DOWN UNDER - Snowball Earth - page 72

ground that was repeatedly cracked by freezing and thawing, and the shark's teeth are slices through
the solidified sand wedged into the cracks.
So Mount Gunson must have suffered from repeated episodes of freezing and thawing. Why
should that be a problem? Mount Gunson was an island then, surrounded by a frigid ocean. Over to the
east are the ice deposits of the Flinders Ranges, where fleets of icebergs dropped their load of stones
and boulders into a shallow sea. Freezing the ground thereabouts should have been easy.
But to get sand wedges doesn't just require cold temperatures. It also takes warm ones, followed
by sudden, repeated temperature drops. Sand wedges need cycles of freeze and thaw—in other words,
seasons . The problem is that Mount Gunson was close to the equator when the ice was present. And
at the equator, seasons simply don't happen.
We have seasons because our planet is tilted. If the Earth remained bolt upright in its progress
around the sun, there would be no such thing as summer and winter. All year round, every place on
Earth would experience the climate that its local position deserved. Close to the equator, where the sun
is fierce overhead, the climate would be hot. Close to the poles, where the same amount of sunlight
spreads over a larger area, the climate would be cold. January or June, there would be no difference.
But the Earth's tilt makes life more interesting. Superimposed on the overall pattern of cli-
mate—hot equator, cold poles—is a seasonal shift. In January the southern hemisphere is thrust out
towards the sun. Australians and South Americans head to the beaches. Antarctica basks in the mid-
night sun, and temperatures there can stay above freezing for days. By June, halfway again around
the Earth's annual orbit, the northern part of the Earth receives more than its share of sunlight. Now
Antarctica is shrouded in permanent darkness, and northerners take their turn in the sun.
The equator is the only place on Earth to escape this annual cycle of hot and cold. No matter which
hemisphere is grabbing its extra share of sunlight, the equator feels it, too. Equatorial regions muscle
in on everyone else's summers.
So how to explain the Mount Gunson sand wedges? Cast-iron magnetic evidence says that Mount
Gunson was at the equator when the ice came. Nobody doubts this. But the wedges seem to show sea-
sonal changes. What gives?
George Williams thinks he knows. Today the Earth's axis isn't tilted by much, just twenty-two de-
grees, a sixteenth of a full circle. But, says George, what if it used to be much more tilted? Perhaps, in
“Snowball” times, the Earth had tipped over on its side, by something closer to a full quarter-circle.
If so, everything we now know about the climate would be turned upside down. Poles and equator
would swap characteristics with the sun blazing directly overhead at the poles, and spreading feebly
out at the equator. And this, George felt, would nicely explain two of the main conundrums of the ice
era without requiring a frozen Earth. The Arctic and Antarctic regions would be balmy, and the equat-
orial regions frozen, which would explain the Australian evidence for ice at the equator. What's more,
the sun would now be wreaking seasonal changes on to the frozen equator. That would explain the
sand wedges. If George is right, the Snowball simply didn't happen.
George had been talking for decades about the Big Tilt. 2 But until Paul came along, nobody was
particularly listening. When George heard about Paul's Snowball, however, he went immediately on
the offensive. In a magazine called The Australian Geologist , he published a ten-point criticism out-
lining exactly why he thought Paul was wrong and the Big Tilt was right. (George called this criticism
“Has Snowball Earth a snowball's chance?” 3 Paul's immediate response was wryly titled “Tilting at

Snowballs”. 4 )