The epic geologic saga of Bacchus Marsh

How did the landscape at Bacchus Marsh form? It’s an area rich in geological history, believe it or not. The Werribee Gorge in particular exposes a lot of this history in the rocks it has cut through. This short history is based on material gathered in a couple of geology subjects I took. A bit of geological jargon is introduced in italics, with some explanation for the more obscure terms. Look it up on Wikipedia if you need more explanation!

It all started in a primeval sea

How far back do you go to tell a story? If we want to introduce the characters and setting, perhaps the Ordovician period of the Paleozoic era is a good time for this story. This period started 485 million years ago (485 ma in geological shorthand) and lasted for 41 million years (that is, until 444 ma).

You should be aware that since the discovery of plate tectonics last century, we know that the lumps of rock that make up continents are pushed around the globe over many millions of years, floating on the slowly churning, semi-fluid magma beneath the Earth’s crust. Well, in the Ordovician, the main part (that is, the craton) of the Australian continent was part of the Gondwana supercontinent, and the Australian plate was then located about at the equator.

At this time, most of what’s now the eastern states of Australia wasn’t there. Primeval mountain ranges, around where today’s Flinders and Barrier ranges are, were eroding a steady stream of sediments into the ocean off the coast. These sediments created large deposits – turbidites – off the continental shelf.

These turbidites were subsequently jammed hard (in slow, geological motion) into the side of the Australian continent, either by the steady movement of the sea floor towards the continental shelf or possibly by collision with another tectonic plate. This cause uplifting and folding in a process of mountain-building (orogeny) and continental accretion, adding new material to the already-ancient craton that had been floating around since the first Archaean era of Earth, a few billion years previously.

Graptolites in shale. From Bullengarook.

Small colony-forming sea animals from this era called graptolites can be found fossilised in shale deposits here and there. An old slate quarry at Bullengarook is one of the best places around to find them. At this point life on land was just taking its first tentative steps.

A fold of fire and ice

Through the Devonian period (from 419 ma to 350 ma), what has been labelled the Tabberabberan orogeny compressed this region of continental accretion from east to west, creating a pattern of north-south folds that can be seen in many places (such as the synclines and anticlines in rock faces in Werribee Gorge and Anakie Gorge).

Lion’s Head rock, Werribee gorge, showing syncline fold curves in rock layers.

During this process, molten magma intruded from deeper down formed the granodiorite rock formation of the Ingliston Granites (at the west end of Werribee Gorge), and seen most spectacularly and massively at the You Yangs range. These formations cooled deep underground, enabling the large crystals that make granitic rocks to form. They have subsequently been uncovered by erosion.

By the Permian era (299 ma to 252 ma), what we now call the Australian continent had drifted south (still as a part of Gondwana), and our area was covered by a vast continental ice sheet (like those on Antarctica or Greenland) that appears to have advanced and receded over the area several times. The glacial deposits of tillite rock that can be seen around Werribee Gorge and elsewhere locally comprise the mud and gravel, cobbles and even boulders that were dropped out of glaciers, as the ice melted into lakes and rivers. Although they are unmistakeably tillites, geologists who said so were laughed at, prior to the understanding of plate tectonics: how do you get a glacier in temperate, non-mountainous Victoria? They didn’t know our continent had sailed here from Antarctic latitudes!

Fractured tillite showing embedded gravel and cobbles. Werribee gorge.

The Permian marks the end of the background to our local landscape, as well as marking the end of the Paleozoic era in geologic history. The Permian also notably saw a global mass extinction of species. The causes appear to be linked to changes in climate, but the details are fuzzy, like the fossil record. Over 90% of marine species appear to have become extinct, and most land species as well. Yikes.

Dinosaurs left no footprints (that now remain)

Geologic history goes through periods of building up (orogeny or sedimentation) and tearing down (erosion). The Mesozoic era, famous for dinosaurs, was largely defined by erosion in our area, so there are very few rocks of this age present. A rare fragment of Triassic period sandstone (from just before the dinosaurs proper arrived) can be found at the Council Trench geological reserve in Tramway Lane at the back of Darley, deposited there by an ancient river. You can visit the reserve, for the view of the Pentland Hills or even the interpretative geological signage!

Pentland hills in their winter green, looking across Korkuperrimal Creek (Mt Blackwood in the distance) – from Tramway Lane

The Cretaceous period – both heyday and last days of the dinosaurs – was a period of geologic fragmentation, as Gondwana began to split apart (having earlier split from the supercontinent Pangaea). Massive, cool climate forests probably covered large parts of the continent. The ancestors of today’s Wollemi pine and others of the Araucariaceae family, and many more non-flowering plants like cycads, ginkgos, pines and ferns would have thrived. Little mammals evolved from a few evolutionary odds and ends that had arisen around the Triassic, after the Permian mass extinction. Flowering plants began to proliferate, and one lineage of the dinosaurs – birds – started to diversify.

The current era, the Cenozoic (sometimes spelt “Cainozoic”), occurs from the end of the Cretaceous period, when a giant meteor appears to have sealed the fate of the dinosaurs in another mass extinction. It wasn’t as dire as the Permian mass extinction, and cleared the way for the mammals and birds and flowering plants (although they may have already been ascendant – and the dinosaurs may have already been approaching their evolutionary use-by date – according to some recent research).

In the Cenozoic, the region as we know it formed. In the earlier part of this era (the Paleogene period), volcanic eruptions deposited what is now rich volcanic soil, known as the “old volcanics”, on the Pentland Hills. Much of this is now eroded, and the eruption points of the volcanoes are presumably lost in the mists of time.

Other parts of the Bacchus Marsh area – like the low hills between Goodmans Creek in Coimadai and the entrance to Lerderderg Gorge – are sandy sediments also deposited around this time, I guess from erosion off the ranges (which are mostly Ordovician shale).

The climate was much cooler and wetter then: the northern tip of the Australian landmass was around where Bass Strait is now.

Continental shifts through the eons. Borrowed from here.

Thick forests of plants like today’s Myrtle Beech Nothofagus cunninghamii and its ancestors probably covered much of the continent. Now they have been reduced to small populations in the wet highlands and coastal rainforests like the Otways; and beds of brown coal like that which underlies the plains south of Bacchus Marsh.

Antarctic beech Nothofagus moorei forest at Point Lookout, NSW. 50 million years ago, Bacchus Marsh may have looked like this!

A ring of fire and a fault line

More recently, the area became a centre of volcanic activity again – and tectonic faulting. The “Newer Volcanics” include many volcanoes on the plains of western Victoria that erupted in human times: Indigenous people have stories of them. The most recent were only 5000 years ago (or so). The volcanoes around our area were earlier, pre-human, about 3-5 ma. But they must have been impressive.

Mt Cottrell, south of Melton, was a shield volcano: a gently rising cone with large amounts of lava flowing from its sides You can still see the general outline of it from a good vantage point such as the You Yangs or Mt Blackwood’s summit. Lava flowed in sheets, cooling to form the basalt plains over older sandy sediments (such as you still find around Port Phillip bay). Other eruption points were in the ridge to the south of Parwan and around Balliang East, and north at Bullengarook and Mt Blackwood. To the west eruptions were near Ballan (Mt Darriwill and Mt Gorong) and Glenmore (The Bluff): a ring of fire (although we should note that these volcanoes were probably not all active at the same time).

Myrniong Creek flanks one side of a thick lava flow from Mt Blackwood, a scoria cone (sometimes called cinder cone) volcano; the other side is traced by Korkuperrimal Creek. The long spur (or plateau) of a lava flow from Mt Bullengarook that is followed by the Bacchus Marsh-Gisborne road is flanked by Goodmans Creek on the west, and Coimadai (a.k.a. Pyrite) Creek on the east. These are examples of inverted topography: what was a valley is filled with volcanic rock, and the softer landscape on either side wears away to become new valleys.

Around the time that these volcanoes were active, the Rowsley Fault also was active. The abrupt escarpment of the Brisbane Ranges and Pentland Hills rose up over the Port Phillip plains (or did the plains sink?) around 2.5 million years ago, probably fairly gradually. Some of the lava flows have been deformed by the fault movement, showing that eruptions were occurring both before and after the fault began. Many such faults criss-cross Victoria, as the continental plate stretches and compresses while it continues its northward movement (currently travelling a few centimetres a year). The Rowsley fault is still active, minimally. The last earthquake in the area was in 2015.

Rejuvenated rivers… and the Great Bacchus Marsh Lake?

The massive movements of earth from the fault created a steep gradient for the streams flowing across the escarpment. This stream rejuvenation can be credited for the spectacular gorges coming out of the ranges: Anakie Gorge, Werribee Gorge and Lerderderg Gorge, carving through the Ordovician bedrock and even (in Werribee gorge) the hard Devonian granites. Where rivers encountered softer sedimentary soils, wider valleys such as Werribee Vale, Bacchus Marsh and the Parwan valley (between Rowsley and Glenmore) were created.

g_Werribee Vale
Werribee gorge in the foreground opens up to the alluvial flats of Werribee Vale Road farms. Mt Cottrell on the horizon at left.

The area around Underbank Boulevard is one fragment of a lava flow, cut off from the rest, along the ridge up Ironbark Rd along the train line toward Werribee Gorge. The flat plateau along Flanagan’s Road toward Lake Merrimu (and ultimately Mt Bullengarook) has already been mentioned. Hopetoun Park and Parwan are basalt plain. Typically, at the edge of the plain, a steep escarpment, sometimes with basalt boulders or cliffs is apparent, and older rock layers may be visible on the sides of the escarpment. These older rock layers are evident in the low hills around Goodman’s Creek, Coimadai, and at Long Forest.

Bacchus Marsh itself is a plain of alluvial (river-deposited) sediments. Perhaps even lake-deposited (lacustrine)*. To the east, the combined forces of the Lerderderg and Werribee rivers have cut through the thick lava flows at Parwan. Perhaps this basalt formed a natural dam at some point. It could have created a huge lake where the town now is, until the Parwan gorge was slowly worn down to its current level.

*EDIT (Oct 2016) I have not heard this anywhere else, it is purely my speculation.

g_Parwan basalt
Parwan gorge is weed infested (just about every plant in this picture is a weed), but it’s still magnificent. This is the basalt cliff atop the south-facing slope.

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