AT rock hound Kerry van der Jagt steps back in geological time to investigate Australia’s Most Famous Rock.

SHORT ANSWER // Uluru is a bloody big rock that has been exposed due to weathering of the surrounding land over gazillions of years.

LONG ANSWER // Uluru is in the Uluru-Kata Tjuta National Park about 335km southwest of Alice Springs. It rises 348m above the sand plain and is 9.4km around the base. Its geological history spans hundreds of millions of years and is almost beyond comprehension. A saga of unimaginable upheaval, of inland seas, erosion and folding.

Over 550 million years ago (during the Cambrian period), erosion from the Peterman Ranges to the west of Kata Tjuta led to huge alluvial fans at least 2.5km thick being built up by deposits of Arkose sands from the eroded materials. Later (500 million years ago), the alluvial fans were covered by sediments when the entire area became a shallow sea. The layers contain many fossils of marine life and are the source of the oil and gas fields in Central Australia. A few million years later still, the inland sea retreated and sediment deposition halted. Then, around 400 to 300 millions years ago, another major mountain building event, named the Alice Springs Orogeny, produced massive lifting and folding in the region. This caused the formerly horizontal strata of the Arkose sandstone (which makes up Uluru) to be folded nearly vertically. The sand fan tilted almost 90 degrees!

Over the last 300 million years the softer deposits, which covered the Arkose sandstone, have eroded away. To put it in simple terms, Uluru stands above the surrounding desert because it’s more resistant to erosion than the former rocks which covered it. Continued weathering to this day has produced the characteristic plunge pools, valleys, ribs and the red flaky surface (the result of iron oxide formation after the grey arkose has been exposed to oxygen).

You probably knew // Uluru is the tip of a huge slab of rock that continues below the ground for, it’s thought, up to 6km.

But did you know // The world’s largest monolith is WA’s Mt Augustus – not Uluru. It’s 2.5 times larger in mass and stands at 717m above the surroundings (see page 84 for more info).

And did you also know // Bit by bit over the years, people (mostly German tourists, wouldn’t you know it) have been returning bits of rock and dust taken from Uluru, with notes of apology. One, from South Australia, weighed 32kg. Another was from 40 years ago. Staff at Uluru receive roughly one of these packages of “sorry rocks” per day.

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