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Saturday, February 27, 2021
It’s been a sacred site for thousands of years known to the Kaytetye aboriginal people of Australia as Karlu Karlu and is known as such in four local languages. When first seen by Europeans in the nineteenth century, however, it was not long before the place had been given a new name – The Devil’s Marbles.
Many of the ‘marbles’ are poised, almost at tipping point it seems and with spectacular visible effect.
Situated in the Northern Territory, just over a hundred kilometers from the nearest town of any size, Tennant Creek, the broad, shallow valley in which the stones can be found is littered with these huge, sometimes almost spherical granite boulders.
Essentially there were two steps in their geological formation. Magma within the crust of the earth hardened millions of years in the past and so the granite was formed. The layers of sandstone atop the granite place immense pressure upon it and as the Earth's crust folded the granite was lifted upwards.
The sandstone surrounding the granite erodes, when exposed to the elements, much quicker and eventually the granite was almost above ground. Without the sandstone on top and around it the pressure on the granite lessened, which meant that it expanded. As it expanded it cracked, forming huge square blocks.
The second step is the inevitable exposure to water, causing the rocks to crumble, little by little. This formed a layer of very loose material around the base of the blocks which, when the rocks had surfaced completely, was washed and blown away.
As the millennia passed the rocks were weathered both chemically and mechanically. The rocks were rounded as chemical processes act with more effect on the edges of huge square boulders. This gives some of the rocks at least the look of an onion, with different layers exposed. The process is known as spheroidal weathering.
Some of the rocks have been split due to a process known as solarisation. When the sun sets the extreme heat of the day gives way to a bitter cold. So, the rocks expand as the run rises and shrink as it sets. This pressure has caused many of the rocks to crack, and some have even split in half.