leton's particular part of South Australia at about that time, and Joe gave him a compass. “Pick me up
a sample or two,” Joe said. “Nothing fancy, just hand samples. But check their orientation when you
chip them off the outcrop.”
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USTRALIAN
outback has many guises. There's the famous dry red centre that houses Uluru and
Alice Springs and the weird red mining town of Coober Pedy, the world's biggest producer of pre-
cious opals. The summer's heat is so fierce there that half of its meagre population lives below ground
in mud “dug-outs”, and its post-apocalyptic scenery is the darling of movie producers. But there's
also a subtler wilderness, several hundred miles south of Coober Pedy, on the way back to Adelaide,
and civilization. There lie the Flinders Ranges with their dusty valleys and row upon row of rounded
mountains. They are softer than the red centre, their colours more muted. And they have had many
parts to play in the Snowball story.
The main route to the Flinders winds through a narrow valley, at the foot of a sharply pointed
mountain called Devil's Peak. Here, at Pichi Richi Pass, is where Joe's rock samples came from. To
get to the outcrop, you climb over a small wire fence on to a patch of bare, rocky ground scattered
with tufts of dry grass. Winter's the season to go there—in a southern summer, the heat would be un-
bearable. Even in March, the temperature quickly climbs to 90 degrees F. or more. At least there is
merciful shade to be had among a stand of eucalypts with their peeling bark and graceful bone white
trunks, and the odd desert oak with its black spiny fruit shells still clinging to the bare branches.
Through the trees, a hillside slopes down to the floor of a dry gully. There are no trees on the hill,
just patches of golden grass, and at its feet lies a jumble of mud-coloured rocks with a strange pink
tinge. The rocks are stranger still up close. All of them are shot through with rhythmic dark lines, as if
they had been painted in neat, careful parallels.
The lines, though, pre-date artists by hundreds of millions of years. They are the last remnants of
ancient tides. Once this area was underwater, just offshore from an estuary. The tide flooded in to land,
carrying with it a slurry of fine sand. As the tide ebbed, sand was washed back out to sea and deposited
gently right at Pichi Richi. In and out, ebb and flow, as sand landed periodically on mud, the rhythmic
patterns built up. In the end they solidified, becoming sets of regular dark lines in a pale reddish mud-
stone. Such rocks are called “tidal rhythmites”, and they are very rare. A few good waves will destroy
the pattern completely before it can harden into rock. But somehow the seafloor thereabouts was pro-
tected from waves, and the layers survived.
George Williams, the researcher whose paper had caught Joe's eye, had already used the
rhythmites to work out exactly how long days lasted when the Earth was young. One day hasn't always
passed in a little over twenty-four hours, as it does now. Since the whirlwind days of its youth, the
Earth has been steadily slowing on its axis and days have been getting longer. Those short early days
are frozen into the rhythmic patterns of Pichi Richi's rocks. From the monthly tidal cycles that he
measured there, Williams figured out the number of days in a Precambrian month, and the number of
months in a year. When his rhythmites were still mud and sand, a little over 600 million years ago, a
year lasted thirteen months, and a day less than twenty-two hours.
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