Voodoo is a complex rock platform joined to the land and sloping rapidly off into deep water and heavily exposed to the south east. It draws any swell from this direction along a reef which is separated from the land surface. Sydney's most common swells are from this direction. When regional beach swells are 2M – 2.5M, Voodoo can be near twice the size and pack a serious, Hawaiian-style punch. The rock platform is sheltered from the north by a substantial cliff and thus is happy in any wind from that quarter and including the classic Sydney north-east seabreeze. Surfers say it's optimum is at low to mid tide.
Geology.
Our local and regional environment owes so much to its geological heritage. We live in the Sydney Basin which is lying on a continental shelf of sedimentary rocks several kilometres thick that was laid down between 300 and 200 million years ago. Our present landscape was created by tectonic uplift (the Earths plates coliding with each other) which could explain the slope of the platform and reef.
The rocks around and about are generally thought of in simple terms to consist either of sandstones, shales or volcanic rocks. There are Wianamatta shales lying above Hawkesbury Sandstone which in turn rests on Narrabeen shales. The hardness of the sandstone is often related to the iron or other mineral content and erosion of its strata (layers in the ground) can be varied because of this.
Formation.
Rocky flat platforms along the coast like this one are formed by erosion of softer material above a hard layer. From the rock platform at the surfing site, a reef runs out south east for quite a long way, exactly 1.1 kilometres. The waves build up along this and eventually apply the relentless force of the waves to the surface and cliff behind the platform. Basically, wave formation along the coast is the result of wind interacting with the water surface and the depth of water. Wind causes the formation of crests and troughs which give a wave shape.
Note the change to the wave shape as it brakes. Together with wind and rain hitting the platform, it gradually wears down. Often, the back of the rock platform (the bit which hasn't been eroded yet), forms a cliff, while the ocean edge of the platform steps down into the water. These erosive actions create a maze of cracks and holes that while attractive to marine wildlife provide different rock types to weather in different ways.
Activities.
The platform provides surfers with interaction above the surface. The main wave is a left reef break located 500 metres South of Potters Point. Best around half tide or low tide when the swell is small. On higher tides the left isn't as hollow, though a shorter fun right breaking from the southern end of the platform is available. When you get into the water you instantly feel the pressure of waves. It is this pressure that contributes to the erosive forces of the waves. Even on a small day (1.2M from the SE) it will still be a scary spot. When Voodoo works, it is a heavy left breaking wave that builds up to the rock platform and in the right SE swell it holds up to 6M and is at its greatest during the Spring months becoming a consistent 1M – 2M between Dec-Mar.
The reef is generally flat and uninspiring, until it reaches the sand. Here the reef drops from about 20 metres to over 28 metres. The same change in depth (about 8M) is what forms the waves the surfers ride to the rock platform and provides the forces that have formed the platform. It is believed that this was a previous cliff formed prior to a rise in sea level. The reef area away from the platform provides divers with a safe site composed of a flat top with some small cracks, a small wall of a metre or so and then a quickly slopping reef to the sand. All in all, an excellent dive for the more experienced diver.
It's time to look around and provide your answers to a couple of observations.
Q1. Waves are drawn to Voodoo along a reef. From how far away do the waves build up?
Q2. What causes the waves to form and come ashore at the Rock platform?
Q3. Walk onto or as close as possible to the rock platform and determine which direction it slopes towards?
Q4. Add a photo of something to identify you viewing Voodoo near to the Boat Harbour Aquatic Reserve.
I hope you enjoy your time here as much as I do. You can log your find online immediately. If I fail to receive the answers from you within 14 days or so, the log may be deleted!