The Yarra River

YarraTrailVT

Follow this link for a 360 degree view of the Plenty River near the Yarra River and Rosanna Golf Course

GEOLOGY OF THE MIDDLE YARRA RIVER

The type of rock underlying a landscape helps determine the shape of the land, the nature of the soils, the location of the watercourses and the vegetation communities.

In the Banyule Flats area, the underlying rock / soil is river floodplain alluvium (Quaternary alluvial flats and terraces), overlying the Silurian bedrock of Melbourne.

See below for a brief explanation of the geology of the Middle Yarra. This material has been largely extracted from  Chapter 2 'Geology of the Middle Yarra' of Still Glides the Stream (1).

The Silurian bedrock of Melbourne
Underlying the entire Melbourne area is an ancient bedrock of sandstone and mudstone. It was deposited in layers under the sea during the Silurian period, about 400 million years ago, and was later subjected to tilting and folding. The bedrock comes close to the surface throughout the greater part of the eastern and north-eastern suburbs of Melbourne.
The Silurian bedrock is usually covered by soils of low fertility and typically consisting of loam overlying clay.

The Tertiary sands     (The Tertiary period began about 65 million years ago)

The folded Silurian surface was lifted up above sea-level, underwent a long period of erosion and became a gently undulating 'peneplain'.  Eventually, in the Tertiary period, Port Phillip Bay and much of the surrounding land sank below the sea and new series of deposits were laid, both under the sea and on the low-lying adjacent land. These deposits were generally of sand, silt and clay, brought down by flooding rivers. These Sands still extend throughout the south-eastern suburbs from South Yarra to Mordialloc and Dandenong. They also occur in large patches of Camberwell, Heidelberg and Essendon. But in between these patches, the land has been eroded right down to the Silurian bedrock.
The soils formed on the Tertiary sands might typically consist of sand overlying clay, to a depth over one metre. Such soils are generally poor in nutrients.

The Newer Volcanics and recent Alluvium    (Quaternary basalt and Quaternary alluvial flats and terraces)

Beginning about 4 million years ago and continuing upto about 820,000 years ago, lava flows of the Newer Volcanics began to erupt from many points to the north and north-west of Melbourne. They flowed down the old valleys of streams, including the Merri Creek and Darebin Creek and down the lower Yarra Valley. The eastern boundary to the lava flows was formed by the higher land in the present sites of Preston, Alphington and Kew. The lava flows caused part of the Yarra River course to shift to the east and cut a new valley.

The lava also dammed the Yarra near the junction of the Darebin Creek and Yarra River - making a huge lake of the river flats above, upstream to Templestowe.  A new floodplain was then formed via progressive sedimentation of the lake. When the river eventually cut through the damming bedrock, the lake was drained and the river now meanders through the floodplain of deep alluvium.

Basalt soils of the Newer Volcanics are typically heavy clays, relatively rich in nutrients. In contrast, the soils of the alluvial flats are dark loams, clays and sands, fair to poor in nutrients.

Geology of Middle Yarra

Picture Credit: This map is copied from the book Still Glides the Stream (1) page 18. Credit for these maps, given in Still Glides the Stream is given to John Waddingham who produced the maps.

 

STREAMS AND WETLANDS OF BANYULE FLATS

(Information for this section has been mainly taken from the source Plenty River and Banyule Creek - a landscape study (3) and from Chapter 5 'Banyule Flats and Warringal Parklands, Heidelberg' of Still Glides the Stream (1) )

Both the Plenty River and Banyule Creek flow into the Yarra River in the area around the Banyule Flats. The Plenty River has a catchment area of 352 square kilometres. In the Banyule area it is a relatively mature, incised meandering stream. The sediment and turbidity loads in the stream may be related to the erodible nature of the Silurian sediments which constitute the bed of the stream.
In comparison with the Plenty River, Banyule Creek is a much smaller watercourse, with a catchment area of only 4 square kilometres.  Banyule Creek rises in the area of the Simpson Army Barracks just north of Viewbank.

Banyule Billabong is about 800 metres long and roughly L-shaped. Beginning from its wider western end, it runs east for about two-thirds of its length and then turns abruptly southwest, the southern tip being close to the river. At the bend there is a shallow secondary branch that creates a small island when the billabong is full of water. The wetland is filled when the river floods and the water reaches a sufficient height. This occurs irregularly.

Just to the north-west of the billabong is Banyule Swamp - a broad, shallow wetland. It is now fed from an urban stormwater drain - entering at the north-east of the swamp - and nearly always contains water. The swamp has an elongated shape, with a very shallow northern portion and a deeper southern portion.  In 1984 Heidelberg Council converted the site to a wildlife sanctuary. In 1999 Banyule Council blocked the southern drain and built up the level of the west bank, enabling the swamp to hold water permanently, at a higher level than before. The effect of the new hydrology and the revegetation is a very beautiful wetland, extremely rich in bird species.

Banyule Flats Map

Picture Credit: This map is copied from the book Still Glides the Stream (1) page 36. Credit for these maps, given in Still Glides the Stream is given to John Waddingham who produced the maps.



Sources  / References:

  1. Lacey, Geoff (2004) Still Glides the Stream (The natural history of the Yarra from Heidelberg to Yarra Bend)
    (Australian Scholarly Publishing Pty Ltd)
  2. Presland, Gary (2008) The Place for a Village - how nature has shaped the city of Melbourne. (Melbourne: Museum Victoria Publishing)
  3. Plenty River and Banyule Creek - a landscape study
    (Yarra Valley Library Reference  LH 333.91 PLE
    (where LH means Local History referencable at Ivanhoe Library))
  4. Otto, Kristin (2005) Yarra - A Diverting History of Melbourne's Murky River
    (The Text Publishing Company)



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