Research paper written by Peter
Dahlhaus and reproduced with permission
Dahlhaus Enviromental Geology Pty. Ltd.
The origins of the peninsula can be traced
back to the Cretaceous period when Australia split from Antarctica in the
final break-up of Gondwanaland. During that time, a rift valley formed
between the two continents and was filled by sediments deposited by fast
flowing, braided river systems.
The sediments, which were derived from the erosion of large volcanoes, now
form the grey to greenish grey fieldspathic sandstone (greywacke or
arkose) and mudstone 'bedrock' of the Bellarine Peninsula. These rocks,
exposed in outcrops from Clifton Springs to Portarlington, underlie the
Following the continental break-up, the Australian continent moved
northwards. As a result of the split, the southern edge of the Australian
continent experienced uplift, creating the South-Eastern Highlands. The
erosion of the highlands provided a source of sediment that was deposited
into the widening basin between the two continents.
The sea had inundated the entire peninsula by the mid Miocene, when the
coastline was situated as far inland as Meredith. As the sea retreated
during the Pliocene, it left behind a thin deposit of sand - the Moorabool
Viaduct Sand -much of which covers the central part of the Bellarine
The collision of the Australian Plate and the Pacific Plate during the
Miocene created the SE-NW compressional stress field that Victoria
currently experiences. This stress field activated faults at right angles
to it, including the Bellarine Fault and the unnamed fault east of Point
Henry. The continued activity of these faults has lifted the Bellarine
Horst to its present level.
Shortly after the uplift in the Pliocene, the coast between Torquay and
Ocean Grove comprised a large funnel-shaped embayment, narrowing towards a
wide channel that ran to Corio Bay. The embayment was shallow, with a
floor of Tertiary (mostly Miocene) sediments. Changing sea levels
associated with the continued retreat of the sea constructed a series of
small barriers and lakes throughout the embayment, resulting in the
deposition of a series of dense shell beds and minor sands, which now
underlie the Connewarre Lake system.
In the Late Pliocene or Early Pleistocene, volcanic eruptions commenced at
Mount Duneed and lava flowed into the shallow embayment. As the lava
progressed eastwards, it divided into three separate flows, a westerly
flow along the Tait's Point - Fisherman's Point ridge, a central flow
northeast of Pelican Rocks, and a larger southerly flow from Black Rocks
to the Bluff. Lagoons formed behind these lava barriers, as the Barwon
River was dammed for some time. Eventually, the Barwon cut through the
Taito's Point - Fisherman's Point barrier, then the Shook Point- Pelican
Point barrier, and continued along the edge of the southerly lava flow
between the Bluff and Ocean Grove.
From the mid Miocene onwards, the polar ice caps formed and sea levels
began to fluctuate more dramatically in response to climate changes.
Sea-level changes over the past hundred thousand years are responsible for
the development of the present day landscape and features of the Barwon
Heads - Ocean Grove area. During the last inter-glacial period, about
125,000 years before present (BP), sea levels were generally about 6
metres higher than present. As a new glacial period (ice age) developed,
the sea levels trended downward, but with a series of reversals during
short warming episodes .
As the sea levels dropped much of the continental shelf between
Victoria and Tasmania became exposed. The colder, drier climates were
accompanied by periods of high winds, which mobilised the sediments on the
exposed sea-floor to form dunes along the emerging coast. During this time
a large sand bar barrier the Nepean Bay Bar - formed across the front of
the broad depression of Port Phillip Bay. The barrier was built up by
overlapping dunes and beach deposits with a high calcium carbonate
content, to form aeolian calcarenite. At Barwon Heads, this material
accumulated on the basalt and now forms the cliffs exposed at the Bluff.
During the intervening warmer episodes, wetter climates and
vegetation growth were probably responsible for the development of soils
on the calcarenite dunes, Calcium carbonate mobilised from the topsoils
was translocated down the profile to form calcrete horizons. The plant
roots provided pathways for percolating groundwater to form soil pipes. At
the onset of cooler and drier climates, aeolian activity recommenced and
the soils were buried, forming palaeosol horizons in the strata.
As the shoreline receded, the Barwon and other rivers lengthened their
course and deepened their valleys to reach the ever-distant sea. The
rivers cut through the Nepean Bay Bar and other dunes that had formed on
the floor of the exposed Bass Strait. At the glacial maximum, around
18,000 BP, sea levels were about 120 to 150 metres below present day
After the commencement of deglaciation (- 17,000 BP) sea levels rose
rapidly to reach present day levels by 6,000 BP. The rapidly rising sea
levels must have resulted in extreme rates of shoreline migration,
estimated for Australia as 20 metres per year or 40 centimetres per week!
On the Bellarine Peninsula, shell beds dated at 6,000 years suggest that
sea levels were one or two metres higher than present day. Although they
have since dropped to present levels, it is believed that they have risen
slightly over the past century.
Previously described by Daintree (1862),
Griffiths (1893), Rosengren (1977, u.d.), Marsden (1988) and Bird (1993),
the Bluff is a remnant of the Nepean Bay Bar aeolian calcarenite
unconformably overlying the basalt of the Mount Duneed lava flow.
The basalt, exposed around the eastern base of the Bluff and shore
platform, is fine-grained and sometimes vesicular, composed principally of
olivine, augite and plagioclase. On the north eastern side of the cliffs,
the basalt exposed at the unconformity is weathered to a mottled red-brown
and grey clay, being the ancient soil developed on the lava flow before it
was covered by the dunes. This palaeosol shows highly weathered basalt
boulders (corestones or 'floaters') in a matrix of clay, the upper part of
which has been oxidised (red-brown) and is gleyed (grey) lower in the
profile. In places, the weathered basalt and palaeosol has been
preferentially eroded, resulting in overhangs of up to approximately 3.5
metres of calcarenite .
Above the basalt on the northern side of the Bluff, the fawn to pale
yellow-brown aeolian calcarenite forms steep cliffs. The aeolianite is
composed of crossstratified beds of cemented well-rounded quartz sand and
shell fragments. The foreset beds that formed on the lee side of the
dunes, are those with the steepest dip. They indicate that prevailing
southerly winds, bringing sand from Bass Strait, formed the dunes. On the
southern side of the Bluff, the predominant dip of the
cross-stratification is not as obvious (since it dips north), but the
variations in wind direction remain clearly recorded by the changes in
inclination of the strata.
Infiltration, derived from both rain and ponding surface water, has formed
calcretes and solution pipes, especially in the upper levels of the cliff.
The percolating water, acidified by carbonic and humic acids, dissolves
the calcium carbonate (CaC03) in the rock and then redeposits it when the
water evaporates. In this manner the CaC03 is translocated down the
profile to form calcrete layers. These are most obvious as the pale yellow
to white layers of hard limestone that cap the Bluff. In places the
calcretes have been eroded, broken, and recemented to form a thick layer
of breccia. Griffith (1893) noted the presence of some basalt in the
breccia and concluded that there was an alluvial component, attributed to
the Barwon River. Solution pipes, often lined with calcrete, formed as
root channels, plant stems and tree trunks covered by the dunes, provided
preferred pathways for the infiltrating water. The pipes project down into
lower layers, and some have been enlarged by infiltrated water, filled
with sand or filled with rubble that has been recemented to form breccia
At least six palaeosols are layered in the calcarenite, each representing
a period of stability in dune building when soils (and presumably,
vegetation) developed on the surface. In some exposures the palaeosols
have been preferentially eroded, forming overhangs of calcarenite or