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As Leon Costermans (see references) suggests, geology is about working out the story of what has happened to the landscape. In our case, the geology has had a strong effect on the vegetation, which in turn affects the fauna found at each site.
Further down the page is a guide to visiting some of the sites of geological interest in the valley.
Victoria 's current landscape is quite young - only about 500 million years old (there are parts of WA that are 3,500 My old!). At that time the future site of Melbourne was a shallow sea; over the next 100 My or so sediments washed in from eroding ranges to the west. These sediments were compressed and eventually uplifted to form the eastern side of Plenty Gorge.
There followed period of igneous activity, during which slow underground cooling of magma formed the granodiorite masses which were later exposed by erosion - forming Quarry and Granite Hills. Then a long quiet period with glaciation (Australia then being somewhat further south than today); from about 165 Mya the new continent separated from Antarctica. More sedimentation followed until about 60 Mya (just after the demise of the dinosaurs).
More volcanism brought us up to 6 Mya; there are patches of this still extant just north of Greensborough. Then a relatively tumultuous period: major volcanism on the western side of the river, and drastic uplift and faulting of the old sedimentaries on the east side. The last lava flows were only 800,000 years ago - practically yesterday in geological terms - from Mt Fraser at Beveridge, and reaching the current course of the Plenty River north of Mill Park. (The old course of the river was further west, buried as the lava flow filled the valleys.)
So in summary:
1.Laying the foundations - sedimentation 500-400 Mya
2.Igneous activity 380-360 Mya
3.Long quiet period with glaciation 340-165 Mya
4.Separation from Antarctica, sedimentary infill 146-60 Mya
5.Volcanism and sedimentation 60-6 Mya
6.New volcanics, faulting and sedimentation 6-0 Mya
So this is what it looks like now:
•Light blue - Dargile Formation = Melbourne Formation (444-419 Mya)
Yellow – Morang granodiorite (383-359 Mya)
•Pink areas south of Mill Park = Greensborough Basalt (older volcanics – 23-20 Mya)
•Orange - Brighton Group = sub-basaltic sediments (23-5.3 Mya)
•Green – alluvial terrace deposits (2.6-0 Mya)
•Other pinks – Newer Volcanics (800 kya)
Map from Drought Response Plan, Plenty River Catchment: Melbourne Water 2017
The Plenty River once flowed several kilometres further west. As the lava flows filled up the valleys, it was pushed to the edge of the new basalt (a hard rock), where it carved a new route through the softer old sedimentaries.
So from roughly Mernda to Mill Park, the west bank of the river is a relatively flat basalt plain, while the hilly east bank is old Silurian (about 420 My) sandstones and siltstones.
The interesting thing here is that the basalt plains support a fairly open River Red Gum woodland. The sedimentary areas, with their thin stony soil, carry box/ironbark woodland, often with a denser understory (see the Environment page). You can look along the river and see a distinct difference in the colour and texture of the Eucalypt foliage on the two banks.
The different vegetation complexes (and they really are much more complex than I've described!) support slightly different faunas. White-winged Choughs, for example, are ever-present on the east side of the river, but rarely cross to the west. The thicker and more varied woodland of the east carries a wider variety of small bush birds - whistlers, thormbills, honeyeaters and so on - than the more open and uniform woodland of the west.
These sedimentary strata in Yellowgum Park have been twisted 90 degrees into the vertical, and bent and fractured by the tremendous forces acting on them.
In places the originally-horizontal sedimentary layers, formed about 420 Mya, have been bent into an arch (an anticline) in the violent upheavals of the last 6 My.
This area in the north-east of South Morang Wetlands shows River Red Gums growing in the basalt pavement. The trees in the background are boxes growing on the old sedimentaries on the other side of the river.
Looking west from the Ashley Rd area towards South Morang. The basal sedimentaries can be seen; beneath the vegetation above them lie the much younger basalt layers. These two photos are almost directly opposite each other.
Looking north-west from the top of Quarry Hills, you can see some of the hundreds of volcanoes that covered most of western Victoria with lava in the last 6 My or so.
Exploring Plenty Valley Geology
If you're interested in finding out more, and seeing the various components for yourself ...
Check the references given below. The app is really helpful in locating exposures and boundaries.
Here are some key sites worth visiting.
IMPORTANT: information below is as correct as I can make it at the time of publishing. Please take care and exercise caution near unprotected edges and in areas of loose rock. Ensure you explore within the bounds of your own skills! You are responsible for the safety of everyone in your party.
References - these are all highly recommended!
Victorian geology: Stories Beneath our Feet, Leon Costermans & Fons VandenBerg, Costermans Publishing 2022. A solid tome that will give you a real handle on Victorian geology. Lots of background, as well as specific places to go and how to interpret what you see there. Essential.
600 Million Years: Victoria Evolves (Museum of Victoria permanent exhibition, and online): https://museumsvictoria.com.au/melbournemuseum/resources/600-million-years/. Lots of online content, including maps of Australia's location through the ages, but worth seeing the exhibition in person too.
Australian Geology Travel Maps app – Trilobite Solutions (app free, about $12/year subscription gives access to Australia-wide geological maps). Shows you exactly what you're standing on.
Victorian geological maps downloadable from this site.
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