History of Bunbury's natural landscapes

Bunbury's natural landscapes: The view from hill to another. Photo: Bernhard Bischoff.
Bunbury's natural landscapes: The view from hill to another. Photo: Bernhard Bischoff.

Bunbury's landscape is made up of plains and hills.

Their history is beautifully laid out in southern Bunbury between the Preston River and the ocean.

The seven kilometre long continuous corridor of original bushland which still exists there represents a unique cross section in the Swan Coastal Plain.

Expert botanists Bronwen and Greg Keighery confirmed in 1999 that the 'sequence of vegetated landforms (Quindalup/Spearwood, Bassendean Dunes - Pinjarra Plain) encompassed in the corridor is unusual in the Swan Coastal Plain and does not appear to occur elsewhere'.

Being an East-West aligned cross section it tells a great deal about the history of this part of the Coastal Plain.

It reveals that late in its history there was quite a battle between the eastern alluvial and western eolian influences, between fluvial and eolian sedimentation.

The ancient Preston and Collie rivers were the dominant force for over 200 million years as they carried sediments down from the eroding crystalline hinterland filling the 5 kilometre deep trough along the Darling Scarp.

These piles of sediments we know as Jarragadee and Leederville formations.

The period of alluvial sedimentation ended with the laying down of fine sandy clayey and badly draining Pinjarra Plain sediments.

By about 2 million years ago the earth entered a cycle of warm and cold periods which were responsible for the Ice Ages on the Northern Hemisphere.

There was no glaciation here but the wildly fluctuating sea levels caused by the buildup and melting of continental ice resulted in the dune formations that characterise the coastal strip.

In Manea Park we find low sandy dunes interspersed with wetlands which shows that the first the Bassedean dunes had a hard time to establish themselves being often washed away and spread over the plain by the river.

Further west is the zone of Spearwood dunes.

The ridges of College Grove the former Rifle Range and the Usher and Shearwater areas are the remains of the Spearwood Dune formation.

That is characterised by limestone known as Tamala either deeply buried under sand as in College Grove or close to the surface as on the Usher and Shearwater ridges.

This Tamala limestone forms when lime is dissolved by rainwater percolating down through tall sand dunes and recrystallises at the base or near the groundwater horizon.

The Maiden's dunes at the ocean end of the corridor belong to the Quindalup system.

These dunes are only about 12000 - 10000 years old and therefore still largely unconsolidated meaning that they are prone to blow-outs and need special protection.

But there is another major event that the cross section can tell us: a marine transgression

The steep western slopes of the College Grove ridge down to Bussell Highway as well as the steep drop of the Maidens down to the beach allow the conclusion that some 6000 - 4000 years ago the ocean rose 3-4 metres above today's levels and rushed into the existing dune landscape.

Its erosional force is responsible for the 5 -Mile Brook and Hay Park plain and other low-lying parts of Bunbury.