Last Glacial Maximum & Sea-Level Rise: The Inundation Of The Rottnest Shelf

1. Paleogeography of the Rottnest Shelf During the Last Glacial Maximum

During the Last Glacial Maximum (LGM, approx. 21,000–18,000 cal BP), global sea levels fell to roughly 125 to 130 metres below present levels as vast quantities of ocean water became locked in continental ice sheets.¹ In southwestern Western Australia, this profound regression transformed the marine environment, exposing the entire continental shelf—the Rottnest Shelf—as a vast, subaerial coastal plain extending up to 35 kilometres west of modern Perth and Fremantle.² Wadjemup (Rottnest Island) and Meeandip (Garden Island) were not offshore islands, but prominent terrestrial limestone ridges and elevated hills rising above an expansive savannah-woodland landscape traversed by the ancestral courses of the Derbal Yerrigan (Swan) and Canning rivers.³ This vast drowned country represented integral clan estates of the Bibbullmun Sovereign Nation.⁴

2. Post-Glacial Sea-Level Transgression and Coastal Inundation Dynamics

The termination of the Pleistocene witnessed catastrophic global meltwater pulses (MWP-1A and MWP-1B) that initiated rapid marine transgression.⁵ Between 15,000 and 7,000 cal BP, sea levels rose at average rates exceeding 1 to 2 meters per century, steadily submerging thousands of square kilometres of low-gradient coastal plain.⁶ Hydrodynamic models indicate that during peak transgression, horizontal coastline retreat across the Rottnest Shelf occurred at rates of several meters per year, drowning familiar hunting grounds, freshwater river channels, and lithic quarry outcrops.⁷ Wadjemup was permanently severed from the mainland around 7,000 to 6,500 cal BP, isolating the island and forever transforming regional territorial geography.⁸

3. Submerged Archaeological Potential of the Wadjemup Shelf

Recent marine geoarchaeological surveys and high-resolution multi-beam bathymetric mapping have confirmed that the submerged Rottnest Shelf contains exceptionally well-preserved paleolandscapes.⁹ Acoustic sub-bottom profiling has mapped drowned river channels, submerged limestone karst features, and drowned coastal barrier dunes that provided resource-rich habitats during Pleistocene occupation.¹⁰ Because rapid marine inundation can seal archaeological deposits under marine silts without destructive wave abrasion, these drowned landscapes represent a vast, intact underwater cultural heritage repository containing ancient hearths, shell middens, and lithic assemblages that bear direct witness to deep-time Bibbullmun custodianship.¹¹

4. Deep-Time Oral Traditions: Indigenous Memory of Drowned Continental Boodjar

The catastrophic inundation of the continental shelf is vividly and accurately preserved in the ancient oral histories and songlines of the Bibbullmun Sovereign Nation.¹² Elder testimonies recorded in the 19th and early 20th centuries recount how ancestors walked on dry land to Wadjemup and Meeandip to hunt wallabies and collect emu eggs before a great rising of the ocean severed the land, turning hilltops into islands and creating the sea passages known today.¹³ These traditions represent empirical oceanographic observations transmitted across more than 300 human generations, confirming that Bibbullmun customary law and oral archives constitute genuine, scientifically verifiable histories of sea-level rise and landscape transformation.¹⁴