Absent Archaeology
There is good evidence for Aboriginal occupation of Victoria's South West Coast after the Last Glacial Maximum circa 20,000 years ago. In the west around Discovery Bay and Cape Bridgewater, archaeology begins at 13,250 years before present (ybp) at South Cave. In the east around Cape Otway it starts later in 1,420 ybp at Seal Point.
What seems to be missing, however, are reliable signs of human presence before the cold dark days of the last Ice Age. It is generally accepted that people first crossed over into the continent of Sahul about 50-55,000 years ago. They then proceeded to colonise as much of of it that was accessible and hospitable by about 45,000 years ago.
The pattern of exploration was one of following the littoral, with its bounty of shellfish and sea-wrack. Given this strategy, there was no apparent reason why the pioneers of this migration did not eventually come to the shoreline of western Victoria in Australia's south east.
There have been some elusive clues, a "millstone" found near Terang, a "stone axe" from Tower Hill, and "hearth-stones" at Point Richie, Warrnambool, which seemed anomalous in their stratigraphic settings. But extensive research, particularly at Point Ritchie, has failed to verify them as human artefacts from the pre-Glacial period. There is, so far, nothing to indicate regular, or even seasonal, habitation of this coastline during the first 30,000 years of the human era in Australia.
This does not necessarily mean that Aborigines were never here, or that substantial evidence might not yet be discovered. One plausible explanation for the absence of archaeology may be that the coastline that faced arrivals 40-50,000 years ago was not the current one.
From approximately 50,000 to 30,000 ybp, sea levels fluctuated between 70 and 90 metres below present (mbpsl), with the average trending to the lower end of this range. In 31,000 ybp, and over the next 3,000 years, the ocean dropped sharply by 30 metres, before slowing its rate of fall and reaching its nadir of 125 mbpsl about 22,000 ybp.
Geological time is a much condensed version of human or historical time. These changes to the location of the coastline might have been barely noticeable within a person's lifetime. Over a number of generations, though, alterations to the shape and richness of foraging territory would have demanded adaptation.
An additional factor is that accompanying the falls in sea level was a deteriorating climate. Human migration to Australia and subsequent occupation of the littoral and interior regions was undertaken in warmer and wetter conditions. As continental ice sheets accumulated elsewhere in the world (and glaciers advanced in the Snowy Mountains), the weather became significantly colder, drier, and windier. Retreat to more sheltered environmental refuges probably became essential for survival during the LGM, which was at its most intense from 22,000 to 18,000 ybp.
The relevant part of this story is that where Aboriginal activity was most likely to have occurred in the South West, many kilometres off the present coastline, has long since been drowned by the sea as it recovered its former ground in the Last Interglacial (from 18,000 to 6,500 ybp).
Opportunities for exploitation of the territory beyond the current shoreline would have varied. Distance to the edge of the continental shelf is now about 70 kms. The extension of pre-existing rivers and estuaries to the 50 or 60 metre contour line, roughly 8 to 20 kms, would have produced habitable and productive valleys and headlands. But beyond that might have been an intermediate zone.
Sea levels rose and fell during the early and middle "human period", with intermittent plateaus or "high stands" occurring that allowed new coastlines to form and stabilise. Such sites could have supported opportunistic visits once edible vegetation and shellfish became available, but circumstances could also change as the sea went back and forth over time.
The farthest region, past 90 to100 mbpsl and where sea levels finally dropped to their LGM low point, probably remained bare, bleak, and barren. Calculations of the mass of ice involved have suggested that 135 mbpsl might be a more accurate measure than the more commonly reported estimate of -125 metres. The revised figure includes adjustments for a degree of "global oscillation".
It is argued that the exchange of water between ocean and ice resulted in a change of "surface load" on individual continents, some sinking beneath their frozen burden while others like Australia became lighter. If this is right, the outer limits of exposed land could have extended to the edge of the Otway Shelf, where the sea floor plunges kilometres down to the Abyssal Plain.
The effect of the Southern Ocean upwell as it collides with the continent and forces its way through deeply incised canyons is very powerful. It is likely that the last 20 or 30 kilometres uncovered during the 10,000 years after 31,000 ybp remained hostile to humans. Dangerous, saline, wind and tide swept, and fundamentally uninviting.
A startling series of onshore events may also have contributed to Aboriginal decisions about occupying the South West Coast. Sometime before the start of the long winter of the LGM in 31,000 ybp, there was a spike in volcanic activity on the western plains of Victoria. Three new vocanoes erupted, forming Tower Hill, Mount Eccles, and Mount Napier.
Dates for these events have gradually gotten older as scientific methods have become more refined. Recent eruption time estimates for Tower Hill are 35,000 years ago (+/- 3ka, Sherwood et al, 2004), and 36,800 years ago (+/- 3.8ka, Matchan et al, 2020). The dates for Mount Eccles are 37,000 years ago (+/- 5ka, Gillen et al, 2010) and 36,900 years ago (Matchan et al, 2020). Mount Napier dates are 31,900 years ago (+/- 2.4ka, Stone et al, 1997) and 36,000 years ago (+/- 3ka, Gillen et al, 2010).
[The other massive lava feature on the map is Mount Rouse, which is much older at 300,000 ybp and not relevant to this discussion].
The most current of these estimates tend towards the three eruptions all occurring around 37,000 years before present, although with large margins for error. If these measurements are correct then this was a momentous millennia for any humans populating the region.
Tower Hill was originally a maar volcano. As hot magma rose through vents in the earth's crust it met underground aquifers. As these water tables were vapourised by the high temperatures, pressure increased within the limestone and caused a cataclysmic explosion. Clouds of shattered debris then fell in layers of ash and pulverised rock around the volcano crater, forming a "tuff ring". Later extrusions of magma generated high fire fountains and the emergence of scoria cones within the basin, creating the present day feature of a large "nested maar".
Mount Eccles and Mount Napier were predominantly lava producing volcanoes. Mount Napier's Harman Valley Flow spread east and south through spectacular lava tunnels for 20 kilometres. Mount Eccles' Tyrrendarra Flow followed natural drainage lines to the sea, a distance of 40 kilometres from its source. Their lava caves and "stony rises" are still an impressive sight. The sheer scale, the volumes of lava and volcanic energy required, would have made a awesome impression on people who experienced them in real time.
The coincidence of these three displays of natural power, happening at more or less the same time, may have discouraged human activity, at least in the short term when it was actually dangerous to be close by. When disaster is repeated it also has implications for cultural memory and future practice.
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REFERENCES
T Richards, 2012, ' An early-Holocene Aboriginal coastal landscape at Cape Duquesne, southwest Victoria, Australia', Terra Australis 34, Peopled Landscapes, ANU E Press, 64-95
L Niewojt, 2009, 'Gadubanud society in the Otway Ranges, Victoria: an environmental history', Aboriginal History, 33, 175-196
Brooke, Nichol, Huang, Beaman, 2017, Paleoshorelines on the Australian continental shelf: Morphology, sea-level relationship and applications to environmental management and archaeology', Continental Shelf Research, 134, 26-38
Lambeck, Rouby, Purcell, Sun, Sambridge, 2014, 'Sea level and global ice volumes from the Last Glacial Maximum to the Holocene', PNAS, 111(43), 15296-15303
Sherwood, Oyston, Kershaw, 2004, 'The Age and Contemporary Environments of Tower Hill Volcano, Southwest Victoria, Australia', Proceedings of the Royal Society of Victoria, 116(1), 71-78
Gillen, Honda, Chivas, Yatsevich, Patterson, Carr, 2010, 'Cosmogenic ²¹Ne exposure dating of young basaltic lava flows from the Newer Volcanic Province, western Victoria, Australia', Quaternary Geochronology, 5, 1-9
Matchan, Phillips, Jourdan, Oostingh, 2020, 'Early human occupation of southeastern Australia: New insights from 40Ar/39Ar dating of young volcanoes', Geology, 484, 390-394
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