Cracks, peds, and their surfaces in some soils of the Riverine Plain, New South Wales

The nature and arrangement of cracks (planar voids) and peds in two redbrown earth profiles and two grey and brown soils of heavy texture profiles are described and their possible origin is discussed. The planar voids are classified and six patterns recognized. The vertical sequence of these patterns in each profile appears to be mainly controlled by the wetting and drying history, but in all profiles vertical cracking tends to predominate in the uppermost clay horizons and horizontal cracking becomes more significant with increasing depth. Specific modifications on natural surfaces (cutans) are not associated with particular kinds or patterning of planar voids, but are probably influenced to some extent by the swelling-shrinkage behaviour of the material. Peds have been redefined on the basis of recognizable natural surfaces as distinct from the occurrence of populations of units of similar size and shape; certain of the crack patterns separate peds of variable size and shape.