The Narrabeen Group is a thick sequence of bedded Triassic sandstones which occupies the stratigraphic interval between the Permian coal measures and the Hawkesbury Sandstone in the Sydney Basin. In the northern Sydney Basin it is laterally extensive, extending throughout the southern Hunter Valley from the Central Coast to beyond the Great Divide. In the Watagans region, a unique situation arises where a disconformity causes the Narrabeen group to be underlain directly by Permian upper marine sediments. The result is a dramatic occurrence of ancient massive landsliding, leading to steep sandstone slopes that break to an elevated detrital plane, of considerable lateral extent. This paper describes the unusual geomorphic features associated with the landslide mass and interprets a variety of individual mass-movement and rockfall mechanisms which have contributed to these impressive features in the various stages of its development.