The Seascape
​Many surfers have pondered the stark contrast between Waitpinga and Middleton, only 20 km to the north-east. Waits and Parsons are steep, powerful beach breaks that often barrel and form regular rips and banks. The sand is blonde and coarse and the ocean waters clean and green. Whereas Middleton is a broad, gently sloping beach of grey sand lapped by similarly grey, silt-laden waters. The waves are unusual compared to almost all other Australian beaches. The broad flat beach does not readily form rips and gutters. Waves typically form spilling peaks that roll slowly across a broad surf zone. Steep wave faces are rare and short-lived, barrels rarely heard of.
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The contrast between Middleton and Waitpinga could not be more extreme. So what's going on? It's all about sediment supply. The beaches of Middleton are the western end of the mighty Murray River mouth. Despite the Murray waters rarely reaching the sea, the river has dumped incredible amounts of sand and silt over the eons. The relatively high wave energy of the Southern Ocean is constantly reworking these sediments and has laid them out in a broadly sloping beach that extends for over a 180km of the Younghusband Peninsula and Coorong. More significantly, a gently sloping ramp of sediment extends a great distance and depth offshore of the beach. At Day Street it is over 10.5km offshore before you reach the 30 metre depth contour (isobath). Conversely at Waitpinga and Parsons the beach is derived of coarse quartz sands derived from nearby granite and metamorphic rocks. With relatively limited sediment supply, the beaches are comparatively narrow and exceptionally steep, despite a similar wave climate. Just looking at the waves you get the feeling they are coming out of deep water and detonating on a steep beach. Unsurprisingly it's only 1 kilometre from the beach to the 30 metre isobath.
It has been suggested that the deep channel of the Backstairs Passage between Cape Jervis and Kangaroo Island was carved by glacial action. If you go back 250 million years ago to the Permian period, Australia was still part of Gondwana and the land was over 70 degrees south. The planet was gripped by multiple ice ages and ice sheets spread across the land. There is certainly evidence of glaciation with some basement rocks carved with deep grooves and glacial sediments deposited in numerous locations. In fact the granite outcrops at Knights were passed over by Permian glaciers and sediments in from the glacial lakes can be found in the cliffs. However this has little to do with the incredible wedge that rebounds of the cliffs.
The Encounter Bay granites are heavily jointed and intruded by dykes. This joints are oriented SSW/NNE and hence the blocky granite forms a ramping wall perpendicular to the swell, resulting in some wave energy rebounding as a wedge travelling at an oblique angle to the main swell. The result is plenty of amusement for the bodyboarders and their orthopaedic surgeons.
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Adapted from Chris Buykx's Swellnet series on Coastal Creationism.
