Geology

The information in this section has been derived from a range of sources including publications by the Geological Survey of Victoria (1974), Rosengren (1986), Douglas and Ferguson (1988) and Leonard (1992).

The most widespread geological material outcropping in the study area is that the basalt of the Newer Volcanics, ranging from 2.5 to 5 million years old. The bulk of the volcanic material is tough, dark, strongly jointed olivine basalt. The Late Tertiary to Early Quaternary volcanoes produced abundant flows of basalt lava that spread rapidly from the vents and buried existing landscape features. Individual flow thicknesses varied from 2 to over 10 metres and the entire sequence is, in places, almost 100 metres thick. The flows originated from numerous vents and fissures and many of the eruption points remain as conspicuous landscape features. Those identified on the 1:63 360 and 1:250 000 geological maps are shown on the map prepared for this report.

Since the extrusion of the basaltic lava, a drainage pattern had been established which is described in more detail under Geomorphology. Quaternary and Recent alluvium has been deposited along sections of the Werribee River and the Little River and the deltas of both rivers comprise extensive sheets of Quaternary alluvium.

The basaltic lava overlies a sequence of Tertiary sediments that are subdivided into the Werribee Formation, which is the oldest, and the Fyansford Formation. These sediments overlie basement rock, either Devonian granite or Ordovician sediments. At Green Hill, fragments of the underlying Tertiary sediments may be found which contain casts of marine fossils. The fragments were carried upward by the eruptive gases and are present as inclusions in the lava and scoria bombs (Rosengren, 1986)

References:

Leonard, J.G. (1992). The Port Phillip Region Groundwater Resources. Department of Water Resources, Victoria.

Rosengren, N.J. (1986). Sites of Geological and Geomorphological Significance in the Western Region of Melbourne. Dept. Conservation, Forests and Lands.

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