The name refers to a remnant of ancient weathering on the land surface before the existence of the Mount Lofty Ranges, dating back almost 1000 million years. It is believed that a Major W.H.Edmunds, a Boer War veteran, gave the site its name as the flat, treeless plateau reminded him of field gun placements used in South Africa.
Recognised as a significant geological monument by the Australian Geological society, the Gun Emplacement is an almost perfect record for the history of the Anstey Hill park area. Though over time there has been differing proposals as to the exact origins of the Gun Emplacement, its importance and preservation is agreed on nevertheless. With its views of the plains, the Eden fault and the various gullies, it also has immense value for interpreting the geomorphology of Adelaide.
Fenner (1939) put forward the hypothesis that the Gun Emplacemet beds were part of a vast series of alluvial deltas. He proposed that the beds were composed of materials from two distinct types of origin: relics deposited by streams running over the old pre-Miocene peneplain and old pre-Miocene peneplain gravels with older alluvial detritus. This detritus was deposited as a result of the widespread faulting and differential uplifting that dominated the area in the post-Miocene times. He then went on to suggest that because there is no other evidence of the alluvial deltas apart from the Anstey Hill relic, these beds were probably uplifted to their present height in late Pleistocene to Recent times, destroying nearly all the other remains. He suggested that the Gun Emplacement beds probably represent the highest part of the uplifted alluvial deltas.
Twidale (1976) disproved this interpretation of the geomorphological history of the Gun Emplacement and stated that it is a weathering surface, which developed upon Eocene sands. The fault angle depressions between the tilted Para fault block and the Eden fault were filled in with these sands, which were then partially stripped away by erosion in later Cainozoic times. Twidale suggests that at this time a ferruginous capping was formed on the exposed sands and proposes a late Pliocene or early Pleistocene age for the surface.
This theory that the Gun Emplacement is a weathered surface with a ferruginous capping was used in the report called "Geological Monuments in South Australia", edited by E.M. McBriar,1977. The information presented in this report was then used in the "Anstey Hill Regional Park concept report" 1983, prepared for the Minister of Environment and Planning.
The Gun Emplacement consists of a leveled surface sloping gently downward from the steep slopes of the hills face zone in the bottom south-west of the park. The exposed 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.