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19 Steeles Rock, Portarlington - Geological Section

This information has been developed from one or more of these publications:

  • Sites of Geological and Geomorphological Significance in the Westernport Bay Catchment (1984) by Neville Rosengren.
  • Sites of Geological and Geomorphological Significance in the Western Region of Melbourne (1986) by Neville Rosengren
  • Sites of Geological and Geomorphological Significance on the Coast of Port Phillip Bay (1988) by Neville Rosengren.
  • Sites of Environmental Significance in the Flood Plain of the Upper Yarra Valley Region (1983) by Neville Rosengren, Douglas Frood and Kim Lowe (as part of a study of Sites of Environmental Significance by the University of Melbourne for the then Upper Yarra Valley and Dandenong Ranges Authority).
Geological heritage sites, including sites of geomorphological interest and volcanic heritage sites, are under regular revision by the Geological Society of Australia, especially in the assessment of significance and values.Reference should be made to the most recent reports. See the Earth Science Heritage section of the Geological Society of Australia website for details of geological heritage reports, and a bibliography.

Location951791 to 955781. Approximately 400 m of coastline including cliffs and shore platforms between Simpson Street and Gellibrand Street, Portarlington. The site includes the reefs and low rocks known as Steeles Rock.
Image: Sites of Significance Port Phillip Bay
Site 19, Unconformity (arrowed) between Older Volcanic basalt (A) and Moorabool Viaduct Formation (B), Portarlington

Access
The Esplanade, Portarlington.

Ownership/Managing Authority
Crown land (Portarlington Foreshore Committee of Management and Department of Conservation, Forests and Lands).

Site Description
Continuous outcrop along a 200 m section of coastal cliff displays a variety of volcanic and sedimentary materials. The cliffed section, reaching 4 m high, extends from just west of the white beacon westward to near the toilet block, where the cliff swings inland to be a low bluff which turns coastward as a rocky promontory. Weathered basalt outcrops in the west of the section (from the promontory to beyond the toilet block) and is overlain with marked angular unconformity by ferruginous and calcareous and gravely sediments. The basal beds of the sediments are coarse sand and small gravels, passing upward into ferruginous cross-bedded sands and grits. Towards the east, the beds have a pronounced dip eastward of about 30o opposite the white beacon. The upper metre of the section is a fossiliferous calcareous bed which in places has been further cemented by secondary calcite deposition. The fossils are all of living species of bivalves or gastropods.

The structure and stratigraphical relationship of the beds has been the subject of much discussion. They have been correlated with either the Moorabool Viaduct Sand (Upper Pliocene) or regraded as younger Pleistocene beds. At the western edge of the site, the geology is also complex with indurated dipping sandstones forming rugged low headlands and shore platforms. Some of the outcrop on the western platform may be of Cretaceous sandstone which occurs inland but is not mapped as coastal exposures.



Image: Sites of Significance Port Phillip Bay
Site 19, Steeles Rocks, Portarlington
Image: Sites of Significance Port Phillip Bay
Site 19, Dipping ferruginous beds overlain
by calcareous beds, Steeles Rock
section, Portarlington
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