A wave-cut platform (also known as a shore platform) are expansive, narrow flat areas projecting seawards from the base of sea cliffs. They are created by the process of erosion between high and low tide levels, where the waves have the most energy. Wave-cut platforms are usually only visible at low tide as huge expanses of flat rock. In the case of where you are standing, the landward side of the platform is covered by sand forming a beach.
The Newcastle coastline consists of layers of sedimentary rocks – sandstone, conglomerate, shale, coal and tuff, deposited in the Permian, around 250 million years ago. These sediments were originally deposited in river sands and gravels, floodplain muds, peat swamp and volcanic ash. The wave-cut platform in front of you consists of hard and resistant sandstone, with minor softer shale layers. Wave erosion along horizontal to gently dipping bedding planes (the sedimentary layers) results in the flat to gently sloping surface of the wave-cut platform. You will also notice vertical intersecting joints (cracks in the rock) in front of you. These are areas of weakness within the rock where, through the process of erosion, rectangular rocks ‘break off’. These joints control the shape and orientation of the platform margins.
Origins of Wave-Cut Platforms
Over the last 1.8 million years, the sea level has risen and fallen due to increases and decreases in global temperatures. During glacial periods, sea levels were low as ocean water was frozen in huge icesheets. During warm interglacial periods the icesheets would melt, resulting in increases in sea levels. 18,000 years ago the Earth was in the last peak of glaciations where sea levels were around 120 metres below their present level. The Newcastle coastline consisted of a series of hills and valleys and was situated around 20 km east than it is today. Rivers flowed through the valleys cutting deep channels into the Permian bedrock and the hills were rounded by erosional processes.
Towards to the end of the glacial peak, increasing global temperatures melted the icesheets which resulted in an increase in sea levels. The rising sea levels flooded the valleys and crept up the hillsides. Wave-cut platforms began forming around 6,500 years ago, when the sea reached its modern day level. The continued wave action eroded small cliffs on the hillsides.
Wave erosion of the hillsides is caused by the hurling of tonnes of water against the rocks with great force. The energy produced by the waves compresses the air within the joints of the rocks which results in the widening of the joints. At the same time, the wave energy also moves broken rocks, sand and other particles against the eroded surfaces which results in scour erosion. Over time, the wave energy cuts deeply into the cliffs, undercutting the rocks above, and resulting in the formation of a notch. The overhanging rock collapses, which results in retreating cliffs and wave-cut platforms.
Above: An example of gutters and rock pools that have formed in joints by wave erosion
Geological Features of the Wave-Cut Platform
This wave-cut platform contains some very interesting geological features. This part will require you to take a little stroll around the rocks to view some of these features. Please be aware of your footing and don’t attempt this when the tide is high or the surf is very rough.
Located at S32 55.623 E151 47.523 is an igneous intrusion – a basalt dyke. The dyke formed around 100 million years ago when molten basalt from the Earth’s mantle (at a depth of ~100km), was forced upwards through cracks in the overlying sedimentary rocks. The molten basalt solidified as a wall-like intrusion within a joint in the surrounding sedimentary rock. If you look closely at the edges of the dyke (called the chill margins) that the basalt is finer grained than the middle. This is due to the more rapid cooling of the edges compared to the middle. Another interesting feature of this dyke is that there are two different chill margins, one within the other, as a result of a second pulse of molted basalt being forced through the joint. The dyke is visible on Google Earth - change the bottom map on this page to satellite and zoom into the dyke's waypoint.
At S32 55.704 E151 47.477, S32 55.711 E151 47.473 and S32 55.710 E151 47.480 you will see some examples of petrified tree stumps in their original growing positions.
Fallen trunks and branches are abundant in this area and lay along the bedding planes of the sedimentary rocks pointing in the direction of the paleoenvironment water flows. The fossil tree remains are a brown-red as they now consist of limonite. These trees are Glossopteris flora that grew in the Newcastle region around 250 million years ago. Where you are standing right now would have been a muddy floodplain with a forest of Glossopteris trees. The trees were more than likely buried under a flood of sandy water. The broken limbs would have floated along on the water currents until they became water logged and sunk to the bottom.
The fossilisation would have begun soon after the trunks and branches were buried. This process involves minerals replacing the woody tissues, turning the wood into stone. The weight of over laying sediments would squeeze water through the sedimentary layers, which carried dissolved minerals. When the water encountered woody tissue the minerals would leech into the wood, replacing the woody tissue molecule by molecule preserving the detailed cell structure, growth rings and the bark texture.
Have you ever been to the beach and wondered how this rock honeycomb forms? This is a form of weathering appropriately named honeycomb weathering. This honeycombing is in the sandstone on the wave-cut platform S32 55.698 E151 47.466 the honeycombs are small, closely spaces depressions up to a few centimetres in depth and diameter. Sea water pools in depressions in the sandstone and within the poor spaces, as the water evaporates the salt is left behind and formes growing crystals. The growth of the crystals dislodges sand particles in the rock. These dislodged sand particles are then blown away by wind leaving small holes. As the wind blasts the rock, it swirls sand particles around within the holes, enlarging the holes.
Have you ever seen the above brown, red, orange and yellow coloured layered iron deposits within the sedimentary rocks and wondered what it is? These deposits consist of the mineral limonite (just like the fossilised trees). Limonite is a hydrated iron oxide and is commonly found in joints, along bedding planes and replacing plant material as in the case of the petrified Forest. The reason that you can see a lot of these iron deposits on the weathered sandstone is that limonite is more resistant to erosion than the sandstone. You will noticed that the raised zones occur parallel to the joints, and there are resistant caps on the softer sedimentary beds as well as the petrified forest sits above the wave-cut platforms surface due to erosional resistance.
Now for a quick geology lesson.....something we geologists do on a regular day out mapping in the field – measuring the strike of a geological feature. For this a compass is required. The strike is the line or orientation of a bed, fault of any other planar geological feature and is measured in degrees or bearing. To find the strike you put your compass parallel to the bedding structure that you wish to measure and read off the orientation. I have created a video of how to do this if you need help:
How to Measure Strike
Now to earn your log!
1. What is the strike direction of the petrified wood grains located S32 55.704 E151 47.477, S32 55.711 E151 47.473 and S32 55.710 E151 47.480?
2. What is the strike of the dyke?
Please email me your answers (using the 'Contact Member' feature when you click on our Team Name) before logging ypur visit and wait for confirmation. For the answers I only want the bearing in North-South, East-West etc and not the actual degrees but you can include this if you want (N25°E for example). This is so as not to discriminate against people who only have a basic compass or are using a map to determine the strike direction.
Lapidus, D.F. 2006. Collins Internet-Linked Dictionary of Geology. HarperCollins, London.
Nashar, B. 1964. Geology of the Hunter Valley. Jacaranda Press, Sydney.
The City of Newcastle & Hunter Central Rivers Catchment Authority. "Geology of Rock Platforms - Newcastle Coast" Pamphlet.