2. THE TERRIGAL FORMATION
(Previously named "Gosford Formation")

The Terrigal Formation underlying the Hawkesbury Sandstone is a series
of sedimentary rock beds, laid down in the early Triassic period
before the Hawkesbury Sandstone. It consists of layers deposited in the
Sydney Basin mainly from the north-east and south-west, New England and
Lachlan Fold belts.

The total thickness of the Terrigal formation is about 200 metres,
but only the top layers are exposed between McMasters and Box Head.
Horizontal bedding dipping slightly to the south can be seen on the headlands,
cliffs and wave-cut platforms at sea-level.

The sedimentary rocks of this formation (with the exception of conglomerate)
have finer grain and are more prone to erosion than the Hawkesbury Sandstone.
They give more fertile soils and less precipitous slopes.

General view of the bedding planes of the Terrigal Formation rock layers.
The bedding planes dip slightly to the south on the wave-cut platforms.
Shows alternating layers which were deposited
in the early Triassic period before the Hawkesbury Sandstone
(Click on the image to see more pictures)


Sedimentary rocks are formed by the consolidation of sediments
settled out of water, ice, or air, and accumulated on the Earth's surface.
They are derived from weathering and erosion of pre-existing rocks.



The Rocks of the Terrigal Formation :-
The Terrigal Formation consists of many layers
of different kinds of sedimentary rocks:-
  • Sandstones
  • Shales
  • Siltstones and mudstones
  • Conglomerates

Sandstone

Terrigal sandstone is a 'lithic' sandstone with up to 75% quartz grains
together with rock fragments bound with a matrix of
clay, silt, and iron compounds. It weathers more readily than the
'quartzose' Hawkesbury Sandstone which has a higher proportion of quartz.
(Click on the image to see more pictures)


Shale

A fine-grained rock formed by compaction of silt, clay, and/or sand
that accumulates in deltas or lakes and ocean bottoms.
Shales may be black, red, grey, or brown,
and have excellent fissability (layering).

(Click on the image to see more pictures)


Siltstone

Contains less clay than shale and is not finely laminated.
It occurs in bands which contain more silica but less alumina than
sandstones or shales.
(Click on the image to see more pictures)


Mudstones

Fine-grained rock consisting of particles of sandstone,
claystones, shales, etc, but not bound together in layers.


Conglomerate (pudding stone)

contains more or less rounded fragments
or pebbles set in a fine-textured matrix of sand or silt.
(Click on the image to see more pictures)


Weathering of Sandstone:

The white parts of the rock represent Kaolinitic sandstone
which is mainly aluminium-rich compounds,
and the red and purple colours represent various oxides of iron.
(Click on the image to see more pictures)


Honeycomb Weathering:
(Click on the image to see more pictures)


Ripple Marks: At the south (surf club) end of the beach beside the
rock pool (Bogey Hole) there are ripple marks on the rock platform.
The ripples were formed under shallow waters 200 million years ago.
At that time the area was a sandy tidal flat at sea level.
(Click on the image to see more pictures)


Tesselated Pavements on the Hawkesbury Sandstone

Tessellated pavements are formed by weathering and erosion.
An old rock surface is slowly dissected, ie separated into
small sections, most probably by expansion and contraction.
It is not understood why this occurs on some surfaces
and not on others, but tessellations tend to form more often
on homogeneous sandstones.
(Click on the image to see more pictures)
A good example of a tessellated pavement in Bouddi National Park
is just beside the walkway from Putty Beach to Bullimah Beach.
It is on rocks of the Terrigal Formation sandstone on a cliff's edge
about 10 metres above the breaking waves.
It was probably formed after the removal of the rock strata above.
This would have released pressure on the lower rocks, after millions
of years. This then caused expansion of the sandstone and cracking
of the surface to produce a geometrical pattern. Iron staining has
given it rich rust-coloured bands.


Fossils in the Terrigal Formation:

At the base of the cliff at the southern end of Putty Beach
there are plant fossils in the sandy shale beds in the rock platform.
This indicates that the area was probably a swamp long before
the sea level rose to its present level.
(Click on the image to see more pictures)