Dunkerley, D.L., 1976. A study of long-term slope stability in the Sydney Basin, Australia. Eng. Geol.; 10: 1-12. A study of slope characteristics in part of the Sydney Basin indicates that landslide occurrence and long-term slope stability are both governed by the residual shear strength of the clay soils which weather from the Triassic shales of this area. Both the present distribution of landslides and the form of the slopes themselves lend support to this conclusion. It is found that on the Wianamatta shales forming the Razorback Range southwest of Sydney, landslides may only occur on slopes whose inclination exceeds 11°. The average value of φ′r measured in direct shear was found to be 22°. Values of C′r were found to be very small but positive.