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Today Settlement Point arches out into the oceanic waters of Bass Strait.

For much of the last 2.5 million years however this would have looked out towards a sheltered inland bay where marine animals called forams lived in the shallow relatively warm waters. These provided the source of calcium that blew up onto the island and created the limestone we now see here.

motif Settlement Point panorama

The easiest way to appreciate the landscape at Settlement Point is to think of a thickly iced fruit cake.

The cake is the granite which was cooked up and cooled deep underground around 380 million years ago. Over millennia this rock was brought to the surface.

In the last few million years it has been ‘iced’ with a layer of limestone. This was created when shells washed up on shore and were blown inland.

There they formed calcareous dunes up to 10 metres thick, and later cemented together to form limestone.

Settlement point diagram

Looking at the dunes behind the beach you can see how sand accumulates. The limestone stacks sticking up from the water formed as calcareous sand dunes as the sea level rose after the last ice age.

Notice the clear angled bedding planes in the rock and how it is easily eroded to form arches and sea stacks with wave cut notches.

... the granite ‘fruity’ bits

The granitic rock at Settlement Point formed as a molten magma about 400 million years ago.

In contrast to most of the other granitic rocks on Flinders Island, this rock has a greater abundance of black iron-magnesium rich minerals such as hornblende and biotite. Geologists call this type of rock a ‘granodiorite’.

The small black blobs [below] that occur within the granodiorite are clasts of older igneous and sedimentary rocks that have been fragmented and captured by the magma as it intruded through the crust.

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motif geological map detail of Settlement point area

The rock at the boat ramp ...

At the boat ramp we’re looking at the mafic inclusions. granite detail

If you can imagine putting some oil into a cup of water and giving it a stir, the oil seems to break up into little round globules - the oil and water being immiscible which means they won’t bind when stirred. The same thing happens in a granite melt.

granite detail

You’ve got the granite melt here which is the white phase and then the mafic phase has intruded into the granite phase and the two are immiscible when stirred and so the mafic inclusions are like the oil droplets.

The mafic inclusions start as a basalt. Some are basalts some are hybrids with the granites.

granite detail

This grey example is one where you can actually see some basalts in it.

In effect there’s the granite phase and then there’s the basalt phase, so this is probably a 50 50 mix of granite and basalt.

geological map detail of Allports Beach area
motif granite detail motif