Siltstone Rock Platform

October 28, 2006

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Provided and copyright by: David Morgan-Mar
Summary authors & editors: David Morgan-Mar

The photo above shows a siltstone rock platform on the coast of Tasmania (Eaglehawk Neck, Australia). This siltstone was fractured 60-160 million years ago, creating stresses in the rock that eroded at different rates. Where waves frequently wash over the siltstone, the fractures erode more quickly, due to the abrasive action of water and sand, leaving in places characteristic cobblestone-like "loaf" patterns. By contrast, well above the low water mark, the fractures erode more slowly. The principal cause of erosion here is micro-flaking, caused by salt crystallization, which occurs predominantly on the flat areas away from the fractures. This creates an equally characteristic, but very different "pan" formation, seen on the inland side. Note that on the above photo you can see three parallel sets of fractures; two almost perpendicular to each other and the third at an oblique angle.

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