Why do some sections of coast get better reefs than others?

In previous articles we looked at what makes a good reef shape, and then what shapes on the coast - that is, the coastal features such as headlands - create good surf set-ups. In this article we look under our feet to the bedrock geology and what this means on both a regional and local level for the type of reefs and the surf they create.

The geology underlying any reef is usually part of a greater regional sequence of rocks, such as a sedimentary basin or granite provence. This means that particular geological provinces can lead to a grouping of similar reef shapes. When the elements come together you get a cluster of world class surfing destinations. Over the next few articles we will look at a few well known coasts and their waves, and then observe how this is shaped by the underlying geology.

We've already noted that good surf reefs needs to present a well-graded, convex slope to the prevailing swells. On Australia’s surfing coasts, the most likely geology to create these shapes is horizontal (or nearly horizontal) bedded sedimentary rocks. This is because the weaknesses in the rock are often along those horizontal bedding planes. Powerful swells can help shape the reef, but most importantly any shapes will be gradual; if the steps in the reef slope are equal to the thickness of each bed of sedimentary rock, then for the right thickness of bedding, the reef can form a nicely convex shape. For example, areas with gently sloping sandstones in regular, medium thickness beds seem to get excellent reefs.

The ideal shape is a nearly horizontally bedded reef of rock, eroding to broad flats in the shallows but with a steeping (convex) slope into deeper water. Essential to creating this shape is avoiding large vertical breaks or steps in the shape. These reef shapes are best found where the bedrock is nearly horizontal and of medium thickness (beds approx 0.2 metre – 1 metre thick). It helps if the beds are of relatively hard but not too blocky rocks, interspersed with thin beds of softer rock like shale.

The Sydney Basin is actually a sequence of sedimentary rocks stretching from Batemans Bay in the south, Newcastle in the north, and as far west as Lithgow. Although TV weather people may actually confuse the 'basin' with the Sydney Metro region which only extends to the base of the Blue Mountains. In that context it's a geographical feature, not the basin of sedimentary rocks that actually creates the name. The Sydney Basin rocks are mainly shales, sandstones, and coal measures and usually nearly horizontally bedded. The variation along the coast is in the hardness of the rocks and the thickness of the beds.

For the nerds out there, these sediments were laid down over a period from the Permian (around 250 million years ago) with shallow marine and some glacial sediments overlain by the coal forming swamps and forests. Later these were overlaid by Triassic (180 million years old) sandstones and shales of the Narrabeen and Hawkesbury Groups, laid down by large rivers spreading huge thickness of sand across broad braiding river channels. These were later buried by shales and mudstones in floodplains and lakes.

Example 1: The Sydney Basin - the good reefs

Some parts of the Sydney Basin offer a high density of good reef breaks (eg: the South Coast) whereas other parts of the Sydney Basin are characterised by paucity of reef breaks (eg: The Eastern Suburbs). Lets look at what works first. The Coal Coast, Shellharbour and Ulladulla regions offer a high density of good-to-great surfing reefs, every headland has something surfable, and sometimes epic! Some spots are truly world class and a lot of points have waves on both the north and south sides - an unusually wave rich situation. What is happening that allows almost every point to form a surfable reef?

The answer lies in the geology of the region, being mainly thinly bedded sandstones, siltstones, shales and coal seams of the Illawarra Coal Measures. All these amazing reefs are formed where horizontally bedded shales and sandstones of moderate hardness have eroded to form convex smoothly graduated reef slope. 

In other parts of the Sydney Basin geology where the sandstone beds are thicker and harder then the reefs can have significant steps in their profile. For example the Cronulla Region reefs are formed by Narrabeen Group and Hawkesbury Group sandstones. These are characterised by horizontal beds of harder quartz sandstone between 0.5 metre and 3 metre thick, interspersed with thinner beds of softer siltstones, shales and mudstones. Due to the thick hard beds layered with thin soft beds, the reefs will often have abrupt steps in their profile, creating a distinct ledge. It is no accident that surfers talk about many of Sydney’s sandstone reefs as ‘ledges'. The Cronulla Region has been blessed by the fact the Hawkesbury Sandstones are not too hard or too thickly bedded. The reefs are convex, with the odd ledge or two (Shark Island anyone?).

Other regions of Australia characterised by horizontally bedded sedimentary rocks include Victoria’s Surf Coast, although the rocks are limestones and not sandstones.

Example 2: The Sydney Basin - coasts with no reefs

In other parts of Sydney (eg: the Eastern Suburbs) the predominantly Hawkesbury Sandstones are too thickly bedded and blocky to form good reef breaks. From the north side of Botany Bay to North Head the coast is characterised by cliffs of blonde Hawkesbury sandstone. If you look at the profile of the cliffs, the ledges are usually 2 to 3 metres thick, and sometimes more. This situation is repeated near Jervis Bay where the hard Nowra Sandstones are too thick and hard to create good reefs. Conversely the hardness and uniformity of the Nowra Sandstones have created world class rock climbing at Nowra and Point Perpendicular.

These thick, hard sandstones extend under the sea surface and result in vertical and blocky underwater topography, hardly conducive to the well-graded covert reef profile of a good surfing reef.

It is only north of Curl Curl that the Narrabeen Group Sandstones start to outcrop once again. With more softer siltstones and shales between the sandstone beds and generally fewer thick beds of uniformly hard sandstone, the reefs are more likely to form the convex profiles and less likely to form steep blocks and steps. The Narrabeen Group rocks start to appear north of the base of the cliffs at Dee Why, hence surfable bedrock reefs at Dee Why Point and Long Reef and some half decent reefs further north

There are many different types of rocks and geologies that shape coasts. Over the next few articles we will look at some of the important characteristics of Australia’s best known surfing coasts.

Coastal Creationsim is an eight part 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 though he's currently doing a lap of Australia with his family. Read past articles:
Part 1: Basic reef shapes
Part 2: Complex curves
Part 3: The good, the bad, and the ugly of coasts