Images of Paleozoic glacial erosion in Hallet Cove, Australia

by Alex Shepherd,  shephera@geophy.curtin.edu.au

Introduction (location-map.jpg)

About 280 million years ago (Permian Era), Australia formed part of the great southern super continent of Gondwana, which was centered over the South Pole. The Earth had plunged into a major ice age and an ice sheet similar to that in Antarctica covered most of this landmass. Near the end of this ice age, the ice sheet began to melt as the climate warmed, forming a cold lake with rafts of melting ice. As the ice sheet retreated, plant and animal life began to recolonise the area.

Photograph 1 (1-glacial.jpg; thumbnail-1-glacial.jpg) and Figure 1 (fig-1.jpg)

As you walk along the cliff top in this vicinity you are passing sediments deposited on the bottom of an ancient glacial lake. This layered sequence of multicolored silt, clay and sand was deposited towards the end of this ice age.  As warmer conditions returned, the ice sheet began to melt, forming several lakes. A lake here was dammed against the ice margin which was probably just west of the present coastline.  Meltwater streams feeding the lake deposited fine glacial sediments in layers on the lake bottom. Floating icebergs also carried larger rocks and boulders which fell into the lake sediments as the ice melted. These are known as dropstones or erratics.

Photograph 2 (2-glacial.jpg; thumbnail-2-glacial.jpg)

This polished and scratched rock surface (glacial pavement) has made Hallett Cove, Australia famous.  It was caused by a moving ice sheet and is one of the few places in the world where such ancient markings are so well preserved.  As the ice moved northwards over the ancient landscape, sand and silt in the base of the ice polished the bedrock to a smooth finish like a giant piece of sandpaper. Larger rock fragments made the long straight scratches known as glacial striations.  You are looking at a part of the landscape as it was covered by ice about 280 million years ago and exposed when the ice melted. Other polished and striated surfaces can be seen along the cliff edge.

 [Environment and Natural Resources, Mines and Energy, Australia]

 Thumbnail images