See 2 inter-glacial periods – right here!

Recently the Naturalist’s Club had a presentation followed by a coastal walk, conducted by Vic Semeniuk. For the walk the group met at Cottesloe’s Mudurup Rocks. Vic took us through a quarter of a million years of fascinating geology! Many thanks to Vic and the Nat’s Club for a fascinating morning. See Mike Gregson’s notes below, thanks Mike!

(As we were heading home Leeuwin sailed past in all her splendour.)

Dr Vic Semeniuk is a specialist in coastal formations. He gave us a fascinating talk on Perth’s beaches, dunes and rocky shores, describing a great cyclic pattern of sedimentation and landform development that occurred during the Pleistocene ice ages and interglacial periods, and which is still occurring today. He followed it up the next day with an excursion at Cottesloe’s Mudurup Rocks, where the geological processes he described are illustrated in the sand and rocks we were standing on.

Vic began by describing the way the waves wash sand up onto the beach, forming laminations. Sand grains, differing in size and density, are sorted by the variable energy of the waves. And because wavelength varies, so the beach changes hourly. This leads to a complex pattern of layering. As the tide recedes, bubble sand can be observed, caused by the waves interacting with tides. On the Saturday, Vic demonstrated the bubble sand that waves had created only a few seconds beforehand, and the layering of different particle sizes that had also recently been built by the waves. Layering patterns are not only the result of waves and tides, but also wind, storms and the slope of the beach.

Waves form ripples on the near-shore sea-bed. These ripples vary in size, depending on wave orbital velocity and water depth. They range from tiny ripples to the mega-ripples that we experience as sand bars that we wade over on the way out to the surf. Vic’s diagram of wave orbitals showed us how ripple size varies with depth.

During storms, flotsam is dumped in a chaotic and non-layered manner high on the beach. At Cottesloe we saw seaweed, cuttlefish-bones and Rams-horn Shells that had been stranded at the high-water mark in a recent storm.

Another feature of sandy shores is that sea breezes move fine and medium sand out of the beach to form dunes. As more sand is added to the beach, an “upward-shoaling sequence” occurs, and all the features described here – the ripples, the layering, the bubble sand, the chaotic deposition and the dunes – move seawards in order.

During the Pleistocene glacial periods, the sea receded to the edge of the continental shelf. With the sea withdrawn, all the features of the sandy shores that had been created in the interglacial periods became indurated by calcite cementation to form limestone. Vic showed us fossilised mega-ripples in the limestone cliffs at Mudurup. Above that we were shown fossilised bubble sand and fine layering. Above that again we saw a fossilised chaotic layer containing cuttlefish-bones, and near the top of the cliff, the layering pattern typical of dunes, also fossilised. There, preserved over tens of thousands of years, were the results of processes we could see occurring right now in front of us.

In the interglacial period after the ice age, the sea may return to the old shoreline. If it meets limestone there, it cuts a rocky shore, forming platforms, notches, benches and cliffs. Animals and plants live on these platforms and cliffs, causing erosion, and forming potholes which widen out and eventually fill with sediment.

Vic then took us in our imagination to the last interglacial period, tens of thousands years ago. The features of the eroded rocky shore from that time, such as sediment-filled potholes on a wave-cut platform, are preserved in the rocks that have subsequently been overlayed by a sandy shoreline that has itself been fossilised! And so at Cottesloe we can see (with some prompting by a scientist with an experienced eye) the record of processes occurring in two successive interglacial periods, one above the other. The upper sequence is a record of sandy-shore processes and the lower one is a record of processes acting on a rocky shore.

This topic beautifully illustrates the concept of uniformitarianism – that the present is a key to the past. The processes occurring today on our local shores can help us interpret the geological record to discover the history of the earth. Our thanks to Vic for shining a light back in time to show us the great climatic cycles that formed our present coastline, and for demonstrating how those processes are still occurring at this very minute.

~ Mike Gregson