This small section of the rock platform serves as a good analogy for the ancient sub-surface topography of Sydney Harbour.In simple terms the harbour is formed by the erosive path that the Parramatta River has carved on its journey out to sea through its base of Hawkesbury sandstone and into which has flown the rising sea since the last ice age.
The image below shows a bathometric chart describing the path that the Parramatta River created for itself in the last ice age, when the sea level was much lower and the coast was consequently much further eastward ~15 or more kms.
The Parramatta River turned to the south at the Heads, travelling down to meet the "Bondi River" and then reorienting itself towards the east and before emptying into the ancient sea.
In the last ice age the view towards the eastern horizon from a position standing on either of the Heads would have been not unlike the present view from Katoomba towards the Jamison Valley. That is, sheer sandstone cliffs falling vertically into a cool climate forest v-shaped valley at the bottom of which travelled a modest river.
Source : Cainozoic Morphology of the Inner Continental Shelf near Sydney, NSW.A.D.Albani et al. 1988
The chart above describes the path of the "Bondi River"which has been interpreted as the original way in which the Parramatta River found its way eastward - through what is now the Rose Bay Golf Course - before silting itself up and precipitating the carving of the modern river's path out to sea via the Heads. (See an earlier post on this topic).
There are other intriguing geological phenomenon that are observable above the current sea level along this section of coastline. When east Gondwana began to break up under techtonic forces brought about by under-plate felsic intrusions around 120 million years ago there occured a series of near vertical "strike-slip" movements in the techtonic plates. In Sydney these strike faults took place along a north-northeast direction. The strike faults belonging to the "Watsons Bay Fault Zone" are clearly visible on the surface of the lower rock platform on the left side of the image shown above.
The southerly extent of this fault zone is visible at South Era Beach in the Royal National Park (see coloured chart below). This fault swarm is also clearly visible at Bottle & Glass Island and Shark Island.
Subsequent to the faults breaking up east Gondwana, igneous dykes exploited the weaknesses of the north-northeast strikes about 50 million years ago. A number of these dykes, including the examples at Salmon Cove and North Bondi, travel at an opposing angle to the faults - in a north-west orientation.
