Bunbury: A city on basalt

On the water: Basalt formations can be seen in various locations along Bunbury's picturesque coastline. Photo: Supplied.

On the water: Basalt formations can be seen in various locations along Bunbury's picturesque coastline. Photo: Supplied.

A spectacular formation of black rocks can be seen and enjoyed along the northern ocean beaches of Bunbury.

Its northern most point is Point Casuarina, hidden ever since the construction of the land-backed wharf.

It was noticed by the crew of the Naturaliste - one of the two ships of the French expedition under Baudin - as the entrance to an inlet in June 1801.

However, it was more closely investigated in March 1803, when Baudin anchored the Geographe off the point to have a brief survey done of the inlet, which he named after the expedition's botanist Leschenault de la Tour.

As a significant geological phenomenon, the basalt tells of plate techtonics, of continental drifting, and more specifically of the moving away of the Indian plate from Western Australia as part of the breaking up of the Gondwana supercontinent.

The Gondwana supercontinent encompassed all of the southern continents.

The basalt appears to have flowed in several episodes from volcanic cracks into valleys of the Yarragadee landscape some 136 million years ago.

That is, during the later stages of the break-up which started 450 million years ago.

The hard basalt rock would have been a dominant feature in the landscape for tens of millions of years.

It also would have been an obstacle for the combined Preston and Collie River, forcing it to turn sharply at the western end of the inlet to exit into Koombana Bay.

For the Aborigines, this basalt would have been a highly significant landform.

You may wonder about how the basalt effected the township of Bunbury apart from protecting it against the powerful Indian Ocean and creating the wide, sandy beaches - once black with ilmenite and other heavy minerals - of Koombana Bay.

Because the basalt resisted many attempts to drill through and access fresh water from below the basalt, Bunbury townspeople had to cope with private shallow wells that were easily polluted and tanks.

Both of which often ran dry, until a public water supply became available in the new century.

One could argue that it is because of the basalt that there was such a number of hotels and other licensed premises in a town of only about 3000 inhabitants.

Later on, two breweries and two aerated water factories were established.

The basalt also created problems for major structures like the timber jetty and the wheat silos.

It still creates problems for the inner harbour, where plans to deepen the harbour basin have been put on hold because of the cost of removing the basalt.

The piles for the timber jetty were fitted with iron spikes for a firm hold on the basalt, while stability was achieved by at least doubling the width of the jetty and bracing the piles with massive jarrah beams.

Because there was only sand over basalt, the 1937 historic wheat silos were constructed on the unusual foundation of 3000 jarrah piles.

Today, the basalt is more and more recognised as a uniquely Bunbury attraction, existing only minutes from the city centre.

It is an ever-changing water playground, a wonderful object for sightseeing and picture-taking.

The backwash wave patterns and the oncoming waves meet in an explosive clash.

Do you have any tales of Bunbury's history? Email them through to editor.bunburymail@fairfaxmedia.com.au