The influence of weathering on the physical, chemical and geotechnical characteristics of soils has been examined for both unstable and adjacent stable ground at a site in North Island New Zealand where earthflows have developed in regolith overlying Tertiary calcareous mudrock of marine origin. The same sequence of weathering zones was visually identified during sample extrusion for both the unstable and stable ground, although the earthflow sites were characterized by extension of weathering to greater depths. Both the characteristics of the weathering zones and the relationships between calcite content and geotechnical index properties show significant similarities to those reported for the Fuller's Earth in the UK. Variations in density and moisture content throughout the profiles are attributed to variations in the extent of stress relief fissuring, augmented by a calcite content which varies with depth and position on the slope. If the variation in dry density (and moisture content) due to calcite content alone is removed, the depth at which stress relief fissuring becomes prominent can be readily discerned and appears to be associated with the zone of basal shear in the earthflows. Physico-chemical swelling appears to be absent, even though the soil mineralogy is dominated by Casmectites. Post-failure weathering, clay mineral alteration and calcite removal are expected to provide a mechanism for promoting gradually increasing earthflow instability.