The Geology of the
Hallett Cove Conservation Park
At Hallett Cove, evidence of episodes of the Earth's history from the last 600 million years can be seen, from a total of Earth age of about 4,600 million years.
Much of the evidence has been eroded from the Hallett Cove landscape and lost from the geological record. However, some results of the Earth's continuous geological change remain for us to see and piece together.
The following brief summary of these ongoing and complex events has been sourced from The Field Guide to the Geology of Hallett Cove and A Field Guide to the Coastal Geology of Fleurieu Peninsula, both produced by the Field Geology Club and available by writing to PO Box 28, Marden SA 5070
About 600 million years ago the area now occupied by the Mount Lofty and Flinders Ranges was part of a long "trough" in the seafloor (the Adelaide Geosyncline) where sediments such as silt and sand were deposited and buried.
At about 500 million years ago this region was squeezed by movements in the Earth's crust and these sediments were deeply buried and hardened to form rock (siltstone, sandstone and quartzite). The compression continued, folding the rocks and uplifting them to form an enormous mountain range called the Delamerian Highlands.
These Highlands were eroded down to form a hilly landscape. The eroded silt and sand was washed out into the sea to the east and we now see them as some of the rocks in New South Wales, Victoria and Tasmania.
At Hallett Cove we can see the folded siltstone, sandstones and quartzites of the Delamerian Highlands at Black Cliff and in the cliffs and shore platform to the north.
Permian Ice Age
At about 280 million years ago Australia was part of a supercontinent called Gondwana which included all the (present) southern continents and India. Gondwana was then moving over the South Pole and was covered by an icecap.
That icecap covered the southern two-thirds of Australia from about 280-270 million years ago (Permian times) and was at least one kilometre thick. The icecap moved in a northerly direction over Australia.
Evidence left by the ice includes chatter marks, crescentic gouges, scratched and polished bedrock and the remains of a U-shaped valley can be seen along the coast, south of Hallett Cove Beach. The glacial striations on Black Cliff indicate that the ice sheet at Hallett Cove moved in a north west direction.
About 270 million years ago the climate warmed and the ice sheet melted. Meltwater laden with fine rock flour and coarser material accumulated in layers on the floor of lakes, and boulders in icebergs were released and dropped into very fine sediments. This evidence can be seen throughout Hallett Cove Conservation Park.
Late Pliocene Times
The Gondwana supercontinent later broke up and the fragments dispersed, with Australia finally separating from Antarctica about 43 million years ago. The next geological evidence to be seen at Hallett Cove relates to a landscape about 4 million years ago.
In a much warmer climate, Gulf St Vincent lapped against the ever uplifting (and eroding) Mt Lofty Ranges. Shellfish proliferated to such an extent that their shells accumulated on the sandy shallow sea bed and over time formed a fossiliferous sandstone. A shelf of this sandstone can be seen at a number of spots around the Park.
Pleistocene to Recent Times
Again sediment was removed and the next evidence relates to about ½ million years ago. Wet and dry seasonal climatic changes caused a calcrete layer to develop in surface soils and this rubbly limestone can be seen around the rim of the Amphitheatre. Climate over the longer term also varied greatly, causing long periods of alternating wet and dry and warm and cold weather.
100,000 years ago now-extinct megafauna roamed the area; Diprotodon and giant kangaroo remains have been found in the banks of the Field River inland from the southern end of Hallett Cove beach.
The Amphitheatre was eroded into its present shape about 10,000 years ago, revealing glimpses of 270 million years of geological time. The Hallett Cove region and the Mount Lofty Ranges are still being uplifted and the resulting processes of erosion and sedimentation continue.
While preserving the unique geological evidence at Hallett Cove is the Park's primary purpose, there are other important environmental aspects to be conserved and explored.
The Park has the largest area of publicly accessible remnant coastal vegetation along the metropolitan coastline and it is a sanctuary for birds, reptiles and insects.
Removal of rocks, fossils
or biological specimens is not allowed
Cooper, H M et al (1970): Hallett Cove, a Field Guide, South Australian Museum, Adelaide.
Giesecke, R. (Ed., 1999): A Field Guide to the Geology of Hallett Cove and Other Localities with Glacial Geology on Fleurieu Peninsula (1999), Field Geology Club of SA.
Hasenohr, P. & Corbett, D. (Eds., 1986) A Field Guide to the Coastal Geology of Fleurieu Peninsula, Field Geology Club of SA, Adelaide.
Hallett Cove Conservation Park Management Plan (1986),
SA National Parks & Wildlife Service, Adelaide.