HALLETT COVE: Matthew Flinders & the Coastal Landforms of SE Australia

'Fire seen ... Mud & sh. ... Fine ... Gulf of St Vincent ... ' - extract from Matthew Flinders chart Hallett Cove Conservation Park - glacial features
The site On the coastal cliffs south of Adelaide is an area that is extremely important to the 'story' of landforms. Numerous signboards (and publications from relevant sources) explain the processes and results of previous glaciations upon the development of landscapes here.	Photo 1: Signpost, southern entrance to park
This site has been identified as of national and international significance to geology and geomorphology. Hallett Cove Conservation Park is maintained by the National Parks and Wildlife Service of SA. This page describes just a few of the important features developed in the area under the Permian (Gondwanan) glaciation. Note that this location shows direct and indirect indications of an earlier glaciation (the Sturtian, about 750my ago) and a later glaciation (the Pleistocene, peaking about 18,000 years ago).
Glacial striations In the Permian Period, about 280my ago, a thick ice sheet covered southern Australia and adjoining portions of Gondwana. The glacial striations (or 'ice scratches') show that the general direction of ice flow at Hallet Cove was northwesterly, although this varied according to local topography. It may seem quite amazing to those who know Australia as a hot, dry land that much of the continent has undergone considerable glacial action! Scouring of the rock surface (siltstone, in this case - see photo) took place when ice with coarse rock fragments rocks embedded in it rubbed across the rock surface. (Had fine materials contacted the rock, it would have become smoother.)	Glacial striations on siltstone Photo 2: Glacial striations on smooth, metamorphosed sediments
In the photo (above right) the cliff-top rock is siltstone, deposited in a large trough as fine sediments (silt = mud + clay) about 600my ago (in the Proterozoic era), and folded and altered (metamorphosed through heat and pressure) about 500my ago. It became scratched and partially smoothed by moving ice and ice-borne materials (moving from foreground to background). It was then covered by materials that proteceted the surface from further change, and then, more recently exposed by erosion, to reveal the scene of today.
Glacial erratics The Permian Period ended when about 10my of glacial conditions drew to a close. As the great ice sheet began to break up, huge meltwater lakes covered vast areas. The icebergs floating in these lakes melted away and their mud, sand and rock loads were dropped onto the lake floors. Some of these dropped rocks - called glacial erratics - are now exposed. Many were transported large distances by the ice from their sources. The boulders shown (right) are of metamorphic quartzite, and are different in composition to the local bedrock. It is believed that some rocks were carried more than 50kms from the south-east. Softer sediments surrounding these erratics have been removed by subsequent erosion.	Photo 3: Glacial erratics, Hallet Cove
A number of large erratics are found on the nearby beach. While brought there by ice action during the Permian (280my), one erratic is composed of Sturt Tillite (metamorphic pebbles embedded in mudstone). This rock is from the much earlier Sturtian glaciation (about 750my).
The Sugarloaf Perhaps the best known feature of Hallett Cove Conservation Park is the Sugarloaf. It is a residual landform, exposed as surrounding material has been weathered and removed. It sits within a depression called the Ampitheatre. The coloured layers hold the key to understanding the original formation of these features.	The Sugarloaf Photo 4: The Ampitheatre and Sugarloaf, Hallett Cove
Photo 5: The Sugarloaf, Hallett Cove
The Sugarloaf is made of sediments that represent a considerable time difference; they have been exposed as surrounding material was eroded away. The brown capping is Pleistocene alluvial silt and clay (deposited during the beginning of the most recent glacial period). This layer is hard enough to help slow down erosion of the soft, older layers underneath. Between the clay layer and the white sand layer is an unconformity, representing a 'break' in the age of the layers of almost 300 million years. Below the unconformity are several visible layers of Permian sediments, each representing a distinct depositional phase. The upper Permian layer is a soft, mainly white sandstone, with small dropstones. Beneath that is a red-brown silt, sand and clay glacial bed with pebbles and numerous dropstones (erratics). The lowest is composed of white glacial sand, gravel and some dropstones. Beneath all of these layers, but not visible at the immediate vicinity of the Sugarloaf, is the old metamorphic bedrock upon which these layer were deposited during the later Permian and Pleistocene times. In spite of intensive study, aspects of the palaeogeography of the Sugarloaf are still not fully understood. These include the role of wind in forming the white layers, and the origin of the red glacial layer.
Reasons why one must be careful at all cliffs! (see sign right) Note: Take extra care at the summits and bases of cliffs. Do not climb fences, nor climb to dangerous spots, to collect rocks or to view landforms.	Photo 6: Halletts Cove - top of cliff