This paper considers the deformation behaviour of four geologically aged, medium-plasticity, heavily overconsolidated stiff clays that affect a broad swathe of infrastructure projects in the SE of the United Kingdom. Static triaxial and hollow cylinder stress path experiments on high quality samples are examined along with dynamic multi-axial bender element and resonant-column measurements. Patterns of undrained shear strength anisotropy are revealed that are governed by the clays' meso and micro-structures. The clays are brittle in shear and their stiffness characteristics are shown to be markedly anisotropic, highly non-linear and pressure dependent. The results obtained have many implications for practical geotechnical engineering.