Submerged landscapes

Submerged landscapes project

This project used published sources, and previously collected raw data to investigate the potential for the survival of ancient land surfaces beneath modern marine sediment and sea water in Port Phillip Bay.

Based on sub-bottom profiler data, pollen data, archaeological and historical evidence it was possible to reconstruct the drowned landscape and ancient environment of Port Phillip Bay area around 10,000 years ago, before the Bay was flooded by rising sea levels.

This project is the first of its kind in Australia, and hopes to demonstrate the value of this multi-disciplinary approach to investigating submerged ancient landscapes in Australian environments.

The landscape reconstruction aims to provide an idea of what the ancient landscape might have looked like, and uses accurate topographic information and vegetation information from pollen data. The activities of the people represented are based on a combination of archaeological information and historical illustrations.

If you have any questions about how the reconstruction was developed, or information which could improve this illustration please contact  hanna.steyne@dpcd.vic.gov.au

The research has been funded by Heritage Victoria through Victoria’s Heritage Strategy.

How do we know the Bay was dry land?

Since the earliest occupation of Australia, dated to around 60,000BP (Flood 1994:1), global changes in climate have had dramatic effects on sea levels, exposing and covering vast areas of land. Periods of intense cold have frozen sea water in glaciers and ice sheets, resulting in sea levels dropping up to 150m below present levels (Chappell et al 1996).

Modern seabed depths have been used to create rough estimates of the location of ancient coastlines. Figure 1 shows the effect a 130m drop in sea level had on the Australian coastline. A drop in sea level of this scale last happened at the peak of the last Ice Age around 20,000 years ago. It is estimated that Tasmania and the Bass Strait islands was joined to mainland Australia until around 14,000 BP, when the sea level was approximately 50m below present levels (Lambeck & Chappell 2001). Current seabed depths suggest that around this time, the coastline might have been about 7km south of Point Nepean, with the Yarra River running through the area now known as Port Phillip Bay. The area beyond the Port Phillip Heads may have become more like a river delta, as the freshwater ran southwards to meet the sea, but the effect the narrow gorge at the Rip had on the water flow is uncertain. It is possible that a waterfall could have formed due to the height difference between the Bay and the land beyond the Rip. Without the assessment of additional geological data from this area, it is not possible to draw definite conclusions.

It is estimated that Port Phillip Bay was flooded by rising sea levels around 8000 years ago (Holdgate et al 2001)

How do we know what the landscape was like?

Sub-bottom profile data, taking slices (profiles) through the seabed, shows us the cutting of river valleys and later accumulation of sediments through time. Figure 2 shows one of the profiles through Port Phillip Bay where large valleys are visible, but now filled in with sediments.

By combining position and depth information from a number of profiles, a 3D model can be created (figure 3) which provides an idea of what the topography would have been like prior to flooding. The sub-bottom profile survey lines are approximately 2-3km apart, making this model very rough, but figure 3 does suggest that the Bay area was generally low lying, with wide valleys running through it in the northern part, and a number of narrower valleys in the southern part.

Where was the river?

Sub bottom profile data provides details about the nature of the river valleys. Figure 4 and figure 5 show that the northern valley is up to 3km wide and around 12.4m deep (Holdgate 1981:122). In the southern part of the bay the channels divide into 6 narrower but more deeply cut channels figure 6. The channels could not be traced any further south due to the type of seabed sediments, which prevented deeper penetration by the sub bottom profiler. It is thought that the smaller channels re-joined into one large channel in the Capel Sound area, then flowed along the route of modern South Channel then out through a deep channel or waterfall at the Rip (Holdgate 1981: 128).

What trees & plants were here?

Generalisations about vegetation at the end of the last Ice Age suggest a thin and broken band of woodland along the eastern and south-eastern coast Australia, whilst the Bass Strait Islands were dominated by open scrub and heath, with some forested areas (D’Costa & Kershaw 1997). It has been suggested that the windy and dry climatic conditions of the period may have reduced woodlands to localised favourable sites, such as river valleys, and that grass and scrub covered much of the eastern coast (Adams & Faure 1997).

This vegetation reconstruction is based on pollen information collected from buried sediments in Port Phillip Bay. Buried sediments from core samples contained pollen from ancient trees and plants. The pollen of each plant and tree type is unique and can be seen under a microscope allowing the identification of plant and tree species which were alive in the area in the past.

Sediment and pollen from the top of the core is the most recent, whilst the sediment and pollen from the bottom of the core is the oldest. Core 7D, collected by Guy Holdgate in 1971 from the central part of Port Phillip Bay, has been analysed for its pollen, and has been dated. An oyster shell from 1.5m down core 7D was dated by radiocarbon to 6273 ± 36 BP (Wk-23494). The pollen from the lowest sample (3.75m) produced evidence of Sheoaks (Casuarinaceae), daisies (Asteraceae), a small amount of grass (Poaceae), saltbush (Chenopodiaceae) and some fern, moss and liverwort spores (Barbara Wagstaff pers comm.). As the pollen comes from a layer below the radiocarbon dated oyster shell, the pollen must be older than 6273 ± 36 BP (Holdgate pers comm.). A summary of the pollen identified in core sample 7D is in figure 7.

What the weather was like?

The climate, along with sea levels, has changed hugely between the last Ice Age and the modern period.

A range of information supports the suggestion that, with the end of the Ice Age, came a gradual warming of the climate. It is thought that in this post-glacial period, around 10,000 years ago, that the climate in southern Australia would have been moister than today, and with similar, or slightly cooler temperatures (Adams & Faure 1997).

How do we know that people were here?

Dreamtime stories passed down through the generations tell the stories of groups living in the Port Phillip Bay area. The stories describe the formation of the landscape, the daily lives, customs and rituals of the people living here. (link through to the dreamtime stories)

Archaeological excavations from around Port Phillip Bay have found evidence of people living in the area continuously from around 32,000 years ago. Evidence from archaeological excavations at Keilor and Pakenham Lakeside provide information about daily life in campsites with hearths and stone tool working areas. Burials at Green Gully, Brimbank Park and on the Werribee River provide evidence of people ending their lives in the area, being cared for by their surviving relatives and of customs and rituals being practiced in the area.

How do we know what people were doing?

Archaeological sites have revealed evidence of stone tool making, fires and burials within areas identified as campsites. The tools could have been used for hunting or working animal hides or wooden objects. The fires would have been used for cooking food and keeping warm whilst the burials hint at one part of the belief systems and rituals the local people had. Dreamtime stories (link to full stories) also tell us about the everyday activities, such as collecting plants, children playing but also relationships between people. Historical records and traditions still practiced can hint at tool kits which might not survive archaeologically such as basketry which would have been used to make a range of items from small bags to fish traps and decorative items.

How did people keep warm?

The only direct information we have about how people kept warm in this damper and colder climate is the archaeological remains of hearths. We know from historical records such as photographs and diaries that the local people used Possum skin cloaks to keep dry and warm. In addition, a number of historical drawings and photographs show that bark huts were used as shelter, however it is unlikely that Sheoaks would have provided suitable bark to roof these structures. It is possible that the shelters could have been roofed with grasses, reeds or rushes or even animal hides.

How do we know what people were eating?

This is based on a combination of knowledge about the likely animals, birds and fish in the area, the tool kit of people at the time and what they would have been able to hunt, and aboriginal knowledge of food sources in the Port Phillip Area which has been passed down through the generations.

Glossary

Ice Age - The general term "ice age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. In this case, the last Ice Age refers to the most recent colder period which peaked – when the ice sheets reached their greatest extent – about 20,000 years ago.

Sub-bottom profile - Sub bottom profilers are powerful low frequency echo-sounders developed for providing profiles of the upper layers of the seabed. Echo-sounders send an acoustic directly downwards to the seabed and record the returned echo. The time for the signal to return is recorded and converted to a depth measurement by calculating the speed of sound in water.

Core sample – A core sample is a cylindrical section of sediment or rock obtained by drilling into the seabed, with a hollow tube called a corer. In the coring process the sample is pushed more or less intact into the tube.