The Triassic period

220 million years ago, Hawkesbury River sandstone forms from accumulated layers of sediment, bedded at the delta of a wide, swiftly flowing river. The cliffs of the South Head consist of horizontal bedding layers of this sandstone, made up of quartz material carried from mountains of an earlier geological age located to the south-west of present-day Sydney. The quartz, bound together with clay and other ‘cementing’ materials, forms a resistant mass. Good examples of this ‘bedding’ formation can be seen in the cliff face at Cliff Street, Watsons Bay, opposite the Short Street intersection.[1]

The Jurassic period

160 million years ago, volcanic activity subjects the sandstone coastline to explosive pressures, the ultimate results of which may be seen today in various ‘Chines’ or chasms - such as the spectacular 2 metre-wide cleft in the South Head cliff-line known as Jacobs Ladder. Immense heat and pressure is generated, the force of which parts the sandstone strata along its natural joint lines, creating a fissure which is then filled by molten igneous material – largely basalt. Subsequently, this volcanic ‘infill’ (dyke) has eroded at a faster rate than its sandstone surrounds, leaving dramatic clefts in the cliff face.[2]

Jacobs ladder from the south 2000.
Photograph Woollahra History and Heritage Society.

The Pleistocene period

1 million years ago, an ice-age river, flowing 60 - 85 metres below the present sea-level, cuts through the uplifted coastal sandstone to bedrock.[3] Watercourses feeding the river cut deep valleys, subsequently filled by sand and mud, forming in their wake the beginnings of beaches and lagoons.[4]

Recent Geological history

130,000 years ago,inter-glacial warming causes the sea-level to rise some 6 metres above present levels. The South Head region becomes an island, separated by water from higher land to the west. Rock platforms are created around the harbour shoreline and ocean cliff-frontage from wave action during this period.[5]

20,000 years ago, during the last ice-age period, Port Jackson, now inhabited by early indigenous peoples, is an arid and rocky valley incised by freshwater streams.

8,000 years ago, the sea rises to its present level, and beaches and lagoons take on their present natural forms prior to European engineering intervention such as the piping of natural watercourses through storm-water systems. The land encompassed by the South Head region supports communities of Eora-speaking peoples of the Dharug language group. At the time of European settlement the Birrabirragal were based along the harbour shoreline of Vaucluse and Watsons Bay. Evidence of this early occupation remains today in the form of rock carvings and shell middens.[6]