January 23, 2007
The interesting rock formation shown above is found at Eaglehawk Neck in Tasmania, Australia. This inter-tidal rock platform resulted from an unusual set of geological circumstances. Flat-lying siltstone was cracked by stresses in the Earth’s crust, possibly between 160 million years ago and 60 million years ago. The resulting cracks (joints) are seen as three main sets, one aligned to the north-northeast, a second to the east-northeast and the third to the north-northwest. This jointing, exaggerated by processes of erosion, has created the ‘tiled’ appearance. When seawater covers the rock platform, fragments of rock are carried away. Near its seaward edge, sand is the main cause of the erosion. When combined with wave action, the erosional process causes ‘loaf’ or ‘pan’ formations. Away from the seashore, the pavement dries out for longer periods at low tide and this allows greater development of salt crystals. Salt forms on the pavement and thus erodes the pavement’s surface more quickly than it does the joints. Therefore the surface of the pavement is lowered, while the ‘joints’, which erode more slowly, become rims.