Western Region of Melbourne
‘The surface geology of much of the region is dominated by the remnants of Cainozoic volcanism, and there are numerous eruption points, and broad, moderately thick lava flows in the northern, central and western sectors.’ ‘The major lava-free areas are those such as the You Yangs, which had sufficient elevation to escape burial’
‘The most widespread geological material outcropping in the area is the basalt of the Newer Volcanics, ranging from 2.5 million to 5 million years ago. Individual flow thickness varies from 2 metres to over 10 metres, and the entire sequence is in places almost 100 metres thick.’ The flows originated from Mt. Cotterell (lava cone), Mt. Kororoit (scoria capped by lava), Greek Hill (mixture of basalt and scoria), Black Hill (Cowies Hill), Green Hill and Mt. Mary (lava and scoria cone) and Spring Hill (lava dome). Some of the tongue lava -flows from Mt. Cotterell were up to 10 kilometres long. ‘The bulk of the volcanic material is tough, dark, strongly olivine basalt. Scoria and and tuff (fragmental lava) is less common and is associated with higher and steeper volcanic hills such as’… ‘Mt. Bullengarook’.
‘Although classified as Newer Volcanics, these volcanoes are of the Pliocene age, and generally are much older than the Newer Volcanics of Western Victoria. Original volcanic features, such as stony rises and crater, have thus been modified by weathering and are less preserved here.’
‘Site Description: Mount Cottrell is a volcanic cone formed by the radial eruption of numerous lava tongues.’ Height is 205metres. ‘ The massive nature of this basalt cone is not readily appreciated when viewed from close quarters, but is readily apparent when seen from distant points such as the You Yangs.’ ‘The slopes are of low angle, apart from those at the summit where a hard dense basalt bluff 5-6 metres high surrounds a shallow crater.’
‘Significance: Regional. There ar two aspects to the significance of this site - the specific nature of the bluff and crater at the summit, and the extent of the radial flows. The cone is the most massive of the Werribee Plains volcanoes and gave rise to a number of distinct lava flows. The basalt rim is of particular interest as it has similarities to the cappings of the volcanoes of the Gisborne area.’
‘Geological Significance: Highly significant geological feature. The Mount is part of the Register of the National Estate, categorized as having State Significance by the National Trust of Australia (Victoria) and the Geological Society of Australia (Victorian Division),
1994. Mount Cottrell is the best example in Victoria of a lava shield with lava cone as summit. Quoted from the Rosengren (1994) in his inventory and evaluation of scientific significance,
The site was rated as having state significance due to its significance’ Rosengren 1994
‘The flows from this point’, this being Mt.Cotterell. ‘may have influenced the course of the Werribee River.’ Reference: Condon M.A. (1950)
‘The Werribee River downstream of Bacchus Marsh basin has cut a gorge through an extensive lava field. For the succession of basalt flows and interbedded pyroclastics the name ‘Exford Volcanics’ is proposed’. Reference M. A. Condon 1950.
Bracketed quotes from:
Sites of Geological and Geomorpholgical Significance in the Western Region of Melbourne N.J. Rosengren 1986 unless stated otherwise.
http://lifesites.com.au/pinkertonforest/mtcottrell-background/mtcotterell-geologicalhistory#sigProGalleria81e43ea7c3
