Drowned Palaeoshorelines & Submerged Shelf Geomythology: The Geochronology Of Aboriginal Oral Tradition

1. Eustatic Sea-Level Rise & Rottnest Shelf Inundation (18,000 to 6,500 BP)

During the Last Glacial Maximum (LGM), which peaked approximately 21,000 to 18,000 years ago, massive continental ice sheets locked up vast volumes of the Earth's water, causing global eustatic sea levels to drop to approximately 120 to 130 meters below present levels.^[4][5] Along the western and south-western margins of Australia, this sea-level retreat exposed hundreds of thousands of square kilometres of the continental shelf as a dry, vegetated coastal plain.^[1][2]

During this epoch, the islands of Wadjemup (Rottnest Island), Mewstone, and Carnac Island were not islands at all; they were elevated limestone ridges and granitic hills standing high above an extensive plain extending up to 25 to 30 kilometres west of modern Fremantle.^[1][3] The Swan River (Derbal Yerrigan) flowed through this exposed plain, cutting deep underwater canyons that remain visible today in bathymetric surveys.^[2][3] Between 14,000 and 6,500 BP, rapid post-glacial melting initiated the Flandrian Transgression, causing sea levels to rise at rates exceeding one to two meters per century, permanently drowning the continental shelf and isolating Wadjemup by approximately 6,500 to 7,000 BP.^[1][5]

2. Geomythology: Custodial Oral Histories of the Flooding of Wadjemup & Derbal Yerrigan

In a pioneering geomythological investigation published in Australian Geographer, Patrick Nunn and Nicholas Reid documented that Australian First Nations oral traditions preserve accurate, eyewitness historical accounts of post-glacial coastal inundation dating back over 7,000 to 12,000 years.^[1]

Bibbullmun oral records collected in the nineteenth and early twentieth centuries by Daisy Bates and colonial observers record explicit accounts of the drowning of the coastal plain.^[6][7] Elders recounted that in ancestral times, their forebears walked on dry land from the mainland to Wadjemup (Rottnest) to hunt emu and kangaroo.^[6] The tradition recounts that a cataclysmic surge of ocean water broke across the limestone ridge, transforming the hills into islands and submerging sacred hunting grounds beneath the sea (Wardun).^[6][7] Because writing was absent, this geochronological event was preserved through rigorous, verbatim oral transmission across more than 300 generations—constituting the world's oldest surviving verified oral history.^[1][6]

3. High-Resolution Bathymetric Evidence: Drowned River Canyons and Submerged Caves

Modern marine geophysics and underwater archaeology fully substantiate this oral geochronology.^[3][5] Multibeam sonar bathymetry along the Rottnest Shelf reveals drowned paleochannels of the ancestral Swan River, submerged coastal wave-cut notches, and drowned sinkholes (cenotes) in the Tamala Limestone formation.^[2][3]

Underwater archaeological investigations off the Western Australian coast—such as those published by Benjamin et al. (2020) at the Dampier Archipelago—have proven that ancient Aboriginal stone tools, hearths, and grinding stones survive intact on the submerged continental shelf beneath marine sediment blankets.^[3] Similar submerged stone artifact scatters are preserved across the Rottnest Shelf, representing an untapped archaeological archive of Pleistocene Bibbullmun occupation.^[2][3]

4. Submerged Sea Country Jurisprudence: Maritime Boodja Title to the Continental Shelf

The verification of drowned palaeoshorelines carries immense legal significance for native title and maritime sovereignty.^[1][3] Under classical Bibbullmun law, the drowning of land by ancestral waters did not extinguish property title (kalla-boodja); rather, the drowned estates transformed into Sea Country (Wardun Boodja), governed by specialized maritime customary law and totemic custodianship.^[6]

When the British colonial regime invaded in 1829, it presumed that Indigenous territory ended at the high-water mark, claiming the continental shelf as crown waters.^[7] However, scientific geomythology and marine archaeology prove that Bibbullmun estates encompass the entire continental shelf.^[1][3] The ancestral oral accounts of the flooding of Wadjemup provide an unbroken evidentiary title deed linking modern Bibbullmun custodians to the submerged geography of their ancestors.^[1][6]