The article analyses the possible variants of natural infi lling for the foundation pits of medieval buildings, gives a general characteristic of the infi ll soils, and a concise description of the basic natural displacement processes pertaining to the archeological perspective. The article gives the morphological characteristics for individual layers and the infi ll in general (color, composition, shape of layer, inclusions, etc.), which can indicate the natural origin of the infi ll layers in the foundation pit. The main focus of attention is on the morphological properties of the soil, which archaeologists can use in their work and which do not call for any specialized training in the natural sciences. The author shows than intensive infi lling begins after the building ceased to function, and can follow several scenarios depending on the presence of covering, reinforced walls and land water. At the initial stage of infi lling, the main processes in automorphic conditions are collapse, caving and deformation; in hydromorphic conditions the main processes are suffusion.