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Devil's Marbles
20° 33'S  134° 16'E.
Devils Marbles
Devils Marbles  

In the Davenport Ranges north of Alice Springs, the Stuart Highway passes close to an unusual rock formation first recorded while the Overland Telegraph was being built in 1872. This formation, containing many rounded boulders, is known as the Devil's Marbles.

The Devil's Marbles began forming about 1700 million years ago when a layer of molten magma was deposited below a layer of sandstone and cooled into granite. Shrinkage formed a pattern of right angles cracks, called joints, in the cooling granite and, during succeeding years the porous overlying sandstone allowed groundwater to percolate down to the granite and into the joints. Groundwater filtering in the joints reacted with some minerals in the granite to form clays; a warm and humid climate helped this weathering process which occurred more quickly on the corners of blocks because more surface of the rock was exposed there.

Eventually the overlying sandstone eroded away, exposing the granite to wind and rain. Clay and softer weathered corners and edges of previously buried granite blocks were washed away leaving boulders on top of each other and scattered over rock platforms.

Devils Marbles - page 2
Once the naturally grey coloured granite was exposed to the atmosphere, water filtering through the boulder surface rusted iron minerals in the rock and changed the surface colour to the red-brown so obvious today. Whenever a rock splits naturally the native grey colour is exposed.

Erosion of exposed boulders continued in the form of "onion-skin" weathering where layers of rounded surfaces have lifted away from the boulder. This exfoliation is caused by water seeping into the boulder surface and reacting with feldspar minerals altering them to soft clay which is much weaker than the surrounding rock allowing the outer layer to peel off along fine fracture lines and fall away. Layers of exfoliated rock are visible in the second photograph from the bottom.

A more obvious change take place when a boulder splits completely and one, or both, parts fall away exposing a flat, grey surface (see bottom photograph). In this case rainwater penetrates into fine cracks in the rock and converts minerals in the granite into clay weakening the rock at that place. Eventually, as the weakness spreads, the rock breaks under its own weight.

Erosion of exposed boulders continued in the form of "onion-skin" weathering where layers of rounded surfaces have lifted away from the boulder. This exfoliation is caused by water seeping into the boulder surface and reacting with feldspar minerals altering them to soft clay which is much weaker than the surrounding rock allowing the outer layer to peel off along fine fracture lines and fall away. Layers of exfoliated rock are visible in the second photograph from the bottom.


Information.
   The text above is based on displays at the Devils Marbles.


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