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Introduction
The steep cliffs, rock stacks, caves, headlands and bays that form the Port Campbell coastline are the result of continuous marine erosion on generally flat lying limestone rocks formed some twenty million years ago.
The south-western coastline of Victoria was not always land. The layered rocks that can be seen along the coastline were formed in a shallow sea about twenty million years ago (the geological time period known as the Miocene). During the Miocene the coastline of southern Victoria was much further inland from its present position, in fact, it was located along a line that joins Sunbury to Ballarat-Hamilton and to just north of Mt. Gambier.
An abundance of marine algae and animals lived in this ancient shallow sea. Calcium-rich algae and shellfish and bryozoans lived on the sea floor. Bryozoans are similar to corals (like the ones in the Great Barrier Reef) in that they grow in colonies, except that bryozoans tend to live in cooler waters.
The shellfish and bryozoan colonies formed close to the shoreline in this ancient sea and the wave action along the shore broke up the shells and bryozoans into small, sand-sized pieces that were swept short distances and over time built up large sandbanks in the shallow sea.
Around five million years ago (in the Pliocene) this shallow sea became land and the vast sandbanks of broken fragments of marine animals compacted to form rock known as limestone.
Throughout the last two million years (in the Quaternary), sea levels rose and fell for short periods of time. As a consequence, during these times the coastline shifted further north and south of the present day position. For most of the time however the limestone was exposed to the atmosphere and the attack of rainwater and marine erosion.
The flat-layered limestones seen in the present coastline vary in hardness and as a result weather and erode at different rates. More rapid rates of erosion also occur along deep vertical cracks (known as joints) that run south-west to north-east throughout the limestones. The location of these joints through the rock has strongly influenced the position of gorges and inlets along the coast.
The coastline is continuously being undermined and eaten away because of the generally rapid erosion of the limestone and the constant action of waves and seaspray on the cliffs. Slabs of cliff occasionally fall into the sea. The sand, gravel and debris that wash into the sea make the waves even more abrasive as they crash against the coastline. This quickens the erosion process.
Ocean currents eventually disperse the sand-sized particles that result from this erosion, and over time the particles settle to form new sandbanks somewhere offshore. These may become part of the land in the future.
The striking geological features of the Twelve Apostles, London Bridge, Loch Ard Gorge, Thunder Cave and many others along the coast of the Port Campbell National Park are the result of the constant attack of the sea on limestone cliffs that are easily weathered and eroded.
This geological process will continue to erode the coast in the Port Campbell region.
This Earthcache
From your vantage point at the published coordinates, you can see a number of typical features of this evolving coastline.
To the east, you can see a new sea cave forming. This may extend inland a hundred metres or more before roof sections collapse forming an arch, a blowhole or eventually a gorge, or it may break through the headland forming an arch and subsequently a sea stack.
To the south east, you can see Bakers Oven Island, with the distinctive arch raised out of the water. Erosional forces from wind and waves have attacked and worn away the weaker rock, probably around a crack or fault, and have cut through the island. Eventually the arch will grow to a point where the roof will collapse, forming two sea stacks.
To the south (and to the right of the island) you can see the bases of two sea stacks that have long since collapse into the sea, leaving only the harder base rock as two shallow reefs.
In order to log this earthcache, please answer the following questions, and email your answers to the cache owner:
1. How many major rock layers can you count on the island?
2. Estimate the height of the island from sea level.
3. Estimate the width and height of the "baker's oven".
4. You will also need to take a photo from the published coordinates, showing the coast line or the island in the background, and including at least your GPSr. This photo should be uploaded with your online log. An example photo is shown below.
No photo, or no email with the answers to the questions above, and your log may be deleted.
Do not visit this earthcache at night - you will not be able to see anything, and therefore you won't be able to answer the questions or learn anything. You also will not be able to take the photograph that is a requirement for logging the earthcache.
Please be aware there are cliffs nearby, but there is no reason to put yourself in any danger in order to complete this earthcache (it can all be done from the carpark). And remember, "take nothing but photographs, leave nothing but footprints"!
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
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