Anomalous jointing of drillcores is sometimes observed in prospect boreholes in the Upper Silesian basin. The drillcore is jointed into discs, slices or porringers that are not resemblant to natural jointing. This occurs in various types of sediments, but especially in brittle rocks. Orientation of jointing planes is perpendicular to the borehole axis and does not depend on the distance from the front of the core run. Anomalous jointing reflects locally increased stress in the layer.