Geological History

Geological History of the Brisbane Ranges

As well as the general history of the park, there is a rich geology history.

The Ranges stretch from Bacchus Marsh in the north to Maude in the south, and have a maximum elevation of 440 metres. Steep valleys and Gorges dissect the eastern part of the Park, while the western portion of the Park is more undulating and more easily accessible.

The Brisbane Ranges mainly consist of sedimentary rocks of early to mid Ordovician age (505 million years ago). These sediments were deposited during a time when the first fish were evolving and shelled organisms lived throughout the seas and oceans. At this time, the sea was much higher than the present level and most of Southern Victoria was submerged. The sediments were deposited by underwater avalanches (Turbidites) that fall due to gravity from the continental shelf to the ocean floor. These avalanches deposited hundreds of metres of sand, silt and mud on the ocean floor.

As deposition of sediments continued over time, the underlying Ordovician sands, silts and muds were compressed forming sandstone, siltstone and shale.

The next major event in time that resulted in the Ranges' current day appearance occurred about 380 million years ago during the late Devonian, a time when the first amphibians were first venturing from the sea to the land. During this time, the earth experienced extensive plate movement that compressed, buckled and folded the Ordovician sediments. At the same time molten magma melted through the sediments to the east of the Ranges. The magma soon cooled and has since been exposed as granite, forming the You Yangs and the Anakies to the east and the site of the Fairy Park and Dog Rocks to the south-east. The compression by the plate movements and the heat from the molten magma metamorphosed some of the shale in the Ranges to slate, which has since been quarried in the north-east corner of the Park.

At the conclusion of the Devonian, the world experienced a time of cooling, which led to the development of ice caps and a lowering in the sea level. This global cooling was not sudden, as it occurred over a period of 150 million years, during which more sediment was deposited upon the folded Ordovician rocks.

The world was now in the Mesozoic (200 million years ago), a time when dinosaurs roamed the land and the first birds and animals emerged. It was at this time that the super continent, Gondwana separated and Australia took on its present shape as an island continent. The sea level receded and the overlaying sediments were eroded away, exposing the You Yangs granite as an island in a shallow sea.

The next 195 million years were "relatively uneventful", except for the extinction of the dinosaurs, the development of mammals and the continuing fall in sea level.

That was until about 4 million years ago (Pliocene-Pleistocene) when Central Victoria experienced considerable faulting. The Rowsley, Hanover and Meredith Faults uplifted the Brisbane Ranges above the plains to the east. During this time, Western Victoria was volcanically alive, as lava flowed from Mt Anakie, covering the Werribee Plains and areas towards Geelong, and lava from Mt Wallace covered the plains to the north-west.

Since the extinction of the volcanoes, the Brisbane Ranges have continually been eroded by wind and water systems such as Little River and Stony Creek, cutting steep sided valleys into the Ordovician rocks along their winding course. The eroded Ordovician rocks and overlying sediments have been deposited as river sands at the base of the Ranges and dispersed out over the basalt plain.

It is through all of the above processes, which originated over 500 million years ago that we are able to enjoy the magnificent panoramic views, the beautiful gorges, and rich flora and fauna diversity of what we now know as the Brisbane Ranges National Park.