A Shifting Coastline

Millions of years ago Australia, along with India, Antarctica, Africa and South America were once joined and helped to form a super-continent called Gondwana (pronounced gone-dwa-na). In forming this super continent Australia and India were once joined, but over time a fault in the earth formed as India pulled away from Australia and began to drift north. This fault, called the Darling Fault, occurred along what we know today as the Darling Scarp. If you were able to stand on the edge of this fault line at the time, it would have been an amazing 10-15 kilometres drop to the land below! Today, the drop is just a mere 300 metres.

Changing Sea Levels

Millions of years ago when Australia broke away from India the sea came right up to the base of the Darling Range forming the first coastline. Today you can still find evidence of this ancient shoreline in the form of shells and old marine deposits left behind as the sea retreated. Over time the area below the fault filled up with sand and marine deposits which were brought in from the west by changing sea levels. From the east came sediments carried by rivers and creeks flowing down from the Darling Range. Let’s just think about this for a minute. Over a very long period of time layers of sediment, up to 15 kilometres thick, have built up on the Swan Coastal Plain! Wow, that’s the same distance as travelling from Perth to Midland!

The formation of the soils on the Swan Coastal Plain has been greatly influenced by changes in the Earth’s climate. Let’s have a look at these impacts. Over the last 2 or 3 million years the icecaps at the Earth’s poles have repeatedly shrunk and expanded as the earth’s climate has either become warmer or cooler. This caused sea levels to rise and fall.

Approximately 130,000 years ago, (that’s a lot of sleeps!), the coastline was about 10 kilometres further inland than today. The grey Bassendean sands were once the sand-dunes which formed this coastline. Have a look at a map of the metropolitan areas and see which suburbs would have once been on the coast and which ones would have been under water. Rottnest Island was not an island but a coral reef about 30 kilometres offshore.
About 18,000 years ago the sea-level was an amazing 130 metres below its present level. This came about because the earth was experiencing an ice age and temperatures were much cooler. The polar ice caps increased in size as water froze. With less water in the oceans the sea level dropped and the coastline moved 40 kilometres further west than it is today. Rottnest Island became part of the mainland and ‘dry’ land stretched a further 20 kilometres west, beyond Rottnest Island. Amazingly you could have walked to ‘Rotto’ instead of catching the ferry!
Over time, temperatures on earth began to rise again causing the icecaps to melt releasing some of the frozen water previously locked away. As a result the sea-level began to rise again until it reached its present level approximately 6,500 years ago.

With each of these changes in sea level, sand and marine deposits have been left behind by the ocean and the wind forming the Bassendean, Spearwood, and Quindalup dunes.

While all of this was taking place down on the plain, the rocks of the Darling Range were being eroded by the forces of nature. This material, consisting of clay, sand and silt particles was transported by a network of rivers and creeks and deposited along the base of the Scarp to form the Pinjarra Plain and Forrestfield landforms.