Today, this ancient Silurian rock can still be seen, exposed to the elements at points along the various watercourses of the region such as the Leigh River and its tributaries including Cargerie, Williamson's, Reid's and Coolabaghurk Creeks. It can also be seen in rather dramatic form in the steep rock walls which make up the narrow gorge through which the Leigh flows at the Leigh Grand Junction Bridge.
|Silurian rock formation at the Leigh Grand Junction Bridge|
As I discussed in my previous post, from the 1860s onwards as the shallow alluvial gold supplies began to dry up, the large mining companies moved in, working large claims around the Ballarat region and extending south onto the Sebastopol Plateau and beyond. These companies were all working "deep leads" - deposits of alluvial gold buried deep underground along the courses of ancient riverbeds. One such lead to the south of Ballarat was known as the Durham Lead. It was buried up to 100m below the surface, under layers of basalt laid down during volcanic events more than 3 million years ago.
This of course raises an interesting question: exactly which ancient riverbed does the Durham Lead follow?
In geological terms, the answer lies back in the Paleozoic era when the Silurian bedrock of the area was first laid down and in the millions of intervening years up to the present. During this time (over 400 million years), the forces of nature exerted their influence. The rocks were subject to weathering and erosion. Gradually, watercourses formed. These were wide, shallow streams cutting their way through the ancient bedrock, forming new rivers and creeks.
Over time, temperatures changed, sea levels rose and coastlines retreated. Rivers, along with the countryside were inundated and their beds filled with the remains of marine organisms by a huge Miocene sea stretching inland as far as the township of Meredith. These calcium-rich deposits would eventually form limestone, tracts of which can still be found scattered across the region at places such as Fyansford where limestone has long been quarried. In the case of the ancient Leigh River, the Miocene deposits extend a,lmost up to the Bamganie area, covering the ancient riverbed and the "drift" which had collected along its course from earlier erosion.
Once again however, with the passing of many more millennia, the region entered a stage of deposition. Sea levels began to recede and more material was laid down during what is termed the Pliocene era. This was followed by a period of erosion where the bed of the ancient Leigh was carved deeper into the bedrock, its channel becoming steeper and the flow of the river, more powerful. These early Pliocene deposits were worn away, surviving as caps on higher outcrops in areas such as the rising ground between the modern Leigh River and Native Hut Creek or the Leigh and Williamson's Creek.
|Depiction of geological events affecting the ancient Leigh River. Diagram adapted from the following|
Victorian parliamentary paper, 1874: http://www.parliament.vic.gov.au/papers/govpub/VPARL1874No64.pdf
New deposits, a combination of the old Pliocene material, the bedrock and of course, alluvial gold once again settled on the riverbed. It was this material which would one day form the "wash dirt" of the Durham Lead, the mud, clay and rocks from which the prized gold had to be extracted, but that was still several million years in the future. At this time (later in the Pliocene era) the region entered a phase of volcanic activity. During this period several distinct volcanic events affected the course of the old river. The earliest, flowed down the course of the river from the north, trapping the sediment deposited on the riverbed under a thick layer of basalt which in turn was covered by another layer of drift material. A second eruption covered the three previous layers, almost filling the bed of the old river before a third eruption (possibly flowing south from Mt Mercer and Mt Lawaluk on the west and Green Hills - aka Mt Collier - to the east) overtopped the channel and spread across the plains below Mt Mercer. In this area, the course of the old Leigh River and any gold it contained was now completely buried beneath the basalt plains.
|Aerial view of the Mt Mercer volcano cone. Click to enlarge|
Further down, below Bamganie, all trace of the lead was thought to have been obliterated by the encroachment of the Miocene sea. From this point, the ancient riverbed was lined not with Silurian bedrock, but with Miocene limestone.
Of course, with the filling of the old channel, the water which flowed from the higher ground above Ballarat needed to go somewhere, so over time a new channel formed, wearing its way through the recently formed upper layer of basalt. Often it flowed along the line where the hard basalt met the softer Silurian rock. In its upper reaches, the new river for the most part followed a similar course to the old, cutting back and forth across the old riverbed until it reached the Mt Mercer flow.
At this point, the lava flow was so great that the course of the old river disappeared entirely beneath the basalt and it became all but impossible for the mining companies to trace the lead. As a consequence, there was little large-scale mining activity below the Leigh Grand Junction Bridge.
For the newly formed Leigh River, it became necessary at this point to forge its own path through the basalt, which it did, winding its way towards Shelford and Inverleigh, forming what we recognise today as the course of the modern Leigh River.
|The Leigh River at Shelford|