Flinders Island and Bass Strait, a geological history

[Now, before I begin this I must make a disclaimer that I am filtering this information through my rather doubtful brain. While I read a little book of essays, Bass Strait: Australia’s last frontier, I may have the wrong end of the stick and be leading you up a very dodgy Bass Strait path. Mistakes are therefore all mine!]

Bass Strait is the body of water between the mainland of Australia and Tasmania, our island state. I never knew that the border between Victoria and Tassie was just south of Wilsons Promontory.

This map of Bass Strait is from Atlas of Victoria, 1982 and I scanned it from Jean Edgecombe’s book “Discovering Flinders Island” 1999

It is, in geological terms, a new strait, for about 350 million years ago south-eastern Australia, right down to Tasmania, was all land. Bass Strait is an isthmus and the peaks on Flinders Island would have been mountain ranges far from the coast. 150 million years ago an east/west trough developed and about 70 million years ago the sea began to “trespass” into the western edge of it — around where Cape Otway is, if you know your Victorian geography. Then the central part of it began to sink, creating the Tamar Valley, the valley where Launceston is now. The Bass Basin was becoming a large estuary.

Bass Strait sea levels over time. (Information from National Parks Service. published in Edgecombe’s “Discovering Flinders Island’)

There was a big invasion of sea 38 million years ago, submerging most of the land mass. However the land bridge has reappeared at least once, due to big shifts in sea levels. The thinking is that the last time it flooded was 10,000 to 15,000 years ago, leaving only the islands poking through. Now Bass Strait is the world’s largest area of shallow water so close to a major city. This map, which is on display in the Museum in Emita, shows how shallow and therefore how treacherous the seas are. The seas are made more dangerous by the tidal pulls of the large oceans either side — the Pacific on the east and Indian to the west.

This map shows the soundings taken to determine the depth the waters around Flinders Island.

Close up of the map, showing the depth of the seas between Flinders Island and Cape Barren Island

Naturally when it was land all living things migrated north and south. However 10,000 years is not long in evolutionary history, so the islands in the Strait share flora and fauna with the mainland and Tasmania. Interestingly it is only the bigger islands (Flinders, King and Cape Barren) that are able to support a wide variety. For example these bigger islands have both tiger snakes and copperheads, while the smaller ones have either one or the other. [I am happy to say that I saw neither!] Kangaroos and wallabies are not found together on any island less than 1000 acres. Jean Edgecombe says this in her little book Discovering Flinders Island

As the seas gradually rose, some animals which had formerly hunted and lived on the broad isthmus….found that restricted island territories were too small to support them. On Flinders, for instance, Grey Kangaroos, Eastern Native Cats and Barred Bandicoots, died out although they are still found in Tasmania.

She goes on to say

Flinders is thus a particularly interesting Noah’s Ark island, representing common territory for the survivors — northern limit for some of the Tasmanian species, southern limit for some to the northern species of of plants and animals.

It is an interesting island in an environmental sense because it wasn’t farmed for quite a while. It was easier to keep stock on the smaller islands because fences weren’t needed. 1952 saw the first allocation of land for the Soldier Settlement Scheme, although there had been settlement and some farming before then. The Island has a variety of habitats that support a variety of plants and animals — granite peaks, coastal heaths, lowlands and lagoons, both fresh and saltwater.
So a little geological history. Next time I want to tell you about more recent history — the exploration of Bass Strait by Europeans. I had often wondered why early explorers didn’t realise that Tasmania was an island. Now I know…..