Wadjemup / Rottnest Island
Geology Explorer
Setting the scene
Rottnest Island — Wadjemup in the Noongar language — is a popular destination for day-trippers and weekend visitors, alike. Yet, how many venture beyond the sandy beaches, alfresco cafés and ice cream kiosks to explore the natural history of the island in more depth?
Rottnest Island seen from satellite — a ragged postage stamp of an island that's a welcome retreat from the hustle and bustle of Perth
There is a story to be told in the rocky points that jut out into the sea, dividing the beaches and providing homes to myriad small creatures; a story writ in the landscape itself, and in the ancestral memories of Aboriginal Australians.
We can uncover those stories, and others that stretch farther back in time, by looking more closely at easily visited sites around the coast and between the lakes.
They tell of a low-lying lump that was not always separate from the mainland, and of a climate that has waxed and waned over the multi-millennia.
Acknowledgement of Country We acknowledge Aboriginal and Torres Strait Islander peoples as the Traditional Custodians of the lands on which we deliver our services. We pay our respects to elders and leaders past, present and emerging.
Getting there
Rottnest Island is easily reached by ferry from several locations — there are ferry terminals at Perth City and Fremantle, and farther north along the coast at Sorrento/Hillarys Boat Harbour.
Getting around
The simplest and most fun way to tour the island is by bike. Bring your own, or hire one when you reach The Settlement at Thomson Bay
Hire bikes and a visitor's bike at The Basin
Geoheritage
Rottnest Island is a Class A Reserve (R 16713) . Within the reserve are nine State Geoheritage Sites with geological features considered to be either unique or of outstanding scientific and educational value within Western Australia. Special care should be taken not to litter, camp, drive or ride through, collect rocks or damage outcrops at those sites.
Geoheritage sites are shown with green rectangles
There is more information about the status of Geoheritage Sites — and the more stringently protected Geoheritage Reserves — on the Geological Survey of Western Australia website . If you visit these sites, please value their significance by treating them with respect.
Exploring the geology
The young geological history of Rottnest Island can be pieced together from clues preserved in the rocks. We'll explore the geology, landscape and fluctuating fortunes of marine life around Rottnest Island by visiting 11 sites showing features that reveal a story of past shorelines, sea-level change, coral reefs, and dunes.
Use the map to visit each of the sites. You can either click on a marker in the map to go directly to that site, or scroll down the screen to tour the sites in order. At each locality, scroll right on the image to see more photos or a sketch for further explanation.
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1
Thomson Bay
Wadjemup/Rottnest Island tells the story of sea-level change over the past 150 000 years or longer, during which time sea level has been both higher and lower.
The Settlement at Thomson Bay is a safe harbour protected at its northern end by a low bluff of Tamala Limestone. Most of the island is built on this sedimentary rock, formed from coastal wind-blown limesand dunes when sea level was about 100 m below today's level, and the coast may have been 12 km, or more, west of the island.
West of Thomson Bay are several inland salt lakes, and farther west near the centre of the island, the highest point is Wadjemup Hill (scroll right for a photo). The lighthouse there is embedded in a landscape of younger, rolling, steep-sided sand dunes on top of older, low-relief dunes of variably consolidated Tamala Limestone.
2
The Basin — shoreline platform, notches and visors
The Basin is a shallow bay protected at its western and eastern ends by low, rocky headlands. The shore platform is a dark shadow under the water extending seawards from the rocky headlands and beach. This is a nearly flat shelf of Tamala Limestone ~50 cm below mean sea level, and exposed at very low tide. It has a prominent raised rim on its outer edge where it is more strongly cemented by lime and algae.
At the eastern point and the west end of the beach, low bluffs of Tamala Limestone are strongly notched just above the high tide level. This notch is formed by wave action during fair weather conditions, and is overhung by a visor (jutting out like the visor on a bike helmet), on top of which is a narrow bench that shows the active erosion level during storm events (scroll right to see a photo and sketch).
3
Herschell Quarry — shell deposits
As we go inland towards the salt lakes, there are layered shell deposits exposed in a small quarry just below Mount Herschell at the eastern end of Lake Baghdad. The shelly material is similar to that seen on the beaches, but these shell beds are at a height well above the beach at The Basin.
This is the Herschell Limestone, made up of shell beds about 2 m thick, and comprising two distinct layers. The lower layer contains closed pairs of shells in random orientations, suggesting the shells settled into the silt just below wave base, in a quiet-water environment among sea-grass meadows. Shells in this layer have been radiocarbon (C-14) dated to between 5900 and 4800 years ago and, based on the depth wave action typically reaches, sea level would have been about 2.5 m higher than today.
In the upper layer, almost all shells are dislocated single valves resting concave down and aligned horizontally. This indicates relatively strong currents where the shells were deposited, probably in a beach setting or intertidal zone. These shells have been dated to between 3100 and 2200 years ago.
The trend over time from deeper water, silty shell deposits to intertidal or beach deposits at the same location tells us that sea level must have been falling even as the shell beds were accumulating.
We can visualize this by drawing the sequence of accumulating shell beds in two stages (scroll right to see the sketch), showing their thicknesses and depositional environments at 5900–4800 years and at 3100–2200 years.
This location falls within Geoheritage Site - 57, Herschell Quarry, Rottnest Island. The quarry is private property and is not open for visiting by the public.
4
Causeway — elevated shoreline platform with notch
At the Causeway, between Herschel Lake and Government House Lake, a bluff formed of Tamala Limestone has notches and visors cut into it that, at first sight, look similar to the notch and visor pattern at The Basin. Here, however, they are well above present sea level.
The lower surface, from where the photo was taken, is thought to be a relict shore platform about 0.5 m above the average level of the present-day shore platform (scroll right on the photo to see a sketch). In places, the surface of the lower, lakeshore platform has a thin crust of worm tubes on the limestone, similar to that on the seashore platform today. The patchy crust of worm tubes is present all the way up the bluff.
The deep notch immediately above the platform would have been cut into the bluff by wave action, in a similar way as at The Basin. In fact, this is a double notch (scroll right to second photo) with a narrow relict visor between them, about 1.1 m above the average level of the present shore platform (scroll further right to see a sketch).
The horizontal surface at the top of the bluff is also interpreted as a former shore platform, now about 2.4 m above the present shore platform. The pattern of platforms and notches, together with the fossil worm tubes, shows the platforms and notches were formed as sea level rose from about 1 m to more than 2.4 m above its present height.
This location falls within Geoheritage Site - 37, Elevated platform, Causeway, Rottnest.
5
North Herschel Lake — elevated shoreline platform with notch
A relict shore platform and deeply incised notch, like that at The Causeway, is evident along much of the low ridge between Herschel Lake and Lake Baghdad. This platform–notch feature is at nearly the same height wherever Tamala Limestone rims the salt lakes.
The limestone in the notch and on the outer surface of the upper shelf is pockmarked with holes about 1 cm diameter (scroll right to see a photo). These holes were bored into the soft rock by molluscs (shellfish or snails) when these rocks were in the intertidal zone.
This location falls within Geoheritage Site - 36, Elevated platform, Baghdad/Lake Vincent, Rottnest.
6
Fairbridge Bluff — ancient coral reef
At Fairbridge Bluff on the south coast of Rotnest Island, about midway along Salmon Bay, a fossil coral reef, the Rottnest Limestone, is preserved for about 200 m along the shoreline. Here you can see staghorn coral and platy varieties of the coral Acropora, as well as large colonies of brain coral (scroll right on the photo to see close ups of the corals).
Shelly limestones are intimately associated with the reef, and contain abundant gastropods, especially the thick-shelled genus Turbo. Coralline algae encrust many corals and form the framework of the reef. The reef has been dated by the uranium–thorium (U–Th) isotopic method to ~132 000 years before present, and the top of the reef is almost 3 m above the shore platform.
Compared with coral reefs elsewhere along the Western Australian coastline, it’s clear that sea level must have been several metres higher than at present, and water temperatures must have been significantly warmer, when the Rottnest Limestone coral reef was alive.
However, the Rottnest Limestone fossil coral reef is enclosed and mostly overlain by Tamala Limestone which was deposited in a terrestrial setting (see description for South Point).
This locations falls within Geoheritage Site - 42, Fairbridge Bluff, Rottnest Island.
7
Mary Cove — algal polygons
In the low-tide zone here, and also at Wilson Bay and Radar Reef, there is a distinctive pattern of polygons composed of brown algae. Exactly how these algal polygons form is not fully understood, but there’s some evidence the shapes are maintained by the grazing habits of herbivorous reef fish, such as Western Buffalo Bream. The fish observe ‘boundaries’ between neighbouring territories, resulting in the geometrical patterns of algae. At some locations the polygonal patterns can persist over several years.
8
South Point — Tamala Limestone with cross-bedding
Just north of South Point, at the western end of Strickland Bay, the limestone cliff has fallen away to expose one of the best cross-sections of Tamala Limestone on the island. There are at least two stacked sets of layered limestone, in which the angle of the layers alternates between the sets. This alternation of layering is known as cross-bedding and, together with the texture of the sediment, is convincing evidence the rocks originated as windblown (eolian) dunes of lime and quartz sand, and shells.
These limesand dunes would have been several metres high and must have formed in an on-land (terrestrial) environment. The significance of this becomes clear when we realize that, at other places around Rottnest Island, rocks at nearly the same height must have been formed under the sea. The dip of the cross-beds in the upper part of the cliff shows the sand dunes migrated across the area from north to south.
This location falls within Geoheritage Site - 121, South Point, Rottnest Island.
9
Wilson Bay — 'paddy-field' terraces
The features to see here are the 'paddy-field' terraces. The shore platform is divided into curved, stepped spaces resembling rice paddy fields, with rims a few centimetres high that face landward. In this way, the rims define shallow terraces over which the waves cascade as they break onto the platform. Strangely, the terraces step up as one looks out from the shore.
At Wilson Bay, the outer terraces are as much as 70 cm above the general level of the platform. Exactly how these terraces form is not certain, but it’s possible the narrow rims are more strongly cemented, and algae or other biological mats may also play a role.
10
Radar Reef — storm bench, shoreline ramp, algal polygons
Where the shoreline platform meets the limestone cliff there is a gently sloping shoreline ramp backed by a storm bench with an extensive deposit of tempestite. This is an accumulation of poorly sorted cobbles and boulders deposited during storm events (tempests), when the energy and height of the waves are great enough to move the boulders to this level.
The limestone ramp, extending upwards from the intertidal domain to above high-tide level, has two zones. The seaward zone (area of maximum swash) is hackly and extremely rough (it’s painful to walk on with bare feet!) as it is created by saltwater erosion of the limestone. The landward zone has a smooth, well-rounded surface caused by deepening and widening of cracks in the limestone. Blue-green algae cover the landward zone.
In the low-tide zone there is a distinctive pattern of algal polygons like those seen at Mary Cove.
11
West End and Cape Vlamingh — shoreline notches, storm bench
West End, at the southern tip of Cape Vlamingh, is a good place to see several shoreline features. Around this western point of the island, especially at the western and eastern ends of Fish Hook Bay, there is a very well-developed, deep shoreline notch with its overhanging visor cut into Tamala Limestone.
Above this, especially around the headland at the western side of the bay, is a narrow but distinct storm bench partly covered by limestone blocks — a tempestite like that at Radar Reef.
Heading around the point of Cape Vlamingh, the shoreline is notable for its lack of sandy beaches. This end of the island bears the brunt of most of the stormy weather, which drives up from the southwest. One result is that the shore platform here is only about 20 cm below mean sea level, whereas at the eastern end of the island the platform is 50–70 cm below mean sea level. This difference has been seen around Australia: wherever wave action is strongest, the platform is higher.
The narrow storm bench along Cape Vlamingh is backed by a mixed deposit of rocks, some tossed up during storm events and some tumbling down the low cliff.
How do all these features we've seen relate to each other in space and time? The relationships through time are best shown with reference to changes in sea level, explored in the next section (Changing sea level). The relationships around and across the island can be summarized by drawing a simplified cross-section.
We've seen that the relict shore platforms and notches around the inland salt lakes (here shown for Serpentine Lake), and the Herschell Limestone, are at nearly the same height. In the cross-section, we can imagine the sea level at ~5000 years BP 1 – 2.4 m above its level today, drowning the lakes.
The top of the Rottnest Limestone is a little higher still, and we can visualize sea level up to 10 m higher than today at ~132 000 years BP. When the fossil reef was alive, much of the island would have been submerged.
Changing sea level
The geological features seen around and across Rottnest Island tell us about changing environments over time, as a result of changing sea level. The story these features reveal is supported by data compiled worldwide for the past several hundred thousand years. Global sea level has fluctuated as polar ice caps have waxed and waned, and these global changes have also been felt along the Western Australian coastline.
The age of the Rottnest Limestone fossil coral reef, at ~132 000 years before present (BP), corresponds very closely to the last major interglacial period — that is, a time when the Earth’s polar ice caps were even more restricted, and sea level was significantly higher. The fossil coral reef at Fairbridge Bluff confirms that sea level must have been more than 3 m higher than it is now.
Between ~130 000 and 11 000 years ago, the Earth went through a major glaciation, peaking about 22 000 years ago. Water was locked up in the polar ice caps and across the northern continents, and global sea level was at least 100 m lower than today.
During the sea level low stand (~20 000 years ago), the coastline lay 12 kilometres or more west of Rottnest Island near the edge of the continental shelf. With the continental shelf exposed in a cold, dry climate, conditions were perfect for enormous quantities of sand comprising the shells of tiny marine organisms, foramanifera, and shell fragments of other marine species, to be swept up into extensive coastal dunes.
A bathymetric map shows the extent of the shallow continental shelf from which the sediment was derived, and across which the dunes were originally distributed. Those dunes are preserved as Tamala Limestone on Rottnest Island, and elsewhere along the west and southwest coast.
When global conditions changed again, sea level rose until about 5000 years ago and low-lying parts of the island were inundated by the sea, forming embayments and tidal estuaries. This was the environment in which the Herschell Limestone shell beds accumulated, on top of and flanked by Tamala Limestone.
The elevation of the Herschell shell beds, combined with their age of 5900 to 2200 years, confirms that local sea level may have initially risen to a few metres higher than at present. The raised shoreline platforms cut into Tamala Limestone around the lakes at The Causeway probably formed around this time (~5000 years BP). The heights of those relict platforms and notches have been matched to the height and age the Herschell Limestone.
By exploring the geology of Rottnest Island, a picture emerges of past environments buried in the passage of time. The influence of global climate change on sea level in the local setting holds clues for how the landscape might change in the future.
Panorama of Fish Hook Bay. The calm conditions belie the power of the storm surges that batter this coast at times
