Stabilization Of The Swan Coastal Plain: Quaternary Dune Systems & Geomorphic Evolution

1. Quaternary Geomorphic Triad: Bassendean, Spearwood, and Quindalup Dunes

The Swan Coastal Plain forms a complex, low-gradient coastal piedmont stretching from the Darling Fault westward to the Indian Ocean, representing the geographical heartland of the Bibbullmun Sovereign Nation.¹ The surface morphology of this plain is dominated by three distinct Quaternary dune systems of progressively younger age arranged parallel to the contemporary coastline: the Bassendean, Spearwood, and Quindalup dune systems.² The oldest, the Bassendean system, represents heavily leached, quartz-rich Pleistocene coastal barriers dating from the Middle Pleistocene (>500,000 BP).³ Directly west, the Spearwood system consists of lithified eolian calcarenite of the Tamala Limestone Formation mantled by yellow siliceous sands, deposited during Middle to Late Pleistocene interglacials.⁴ The westernmost Quindalup system comprises Holocene carbonate-rich coastal dunes mobilized and stabilized over the past 7,000 years following modern sea-level stabilization.⁵

2. Hydrological Genesis of Interdunal Wetlands and Peat Basins

The juxtaposition of these permeable sand ridges against impervious clay strata of the Pinjarra Alluvial Plain generated an exceptional, high-density hydrological matrix.⁶ Surface expression of the unconfined superficial aquifer (the Gnangara and Jandakot mounds) fills interdunal depressions between dune ridges, creating linear chains of perennial lakes, seasonal swamps, and organic peat sumps.⁷ These wetlands—including Lake Monger (Galup), Herdsman Lake (Ngurgenboro), and the Bibra Lake system—became hyper-productive ecological epicenters that supported dense ancestral Bibbullmun populations.⁸ Water chemistry across these wetland chains varies markedly, from alkaline groundwater-fed lakes in Spearwood calcarenite to acidic, dystrophic peat bogs within Bassendean sands, driving rich biodiversity across plant and animal guilds.⁹

3. Fluctuating Sea Levels and Fluvial Incision of the Derbal Yerrigan

The morphological development of the Swan Coastal Plain was fundamentally shaped by glacio-eustatic sea-level fluctuations.¹⁰ During the Last Glacial Maximum (approx. 21,000 BP), sea levels dropped to 125 metres below current levels, exposing the entire continental shelf out to Rottnest Island (Wadjemup) and beyond.¹¹ During this period, the ancestral Derbal Yerrigan (Swan River) and Canning River incised deep valleys through the Tamala Limestone, carving steep limestone gorges before discharging into the ocean tens of kilometres westward.¹² The subsequent Holocene marine transgression drowned these valleys around 7,000–6,500 BP, transforming them into the expansive estuarine lagoons that modern Bibbullmun people identify as sacred sites created by the Waugal.¹³

4. Indigenous Spatial Ecologies and Geomorphological Custodianship

The stabilization of the coastal plain provided the structural foundation for the complex Bibbullmun six-season ecological calendar (Birak, Bunuru, Djeran, Makuru, Djilba, Kambarang).¹⁴ Clan territories (kalla-boodja) were mapped perpendicularly across the geomorphic zones, ensuring that each family lineage had equitable, contiguous access spanning from the fertile clays of the Darling Scarp foothills through the productive interdunal wetlands to the coastal marine fringe.¹⁵ This cartographic spatial economy demonstrates a sophisticated understanding of Quaternary geomorphology, ensuring sustainable resource harvest across millennia of environmental flux.¹⁶