Geomorphology

The landscape features in the Werribee Plains region are as a result of three main processes that occurred from the Late Tertiary to the present day: volcanism, uplift of land caused by the Rowsley fault and climate and sea level changes. The You Yang’s are an outcrop of Devonian granite (360 to 400 million years old) which protrude above the volcanic plain.

Werribee Region Physiography This map has been adapted from the map: "Melbourne and the Bays: Physiography" map in the Atlas of Victoria (1982). It was originally prepared by J.J Jenkin.
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In the Tertiary period, more widespread land surfaces were developed by fluviatile erosion across a range of rock types. Although subsequently dissected, faulted, tilted, covered by lava flows or partly invaded by the sea (and covered by a veneer of sand and gravel), some of these land surfaces can still be recognised. One of these from the early Tertiary (30-70 million years ago) occurs to the north-west of the Werribee Plains region, in the Blackwood and Brisbane Ranges (Jenkin, 1982).

The volcanism that occurred in the Late Tertiary to the Early Pleistocene (5 to 2.5 million years ago) produced the level to undulating plain surface which is characteristic of the western suburbs of Melbourne and extends to Bacchus Marsh and south and west of Geelong.

Within the Werribee region there are a number of eruption points which have steeper slopes and are higher than the surrounding lave plain (Geological Survey of Victoria, Melbourne Sheet, Scale 1:250 000, SJ 55-5). The highest eruption points in the region include Green Hill, One Tree Hill, Spring Hill and Mount Cottrell. Apart from these eruption points the lava plain slopes towards Port Phillip Bay, i.e. south-east, at an average grade of about 1:150.

The original lava surface was rough and stony resulting in irregular topography and many small depressions. Some of these depressions have poorly defined to no outlet channels. The largest of these is the swamp that occurs immediately west of Edgars Road, three kilometres north of Little River. The present drainage system appears to be controlled by the edges of the individual lava flows and the more prominent edges of these flows have been mapped over part of the region (Geological Survey of Victoria, Melbourne Sheet Scale, 1:63 360, Sheet SJ55-1).

A major influence on the topography has been the Rowsley fault, which resulted in the formation of the Lerderderg Ranges, Brisbane Ranges and the Pentland Hills. The most recent major movements of this fault took place in the Early Pleistocene about one to two million years ago. This uplift caused the streams to be rejuvenated and the Lerderderg River, Werribee River, Parwan Creek and the tributaries of the Little River cut deep valleys into the uplifted plateau and the scarp of the fault. Some of the eroded material was deposited in areas adjoining the scarp, for example, west of Balliang, the upper flood plain of the Parwan River and near the Melton Reservoir. However, much of this material transported directly down the Werribee River and the Little River and redeposited in the Werribee Delta.

Rejuvenation of stream valleys has also taken place during the several periods of sea level fall during the Pleistocene. As recently as 20 000 years ago the sea level was over 100 m lower than today (Jenkin 1968) and this caused the major streams (Werribee River, Little River and Skeleton Creek) to cut into the landscape. Because of their low flow, minor drainage lines were unable to make much impression on the lava plain.

At the time of low sea level, Victoria and Tasmania were connected by a land bridge that extended from west of King Island to west of Flinders Island (Hill and Bowler, 1995). The rainfall was about half that of today, and the weather colder and windier. It is likely that dust and sand storms swept across the area, depositing aeolian material occur north east of the Werribee Delta - where there is an extensive deposit of wind-erodible alluvium (Geological Survey of Victoria, Melbourne Sheet Scale, 1:63,360, Sheet SJ 55-1).

The veneer of wind deposited sand, silt and clay has since been incorporated into the soil by biological activity add some of the resulting soil has been re-deposited by the overland flow of water into the lower part of the landscape. Direct evidence that the area is covered with a veneer of wind-blown material is provided by the fact that the surface soils often have a high sand component. The predominant mineral in sand is quartz, which is a mineral not found in basalt. However, at Green Hill, fragments of the underlying Tertiary sediments which do contain quartz were carried upward by eruptive gases and were deposited locally.

As the sea level rose to just above its present level during the Mid Holocene (about 4 500 to 7 000 years ago), minor alluvial deposition occurred in parts of the stream valleys, the most prominent being the narrow terraces along the Werribee River.

Thus the present landscape in the Municipality mostly comprises a gently undulating to level lava plain with a drainage system often influenced by the edges of old lava flows. Although a thin veneer of windblown material blankets much of the area, a variable stone cover is present. Most of the stony lava area north-east and around Werribee has been stone-picked to improve its agriculture versatility and some of the swampy areas have been drained. Only a few channels such as the Werribee River, Little River and Skeleton Creek have succeeded in deeply incising into the lava plain. The Werribee River, together with the Little River, has formed and extensive alluvial delta as a result of erosion higher in the catchment and the soils here can be used for intensive cropping.

References:

Hill, S.M. and Bowler, J.M. (1995). Linear dunes at Wilson's Promontory and south-east Gippsland, Victoria: Relict landforms from periods of past aridity. Proc. Roy. Soc. Vic. 107: No. 2. Pp 73-81.

Jenkin, J.J. (1968), The geomorphology and Upper Cainozoic geology of southeast Gippsland, Victoria. Mem. Geol. Surv. Vic. 27.

Jenkin, J.J. (1982). Physiography. In Atlas of Victoria. Victorian Government.

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

Sargeant, I. J. (1998). Geomorphology of Wyndham. Report prepared for Wyndham City Council. Ian Sargeant and Associates Pty Ltd.

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