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.
REFERENCES.
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.