Welcome to the Iatest Coastal Creationism, the series that looks at the geological forces that have created our coastlines. Till now, we've been examining particular phenomena such as how the ideal reef shape is formed, or why some stretches of coast perform better than others.

From this instalment onwards, the author, Chris Buykx, is going to retract the viewfinder and take a wider scope. As always, he'll describe how coastlines were created but with an emphasis on broader regional dynamics.

To begin with, Chris sets his sights on the coastline south of Adelaide - from the Mid Coast through to the Coorong.

Gull Rock on the Mid Coast (Photo Shane Daw Westpac Lifesaver Rescue Helicopter service)

South Australia is probably not at the top of most surfers lists as a destination, however the great diversity in coast and waves does give us some helpful insights in to processes at work in the surf zone. In this article I am specifically looking at the coasts south and east of Adelaide because there is a lot happening geologically. It is interesting because there have been a number of important geological events at regular intervals for over 600 million years. Significantly, some of those processes are still happening today.

Most readers would be aware that Australia is a geologically ancient continent, with some basement rocks in WA over 3,500 million years old. Some of the land surfaces in the west have gone over 200 million years with only weathering and erosion. The south and east coasts of South Australia are interesting because of the unusual combinations of ancient basements rocks, extensive glaciation when part of Gondwana, and then a long history of tectonic movements and sediments that cover the period that Australia rifted from Antarctica and right through until present day.

The result of this is incredible diversity in geology, coastal geography, and ultimately the shape of the reefs and sand banks. Adelaide surfers know instinctively that the character of the breaks changes massively between the Mid Coast, Kangaroo Island, Victor Harbor, the Coorong, and the Limestone Coast. This change in breaks is not just a function of aspect and exposure to swell, its also about rocks and sediments that make up the coast. In this article we'll look at each region as we head south from Adelaide and examine the factors at work.

1. The Mid Coast (O’Sullivans Beach to Sellicks Beach):

Many surfers have a love/hate relationship with the Mid. They curse its lack of exposure to swell, but love its abundant peeling reef breaks. It seems criminal that such great reefs are blocked from much of the Southern Ocean swell.

The obvious question is why there are so many decent surfing reefs here but not further south. Well, i'ts a long story that goes back to the time Antarctica and Australia split up. The Southern Ocean was the last major continental rift in the break up of Gondwana. The rift opened like a zipper from west to east, with blocks being uplifted and down-warped with massive faults between the blocks. One block subsided about 45 million years ago as the ocean opened and the resulting basin slowly filled with a variety of shallow marine sediments. The thinly bedded silt stones, fossiliferous limestones and sandstones formed the Willunga Embayment, part of the St Vincents Basin.

Importantly, these bedded rocks have been slightly tilted and now form the gently dipping thinly bedded outcrops discussed in Coastal Creationism 4. These beds erode to form ramps and convex lozenges that are perfect for turning long-lined swell into peeling walls. Interestingly, these same rocks also form the rich, limey soils of the McLaren Vale and some of Australia’s finest wines.

The view south from Sellicks Beach showing the escarpment that marks the end of the Mid Coast

2. Southern Fleurieu Peninsula and Kangaroo Island:

The coast, landscape, and geology abruptly change south of Sellicks Beach, at the fault scarp created by the uplift of the southern end of the Mt Lofty ranges. From the escarpment to Cape Jervis, including much of Kangaroo Island, we see the older folded and hard rocks. These rocks are around 520 million years old and part of the Adelaide Geosyncline that makes up the Mt Lofty and Flinders Ranges. They are generally steeply folded and include very hard beds of metamorphosed sandstones and some granite intrusions. The result is extremely blocky outcrops that erode in to chaotic, steep and chunky reefs that don't tend to form good surfing reefs. The are a few locations on the south coast of Kangaroo Island where the folded rocks dip at a shallow angles toward the ocean, creating rare lozenges and ramps that turn into serious reef breaks on their day.

3. Victor Harbor Region:

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. Despite the tourism hype that Middleton is South Australia’s home of surfing, the waves are unusual compared to almost all other Australian beaches and can be underwhelming for those expecting some power. The broad flat and virtually featureless 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 unheard of.

The contrast between Middleton and Waitpinga could not be more extreme. So what's going on? As discussed in the last Coastal Creationism, 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.

Waitpinga Beach. Note the short, steep beach profile characteristic of this stretch of coastline

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.

4. The Coorong and the Limestone Coast:

From Middleton to Cape Jaffa the entire coast is relatively young, comprised of sediments laid down in the last few hundred thousand years. We have already discussed how these recent ice ages resulted in much lower sea levels and the coast extending out toward the continental shelf. During these periods huge amounts of sediments were laid across the region. When sea levels rose, these sediments were pushed up in to beach barriers and parallel dune ridges. The Younghusband Peninsula is a beach-dune barrier created when the sea rose to its current high levels, stabilising around 6,000 years ago. Behind it is the Coorong, a back barrier lagoon created between current dunes and the dunes formed at the old shoreline approximately 125,000 years ago. The sands are derived mainly from the sediments brought down the rivers. That is, they are quartz, silt and clay, and not readily cemented into rocks unless buried for millions of years.

The same process has been replicated on the Limestone Coast, forming many ridges parallel to the coast. However the coastal ridges are a recent and minor feature. The Limestone Coast is dominated by a broad and thick sequence of shallow marine limestones that were laid down between 15 and 38 million years ago, as the Southern Ocean opened. The limestones are often thickly bedded (>1m thick), as a result the resulting reefs tend to be blocky and a bit random, lacking the smoothly convex ramps found in other geologies. There are a few decent reefs and these occur where thinner beds occur between harder beds of limestone at an appropriate depth in the surf zone so as to form a decent ramp.

Coastal Creationsim is an ongoing series written by Chris Buykx. Chris is a geologist, traveller and lifelong surfer. Specialising in eco-tourism, his passion is interpreting nature and the environment. Chris is a resident of Sydney’s Northern Beaches.