Serpentine Lake - North Shore

Coral at the lake junction

The 1985 management plan identified a spit at the junction of Serpentine and Government House Lakes as one of six fossil sites that could potentially be protected and used for education. It considered this site particularly important because it contained the coral Platygyra magna.

 

A spit is a narrow projection of deposited sediment. The plan’s reference identifies a fossil locality at the lake junction but does not establish that it is the same exposure as Playford’s roadside quarry.

 

Shoreline notches and worm-tube crusts

Playford’s north-shore excursion describes well-developed “double notches” along Serpentine Lake’s northern shoreline, reached after crossing the divide from Pearse Lakes.

 

These are paired recesses in the shoreline rock. Their detailed form and geological interpretation are illustrated in the guidebook’s Figure 53.

 

The same locality contains exposures of a serpulid crust. Serpulids are marine worms that build hard tubes from calcium carbonate, the same principal mineral material found in limestone and many shells. Accumulations of these tubes can form a crust on a surface.

 

The notches and serpulid deposits are separate features, although both contribute to the geological record preserved around the lake.

 

Microbial formations at the western end

Playford records dead and eroded stromatolites along the shoreline at Serpentine Lake’s western end. The dome-shaped formations reach approximately half a metre across.

 

Stromatolites are structures built through the activity of communities of microorganisms, with sediment trapping and mineral precipitation contributing to their growth. They are different from coral colonies and accumulations of mollusc shells.

 

Playford suggests that their growth may have been associated with freshwater seepages from the Oliver Hill groundwater mound. A groundwater mound is an area where the underground water table stands higher than in the surrounding ground, allowing water to move outwards towards lower areas.

 

He also suggests that the formations may have grown when the lake level was slightly higher. Both explanations are presented as possibilities rather than established dates or conditions.

 

Other notes describe fossil thrombolites on the western shoreline as approximately 2,000 to 3,000 years old. Thrombolites and stromatolites are related microbial structures, but the terms describe different internal arrangements: thrombolites have a clotted fabric, while stromatolites have a layered fabric. The age and terminology need to remain tied to their respective sources rather than being applied interchangeably to every formation at the western end.

 

Shells carried inland by birds

The paper Deposits of Shells Transported by Birds records shell accumulations at several inland locations around Rottnest Island, including three associated with Serpentine Lake:

• The elevated peninsula projecting into the lake’s north-eastern end.

• A hill on the peninsula between Serpentine and Government House Lakes.

• The islet between the two lakes.

 

The paper identifies these as deposits transported by birds. Their presence on elevated ground therefore has a different explanation from shells embedded within the fossil-bearing limestone.

 

Across the island, the deposits occurred at heights ranging from just above high-water level to approximately eighteen to twenty-one metres above sea level. Concentrated deposits were generally found on limestone, often collected in shallow hollows on flat rocky promontories. Isolated shells were also observed on recent sand dunes.

 

The deposits commonly contained the opercula of Turbo stamineus, together with broken shells of the same species and smaller quantities of limpets and chitons. An operculum is the hard lid that closes the opening of certain sea-snail shells when the animal withdraws inside.

 

The author observed that Turbo shells were consistently broken, whereas limpets, chitons and other open shells were generally undamaged.

 

Why some deposits contain mostly shell lids

The paper describes how exposure gradually changes the appearance and composition of these shell accumulations.

 

Fresh shells were initially pale. Surface growth of microorganisms gave them a greyish colour, which could darken considerably over time. Physical weathering and chemical dissolution broke down the shell material, and wind carried away smaller fragments.

 

The thick, solid opercula survived longer than the more fragile shells. Older deposits could consequently become dominated by opercula even though the original accumulation had contained a wider mixture of shells.

 

The author describes these as deflation residuals. In this context, deflation means the removal of loose material by wind, leaving the heavier or more resistant pieces behind.

 

The paper considered such residual deposits to be several hundred years old. This was an interpretation based on their condition and weathering, rather than a measured date for each deposit.

 

Army water supplies

Playford records a 1939 decision by the Army to increase freshwater supplies for Kingstown Barracks and the Thomson Bay settlement.

 

An area beside Mount Herschell was to be surfaced with bitumen so that rainwater could run into storage tanks. The sealed surface acted as a catchment, directing rainfall towards storage rather than allowing it to soak into the ground.

 

The Army also installed a supplementary water system supplied by several wells on low ground associated with Serpentine Lake.

 

This records a military use of the freshwater resources around the lake. The passage does not establish a connection between these wells and the separate emergency tank groups developed at Forbes Ridge, Bickley and near Oliver’s Battery in 1942.

 

Proposed tree planting in 1899

The Western Mail on 14 July 1899 reported on the Woods and Forests Department’s activities and proposed planting on Rottnest Island.

 

An avenue of Ficus macrophylla and Araucaria excelsa, commonly known as Moreton Bay figs and Norfolk Island pines, was to be planted around Serpentine Lake and named Lady Smith’s Avenue.

 

The article records the intention to plant the avenue. It does not confirm that the work was completed, where the proposed route ran or whether the name came into use.

 

Fire on the southern shore

A major bushfire reached Serpentine Lake on 31 December 1948. Reporting the following day, The West Australian described approximately sixty volunteers, including holidaymakers, fighting the fire between 10 am and 4 pm. An estimated 400 to 500 acres of scrub burnt, equivalent to approximately 162 to 202 hectares.

 

A strong south to south-westerly breeze drove the fire from Parker Point towards the eastern boundary of the airstrip and the lake’s southern shore.

 

Firefighters attempted to stop its northward spread near the railway by burning a break. The flames crossed the break and continued until they reached the lake.

 

Knapsack sprays and a mobile water tank were used to protect huts near the airstrip and former Army gun emplacements. The newspaper reported no damage to property or tree plantations.

 

North-shore woodland and restoration planning

The 1985 management plan described mixed scrub and woodland along Serpentine Lake’s northern margin as an example of the vegetation it wanted to establish elsewhere on the island.

 

By that time, the area between the western margins of the lakes and the eastern boundary of the water catchment had already been substantially replanted. A further five-year reafforestation program was underway inland from Porpoise Bay.

 

The intention was to connect separate plantations into a more continuous woodland and scrub zone. Proposed plants included Melaleuca, Callitris, Pittosporum and Templetonia, together with suitable shrub species.

 

Woodland south of the lake

The 2014-2019 Interim Recovery Plan for Callitris preissii or Melaleuca lanceolata forests and woodlands identifies an occurrence known as ROTTNEST02, also numbered Occurrence 47.

 

Serpentine Lake forms its northern boundary, placing this particular woodland occurrence south of the lake. Cleared bushland and unsealed tracks were described along its western, eastern and southern sides.

 

The community is associated with Rottnest Island pine and Rottnest teatree and is classified as Swan Coastal Plain community type 30a. The recovery plan recorded it as a threatened ecological community ranked Vulnerable in Western Australia at the time.

 

An ecological community is an assemblage of organisms occurring together in a particular environment. Its conservation classification concerns the community as a whole, rather than automatically assigning the same status to every individual tree.

 

The plan also identified a Priority 1 microbial community in Serpentine Lake adjoining the woodland occurrence. This was described as microbialites and microbial mats of Rottnest Island’s coastal hypersaline lakes. Hypersaline means that the water is more saline than seawater.

 

Ruddy turnstones

The 1985 management plan identified Serpentine, Government House and Herschel Lakes as favoured feeding locations for ruddy turnstones.

 

These migratory shorebirds breed in Siberia during the northern summer and travel south to spend the Australian summer on Rottnest. The plan records most arriving in September and leaving in April or May, although some birds, thought likely to be juveniles, remained through winter.

 

Ruddy turnstones search for food by using their bills to turn over stones, weed and other loose material. The stony and debris-covered edges of the salt lakes provide suitable feeding ground. The birds also visit beaches, exposed reefs and some swamps around the island.

 

Sources

• Phillip Playford, Guidebook to the Geology of Rottnest Island, discussions of the Herschell Limestone, Baghdad Member, stromatolites, water supplies and the north shore of Serpentine Lake.

• Rottnest Island Management Plan - Volume 1: The Plan, August 1985.

• Deposits of Shells Transported by Birds - Curt Teichert & D. L. Serventy, American Journal of Science, Volume 245, Issue 5 (1947).

• Western Mail, 14 July 1899, page 5, Woods and Forests Department.

• The West Australian, 1 January 1949, page 3, Rottnest Volunteers Fight Fierce Blaze.

• Department of Parks and Wildlife, Callitris preissii (or Melaleuca lanceolata) Forests and Woodlands: Interim Recovery Plan 2014-2019, No. 340.