A biological origin for gravel mounds in inland Australia

ABSTRACT

More than 1000 near-circular mounds up to 40 m diameter and 2.1 m high, which contain 15–1250 m³ (mean 159 m³) of gravel, have been identified in western New South Wales. Mounds are more pebble-rich than the surrounding surface regolith and contain abundant pebbles to 1196 g weight. Resemblance to smaller gravel nest mounds of the malleefowl (Leipoa ocellata) suggests they were constructed by a similar although larger bird. Pebbles were inadvertently concentrated in the nest mounds during construction and maintenance. Formation by geological mechanisms, as burrow spoil or burial mounds is not compatible with the features of the mounds. The mounds are undated, but partial blanketing by eolian sand suggests they predate the Last Glacial Maximum. The mounds are now spatially associated with Casuarina trees. Pollen records in southeastern Australia reveal that shrub and grassland replaced Casuarina woodland as aridity increased after ca 35 000 yrs BP. Climate change driven habitat loss in the semi-arid zone likely caused the extinction of the apparently Casuarina woodland dependent megapode responsible for the gravel mounds. The mounds partly map the megapode distribution and provide an opportunity to date the local extinction of a megafauna member, compare this to climate and vegetation changes, and determine the duration of overlap with human occupation.

KEYWORDS:

• gravel mound
• regolith
• megapode
• malleefowl
• megafauna
• Casuarina
• vegetation change
• climate change

Acknowledgements

The owners of Booroondarra, Cawker's Well, Churinga, Gambolalley, Merrylands, Netallie, Ophir, Osterly Downs, Pulpulla, Tambua, Tara, Tarella, Witchelina and Yimkin stations are thanked for allowing access to the properties and advising the presence of mounds. The South Australian Department of Environment, Water and Natural Resources is acknowledged for allowing the excavation of mounds in Swan Reach Conservation Park. Dianne Schmidt is thanked for sample processing and general assistance in the field, and Jason Schmidt is thanked for preparation of the figures. Kingsley Mills provided most of the mound distribution data for northwestern NSW. The author acknowledges the opportunity to undertake fieldwork 1972–1992 and discover the mounds provided by former employers Electrolytic Zinc Company of Australasia Limited and North Limited. Bill Platts, Les Davis and Bob Beeson are thanked for reviewing drafts of the paper.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

This work was self funded.