Nature’s breakwater

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You stand on a windblown promontory – and that wind has played an important role in shaping this landscape. Its strength and constancy determines, in no small part, what plants can and cannot grow here. And it is similarly important in the construction of the topsoil, which largely consists of nutrient-poor windblown sand, a further factor influencing the species mix. It is a marvel that vegetables were grown and wheat raised on the Dennes Point spit in the 1850s.

Below the sandy topsoil are beds of dolerite boulders, eroded to the point where only a small remnant is left. These nevertheless account for the precinct’s most interesting geological feature. Along the northern shore is a long barrier of mostly dolerite boulders and cobbles, up to half a metre in size, rounded, graded by size, and forming an arc of several hundred metres east from Dennes Point to and beyond Kelly’s Point.

The barrier is so abrupt and impressive that there is speculation that it may be of human construction – possibly from ballast dumped by ships. This seems unlikely – rounded boulders are scarcely a safe cargo in a wooden vessel pitching about in a heavy sea. In any case, dolerite may well be ‘the rock that makes Tasmania’, but it is comparatively rare in the global context.

Still more conclusively perhaps, dolerite boulder barriers also occur on other nearby foreshores, particularly along the estuary coast between Taroona and Sandy Bay. It seems likely that the arced symmetry that characterises the boulder wall at the northern extremity of Bruny Island is ‘a rare recurve boulder spit’ eroding from those local boulder beds of which only a small portion remains, and being rolled into their present formation by the regular, powerful tides and surges of the sea.

Further reading and image sources

Cromer, W., An Unusual Boulder Spit on Bruny Island, Unpublished report for Friends of North Bruny). The geological map is sourced from Cromer’s report.

Kostaglou, P. (2014), Archaeological Survey – Kelly’s Farm Section, Cap de la Sortie: Final Report, Tas Archaeological Services, Hobart (Report for Friends of North Bruny).