Weeroona Island, also known as Port Flinders and Mt Ferguson, is a distinctive feature jutting out from the otherwise flat landscape along this section of the Spencer Gulf Coastline.
Geology of Spencer Gulf
Spencer Gulf formed when tectonic movement caused the large landmass Pangaea to split into Laurasia and Gondwana. Australia formed from the Gondwana land mass. Sea level changes over millions of years have shaped Spencer Gulf's coastline.
The basement rock of Spencer Gulf comprises three major geological provinces: the Gawler Craton-Eyre Peninsula Block (formed 2300-600 million years ago) on the western side of the Gulf, the Adelaide Geosyncline (1400-560 million years ago) on the north and north-western side of the Gulf and the Gawler Craton-Yorke Peninsula block (2300-600 million years ago) of Yorke Peninsula. Weeroona Island is situated on the Adelaide Geosyncline.
This part of the coastline experienced glaciation approximately 10,000 years ago. Evidence of this includes "erratics"- rocks that were dropped by melting glaciers. These rocks often appear "out of place" along the shoreline.
But is this an island?
It is very unlikely that you crossed water to reach Weeroona Island as you drove along the causeway. The island is a hill (named Mt Ferguson) that is almost always surrounded on three sides by water during normal tidal events. However, during large tidal surges, this hill becomes an island. This type of geological formation is unique to South Australia and is similar in age to the Flinders Ranges, one of the oldest in the world.
Tessellated Wave Cut Platforms
This area contains good examples of wave cut platforms, some of which are highly tessellated.
Wave Cut platforms form after destructive waves hit against the cliff face, causing undercutting between the high and low water marks, mainly as a result of corrosion and hydraulic power, creating a wave-cut notch. This notch then enlarges into a cave. The waves undermine this portion until the roof of the cave cannot hold due to the pressure and freeze-thaw weathering acting on it, and collapses, resulting in the cliff retreating landward. The base of the cave forms the wave-cut platform as attrition causes the collapsed material to be broken down into smaller pieces, while some cliff material may be washed into the sea. This may be deposited at the end of the platform, forming an off-shore terrace.
Tessellation occurs when this platform is cracked by stresses in the earth’s crust. The resulting cracks (joints) are often aligned in distinct directions. This jointing, exaggerated by processes of erosion, has created the ‘tiled’ appearance. When seawater covers the rock platform, fragments of rock are carried away.
To claim a find on this earthcache, email the following answers:
1. How long ago did Pangaea split into Laurasia and Gondwana?
2. What type of stone forms the hill that has created Weeroona?
3. Can you see any erratics near GZ?
4. Observe the tessellations in the wave cut platform near GZ. Estimate the average distance between the eroded cracks.
5. Observe the wave cut platform at the second waypoint. How does this platform differ from the one at GZ?
6. At the third waypoint you will find information about the marine biodiversity in the area. Wave cut platforms are important habitats for what?
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