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Major alluvial diamond deposits develop on or adjacent to cratonic source areas
where there is a favourable interplay between climatic, basin dynamic, and local
geomorphic factors. Regions where humid tropical palaeo-climatic conditions have
alternated with semi-arid conditions are most favoured. The deep weathering of
rocks during humid periods leads to efficient liberation of resistant minerals and the
subsequent stripping of the deep regolith during semi-arid phases leads to the
transportation of the released diamonds. Changes in the base level of a river basin
produce alternating periods of local sediment erosion (degradation) and deposition
(aggradation) that are conducive to the local concentration of diamond. Terraces
also develop as the river cuts downwards to equilibrate with a lowered base-level. A
terrace is a preserved section of river sediment abandoned by a river as it incises
downwards in response to a lowering of its base level (for example by a lowering of
sea level). Terraces may occur at different heights above present day river level, the
higher terraces being the oldest.