Why Does That Rock Stand Alone on a Flat Plain? Everything Else Wore Away
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An isolated hill rising abruptly from a flat plain looks as though it was pushed up. It was not. The plain around it was lowered instead, and the hill is what resisted.
What the landform is
The term describes a steep-sided rocky hill standing in isolation above a surrounding plain with no foothills and no gradual transition, meeting the flat ground at a sharp break of slope. Sizes range from a few tens of metres to enormous, and the most famous examples rise several hundred metres above an otherwise featureless landscape. The defining characteristic is the abruptness, since an ordinary hill grades into its surroundings and these do not. They are commonest in dry tropical and subtropical regions and occur on every continent, typically in granite or similar hard crystalline rock.
How one comes to exist
The formation involves lowering the surroundings rather than raising the hill:
- •Rock at depth is weathered chemically along joints and cracks by groundwater
- •Closely jointed rock rots to a soft mass, while massive rock does not
- •Erosion strips away the soft weathered material at the surface
- •The unweathered masses are left standing where the rest has gone
- •The process can repeat over several cycles of stripping
- •The hill itself then weathers slowly, shedding curved outer shells
Why they last so long
Once exposed these hills persist for extraordinary periods, and several effects reinforce each other to make them durable. The rock is poorly jointed to begin with, so water has few routes into it. Steep bare surfaces shed rain immediately, so water has little time to act, and no soil accumulates to hold moisture against the rock. The absence of soil means no plants, so no roots prise the rock apart and no organic acids attack it. The surface sheds in curved sheets as pressure is released, which keeps it smooth and rounded rather than developing the cracks and ledges that would trap water. Some examples have stood essentially unchanged for many millions of years.
The related landforms
Several similar features are distinguished by how they formed and the names are worth keeping apart. A tor is a smaller outcrop of stacked blocks, typically on a hilltop rather than on a plain, produced by the same deep weathering but without the dramatic isolation. A koppie is a small hill of piled boulders, common in southern Africa. A monadnock is a residual hill standing above a worn-down surface, which overlaps but emphasises the surrounding plain rather than the rock. A bornhardt is the specific domed form with a smooth unbroken surface, which is the type most people picture. A mesa or butte looks similar from a distance but has flat-lying sedimentary layers and a quite different history.
Why people notice them
Standing alone in an open landscape gives these hills a significance out of proportion to their size, and the pattern is worldwide. Uluru in central Australia is the best known and has been central to the culture of the Anangu people for a very long time, with the rock surface and surrounding caves carrying an extensive record of that. Granite domes in Brazil dominate a famous harbour. Ancient kingdoms in Zimbabwe and in west Africa built on and around them for defence, since a steep isolated rock with a single approach is a fortress that needs little building. They also provide water, since bare rock sheds rain to its base and pools collect there in otherwise dry country.
The takeaway
An isolated steep hill on a flat plain records what was left behind rather than anything pushed up, since groundwater rotted the closely jointed rock at depth and erosion stripped it, leaving the massive unjointed bodies standing. Bare steep rock sheds rain, grows no soil and therefore no roots, so it weathers very slowly and can persist for millions of years. Isolated rock with one approach makes a natural fortress.