Site Selection Process and Definitions

Definitions

Locality | Access | Ownership| Geology/Geomorphology | Significance | Management

Sites of geological and geomorphological significance on private land in the Portland Shire have been selected on the basis either that they represent a specific characteristic of the region or they are an outstanding or unusual example of a geological or geomorphological feature or process. As such, these sites are of interest to people engaged in research or teaching in the earth sciences who aim to elucidate the composition, origin and dynamics of the physical landscape.

The sites were selected after a study of available literature on the Portland area including a number of unpublished reports and theses. Discussions were held with geologists and others with a knowledge of the area and a detailed study made of vertical aerial photographs. This allowed the detection of patterns and relief features that had not been previously recorded. Field surveys were undertaken during part of March and April 1981 with the intention of verifying the nature of all sites recorded from the preliminary studies, to delineate their boundaries and to obtain ground photographs. Reference was also made to existing oblique aerial photographs available in the Department of Geography, University of Melbourne.

The report is not divided into distinct geological and geomorphological sections as most sites have elements which are relevant to both these branches of earth science. For example, river or coastal cliffs may provide important stratigraphical or palaeontological information as well as illustrating geomorphological characteristics such as the processes of cliff formation.

The sites were initially delineated on a series of five 1:50,000 topographic mapsheets. Each site has a map number (1 to 5) followed by a site number on the original 1:50,000 sheet. These numbers do not refer to significant rating of the site and are only for indexing purposes.

For each site, a statement has been prepared detailing:

Locality
A six figure grid reference preceded by the map number of the National Topographic Map Series 1:100,000 sheets is given. Some larger sites have been given two grid references to indicate their extent, but these references serve only to locate the sites rather than to accurately delineate their extent. The position of the site in relation to a settlement or a local feature named on the map is given either in the site title or in the locality statement.

Access
As virtually all sites are on enclosed land holdings, only the name of the nearest identifiable road is given - most are reached by private, un-named roads or tracks.

Ownership
A statement is provided only when the site includes both Crown Land and private land.

Geology/Geomorphology
For most sites, these are described together. The name and a short description of the main rock units is provided as well as the nature of any important structures or fossils which the site displays. Landforms and past and/or present geomorphological processes which are of interest are recorded.

Because of the very short time provided for this project, reliance has been placed on existing analysis of geological and geomorphological features supplemented by aerial photograph interpretation at a scale of 1:50,000. All sites were visited to verify their existence but it has not been possible to revise or critically review the analyses provided in the literature - hence numerous errors of fact and interpretation may occur. The classification and sub-division of rock strata into identifiable units - Group, Formation, Member, Lens, Bed, etc - is under constant revision as new data become available. Hence, many names change over time. This applies especially to the classification of the middle and late Tertiary limestones that underly or outcrop over most of the Portland Shire. Cliff sections on the coast, in river valleys, or in landslip scarps are particularly important as type sections or illustrations of the characteristics of rock units. Where possible, these should be retained as clear, debris-free faces. Quarry faces are also an important source of geological data.

Significance
The concept of significance is difficult to quantify satisfactorily. From the geologists and particularly the geomorphologists point of view, the entire surface is significant in that the form and evolution of one part contributes to an understanding of the nature and origin of all other areas. The delineation of a site boundary indicates that a discrete feature or assemblage of features is discernable inside that boundary. However, complete analysis of these is often dependent on investigations of forms and processes that lie outside that site area.

The sites that have been described must be regarded as the minimum required to represent the array of land form and land forming processes that comprise the Portland Shire. Many display features representing geological processes that are no longer, or only intermittently, active. In some cases however, sites are of interest for their dynamics in that the landform is subject to change on a time scale that can be monitored or measured on a short term basis, e.g. the annual movement of a sand dune or landslip.

Site have been selected where they satisfy one or more of the following criteria:

1. Geological

a. An outcrop which is used as a type section - for example Wataepoolan Limestone Member at the Glenaulin landslips.
b. An exposure of the contact between members or formations e.g. at Whalers Point.
c. An outcrop that displays clearly the lithology or structures of a bed or formation, e.g. Glenelg River Cliffs near Drik Drik.
d. A site which displays a geological structure, e.g. Grassdale-Wannon Monocline.
e. Beds containing abundant or rare and stratigraphically important fossils, e.g. the Lower Oyster Bed in the Whalers Bluff Formation at Dartmoor.

2. Geomorphological

a. Sites which show the relationship between landform and the underlying geological materials, e.g. the caves and sinkholes at Drik Drik.
b. Sites which display clearly a geomorphological process e.g. the features on the Tyrendarra lava flow which illustrate the flow patterns and cooling of the basalt from Mt Eccles. These sites may be a result of processes that are no longer active or they may represent active (in some instance rapid) landform change e.g. the landslip sites.
c. Sites were chosen that are representative of the landform units and processes of the Portland Shire ranging from the coastal cliffs, dunes, and swamps to the dissected areas of the Merino Tablelands in the north of the Shire.

Sites have been assigned a significance rating of either International, National, State, Regional or Local. We have identified 2 sites of National significance, 26 sites of State significance, 48 sites of Regional significance and 7 sites of Local significance. Five sites have been assigned an unknown significance rating where there is insufficient data available for a more complete evaluation at this time. Factors considered in determining this rating were:

a. Frequency of replication i.e. the site is a unique, rare or unusual example of a geological formation or landform.
b. Values as a reference or research site which displays classic characteristics or a geological formation and/or a relict or active geomorphological process.
c. Degree of disturbance and/or the clarity of display of the outcrop, structure or landform.
d. Need for further investigation, e.g. where the nature and/or origin of the feature is uncertain or where little detailed investigation has taken place.
e. Where landforms and/or outcrops provide spectacular landscape.

The rating assigned to a site is determined by evaluating the degree two which the five criteria outlined above are fulfilled. A site of National significance is either unique or rare in Australia and has been so little modified that the essential properties are clearly preserved and displayed. The site may represent a major contribution to the understanding of a geological or geomorphological process and contain potential for further research. For example, the surface of the Tyrendarra and Harman Valley lava flows display some of the best preserved late Quaternary volcanic flow features in Australia. The tumuli at Wallacedale are rare volcanic forms and are large and well preserved by world standards.

A site of State significance includes landforms or geological structures which are displayed clearly and/or provides an important contribution to the understanding of stratigraphy and the evolution of the Victorian landscape. The cliff-top dunes at Cape Bridgewater are the best examples in Victoria of a process that has emplaced stranded dunes on most of the prominent headlands.

Sites of Regional significance are clear examples of landform or geology typical of the Portland Shire. The landslips near Tahara and the abandoned quarry in Heytesbury Group limestones near Tyrendarra are examples. Sites such as Mt Deception are classed as of Local significance either because they are of common occurrences, are in a poor state of preservation, or have limited scope for field study and further analysis.

Management
A statement is provided which indicates the extent to which disturbance to the site will alter its geological or geomorphological value. This statement is meant to be a broad guideline and it is emphasised that specific land use proposals must be evaluated in terms of the degree of alteration they would cause to a specific site. The value of some geological sites may be enhanced by controlled excavation. Hence, road widening, new cuttings, or limited quarrying may, by exposing new and fresh sections, be compatible with the maintenance of the site as one of geological significance. However, the total impact of such operations needs to be evaluated. The destruction of strata that contain restricted fossil deposits, or the production of rock debris that obscures an important outcrop or landform may seriously degrade the value of a site.

It is noted that many quarry sites provide important geological sections. The fossil soil horizons and solution pipes in Bridgewater Formation dune limestone exposed in the quarry south of Portland (Site 5-14) could be retained and managed as reference sites if the face was occasionally cleared of weathered or slump debris.