On every continent are examples of isolated stone mountains that are not easy to explain.
© Andrew C. Parnell
Ben Bulben, County Sligo, Ireland.
Mount Augustus in Australia is an example of a sandstone monolith that could be the largest of those monoliths that also includes Uluru. Mount Augustus, or Burringurrah
to the local Wadjari people, is an asymmetric anticline resting on top of what geologists refer to as "very old" granite. Its composition differs markedly from the underlying rock strata.
is an example of another isolated monolith, or tor, that can be found on every continent. They all share similar morphology, although they are composed of different minerals. Mount Augustus, for instance, is made up of rocks and pebbles of various sizes cemented together by hard sandstone. In other areas of Australia, such as Bald Rock
, the great blocks of stone are actually granitic, resting on top of discontinuous strata that does not match the overall structure of the rock.
The geology of Ben Bulben
is quite different, however. It is composed of Darty limestone (which makes up the Darty mountains) and shale, overlying Glencar limestone. Ben Bulben is the headland of the Darty mountains, sloping gradually upward until the flat-topped mesa
and steep, regularly carved cliff face stop at the edge of the ocean. The change from Ben Bulben shale to Glencar limestone
in the monolith is dramatic - the boundary layer is paper-thin.
In other areas of the world, such as Peña de Bernal
in Mexico, or the Rock of Gibraltar
in Europe, the single stones appear to be metamorphic, indicating a powerful energy source that drew the material in the mountains together, as well as melting and reforming their mineralogical composition. The Rock of Gibraltar is composed of metamorphosed limestone and chalk. The Strait of Gibraltar, itself, demonstrates some remarkable topography
and could mark an area where Earth-grazing electric arcs touched-down and sculpted