The slip surface in the D Zone of the Barton Clay

The most dominant of the preferred bedding plane surfaces of shearing in the Eocene Barton Clay coastal cliffs of the Hampshire Basin is that near the base of the D Zone. This is utilized as the basal surface of compound landslides virtually throughout its 2.3 km outcrop from near the cliff top to beach level, an elevation change of nearly 30 m. The shear zone is located along a thin seam of dark chocolate brown clay, which has been investigated by X-ray powder diffraction mineralogical analysis, X-ray fluorescence chemical analysis, scanning electron microscope study of the microfabric and ring shear tests. The dark seam is slightly more clay rich and has a slightly lower value of residual shear strength than the ambient D Zone clay. The reason for its preference during landsliding is discussed. The available evidence suggests that although some previous shear displacement by flexural slip during folding may have occurred, the main displacement results from the lateral rebound response to coastal recession, involving a reorientation of any previous clay particle alignment. Lateral rebound initiates progressive failure, which leads to the compound landsliding of the in situ clay slopes