A conspicuous small plateau known as The Gun Emplacement is located in the south western corner of Anstey Hill Recreation Park. This feature is an erosional remnant of an iron-cemented surface developed on sand that had been previously deposited in a river system about 45 million years ago during the Eocene Epoch. The SA Division of the Geological Society of Australia has designated The Gun Emplacement as a Geological Monument.
The Gun Emplacement is a prominent regolith (layer of unconsolidated solid material) and landscape feature within the Adelaide area that has for long received research attention from Earth scientists, including Fenner, 1939 and Twidale, 1976. As it is a preserved remnant of ancient landscape materials, its characteristics and evolution are important for understanding the long-term landscape evolution of the Adelaide district.
The Gun Emplacement contains a sub-horizontal ferruginous (iron or iron oxide content) capping of interbedded mottled sandstones with weathered lenses (bodies) of ironstone gravels. Sub-horizontal bedding and occasional cross-bedding (layering) occurs within the sandstone and the gravel. The hard erosion resistant capping is between 4 m and 9 m in thickness. It is underlain by between 5 m and 9 m of medium-rained quartzose red sands and conglomeritic lenses (bodies of conglomerate). The quartz grains in both the capping and the underlying sands are sub-angular to sub-rounded. A conglomerate layer of about 2 m thick underlies the red sands, and contains interbedded gravels with ferruginous clasts (fragments) that occur within the zones with a preferential orientation of approximately NE-SW. The conglomerate is polymictic (conglomerate consisting of assorted fragments from different rocks and/or minerals), with extra-formational angular quartz and quartzite cobbles..
The Gun Emplacement consists of a level surface sloping gently downward from the steep slopes of the hills face zone in the bottom south-west of the park. The exposed hard ferruginous capping is several metres thick and is the primary reason the underlying Eocene sands have not been eroded too severely. The ironstone capping is resistant to erosion in its undisturbed state, but when broken through it is vulnerable and the sands underneath are also subject to degradation.
It is believed that Major W.H. Edmunds, a Boer War veteran, gave the site its name, The Gun Emplacement, as the flat, treeless plateau reminded him of field gun placements used in South Africa.
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Ref: THE REGOLITH AND LANDFORMS OF THE ANSTEY HILL RECREATION PARK, WITH PARTICULAR EMPHASIS ON THE GUN EMPLACEMENT. Robert Grzegorzek