Geoscience Reference
In-Depth Information
Most of the trade between Port Augusta and the Australian interior is plied along the narrow Stuart
Highway by terrifying, thundering “road trains”. These are linked caravans of two or maybe three huge
trucks pulled by just one engine, whose driver has probably been up all night, fuelled only by greasy
steak sandwiches “with the lot” (onions, cheese, tomato, bacon, fried eggs, you name it, all oozing
with ketchup and evil yellow mustard) from the occasional wooden roadhouses. The trucks rock your
car with shock waves as they rip past you. They are death to kangaroos, and grey-furred flesh peri-
odically smears the road, picked at by huge, black, wedge-tailed eagles. Even a full-sized kangaroo
won't make much of a dent in a road train. But if you're in a regular car, you'd better not drive at dusk
or dawn when the 'roos are on the move. Evolution hasn't had time yet to equip them with road sense.
They'll lurk in the scrub by the roadside and then leap into your path without warning. Hit one, and it
could well be the death of you, too.
There's little else to distract you during the drive, just the flat, empty, featureless scrub of the Aus-
tralian desert. Eventually a slightly battered sign points to a dirt road on the right. These days not many
vehicles take this turning. In the 1980s, Mount Gunson was a flourishing copper mine, but now most
of the operations have closed down. A few workers' huts remain, with their peeling girly posters. Also
some yellow warning signs—sulphuric acid. corrosive!—next to rusting pipes almost the same col-
our as the dusty red soil. But the rest of the site is scarcely different from the surrounding desert, its
landscape scattered with drab grey saltbushes and Flinders Ranges wattles, fast-growing, fast-dying,
driedout skeletons rubbing up against the vivid bottle-green of the living. When the miners finally pull
out completely, the only sign of the once-frenetic mining will be the great open pits carved deep into
the ground. Now that the mining has ceased, water is no longer pumped out of these pits, and soon
they will become lakes. Then, there will be no more chances to see the geological signs that caused
Paul Hoffman such anxiety.
The Northeast Pit, cut into the Cattlegrid ore body, bears the features that seemed to challenge
Paul's Snowball. This great hole in the ground, a thousand feet long and almost as wide, sinks down
to a flat floor. The sides are steep and layered, dark quartzite near the floor of the pit, paler sandstone
above, and then, in the topmost layer, the ubiquitous iron-rich red of the soil. You crunch along the
floor of the pit on delicate yellow nodules crusted with white crystals of gypsum. Patches of soil are
slotted with short parallel lines, where passing kangaroos have dug in their heels. The sun is invariably
dazzling, glaring off the sandstone. Squinting even through sunglasses, you have to be close to the
walls before you notice anything different about them. But as soon as you pick out one of the strange
structures in the pit's steep walls, you start to see them everywhere.
They are wedges of sandstone, six feet tall and triangular, like a row of massive shark's teeth. Or
perhaps witch's teeth, since they are stained green by the flow of copper-rich water. The quartzite they
sit in has been smashed and broken like a jumbled pile of bricks, but the wedges themselves, and the
rock layers above them, are made of smooth, flowing sandstone. They line the walls of the pit. As
you walk along you see first one, then another, then a whole row of them, strung as on a necklace.
Occasionally you see the outline of a wedge or two above the main row. To an untrained eye they look
bizarre. But to geologists they're classic. They're textbook. Anyone can tell you what they mean.
Sand wedges are the clear signs of a climate pattern called freeze-thaw. Here's how they form.
First, freeze. The temperature drops quickly, and in response the ground shrinks and cracks into regular
polygonal shapes, like the mud cracks that form in the bed of a dry lake. Into the cracks blow sand and
dust. Next, thaw, and the cracks are kept open by the presence of the sand. Next, freeze again, and the
cracks open wider, with more sand tumbling in. Eventually the sand that has wedged into the cracks
solidifies and turns into sandstone. In Mount Gunson's Cattlegrid pit, the broken-up quartzite is the
