Australia: The Land Where Time Began

A biography of the Australian continent 

An A-tent is structure that has an approximately rectangular plan form in which 2 rock slabs are raised a few centimetres above the land surface with their edges touching, the outer edges of each slab touching the adjacent rock. The name derives from the triangular shape of the space beneath them. In North America they are called pop-ups. In Australia they are found in many places across the continent, and have also been found in East Malaysia, central Texas, Labrador and Guyana. Well developed examples are found on areas of granite rocks, though on the Coolamon Plain of New South Wales they are found on limestone country. In the US they have been found on sandstone in Wyoming.

The plan size and slab thickness of A-tents vary greatly. The thickest example found at the time of writing, 58 cm thick, 2.6 and 1.6 m long, 61 cm  and 76 cm wide, and 82 cm above the level of the slope, are on the northern section of Eyre Peninsula, on the western slopes of Wudinna Hill (Mt. Wudinna). Also on the northeastern part of Eyre Peninsula, one is present on the gneissic rocks of the flanks of Carappee Hill that is 13 cm thick. A local expansion of 3-4 % is implied by the A-tents of various sizes, on the northwestern part of the Eyre Peninsula. According to Twidale & Campbell, there is evidence that A-tents are still forming, as seen in a large limestone slab on the floor of a quarry in Ontario, that is 2.4 m high, the slabs extending 15 m on either side of the central fracture. Some time in 1985-1986 a small A-tent formed on Wudinna Hill. In the channel of the Gwydir River, New South Wales, a small A-tent, that was 3-wide, formed downstream from the Coperton Dam.

Blisters, arched, unbroken slabs, collapsed A-tents that form overlapping slabs, as well as dislodged blocks and triangular wedges, both vertical and horizontal, are found in association with A-tents on many of the inselbergs that are composed of granite on Eyre Peninsula.

Twidale & Campbell have refuted a number of mechanisms of formation of A-tents and similar formations. They suggest that if the slabs of an A-tent formed the characteristic raised structure due to solar heating they would probably settle back into place during cool weather, which does not occur, and in Australia, they would be expected to form preferentially on the westerly and northern slopes of hills, which they do not. They suggest that if heating during fires contribute to the formation of A-tents it is only part of the explanation. As the rock is fresh, they do not result from weathering and expansion. The authors claim they can't result from offloading, as the host inselbergs appear to survive because they are in a state of compression, and on any given hill or given area the A-tents are of consistent alignment. If offloading was the cause they should be parallel to the hillslope whatever the overall orientation. The authors suggest there is a possibility that slabs already under stress, such as by heating, may be affected by crustal movements, but they refrain from suggesting this as a possible cause.

According to the authors, A-tents and associated structures are associated mainly with pressure release (crustal compression) that is triggered by earth tremors. They suggest the implied shortening is accounted for by this mechanism, as well as the consistent alignment. They also suggest that the pop-up in Ontario on the floor of the limestone quarry could not result from overburden removal, in quantitative terms. They conclude that A-tents, and associated structures, are the result of crustal stress that causes them to pop up. Triangular wedges, such as those developing on hills composed of granite, are displaced laterally. Both are tectonic landforms on a small scale.

See Source 1 for more detailed information on Australian Landforms

Sources & Further reading

1. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd