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.
The rock at the boat ramp ...
At the boat ramp we’re looking at the maffic inclusions.
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.
You’ve got the granite melt here which is the white phase and then the maffic phase has intruded into the granite phase and the two are immiscible when stirred and so the maffic inclusions are like the oil droplets.
The maffic inclusions start as a basalt. Some are basalts some are hybrids with the granites.
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.