Rocky relationships: the petroglyphs of the Murujuga (Burrup Peninsula and Dampier Archipelago) in Western Australia

Abstract

The petroglyphs of the Murujuga (Burrup Peninsula and Dampier Archipelago) in Western Australia are of exceptional cultural value for the nation. It is Australia’s largest and most significant collection of aboriginal rock art essentially as petroglyphs, with the number estimated at more than one million engravings. The Murujuga is a textbook example of ancient and modern times colliding as it also hosts potentially polluting, major industrial complexes such as iron ore and salt ports; liquefied natural gas, liquid ammonia and ammonium nitrate plants; railway lines, pipelines and rock quarries. The work presented in this paper is underpinned by an annual monitoring study of 10 selected sites including two control sites located on Dolphin and Gidley islands and eight test sites located closer to the industrial areas. The main rock types of the Murujuga, gabbro and granophyre, have been affected by weathering consisting of a cm-thick leached zone capped by a skin of orange and red iron and manganese oxides. The petroglyphs were engraved by removing the few top millimetres of the iron oxide-rich layer and the contrast with the paler leached zone clearly showing the carved motifs. Phosphorus is abnormally enriched in the leached zone and the surface coating, with manganese exclusively on the surface coating, demonstrating the impact of bird droppings and ‘desert varnish’ on the rocks. The colour difference and the hematite–goethite ratio, measured with a field portable reflectance spectrometer, between the background and engraving provides a tool to estimate the relative age of the petroglyphs and the selection of the backgrounds. As the fine-grained granophyres were preferentially chosen from the coarser-grained gabbros to host the petroglyphs, the goethitic (or yellower) backgrounds were prepared as canvas before being engraved. Many petroglyphs register the food items of the area, which changed as the last ice age ended and sea levels rose. We hypothesise that based on their locations and colour difference the constraint for the oldest age of the petroglyphs exceeds 17 000 years BP.