Mass movement and valley side features

Landslips are generally restricted to the north-east of the Portland Shire, on the elevated Merino Tablelands and along the valleys of the Crawford, Stokes and Wannon Rivers and their tributaries. The coastal cliffs of the Portland township, north of Portland Harbour, are also susceptible to landslips. These two areas are considered separately.

1. North-eastern landslips

The occurrence of landslips is related to the geological, topographic, hydrological and land use characteristics of the area. The most common type of landslip is a rotational slide or slump. This occurs when a slope fails by rotation along a curved shear plane, with a characteristic sinking of the head of the slide and a heaving up at the tow. The lower part then often fails further by earthflow. Terracettes occur at several sites, often adjacent to or on landslide masses. These appear to consist of small elongate and repeated rotational landslides. Minor tunnelling has also occurred at the base of some earthflows. Extensive landslips at Glenaulin have blocked the Crawford River to form a lake.

The landslips are generally developed on highly weathered Lower Cretaceous Otway Group sediments on Tetriary Dorodong Sands. These latter sediments are relatively unconsolidated, often being exposed as sands and clays and overlain by profiels described as laterites. These consist of an upper indurated layer of ferric concretions (pisoliths) underlain by mottled and pallid clay zones. It has recently been considered that many weathering profiles described as laterites are not true laterites. The status of the above profiles is thus uncertain and consequently will be described in the report only in terms of their observable characteristics. The hard zone of iron accumulation acts as a cap rock and helpts to maintain steep slopes on the softer underlying rock.

Landslips are primarily geomorphic features, although at some sites the headwalls provide good exposures of the subsurface stratigraphy, e.g. Glenaulin landslips. Where this material is highly weathered, detailed study is required to determine the age and composition of the sediments exposed. This was not possible within the time limits of the study and thus at some sites only a generalised geological description is given.

2. Portland landslips

Landslips in the Portland area are developed on basalt weathered to a stiff fissured clay, which is exposed in the cliffs between Portland Harbour and Dutton Way. Factors contributing to the development of these landslips include the removal of vegetation from the cliff, undercutting of the cliffs by wave action with consequent cliff steepening by gravity collapse adn the fissured structure of the clay which facilitates a high water intake and movement throughout the decomposed rock mass. The landslips are generally rotational slides.

Landslips and other forms of mass movement often pose a substantial threat to property in the Portland Shire. Detailed studies of slope stability are necessaryin areas where there is evidence of prior landslide activity or where changes in land use are likely to initiate slope failures. Study of existing landslips to determine factors involved in their development means that some existing mass movements need to be regarded as reference sites and warrant their preservation as sites of significance. Thus, where minimal damage to property is involved, there is a case to be made for retaining some landslips as active and continuing features. Also, the exposures of underlying material that are displayed in the headwall scarp may be some of the few outcrops of that material available. We have described mass movements from several areas of the Shire of Portland and of criteria outlined above to warrant classification as sites of geological and/or geomorphological significance.

Unless otherwise specified, all landslips included in this report are considered to be of Regional Significance. They display a characteristic of the parent materials (Tertiary or Lower Cretaceous sediments) or are an expression of abrupt changes of slope on the dissected Merino Tableland and the valley sides of the Glenelg, Crawfrod and Stokes rivers and their tributaries.