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6.1 Volcanic plains

6. Western Plains (WP)

6.1.1 Eruption points; maars, scoria cones and lava shields, including associated ash and scoria deposits (Lake Purrumbete, Mt Elephant, Mt Cottrell)
6.1.2 Stony rises (Mt. Eccles, Pomborneit, Mt Rouse)
6.1.3 Plains with poorly developed drainage and shallow regolith (Wingeel)
6.1.4 Plains with well developed drainage and deep regolith (Cressy)
6.1.5 Terraces, floodplains and lakes, swamps and lunettes and their deposits (Lough Culvert, Lower Woady Yallock, Chain of Ponds, Condah Swamp, Lake Murdeduke & lunette

The volcanic plains were built up by sporadic volcanic eruptions over a period of about 5 million years, and are known geologically as the Newer Volcanics, the deposits which form the Newer Volcanic Province of Victoria, which includes parts of the Western Uplands, the Western Plains, and the area across the border around Mt Gambier. Much of the plains were formed from lobes of lava which flowed from the eruption points, overlapping to form a veneer of basalt lava flows. The flow varies in thickness according to both the underlying topography and the present-day surface. The flows are interleaved in places with pyroclastic deposits (scoria and tuff) and discontinuous buried palaeosoils of variable thickness.

Drainage across the volcanic plains is generally poorly developed. The uplift of the Otway Ranges and the disruption of drainage by the volcanic eruptions resulted in the formation of a shallow basin in the central region, where most of lakes, including Lake Corangamite, are found. Fluctuating lake levels during the Pleistocene resulted in lunette formation, especially prominent in the Beeac-Eurack district.
Image: 6.1
Lava from the volcanic eruptions was generally very fluid and flowed along the lowest points in the landscape especially major south-trending valleys, often resulting in blocked drainage systems. The Curdies River and Barwon River were probably the most affected and their present day courses, and those of the ancestral Mt Emu Creek and Hopkins River, have been strongly influenced by the eruptive history of the region. North of Birregurra, the Barwon River has been diverted around the edge of the basalt and was blocked for a time at Winchelsea and Lake Connewarre.

While grasslands are common on much of the plains lava, and on many of the cones, dense trees commonly mark the young stony rise flows, and large scattered Red Gums are found on the deeply-weathered older flows, for example south of Hamilton.

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