It appears as a striking anomaly in a
landscape of low dunes and flat horizons. The dome of Uluru looks unreal as it
seems to be sitting as though it had been dropped from somewhere else onto the
vast and flat desert of Australia ’s
Red Centre. Uluru is, however, not alone. A fraternal twin lies some 40
kilometres away – Kata Tjuta – quite different in appearance but born of the
same mother, the Peterman
Ranges .
As the geologists' story goes, the Peterman Ranges were much higher 550 million
years ago, and erosion of these grand mountains produced two great alluvial
fans, one of them consisting largely of river-rounded stones and the other of
mostly sand. Over the next 50 million years, these fans built up layer upon
layer, becoming kilometres thick. Then the whole area became covered by a sea
and new layers of sediments fell on top of the fans. The weight of the new
seabed turned the fans into rock – conglomerate for the rocky fan and sandstone
for the other. Then some 400 million years ago, the Australian continent
underwent considerable tectonic movement. The sea drained away and the layers
of sediments were folded and tilted. The sandstone fan was tilted so that one
end bent up at nearly 90 degrees. From 300 million years ago and on, the
covering of sand and soil has slowly been blown away, exposing the harder rock
of the ancient alluvial fans to the air. But what you see “sitting” on the
landscape is just the tip of the iceberg, so to speak, or more accurately, just
the tip of the inselberg.
Inselbergs are, according to the
Encyclopaedia Britannica, an “isolated hill that stands above well-developed
plains and appears not unlike an island rising from the sea.” Indeed, the name
comes from German for “island” and “mountain” and was coined by early German
explorers of southern Africa who were
impressed by such landforms they encountered there. Inselbergs are often
granite and other rocks of igneous origin, or other rocks that are more
resistant and whose base lies well below the surface. While the surrounding
soils and sands are gradually removed by the powers of erosion, the harder rock
remains, slowly “rising” above the plains. As in the case of Uluru and its
sibling Kata Tjuta, most of the petrified sediments still lie below the ground,
possibly extending for several kilometres. Only a small portion of the total
mass is exposed, making both landforms the lithological equivalent to an
iceberg.
Incidentally, the sediments are not red in
their unexposed state but rather light grey as the main components are feldspar
and quartz, a type of sedimentary rock known as arkose. The exposed rocks turn
reddish as their iron components oxidize with the air. This brings to mind the
scene in the movie “The Hitchhiker’s Guide to the Galaxy” when for but a couple
of seconds we see workers busy painting the grey rocks of Uluru red in
preparation for the completion of the new Earth. Someone’s shot at a gag was
not so far off from reality.
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