Decalcification is proposed as a cause of the continued movement observed in many natural and engineered slopes in calcareous mudrock formations. It is deemed that such movement may potentially occur as granular calcareous particles, which otherwise maintain high residual shear strengths, are leached from shear surfaces within slipped masses, changing the mode of residual shear from transitional to sliding. Measurements of the residual shear strengths of 42 natural samples taken through the Fuller's Earth succession near Bath over a range of effective normal stresses emphasize the curved nature of many failure envelopes. The results demonstrate that the threshold between the two modes of shear occurs at approximately 25% calcite content. Fuller's Earth Clay with more than 25% calcite content is thus prone to significant decreases in residual shear strength upon partial or total decalcification.