Cape Bridgewater

This information has been obtained from the report: Eruption Points of the Newer Volcanic Province of Victoria by Neville Rosengren. This report was published in 1994 and was prepared for the National Trust of Australia (Victoria) and the Geological Society of Australia (Victorian Division). The review of eruption points was based on an earlier unpublished manuscript Catalogue of the post-Miocene volcanoes of Victoria compiled by O P Singleton and E B Joyce (Geology Department, University of Melbourne 1970).

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 value. Reference should be made to the most recent reports. See the Earth Science Heritage (external link) section of the Geological Society of Australia website for details of geological heritage reports, and a bibliography.

Location:	Stony Hill
	38 23 00S 141 24 30E (external link); 7121-2-1 (Dryden) & 7121-2-2 (Fisherman Cove) 3555310. 17 km W of Portland on eastern side of Cape Bridgewater.
	Heywood
Land Tenure/Use:	Private and public land (Discovery Bay National Park). Public access along coastline via difficult and, in places, dangerous walk. Low tide access only. Grazing.
Type 9:	Composite complex eroded volcano of tuff and lava flows.
	The peninsulas forming Cape Bridgewater, Cape Nelson and Cape Grant display magnificent sections of lavas and pyroclastics. The three peninsulas and intervening bays were described by Boutakoff (1963) as remnants of subsidence calderas that have been eroded by marine action.
	At Cape Bridgewater the coastal cliffs expose pyroclastics (mainly tuff) showing complex bedding and multiple lava flows. The bedding and composition of the pyroclastics suggests phreatomagmatic eruption. Two vertical lava pipes may be the vent(s) of a volcano. The structure was interpreted by Boutakoff (1963) as a collapsed caldera with the upper eastern half removed by marine erosion. The exposure is of variable clarity owing to weathering and slumping obscuring part of the cliff face. Due to steep slope and unstable surface, it is difficult and in places dangerous to climb the cliff face. The whole structure can be viewed across Bridgewater Bay from vantage points on the eastern side. Overlying the volcanic rocks is cemented dune limestone (calcarenite) of the Bridgewater Formation. The calcarenite has formed from parabolic dunes and transverse ridges stranded on cliff tops 30 to 50 m above present sea level and now distant from any sand source. Included in the dunes are areas of karst terrain with sinkhole, closed depressions and little surface drainage. On the south-western coast is an area where columnar and branching calcrete structures over 1 m high have formed from enlargement and subsequent infilling of hollows and pipes in the dune limestone. These have the appearance of a “petrified forest” although they are properly called rhizo-concretions and are not actually fossils. The sea caves in the basalt below the dune limestone contain calcareous accumulations such as speleothems and tufa terraces. These result from the action of ground water, enriched in carbonate dissolved from the dune limestone, leaving evaporite deposits where it emerges onto the cliff face or the roof, walls and floor of the cave.
	135 m; 135 m.
National:	This is the deepest natural exposure of Newer Volcanics material in Australia and an outstanding example of internal volcanic structure. It is the major coastal eruption point exposed in the Newer Volcanics region. It displays explosive and effusive styles of volcanicity with complex bedding structures in lavas and pyroclastic deposits. The area provides a rare opportunity to view a cross section of the inner structure of a major volcanic edifice including the vent, infilling lava plug and associated lava flows. This is the only such exposure in the Newer Volcanics Province and is unusual on a national scale. If the structure is a caldera as interpreted by Boutakoff (1963) this is the major (and possibly only) such example in the Late Cainozoic volcanic formations in Australia. The coastline has a number of rare coastal limestone features including sea caves, blowholes, solution pipes, tufa terraces and the “fossil forest” of rhizo-concretions. It provides an unusually clear example of cliff-top dunes stranded well beyond any sand supply and good sections of the unconformity between the basalt and dune limestone. The site is an important locality in the determination of late Tertiary volcanic processes and Quaternary sea level and climatic changes.
Class 3:	Most of the significant exposures occur on and are viewed from public land. However the hazards of the site (tide, wave action, cliff falls) need to be appreciated by visitors. Apart from natural erosion or cliff collapse and damage by visitor pressure there are no significant threats.
References:	Coulson, A. 1941. The volcanoes of the Portland district. Proceedings of the Royal Society of Victoria 53; pp. 394-402. Boutakoff, N. 1963. The geology and geomorphology of the Portland area. Geological Survey of Victoria Memoir 22. Ollier, C.D. & Joyce, E.B. 1964. Volcanic physiography of the Western Plains of Victoria. Proceedings of the Royal Society of Victoria 77, pp. 357-376 Pope, C. 1983. Palaeovolcanology of the Cape Bridgewater volcano, Portland area, western Victoria. Bsc(Hons) thesis, Monash University, Melbourne (unpubl). Cas, R., Allen, R., Bull, S. & Sukhyar, J. 1984b. The Bridgewater volcanic complex. Volcanics Workshop 1984. Monash University, Melbourne (unpubl).

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