Illuvium is material displaced across a soil profile, from one layer to another one, by the action of rainwater. The removal of material from a soil layer is called eluviation. The transport of the material may be either mechanical or chemical. The process of deposition of illuvium is termed illuviation.  It is a water-assisted transport in a basically vertical direction, as compared to alluviation, the horizontal running water transfer. The resulting deposits are called illuvial deposits. Cutans are a type of illuvial deposit. 
When percolating rain water reaches a drier soil horizon, water from the suspension is removed by capillary action of microchannels, leaving fine deposits (cutans) oriented along percolation macrochannels.