This and the previous photo (and this earlier one) show the characteristic surface details generated by rainfall weathering exposed blocks of limestone.
If one imagines a uniformly smooth sloping surface such as an individual stone block (though the same principle applies to entire hillsides and the formation of stream networks), surface water will initially flow off it as a sheet, but tiny surface irregularities will soon concentrate flow in certain directions. Such channelised flow will cut small rills and, having done so, will then preferentially erode the bed of each rill leaving the intervening areas relatively intact. The sides of each rill will erode too (though at a slightly slower rate) so that adjacent rills will meet eventually at sharp 'catchment divides', as seen here.
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