Some sheets of rock are more rounded on their lower edges than their top. In the past, when they were still covered by soil, the soil deep underground at the base of the slab remained moist and the base continued to weather away. The upper section of the rock - which was covered by the dryer soil near the surface - weathered more slowly. When the surrounding soil was eroded away much later, the inwardly flared rock face was then exposed.
There are no clear examples of flared rocks at Girraween, as the terrain is too hilly. It is possible that the rounded edge of the slab at Underground Creek may have originated this way, before it cracked off.
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Tafoni.
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Underground Creek.
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