_David Witham_
‘Enthusiasm’ was the watchword for the day as around 26 members gathered in the mural room for an initial power point presentation by geologist Ross Cayley. The number in the audience was evidence of this, but it was more than matched by the daylong exuberance of our mentor, who is obviously totally absorbed in his subject and more than willing to enlighten the layman about its latest theories and conclusions.
Ross set about destroying the notion, with which many of us were familiar, that the Grampians sandstones had originally been something like 10 km thick, because the present eroded remnants of this huge pile do not evidence the characteristics to be expected from such pressure, and the increasingly benign climate over the last 400 million years could hardly have caused such an amount of erosion. This idea is backed by the results of modern techniques of magnetic and seismic surveying which indicate the disposition of the rocks and the major faults that lie beneath the surface.
In its place Ross drew a picture of periods of heavy erosion of mountains that no longer exist, which deposited sand into the waters of lakes or seas, and much longer intervals between those periods when much smaller particles were slowly deposited into fairly still water. These processes gave rise to layers of hard sandstone around 150m. thick, which have not eroded significantly since then, interspersed with deeper layers of mudstone approximately 750m thick. Plate tectonics, with the Pacific plate pushing against eastern Australia, have caused the uplift of the Grampians, and horizontal faulting helped with slippage between the layers of rock.
This theory holds to the original belief that the Mt. William sandstones are the oldest, followed by the Serra range and the Victoria Range, but it also explains the large amount of mudstone in the valleys between the ranges that we do not readily see because they are heavily vegetated. Being more easily eroded, they have provided a more fertile medium for the growth of plants.
After an outline of these ideas in his initial presentation, we drove to the summit of Mt. William where Ross showed us trace fossils of worm burrows and much larger creatures in the sandstone rocks. We then stood around the summit cairn poring over a geological map while he explained its meaning. Ross seemed to be the only one impervious to the biting wind. On the way back we stopped to look at road cuttings where the mudstones have been exposed.
After lunch many of us accompanied him part way up the Pinnacle Track from the Wonderland carpark. We looked at cross bedding from old sand dunes in the Grand Canyon, an intrusive dyke at Bridal Veil Falls and the fault line that has created Cool Chamber. Here Ross told us of the time that he and a number of his fellow geologists were casing the area. They all agreed that the Chamber is a great example of a fault but could not agree on the direction in which the two layers of rock had moved over each other. Some of them went on to look at other features, only to come back two hours later to find the rest still arguing the point.
With a wry smile he commented that he prefers to concentrate on the bigger picture! He had certainly managed to do that all day, and we were very grateful to him for the insights he had given us.
Ed: Later Ross emailed me to inform us that the new geology display at the Melbourne museum has some Grampians rocks and also a video of him talking about Melbourne’s geology. The complete display is not yet open , but I saw parts of it last week. You can also see the video on “http://museumvictoria.com.au/600millionyears”:http://museumvictoria.com.au/600millionyears