Skull Rock: the Inside Story
This story is the report written in 1972 for the Victorian National Parks Authority after visiting the island and entering the cave. The cave has been recently re entered by a film crew with climbers and museum staff to film an episode for Australia Coast.
Skull Rock or Cleft Island is a granite monolith 113.4m in height(Ports and Harbours,1959),situated in the Anser Group of islands west of Wilsons Promontory. The main feature of this island is a huge stage-like cave on the western side which gives the rock the appearance of a hollow shell. The floor of the cave is 30m above sea level and there is no easy access. It is quite certain that prior to 1972, no person had entered the cave, and to my knowledge only one person had managed to climb to the top of the rock. The surrounding cliffs form a solid barrier 30m to 60m in height and are quite insurmountable to all but experienced rock climbers.
For all these reasons, it seemed quite likely that the vegetation and fauna of the island would show little of the effects of man that have so changed the other islands of Bass Strait. With this in mind, Digby planned a visit to the island with a party of rock-climbers to survey the vegetation and fauna of the cave. The island is a part of the Wilson‘s Promontory National Park and we were grateful to receive permission from the National Parks Authority to attempt the climb. Brian Greer, then an Assistant Ranger of the Park, and resident at Tidal River, had his own boat and knew the waters around Wilsons Promontory quite well, we were grateful for his assistance and also that of Marjorie, his wife. She is also a doctor and the knowledge that she would be around when we made our attempt was quite comforting.
After many false starts during which the weather blew up and made the landing impossible, we were eventually able to gain a tenuous foothold on the rock on the 10th of November,1972. After several hours of difficult climbing, we were standing above the cliffs looking down on Brian’s tiny boat 113m below us. Having gained so much it was still a matter of an hour’s hard work to descend to the cave on the other side of the rock. This involved a 30m. abseil followed by a pendulum swing of about 10m. across the blank rock face. These techniques enabled us to swing sideways into the cave. We could carry very little in the way of sampling gear, and we could not stay long, so our biological observations are necessarily scanty.Once inside, the full extent of the huge amphitheatre became apparent. The single cavity has no subsidiary channels and the symmetry of the domed roof is nearly perfect. The cave mouth is about 130m. across and 60m high. The cavern goes back into the rock for about 50m. Its sheer size and the restless sea surging against its base 30m. below made us feel very insignificant.
The plants visible from sea level on the lower lip of the cave proved to only grow on the outward side of a frontal dune stretching across the mouth of the cave. Behind the dune the cave floor drops 7m. and is composed of bare sandy grave1. Toward the rear of the cave is a small flat area of dry green algae, on the lowest portion of the floor. This area is a drainage focal point for any windblown spray and rain, as two small drainage channels running down from the dune testify. On either side of this mossy area and against the back wall of the cave are two semi-conical hills, 7m. in height and 10m. in radius. These apparently consist of solid salt and their surface layers are strewn with the remains of many birds. So firmly are these embedded in the salt that any attempt at removal inevitably resulted in their destruction, but remains found elsewhere in the cave are probably similar to those “lost” to the salt. The salt mounds have probably been caused by windblown sea spray, the water evaporating to leave the salt. The biological contents contained within the mounds could be ascertained by means of a sample trench dug into the side. This could lead to information regarding the avian fauna of Bass Strait in past times.
The granitic soil near the mounds is also highly saline, (as I discovered by its effect on a scratched hand) and this is presumably the reason for the lack of plant growth deep within the cave. The salt marsh plant, Arthrocnemum arbusculum was found to be the sole species in the vegetation zone nearest the salt mounds. This plant, noted for its high salt tolerance, has not before been found on the islands in this area although it has been recorded from Big Green Island in the Furnaux Group. (Norman, 1966). Along the top and inside edge of the frontal dune are clumps of tussock grass Poa poiformis, and Pigface, Disphyma australe, but closer to the salt mounds these both become stunted and more scattered. The Poa is dominant over much of the dune surface, generally in clumps of over waist height, and the pigface is dominant on the outer edge of the dune where the gravel borders onto bare granite. Other species of plants distributed unevenly throughout the vegetated area of the cave are: Bulbine bulbosa, Pelargonium australe, and Sonchus oleraceus. Several specimens of the fern Asplenium obtusatum grow in rock crevices. During our visit a pair of Cape Barren Geese (Cereopsis novae-hollandia were on top of the rock and a pair of Pacific Gulls(Larus pacificus) landed within the cave, but no other live birds were seen. At least three nesting sites of the Cape Barren Goose occur amongst the Poa as evidenced by quantities of droppings and feathers. Burrows, apparently of mutton bird size, riddle the frontal dune and a profusion of bird bones litter the floor of the cave. Some bones were brought out and have been identified as belonging to the following species: Silver Gull (Larus novae-hollandiae), Dominican Gull (Larus dominicanus), Short-tailed Shearwater (Puffinus tenuirostris), Fluttering Shearwater (Puffinus gavial), and of Prions (Pachyptila spp.)
The burrows in the dune are likely to be those of the Short-tailed Shearwater but we did not pull any birds out to check. The nearest recorded breeding colonies of the Fluttering Shearwater are in New Zealand, and it is an annual migrant to Bass Strait (Serventy et al, 1971; Simpson, 1972). The presence of a Dominican Gull skull was of interest, for this species is only a comparative new-comer to Australia and only one breeding attempt within Bass Strait is known to date. The species is common in New Zealand and presumably has begun to colonize Australia from there (Simpson, 1972).
No evidence indicating the existence of any mammals was found either within the cave or on the outside surface of the rock. It appears likely that the island is too small to support a viable colony of mammals.
Against one wall we found five or six nodular lumps that looked and felt like iron ore but seemed out of place on a granite island. They proved to be lumps of heavily oxidised and roughly cast iron. The only plausible explanation for their presence in this inaccessible cave is that they were the remnants of a ball fired from a cannon, but by whom must remain a mystery. From the degree of corrosion, they had obviously been there for a long time, but no accurate guess at their actual age can yet be made.
The contents of the cave then, do not form the hoped for reference point for the changes in vegetation types else-where in Bass Strait. Plant growth is extremely restricted and the small mammal fauna characteristic of the other islands of Bass Strait is apparently absent. However this brief survey suggests that the cave warrants further investigation as regards the contents of the salt mounds and shearwater burrows.
This survey would have been impossible were it not for the permission of the National Parks Authority to visit the island and make a reference collection. Brian and Marjorie Greer also made our task far less of a burden with their valued advice and assistance. My climbing partners, Keith McKenry and John Ryder in the earlier, abortive attempts and David Watson in the final successful attempt must also be thanked for tolerating my obsession. The invaluable assistance of Paul Cullen who did the plant identification and Ken Simpson who identified the bird bones and read this manuscript are also gratefully acknowledged.
Norman, 1966. Vic. Nat. 83/ 294-299.
Ports and Harbours 1959. Sailing Directions, Victoria including Bass Strait. Dept. of Public works, Melb.6th.ed.
Serventy,D.L., Serventy, V., and Warham, J. 1971. The Handbook of Australian Seabirds.Reed, Sydney.
Simpson, K. 1972. Birds in Bass Strait. Reed, Sydney.