Selwyn's rock

Early in 2000 I happened to be driving through the Inman Valley, south of Adelaide in South Australia when the sign "Glacier Rock Restaurant" caught my eye. I must confess that I am not intimately familiar with Australia's Quaternary history, but even from my meagre knowledge of Antipodean geography I seemed to be a little too far south of the Flinders Ranges and way too far west of the Snowy Mountains to imagine that this was some glacial relic.

I must confess that curiosity overtook me. Was this just a name given to a restaurant by some displaced New Zealander from the Southern Alps or perhaps some Swiss restauranteur pining away for the sight of the distant Matterhorn?

A few moments with the proprietor, David Hill, revealed the true secret. This was not a Quaternary feature at all, but one that belonged to a far older period of glaciation that affected the southern continents in Carboniferous and Permian times.

Less than 10 minutes later I was descending a steep wooden staircase to the site in question. My understanding is that the bedrock surface below the restaurant was first observed, in a geological sense, by A.R.C. Selwyn, who commented on the glaciated surfaces in 1859. He reported that:

Putting this in an historical context it should be remembered that the "glacial theory" was relatively new at this time. Just 19 years earlier in 1840 Louis Agassiz had suggested that glacially-transported rocks could be seen far from modern glacier limits in parts of Switzerland. Furthermore Agassiz pointed out that the glacially scratched surfaces (seen, for example in Lucerne), were the results of vanished glaciers. The concept of a former continental glaciation was hard for some to follow at that time in Europe. Remember that science was starting to come into conflict with religion and religious dogma dictated that the great biblical flood of Noah was responsible for the "drift" sequences of sediments in northern Europe. However, it was not long before many "natural historians" began to recognise signs of former glaciation in regions such as Wales, Scotland, northern England, Scandinavia and northern Germany as well as in parts of southern Canada and the northern United States. I do not know whether Selwyn had worked in any of these formerly glaciated regions but the preservation of the features on the surfaces of the rocks below the restaurant would certainly be clearly recognised by someone who had observed such areas.

So just what was (or is) Selwyn's Rock? I was never really able to ascertain whether it was the outcrop of the Cambrian surface that was quite spectacularly grooved and striated with patches of Permian tillite in hollows on its surface, or whether it was one other impressive feature. On the side of the outcrop was a huge granite erratic transported by some Gondwanaland glacier during this ancient glaciation. This massive boulder has been transported by ice from the vicinity of Victor Harbour. In this region the erratic was ripped away from the bedrock by the moving glacier and incorporated in the basal ice. It was dragged across the southern part of the Fleurieu Peninsula, probably helping to striate and groove rock surfaces between Victor Harbour and the Inman Valley, until it reached this location. In the process it was gradually rounded and finally acquired the form that you can see today. When this erratic block reached the Inman Valley the glacier's movement had stopped. The ice had slowed and started to stagnate and this granite boulder was destined to remain here. It gradually sank out of the lowest part of the ice and settled into the underlying basal till, deforming and warping the underlying sediments with its weight. Overlying material contained within the ice above gradually settled on top as the glacier melted and a combination of this debris, coupled with meltwater-deposited sediments, likely forms the upper part of the cliff. Similar erratics from the granites exposed near Victor Harbour can be seen as far as Hallet Cove, just south of Adelaide.

One last set of questions remain. When did this glaciation take place, and how many glaciations has our planet seen? It really depends on how you think of this matter. Earth scientists have detected records of at least seven distinct episodes of Earth's history where there appear to be extensive periods of glaciation. In Ontario, there are remains of a glaciation that go back over two billion (2,000 million) years! If you travel into the Flinders Range of South Australia and visit the Brachina Gorge you can see a well-preserved tillite that is about 600 million years old (see the Spring Issue 1998 of Wat On Earth). The glaciation that formed the features at Selwyn's Rock is between ~290 and ~250 million years old, formed in the geological period known as the Permian. It just predates the Mesozoic, the major timeframe that saw the rise and demise of the dinosaurs.

We also know that the continents have moved though time - a slow creep known to you as "Continental Drift". Some 350 to 200 million years ago the southern continents of South America, Africa, Australia, Antarctica (and India) were in close juxtaposition as part of a gigantic continent known as Gondwanaland. This part of Australia was much closer to the South Pole of that time (perhaps between the South Pole and the equivalent latitude of the edge of modern Antarctica). This was part of an area that was extensively glaciated and remnants of that former glaciation can be seen in many parts of the modern continents mentioned above. Not too far from the Inman Valley is the Natural Reserve at Hallet Cove. The rocks here were also scoured by the same glaciation and if you have the chance to visit South Australia you should try to see both of these spectacular localities.

(ABOVE) A striated bedrock surface at Hallet Cove, South Australia. The scratches seen on the bedrock were caused by a Permian glacier that crossed this part of ancient Gondwana.

As a Quaternary geologist these Permian features were easily resolved, but could the public see what I was seeing? I decided to listen to remarks made by other visitors to the site. Most walked around with few comments and left, undoubtedly not realising just what they were looking at. One gentleman made a spirited attempt at an explanation, unfortunately pointing out cleavages in the Cambrian surface as "striations". Almost all missed or failed to comment on the huge erratic boulder in the ancient till. The reason that I was in Australia was to attend a geoscience education conference. Obviously Selwyn's Rock was a good cause to take up, and so - with apologies for intruding on my Australian colleagues' turf - I prepared an interpretative pamphlet that I sent to Dave Hill following my return.

In it I summarised some of the information presented above. I included more detailed photographs taken of the Permian sequence and compared them to similar Quaternary age features seen in Canada.