Exploring the Coastline....
To understand coastal evolution of the Port Phillip region, we must address some aspects of the broader canvas involving global coastal systems. These notes present some of those global features before focussing on specific aspects of Port Phillip and adjacent areas.
1. Sea Level Change
The saga of the global sea level symphony, rhythmic changes of the past 2 million years, embodies one of the great stories of environmental evolution. The rhythms and magnitudes of such planetary changes remain a matter of both fascination on one hand, and central to understanding of the present coastline on the other.
The simple equation
ICE VOLUME = SEA LEVEL VARIATION
conceals an immense drama of global change. The relationship between Pleistocene ice and sea level changes remains a key to our understanding of the coastal zone we know today.
The establishment of global sea level curves over the past 25 years provides new understanding into the history of coasts around the world. Much of this has been the result of isotopic analyses of marine shelly organisms (Fig. 1). These have demonstrated a multi-cyclic pattern of change spanning the entire period of Ice Age times, well beyond the last million years.
It is significant that much of that new understanding of the last Ice Age cycle has been derived from work carried out by Australians. The detailed and brilliant reconstructions by John Chappell (Australian National University) from his study of coastal reefs on the northern side of Papua New Guinea has provided a template for understanding global sea levels of the past 350,000 years (Fig. 2).
Closer to home, the sequence and process of coastal dune building (calcarenites) of southern Australia have provided a similar story. In the Murray Basin, that story spans the past 6 million years (Figs. 3 & 4). For our purposes, two time scales are particularly important:
Before discussing details of sea level change, several aspects of special relevance should be considered.
Coastal Dunes (Calcarenites)
Sequences of calcareous coastal dunes occur from Western Australia across the southern Australian coastline to Wilson's Promontory. They remain conspicuously absent on Australia's eastern coastline.
The dunes, composed largely of carbonate sands formed mainly from comminuted shell fragments mixed with normal quartz sands, have been blown from sandy beaches to form shore-parallel dunes. Although originally formed as mobile dune sands, they are cemented into soft rock be percolating waters which dissolve and reprecipitate the carbonate or lime content They form the dominant coastal forms of the Port Phillip Nepean area today.
The formation of these coastal dune sands has been a matter of considerable controversy. Many workers thought they were formed during periods of low sea levels; that they were related to glacial conditions in which strong winds blew calcareous sediments, exposed by low seal level on continental shelves, into vegetated areas along past shorelines. Others argued they formed from adjacent beaches. In this latter sense, they marked the presence of the shoreline being formed at high sea level, during warmer interglacial phases.
The evidence today points strongly to their formation from high sea levels with cementation and soil formation associated with periods of lower sea levels.
One critical piece of evidence involves the formation of soils. Exposures on eroding cliffs reveal the development of strong red clay-rich horizons separating distinct periods of dune formation. These buried or fossil soils define discrete dune-forming events. They provide a critical method of reconstructing the sequence and ages of dune formation.
In the Nepean area the presence of a sequence of dune forming periods, separated by distinct soils, is clearly evident.
While calcareous coastal dunes form one distinctive feature of the Victorian coastal environments, the presence of some of the world's best developed shore platforms forms another. Excellent examples are developed at Nepean and Point Lonsdale. Not only are these remarkable geomorphic features, but they also provide a virtual habitat mosaic for the great variety of algal, molluscan and other colonies that live in intertidal, or shallow sub-tidal environments.
In profile, shore platforms actually involve the development of two cliffs at the same site (Fig. 5). For this to happen, the local rock type must be subject to moderate erosion rates but must also be durable enough to preserve erosional forms. Rocks of intermediate hardness are most susceptible for such development. Thus, shore platforms are extensively developed on the felspathic sandstone of the Otways and Strzelecki rock types (Lorne, Apollo Bay, Inverloch to Cape Patterson). They do not develop well on granites (Wilson's Promontory, Wollomai) or on softer rocks such as clays and marls (Anglesea, Port Campbell).
The precise development of nearly horizontal platforms on the dune limestones of Nepean and Point Lonsdale are wonderful examples. Here an inner coastal cliff passes out across the abrasion, intertidal platform to an outer cliff below low water level. This outer cliff, often nearly vertical, is sometimes undercut (as divers will attest). For this broad platform to survive, the rate of erosion of the outer and inner cliff must match. If one greatly exceeds the other, the platform will either grow (inner erosion rate > outer), or the platform may be destroyed (outer rate > inner).
'The age and rates of platform development remain little understood. Again this takes us back to the question of sea level change. The almost precise coincidence between the horizontal intertidal platform and present mean sea level remains something of a puzzle.
Global sea level reached its present position approximately 6000 years ago. However, on the Victorian coastline, and in Port Phillip in particular, there is strong evidence of past seal level being up to 2 meters above present. That would effectively submerge the Nepean platform below the low tide zone.
The arguments of past high sea levels were, for many years, strongly disputed. Only some coasts of the world recorded such events. In Port Phillip Bay, many workers had recognised the presence of shell beds to 2 metres above present sea level (in Tootgarook Swamp, Carrum, Altona and near Geelong). Radiocarbon dates consistently put the age of these high beaches near 6000 years before present. But similar studies on the east coast of Australia failed significantly to find any trace of such high levels. And if we believe sea level is consistent around the globe, then we had a real problem.
The conflict was eventually resolved by the recognition that the post-glacial rise in sea level, an event which had put some 150 metres of water loading the coastal shelves, had also involved an adjustment in the earth's crust. This adjustment lags behind the loading factor. Thus in areas where there is a broad shelf, the coastal loading results in depression along the shelf margin with slight uplift inland.
This wedge shaped effect then resulted in emergence of the shorelines of 6000 years ago, lifting them up to some 2 metres above present sea level. On the east coast, where the shelf is very narrow, no such effect is recorded. Depending on the width of the adjacent shelf, this process, resulting from the post-glacial rise in sea level a process known as hydrostatic loading, has variously affected coasts throughout the world.
At Nepean on the other hand, the coincidence of the platform precisely within present intertidal levels would argue for platform development after hydrostatic loading adjustment had ceased a period within the last 4000 years. But could such a huge platform have developed in that time?
Is there a possibility that these platforms are really very old; that they have been inherited from a previously high sea level during the last interglacial some 125,000 years ago? Although these questions remain unresolved, some evidence is advance by the relationship between platform development, sea level stability and tectonic movement.
Sea Levels & Tectonics
A further complication in the Port Phillip region involves the reality of recent tectonic movements. The entire Port Phillip region lies within a geological sunkland, an area bounded on the west by the Rowsley Fault near Geelong and Selwyns Fault in the east crossing the Mornington Peninsula. In a bore near Point Nepean, coastal dune sediments, formed originally above sea level, are found at depths near 100 metres below sea level today. This requires a major downwarp of this part of Port Phillip region within the period of such dune formation, a period within the last two million years.
Given the nature of tectonic movement in the Port Phillip region, the consistency of platform formation across Nepean to Point Lonsdale and beyond, would argue for their development after tectonic deformation. In that sense they would have to develop with the last 4000 years.
Examination of the shore platforms demonstrates a close relationship between the physical processes of erosion and the ecological systems that colonise the habitats thus formed. The rhythmic energy of the tidal zone, sweeping backwards and forwards across the platforms, produces zonal gradients in which the physical processes relate closely to, and often control the biological responses.
Blocks eroded from the inner cliff both protect the base of that cliff and provide detrital materials forming an abrasion zone near the upper, high tide limit. This abrasion zone grades seawards into areas free of detrital material without which the forces of abrasion diminish. Here chemical solution dominates with the formation of lapies, or karstic erosion features. These pass out to the outer region or the surf zone in which high energy concentration results in undercutting and development of the transitional zone to detrital sands.
Platform development is further influenced by structural development in the host rocks. Bedding planes, joints and fracture patterns providing zones of weakness, often result in benches, channels or similar sites of preferential erosion.
One of the popular theories of human occupation of this continent involves special ideas of coastal colonisation. The hypothesis goes like this:
The first arrivals, perhaps some 60-100,00 years ago, had to cross a substantial expanse of ocean to get here. They were therefore a marine oriented culture. After arrival on the northern shores of the continent, and following their marine food sources, they migrated around the coastline long before they colonised inland Australia. Thus ages of occupation will be older near the coast than in inland Australia.
The beauty of this hypothesis is that it cannot be tested unless people arrived at least 100,000 years ago. At any time within the last 100,000 years (the upper and as yet undefined limits of human occupation), the coastline stood offshore from its position today. Any evidence of its occupants lies submerged on the continental shelf. The likelihood of any such submarine evidence surviving, or ever being found, remains infinitesimal.
The only established evidence of coastal habitation by ancient Australians involves the frequent camp sites associated with post-glacial high sea levels. This mostly involves the extensive shell middens scattered along the dunes of southern Victoria. Here, the harvesting of the great variety of shell fish provided a rich source of food for those coastal dwellers of Holocene times (within last 10,000 years). However, their association with periods of glacially low sea levels is certainly a matter for serious speculation.
During low seal levels of the last glacial maximum some 20,000 years ago, the area we know as Bass Strait today, was dry land. It would certainly have been colonised. As the ice caps melted rapidly in the northern hemisphere between 20 to 12,000 years ago, the relatively fast rise in sea levels would have seen the destruction of coastal lowlands on the flat Bass coastal plains. Eventually whole colonies of people were cut off from each other when Bass Strait was formed about 12,000 years ago. Tasmania became permanently isolated from mainland Australia.
That physical barrier remained an extremely important cultural barrier for the next 12,000 years. Thus, many cultural innovations that spread across mainland Australia, never reached Tasmania. The Tasmanians never knew of boomerangs, dingos, or the fine flaking tool-making tradition that found its way across the mainland some 6000 years ago. None of those innovations which entered Australia with the last 10,000 years were transmitted across Bass Strait. This physical barrier remained an impenetrable cultural barrier, preserving a virtual Ice Age culture in the island people.
Despite the relative youth of cultural evidence on coastal sites, one enigmatic occurrence remains unresolved. The origin of a deposit of molluscan shells cemented into cliff-top limestones near the mouth of the Hopkins River near Warrnambool remains to be resolved. The shells date from well before the Holocene. They involve shallow, subtidal molluscan species requiring the nearby presence of the coastline. The last time the coastline stood near its present position (Fig. 2) was more than 100,000 years ago. Were these molluscs collected and transported to the ridge top by humans? If so, they constitute the oldest evidence for humans in Australia today.
This question remains one of major dissent. Archaeologists have tended to dismiss any such claim for human antiquity of this basis. While the absence of any other evidence of human presence (artefacts, fires) is lacking, the matter remains unresolved. However, the failure of any other adequate explanation must also be acknowledged. The circumstantial evidence of human agency therefore remains, with all the enigmatic implications that involves.
Fig. 1. Pattern of sea level changes spanning the last 1.4 million years deduced from oxygen isotope analyses of foraminiferal shells. High negative values correspond to warm interglacial periods. More positive (black) values designate cold glacial, low sea level phases (Shackleton, 1989).
Fig. 2 Detailed sea level curve for last 150,000 years showing pattern of changes through last glacial cycle. Note the relatively short time that the sea has occupied its present position. The shore last stood near its present position some 120,000 years ago (from Chappell, 1990).
Fig. 3. Pliocine strandline ridges in the Murray Basin of southeastern Australia
Fig. 4. Diagrammatic summary of Pliocine sea level changes
Fig. 5. Diagrammatic representation of shore platform in dune calcarenites typical of those in the Sorrento - Nepean area of the Port Phillip region.
Ancient red soils formed during stable periods when sea level was lower, and active beaches had moved seawards. Soft calcareous sands were cemented, then subjected to weathering and soil formation.
Erosion by modem sea levels has produced two cliffs. Platform width is a function of the relative migration rates of the cliffed breaks in slope. Boulder accumulation often protects the inner cliff producing temporary stabilisation. Chemical solution etches the platform into a variety of karstic features.
Biological colonisation is controlled by the physical zonation between the inner abrasion zone and the outer chemically affected zone, often leading to a rampart zone which sometimes marks the outer platform margin.