The fault runs meridionally from well north of Perth to the southern coast. Movement along this fault system in the Tertiary has been postulated, but no modern earthquakes can be traced to its subsurface extension. These duricrusts have since been strongly stripped away in the southern region, exposing fresh crystalline rock. The scarp itself, stepped in places, appears to be a faultline surface eroded back perhaps 1 to 2 km from the present fault trace to the west. Elevations in the Swan coastal plain are generally below 75 m.
Highest points are near the scarp, where alluvial fans have built up a piedmont zone.
Offshore islands are sand dunes built during lower sea level some 5000 years ago. The subsequent rise in this level has cut low cliffs and beaches along the present coast and produced drowned estuaries and lagoons. The scarp itself thus served as a barrier to further inland invasion, but its erosion has been influenced more by fluvial processes than by marine action. Chapter 6 Table of Contents.