Controversy has arisen in the past decade concerning ferricrete genesis by relative (in situ) or absolute (lateral) concentration of iron and aluminium oxides. Some regard laterite profiles as stratigraphic sequences, with crusts being remnants of iron-impregnated valley sediments. In contrast, the traditional notion of laterite formation invokes vertical translocation of minerals under humid tropical conditions on peneplains close to ultimate base level, with incomplete profiles reflecting variable degrees of erosion. Criteria to discriminate the dominant mode of ferricrete origin include topographic and stratigraphic relationships. A transported origin is favoured by unconformable contacts between truncated bedrock structures or quartz veins, overlain by ferruginised materials, whereas preservation of bedrock structures through the complete profile indicates formation in place. Ferricrete macromorphology can also be useful in determining whether iron oxides have moved into an area. Petrological studies suggest that pisoliths with angular irregular shapes and diffuse external borders, and those with similar framework grains within pisoliths and matrix materials, appear to have formed in situ. Formation by physical break up of ferruginised materials, transport and modification is favoured by broken surface coatings, different-sized grains in pisoliths and their matrix materials, different-sized grains in adjacent pisoliths, multiple laminar goethite rinds on pisoliths, abraded compound nodules cemented together by ferruginous, concretionary material, and complex pisolith-within-pisolith structures. Monomineralogy may suggest in situ pisolith formation, whereas transport is favoured by polymineralogical pisoliths resting on materials that could not have provided the elements for their formation. In pisolitic ferricrete, significantly different matrix composition from that of the pisoliths favours interpretation of pisoliths as clastic components incorporated within different materials. Furthermore, maghemitic pisoliths at depth suggest transport and burial and scattering of palaeomagnetic data from individual pisoliths indicates physical disturbance since their formation.
