And the ospreys came too….
The top and most recent layer shows the cross bedding of a fossil sand dune with random criss-crossing of fossil tree roots and larger solution pipes that formed after trees decayed. The next layer down shows the flatter bedding layers of fossil beach sand with embedded shells and bubble sand. Underneath that there is a thinner layer that was formed in the shallow waters on the edge of the beach and is fine grained with shells embedded. Underfoot the beach rock is older and different again showing pot holes in an old wave cut platform, fossil roots and solution pipes.
The different layers have been exposed and then covered as the sea level rose and fell during different ice ages which began 2.5 million years ago. During ice ages water was trapped as ice at the poles and northern continents which meant that the sea level retreated. Ice melting meant sea levels rose. A recent ice age “maximum” 20,000 years ago meant the sea level was 120m below the present level so the coastline of WA was beyond Rottnest. As recently as 7,000 years ago it was still possible to walk to Rottnest from the WA coast. You can read Mike’s notes here.