Geological Sites of NSW banner

The Skillion at Terrigal banner

Southern and eastern faces of The Skillion
Ferruginous zones, resistant to weathering
Here, clay has migrated to fill joints in the sandstone. This site is located at the north-western end of The Skillion, adjacent to the rocky beach
A dislodged rock with a joint line penetrated by ferruginous deposit which has seeped into surrounding rock
Photo of sandstone showing honeycomb weathering
Joint lines running across the rock platform and up into the cliff face. They are deeply weathered where the ocean has access to the joints
Strata in The Skillion's eastern face, from the grey shale at the base to Hawkesbury Sandstone at top

The Skillion at Terrigal

Latitude -33.450994, Longitude 151.451812

Located in the Central Coast Region of NSW, nearest town Terrigal

Source: NSW Geological Survey, Bulletin 26 - R00052006

Link to Detailed Map

The Skillion is a very impressive landscape feature and has plenty to offer those interested in geological processes. The sedimentary rock forming The Skillion (known as the Terrigal Formation) was laid down in the Middle Triassic and is overlain by Hawkesbury Sandstone. The landform in itself is interesting as the surface facing the south has weathered in a different form to the easterly face which exhibits a sheer drop.

Geology

The sedimentary rocks in this location formed in an alluvial environment, possibly in a delta or river flood plain. Examination of the rock faces will show bedding where silt has been deposited over eons. Interpretations of the sediment in the rock indicate that, originally, water flowed to the south-east in the lower sections of the deposition and gradually swung around to the north-east in the upper zones.

Access to the southern side of The Skillion is via a track through an erosion gully formed where the southern part of the headland joins the low cliffs to the south. Make sure it is low tide when you examine this area and be mindful of the possibility of rogue waves breaking on the rocks.

At the base of the southern cliff of The Skillion there is a palaeosol (a former soil preserved by burial underneath sediments and lithified into rock). There are at least thirteen such palaeosols in the Terrigal Formation. Associated with each of them are thin beds of hard, silicified, micro-crossbedded and rippled, very fine sandstone and course siltstone. This silicified hardpan (silcrete) is thought to result from desilication of feldspars and other silicates during weathering, followed by precipitation of silica by various physical, chemical, or biochemical processes. These processes took place as a result of initial high porosity. Additionally, slumped ball and pillow bedding structures can be seen in the overlying siltstone and sandstone.

Also associated with the palaeosols are layers of ferruginous nodules in the bedding. Many layers have been deformed by the growth of these nodules. The formation of nodules was most probably related to fluctuations in the ancient water table.

There are tubular structures in the palaeosol horizons which have been interpreted to be the stems and root casts from the probable remains of a dense cover of rushes on a river flood plains. They have been filled with fine sandstone, have circular cross sections and reduce in diameter and fork apart downwards, replicating the action of plant roots.

Grey band of shale underlying the sandstone

If you walk to the eastern base of the Skillion, you will note a distinct band of grey rock underlying the sandstone along the length of the headland. This is shale.

In the adjacent photo, note the joint lines running across the rock platform and up into the cliff face. They are deeply weathered where the ocean has access to the joints.

There are some interesting weathering sites in the area adjacent to The Skillion and on the next headland north. The following are a selection of photos showing these occurrences.

Unusual dimples in rock platform most likely original shapes on the floor of a shallow seaFerruginous deposit on dislodged rock

© Copyright Cartoscope and Others