27 August, 2011

In the beginning...

It occurred to me recently that I knew very little about the geological origins of the Barwon River, so I set out to redress the issue, but found I was suddenly in some danger of having a great deal of information but very little in the way of understanding. I spent some time sifting through what I could find online and trying to put it in some kind of order. As I understand it, the geological evolution of the Barwon runs very roughly along the following lines:
Cretaceous rock formations at the West Barwon Dam
in the Otway Ranges
Several million years ago, the landscape surrounding the Barwon River looked rather different to what it does today. In fact, the formation of the Barwon dates back as far as the Cretaceous period (70-145 million years ago) when Australia separated from the super-continent Gondwanaland and dinosaurs still roamed Earth. During this period, sedimentary rocks were laid down which can still be seen to the south of the Barwon as rocky outcrops in some places. They also lie off the coast of western Victoria in the Otway Basin.
As recently as 40 million years ago, much of the land through which the Barwon now flows was covered by a wide sea. Early streams flowed from the higher ground of the Otways, northwards to the sea and limestone deposits formed along this ancient coastline.
Limestone outcrop forming the Belmont Escarpment
The Belmont Escarpment overlooking Barwon Valley and the Jerringot Wetlands is one such limestone outcrop, formed between 25 and 30 million years ago, which extends from a large deposit at Waurn Ponds towards Corio Bay.
More recently, during the Miocene period (7-23 million years ago) a series of plate movements and volcanic eruptions saw the coastline extending as far inland as the town of Meredith and the estuary of the Barwon located at Belmont Common. At this time, much of the present course of the Barwon lay under the sea with only a few nearby features such as the ancient granite of the the Dog Rocks at Batesford protruding above the water's surface to form small islands. It was this sea which deposited the limestone which can be found in the Batesford-Fyansford area and which was quarried for use by the Fyansford Cement Works throughout the 20th century. At around the same time, clay and other materials laid down combined with the Batesford Limestone to produce a band of sedimentary rock running down as far as the eastern fringes of Lake Connewarre. This is known as the Fyansford Formation.
Continuing plate movement throughout the Miocene period affected the various fault lines around the Geelong region such as the Bellarine Fault, causing the Bellarine region to lift and then around 20 million years ago, the sea to withdraw, uncovering the land and the current course of the Barwon.
Plioscene basalt rock formation beside the
walking track to Baum's Weir
Subsequent sea movements in the early Pliocene period (3.5-5 million years ago) resulted in the formation of a shallow bay between Torquay and Ocean Grove, covering the lower reaches of the Barwon once again and led to the deposition of shell beds which now lie under this part of the river including Lake Connewarre and surrounds. Sedimentary deposits across the region including the Bellarine Peninsula at this time produced what is called the Moorabool Viaduct Formation. In places it lies over the top of the Fyansford Formation.
It was also around this time that the movement of fault lines pushed the Cretaceous rocks of the Otway Ranges to their current height, giving the Barwon the strength to carve a path to the sea through new lava flows which occured in the later Pliocene era.
It was at this time that volcanic activity around Mount Duneed, Mount Moriac and Mount Pollock led to further changes in the river and the landscape. Until about 2 million years ago, the Barwon followed a course which saw it flow through a gap in the Barrabool Hills south of Lake Modewarre, then down a valley adjacent to the hills before reaching Fyansford. Around 2 million years ago, lava flows north of Inverleigh blocked this course, causing the inundation of the land to the west of Winchelsea before the river once again carved a new path to the sea. The lava flow followed the old riverbed, leaving a deep basalt deposit in this area and contributing to the wide basalt plains which extend across the region, providing extensive farming land today.
Newly-arrived European settlers were quick to see the material value of these basalt deposits, establishing bluestone quarries along the banks of the Barwon. It is this rock which can still be seen in the stonework of many historical homesteads and buildings across the region - the Barwon Paper Mill and its water race being a prominent example. Naturally, this quarrying made a lasting impact on the river surrounds. This is particularly noticeable along the walking track to Baum's Weir where the quarrying exposed the pliocene basalt, leaving high, rocky escarpments which tower over the river below, whilst a number of quarries were located along the river through Newtown which today have been converted to parkland.
Disused bluestone quarry at Baum's Weir showing
pliocene basalt deposits
At the same time, these lava flows extended out into the bay with three branches forming which dammed the lower Barwon in several places and created a number of lagoons - including a lake at Queen's Park, with another blockage below the Belmont Escarpment.
One of these flows produced the basalt ridge line which runs between Tait's Point and Fisherman's Point, separating Reedy Lake and Hospital Swamp from Lake Connewarre and Salt Swamp. As a result, Reedy Lake was fed by fresh water from the now-dammed Barwon whilst Lake Connewarre and the swamplands below the ridge were filled with seawater.
In time however, the barrier at Queen's Park along with that at the Escarpment eroded away, creating alluvial flats downstream. Likewise the lava flow at Tait's Point and another between Sheoak and Pelican Points which had obstructed the river flow were eroded away and water from the upper Barwon and Reedy Lake once more flowed into Lake Connewarre and the lower reaches of the river. This reconnected watercourse now ran alongside the third lava flow which extended to The Bluff and Ocean Grove, establishing the present course of the lower parts of the river and location of the river mouth.
Remains of basalt flow from the late Pliocene era
 forming Tait's Point (right)
It is believed that much of this erosion occurred during times of high flooding - such as 1995 - and as a result of seismic movement. The returning flow of water along with fluctuations in sea levels during the late Pleistocene period (c18,000 years ago) resulted in the build up of sand dunes along the southern basalt flow. In very recent times, with European settlement, this natural rate of sedimentation has been greatly increased by land-reclamation, farming, tree clearing and for a short but intense period, mining in the catchment region of the Moorabool River. All these factors have contributed to sedimentation, causing in-filling which has affected the depth of water in the lakes complex, turning Reedy Lake from an open water system to the swampy complex it has become.
In addition to the various geological and environmental events which shaped the course of the river, across millions of years, continuous erosion and subsequent sedimentation have also influenced the soil of the region, forming the land we see today.

1 comment:

  1. Love your posts, especially those concerning Geelong's local geology and its relevance to the early development of Geelong