Geo-ing on Holidays Part 2

The end of my stay in Australia is rapidly approaching – with only a fortnight to go, I scrambled out from under the reports and papers and data wrangling to take some time to explore the coast of Western Australia. But of course, you can’t escape geomorphology, and the coastline we explored showcased a beautiful variety of present-day features and environments, as well as the evidence of how the coastal landscape has changed over thousands of years.

Travelling from Perth to Exmouth and back with Red Earth Safaris (thanks Victoria!), we covered well over 1000 km each way, from around 31° to 21° south, from the greener, cooler climes of Perth, through the Wheatbelt and into the beautiful red North West, bordered all the time to the west by stunning clear to turquoise waters, white beaches, and the amazing array of wildlife that inhabits them.

The Pinnacles, Nambung National Park – the layering of the original sand dune from which the pinnacles formed from can be seen in the top left of the central pinnacle. Photo: R. Inglis.

First impressions of the landscape were distinctly wind-shaped. Sand dunes the size of multi-storey buildings make their slow progress through scrubby vegetation along the coastal plain between Launceston and Cervantes, but The Pinnacles in Nambung National Park are the remnants of a much older dune system. Deposited around 18,000 years ago, the dunes were later cemented by carbonate precipitation to form part of the Tamala limestone (found throughout Western Australia), which was later eroded and shaped into irregular pillars. These almost eerie formations still preserve the cross-bedding typical of the modern dunes that surround them.

After the Pinnacles, the dune theme (and the Tamala limestone) continued at Shark Bay UNESCO World Heritage Area, at the westernmost extent of mainland Australia. As can be seen in the excellent interpretative panel (below) at Eagle Bluff, the whole of Shark Bay is dominated by a dune system flooded by rising seas during the last 18,000 years, seas that, at one point, rose to even higher than present day levels.

Interpretation board at Eagle Bluff, Shark Bay World Heritage Area. Photo: R. Inglis.

The panel gives some sense of the massive changes in ecology that accompanied these sea level changes. One of the special features of Shark Bay is its high salinity – circulation of water in and out of the bays is inhibited by the underwater landscape (inherited in part from the submerged dunes), and evaporation within the bays increases the concentration of salt. Whilst many species of marine life cannot live in such highly saline water, some benefit from a lack of predators and thrive – such as fragum erugatum, the shells of which make up up to 10 m of deposits of the aptly named Shell Beach. Here I had the first of my flashbacks to the prehistoric shell middens on the Farasan Islands, Saudi Arabia, where past groups made full use of the high productivity of the Red Sea to gather, over time, millions of shells to process and eat. F. erugatum looks a little fiddly to be collecting to eat (even fiddlier than pistachios!) but the beach illustrates the immense potential for marine environments to concentrate and provide resources, and these areas provided a whole range of marine resources that were worth exploiting!

Shells on the aptly-named Shell Beach. Photo: R. Inglis

F. erugatum wasn’t the only organism enjoying salty life in Shark Bay. The stromatolites at Hamelin Pool, Shark Bay have been top of my bucket list since I knew I was headed to Australia. One of the few places on earth where these organisms can be seen living in a marine setting, stromatolites are formed by mats of cyanobacteria that cement tiny sediment particles, slowly building up a ‘layered rock’. These organisms are the oldest living type of organism on earth. Fossils show that they first appeared over 3.5 billion years ago (the Hamelin Pool ones are though to be relative babies at ~1,000 years old) and have been carrying about their business in the same manner ever since. Their business is mainly photosynthesis, the process by which plants convert carbon dioxide, light and water into oxygen and sugar, and the oxygen produced by the photosynthesising cyanobacteria made the development of more complex, oxygen-breathing life forms, such as us, possible. So thankyou, cyanobacteria!

Stromatolites at Hamelin Pool, Shark Bay. Photo: R. Inglis.

The final mainland coastal area of interest was the Ningaloo Reef and coast. One of the largest fringing reefs in Australia, the reef is host to an amazing array of wildlife, including the annual visits of whale sharks, the largest fish in the sea and is another UNESCO World Heritage Area. A wade off the beach for a snorkel or a trip in a glass bottomed boat takes you straight to amazing living coral formations, and the fish that inhabit them. And most excitingly for those interested in sea level change, these live reefs are in many places located within metres of fossil coral terraces that record periods when the sea level was higher than today. I’ve yet to find many references to work on the corals I saw on the Exmouth Peninsula, but they were beautiful to see nonetheless!

After the long drive north to Exmouth and back again, Rottnest Island, or Wadjemup, was my final destination before heading back to Sydney. Drawn by the thousands of quokkas who amble happily about the island checking out tourists, the coastal geomorphology and history of the landscape was an amazing bonus. I’ve since been told it’s a bit of a sea level change indicator hotspot, and of course it took me until I got back to find this excellent leaflet from the WA Dept of Mines and Petroleum, and this more in-depth guidebook.

The island has a history that again seems familiar to  the Farasan Islands and Southern Red Sea. During the Last Glacial Maximum (LGM), around 18,000 years ago, when sea level was ~120m below present, the island was 12 km inland of the coastline of Australia, part of a broad coastal shelf – like the Farasans. Dunes on this plain were deposited, and become lithified into our old friend the Tamala sandstone, before sea level rose again, up to 2.4 m above present day sea level around 4-5,000 years ago. Also preserved (the guidebook now tells me) are fossil coral terraces from the Last Interglacial high sea stand around 125,000 years ago, as well as earlier high sea stands. I’ll just have to go back to visit these…

Textbook wave cut platform (extending into the surf), notch and storm bench at West End, Rottnest Island. The splash in the sea in the centre-distance was made by a camera-shy humpback whale. Photo: R. Inglis.

The coastal features I did spot on my own, however, included some textbook examples of modern shorelines as well as their past equivalents. West Point Bay has an extensive wave-cut platform at present mean sea level, and a wave-cut notch and storm bench above this. Similar notches around now-enclosed saline lakes in the centre of the island mark where these coastal environments once existed there, when sea levels were higher and the lakes were connected to the ocean. These are the very indicators that geologists and geomorphologists use to unravel the past history of sea level change, the histories we need to understand how future sea level change will shape and move our coastlines.

Wave-cut notch in the centre of Rottnest Island, formed at a time when sea level was higher, and the adjacent lake connected to the ocean. Photo: R. Inglis.

So an exhausting but wonderful few weeks in an amazing part of the world. It should have been no surprise that in 3000 km of driving along Australia’s beautiful coastline that there were so many amazing examples of geomorphology in action, both now and in the past, but the coastline of WA was absolutely stunning on so many fronts. All I need now is a new project so I can spend more time there…

Well now I couldn’t just mention quokkas could I… Photo: R. Inglis.


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