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EXCERPT:
William Bay National Park
a safe spot for swimming and
exploring
At William Bay you can again see the core of the ancient alpine mountain range (p.18). As the range was slowly weathered away the Earth’s crust was relieved of an immense load and the remnant core of the range gradually rose to the present day surface. The granite then exposed was weathered to form rounded outcrops and boulders and the sheltered areas they now enclose are safe for swimming.
Recently much of this granite was covered by sand dunes and young limestone.
When the ice melted at the end of the last ice age and the sea level rose
the dunes and limestone close to the coast were not hardy enough to resist
the ocean swells and have been eroded away, leaving the resilient granite.
So most of the hills on land are dunes or consist of soft limestone, whereas
the small exposed islands and headlands are almost exclusively granite.
Elephant Rock (right) is just to the east of William Bay. Here the ocean has cut a small inlet into the granite – but why here?
Whereas granite is almost the only rock type in this area, on the eastern shore of this small inlet is some darker rock. This is dolerite, a volcanic rock originating from below the Earth’s crust and injected into the overlying granite. Here its small crystals are evidence it cooled and solidified quickly at a shallow depth. Also, as can be seen in the photo, the dolerite lies flat and undisturbed upon the granite. The preservation of these original characteristics of the dolerite is conclusive evidence that it is much younger than the granite and was injected into the granite as it slowly rose to the surface.
The explanation for this inlet is therefore that the dolerite intruded into a joint in the granite as the granite was lifted (“buoyed” is perhaps a better way of describing this process) to shallow depths. When these rocks were exposed the “soft” dolerite was eroded away, first by drainage of rain water and then by the ocean’s force, leaving the small inlet.
It is likely that many other gaps and fissures along the coast have been formed this way, but that the evidence – some remaining dolerite – has been completely weathered away.