TV5 Breakneck Hill - Ordovician Sediments, Newer Volcanics

This information has been developed from the publications:

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 Geological and Geomorphological Significance in the Shire of Otway (1984) by Neville Rosengren.

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 (external link) for details of geological heritage reports, and a bibliography.

Location	Toolern Vale - 854391. Shire of Melton. Road cuttings on Gisborne - Melton Road, 5 km north of Toolern Vale.	TV5 - Gisborne Basalt flow overlying Ordovician sediments, Breakneck Hill. Note baked soil horizon (S).
Access	Gisborne - Melton Road.
Ownership	Crown land
Site Description	The road cutting at Breakneck Hill displays a long section in steeply dipping Upper Ordovician sediments (pink to grey and black shales and pale coloured sandstone). Some beds in the sediments have a rich graptolite fossil fauna including species of Climacograptus and Dicellograptus. Overlying the Ordovician in the northern part of the cutting is a section of lava flow from Mount Gisborne.
	Known as the Gisborne Basalt, this rock is a dark greenish grey, vesicular basalt that becomes glassy towards the base of the flow. The heat from the base of the flow has baked the soil that had formed on the Ordovician sediments and has developed cracked and distorted prismatic structures n the upper section of the palaeosol profile.
Significance	Regional. The site is a clear exposure of steeply dipping Ordovician sediments and a good fossil collecting locality. The overlying volcanics are one of the complex of different types that issued from Mount Gisborne, and the contact in the northern part of the cutting is a clear example of the impact of hot lava burying an earlier formed soil.
Management	Class 2. The sites are most likely to be changed due to road widening operations. While this would increase the exposure of the Ordovician sediments it could remove the significant contact of the Gisborne Basalt in the northern part of the cutting. In the vent of road works at the site, consultation between the road construction authority and geologists should take place to determine if a clear section of the contact can be maintained.

Page top