Coastal instability drone surveys – Sydney Northern Beaches

In that time the flow directions of the rivers that delivered most of this sediment shifted from westerly flow in the Bald Hill Claystone and Garie Formation, to easterly flow in the lower Newport, to south-easterly in the middle Newport then back toward the northeast in the upper Newport.

We also fly past two beaches. A short pocket beach at Bilgola that lies across the headwaters of a small stream and the larger Avalon Beach which fills a deeper stream channel and which has high dunes toward its northern end. Behind the beach runoff now flows down Careel Creek (mainly in a concrete channel) out to Careel Bay which contains a large stand of mangroves and very small remnants of saltmarsh.  Vegetation in this bay has changed substantially since the 1940s when saltmarsh was dominant. Although Avalon Beach has more sand than most and should be able to buffer sea level rise longer, when it does fail, it may be that the creek will become a tidal channel cutting off the north end of the Palm Beach Peninsula as an island. That does not appear to be likely in this century but as global warming and sea level rise will continue beyond 2100 it could occur not long after that.

Particular scenes:

0.32 and a few later scenes. The bright green seaweed visible amongst the boulders below low tide is probably the genus Caulpera filiformis.  There is some uncertainty about the origin of this species with one suggestion being that it was introduced from South Africa before 1923, and another hinting that it might be a native which is responding to changing water conditions. Either way it does seem to be more common in recent years and is strongly competing with other species. It is related to a banned seaweed widely used in aquaria Caulpera taxiflora, which is invading the Sydney coast after first being recorded in Port Hacking in 2000.

0.56 A particularly clear boundary about half way up the cliff between the Garie and the Newport Formations.

2 50 to 3.15 A complex of rock falls where the orientation of the cliff face shifts and the vertical joints appear to be closer spaced.

4.52 The cliff face behind Bilgola pool was scaled back and wrapped in mesh in 1968 after rocks fell and damaged an earlier safety fence.

7.10 A distinctive small valley ends at the cliff face and may flow during heavy rain as a waterfall.

7.41 to 8.50 Appears to be the last sign of the purplish Bald Hill Claystone and the nature of the shore platform changes accordingly. This section of cliff is inaccessible.

13.10 A line of fallen sandstone blocks lies on the back of the platform. The blocks come from the uppermost bed of sandstone and have been colonised by blue-green algae making them black. This is an indication of age, the lack of subsequent movement. The nature of this fall where a long scarp has been undercut and collapsed is different from any previously seen and it mat=y relate to a slightly higher sea level in the past. 

17.18 Strong rectangular joint pattern in sandstone of the Newport Formation adjacent to Avalon pool.

18.20 The highest dunes behind Avalon Beach.

2.10 Another large patch of Caulpera.

~22.30 The approximate location of The Hole in the Wall. For an account of this feature and its demise see Pittwater Online News Aug 2017, Issue 325.

23.00 to 24.30 The biggest recent rockfall on the peninsula is just coming into view. More details next time.

 

Notes  Avalon to Palm Beach (Survey 4)

 This survey the drone will take you from Avalon Headland to the south end of Palm Beach. About 3.8km in a straight line but closer to 5.7km as we follow the coast. It is an interesting geological section because it includes the largest recent rockfall, the Hawkesbury Sandstone appears at the top of the sequence, it encompasses two sea caves, and we can see a lot of stratigraphic detail in the high cliffs from Bangalley to Whale Beach which is otherwise almost invisible because access is so difficult.

Particular scenes:

0.00 to 0.12 This is the biggest rockfall on the Northern Beaches in recent years and occurred in the early evening of 11 August 2017. It was preceded by two small earthquakes, one being 9 hours earlier. Were the quakes involved? Maybe!

A number of people saw the event and there are even a couple of photographs of the resulting dust cloud on the www. The headland is 63m high, the scar is about 50x40m and 3 to 4m thick. This volume equates to between 15 and 20,000 tonnes of solid rock.  For a fuller account of the event see Pittwater Online News. Aug 2017, Issue 325

0.35 to 0.52 The highest beds of sandstone have a ‘case-hardened’ surface which can break open and the softer inner sandstone frets out to create a cavern. In these examples the inner surfaces are subject to honeycomb weathering driven by salt crystallisation. 

1.15-1.30 A closer view of the honeycomb weathering.

2.04 The shadowed recess is the entrance to St Michaels Cave. Named by Father John Joseph Therry (1790-1864) who amongst other things was responsible for the construction of St Marys Cathedral in Sydney. In 1833 and 1837 Therry was granted 600ha of land at Avalon including the land above the cave which he named Mount St Patrick – a name that has not survived. 

The cave is formed by weathering of a northwest trending clastic dyke composed of a breccia similar to that found in diatremes (volcanic necks) like the Hornsby quarry.  

2.40 to 2.49 Two welcome swallows share cave insects with a small population of bats. Note the green staining in the sandstone at the bird’s roost. There is just enough light to allow algae to live with the nutrient provided by bird’s faeces.

2.54 As a sea cave it is possible that this feature was initiated at a time of slightly higher sea level than the present. It is no longer within reach of breaking waves as debris from the roof has raised the floor.

3.28 At least two units of a mauve coloured sedimentary rock are visible in the cliff. These are reminiscent of the Bald Hill Claystone but are higher in the sequence. They have been described as fossil soils of Triassic age but are quite unlike modern soils and more like sediments that may have been deposited on tidal flats or in a shallow lagoon. The topic needs to be revisited.

4.40 Very persistent bedding in the middle Newport Formation. Individual beds can be traced much further than usual suggesting extensive quiet water deposition, but in what environment? 

5.50 and 6.36 The orange stained sandstone at the top for the cliff is probably the first evidence of the Hawkesbury Sandstone.

7.05 A complex set of faults and three weathered volcanic dykes intersect the cliff face.

7.22 Closer view of faults and dykes and likely contact between Newport Formation and Hawkesbury Sandstone, regrettably access to that boundary looks terrible! 

7.37 Even closer, notice how the dykes are not perfectly straight and sometimes are offset on horizontal bedding planes.

9.14 Loose blocks of Hawkesbury Sandstone perched uncertainly on the cliff edge. 

9.42 As the underlying shaley rocks break away the sandstone blocks begin to glide toward the edge. They will eventually tumble.

10.29 One of Sydney’s more dangerous fishing spots, the Ovens also known as the Devil’s Cauldron.  Waves surge into this double cavern at almost all tide levels. Is it formed on another dyke or dykes? 

11.14 There is some yellowish staining on the roof but it is not clear that this is a dyke. 

13.42 A slightly higher view of the roofline of the Ovens. The staining appears to be iron and clay washed down an open joint rather than a weathered dyke.

15.58 The prominent rectangular joints on this narrow shore platform are parallel to the Oven’s crevices. Note that several of them have been eroded out by wave action. It seems that dykes are not involved in the creation of the sea cave. 

16.21 Some of the joints in the shore platform are curved and do not go right across the rocks. These may be formed by unloading of that sandstone bed. Two people provide an indication of scale. This is a difficult place to reach and they are welcome to any fish they manage to land. If you are tempted please judge the risk and wear a life-jacket.

21.40 Whale Beach is one of the smallest on the Peninsula and has a very limited sand volume. It was badly eroded in 1974.

22.04  The north end of the beach is backed by an old cliff line.

23.20 Older rock falls with some very large sandstone blocks.

24.08 The two largest fallen blocks on the left have rolled over. The largest is 17x8x3m and would weigh about 1,000 tonnes. 

25.50 The shadowed gap in the low cliff may be hiding a dyke.

 

Notes Palm Beach to Barrenjoey (Survey 5) 

Media Notes for Palm Beach

This video begins in Cabbage Tree Bay at the south end of the Palm Beach isthmus and flies north to Barrenjoey where the lighthouse stands at 102m ASL.  The upper half to three quarters of that headland is composed of Hawkesbury Sandstone overlying Upper Newport Formation, and one of Sydney’s larger dykes outcrops at sea level on the northern edge.

 Barrenjoey is generally referred to as a tied island or tombolo, one of those slightly annoying terms beloved of geographers that does not always tell us much about the feature. The general meaning of tombolo is an island linked to the mainland shore by a body of sediment that is usually assumed to have accumulated as a spit.

 The Pittwater-Barrenjoey link is a tied island in the simplest sense but the story is considerably more complex than the term tombolo suggests. The word is Italian, and although widely used in English, in the original language it more often refers to things to do with crochet or bobbin lace. You do sometimes wonder about translations.

None of the tied islands near Sydney; Kurnell, North Head, Long Reef, and Barrenjoey, are linked to the mainland by spits, they are all connected by barrier beach systems. Palm Beach is a good example and to understand its formation we need to look at bedrock morphology and sediment distribution beneath the waves.

The present day entrance to Pittwater is across a drowned bedrock ridge with a water depth of only 12m between Barrenjoey and West Head. Seismic profiles reveal the presence of two deep river channels eroded into bedrock and partly filled with sediment. One is the ancient channel of the Hawkesbury River that passes between Barrenjoey and Lion Island at a depth of >100m. The other is the former channel of the ‘Pittwater River’ or McCarrs Creek that runs the length of Pittwater then passes under the sand barrier south of Barrenjoey at a depth of 76m. The two palaeochannels join about 4km offshore at 120m depth, on what was the coastline at the peak of the last ice age about 20,000 years ago.

As the global ice sheets melted, sea level rose rapidly, sweeping sand onto the retreating coast. A beach/dune barrier system formed the isthmus but would have been kept open at the northern end by the Pittwater River until sea level reached the crest of the ridge between Barrenjoey and West Head.  At that time the route of the Pittwater River would have shifted to flow north and join the Hawkesbury allowing the sand barrier to close. The distribution of tidal sand and estuarine mud in both Broken Bay and Pittwater, and the presence of deep water (>40m) in the upper reaches of Pittwater confirm the story. Additionally, a number of radiocarbon dates have been obtained from different sediments, almost all of which fall within the Holocene (<12,000 years) showing that these changes took place well within the time frame of human occupation.

Aboriginal people lived in this landscape when Pittwater and the Hawkesbury were deep valleys probably carrying spotted gum forest and pockets of rainforest. Barrenjoey was a lesser peak on a stony ridge extending from Commodore Heights and ending some 4 km east of the present shore. Those people would have observed the sea level rise, saw regular turbulent tidal flows in and out of Pittwater at the foot of Barrenjoey, and the shift of the Pittwater River into the Hawkesbury as the ridge was drowned. This was a bold landscape that must have had important stories attached to it, all of which have been lost.

Particular scenes:

0.23-0.53 Leaving the bedrock headland behind we fly north along the sand barrier toward Barrenjoey with a view across Pittwater to West Head. Dunes rise onto the south side of Barrenjoey and today almost all of the sand is covered in vegetation.

This was not the case for most of the 20^th Century. Photographs from 1912 show that native plants were still intact except for a patch of active dunes at the north end, but by 1927 half of the original plant cover had gone. In the 1940s a golf course and a camping ground were established and by 1947 there was almost no natural vegetation left on the sand.

In 1974 storms waves broke across the barrier and that event triggered a huge conservation effort to stabilise beaches and dunes all along the coast. The foredune was reshaped destroying a large number of Aboriginal shell middens and using a poor selection of plant species, some of which have subsequently become environmental weeds.

 0.54-1.30 Below the lighthouse the southeast face of Barrenjoey reveals two main bands of sandstone. The upper unit is certainly Hawkesbury Sandstone. A quartz sandstone deposited in a major river with its headwaters somewhere in ‘southern’ Gondwana in what is now Antarctica. Some of the lower bands of sandstones are part of the upper Newport Formation but on this exposure it is a challenge to decide where to draw the line.

 2.18 The uppermost sandstone (Hawkesbury) can now be seen to break up as much larger joint blocks several of which are slowly gliding to the slope edge and a few of which are amongst the boulders at the base of the cliff.

 2.33 The eastern point of Barrenjoey has six benches formed on the sandstone beds and there is a very large single block (circa 600 tonnes) lying at an angle on the lowest part of the southern face. This appears to have slid out of a corner of the narrow ridge half way up the cliff. Just below that position the beds show an angular disconformity which is perhaps the best place to identify as the Newport/Hawkesbury contact. This position is consistent with stratigraphic work by Cowan but is a lot lower on the cliff than indicated by the Sydney geological map. This conclusion needs to be tested by fieldwork.

 3.25-3.53 As at Long Reef the most extensive shore platform, or more accurately almost the only shore platform, on Barrenjoey occurs at the eastern tip where waves can approach from south, east, and north.

 3.57-4.20 On the north face there is more rock exposure and the predicted Newport/Hawkesbury contact seems to be confirmed at the base of the orange brown stained sandstone. The underlying Newport lithic sandstones and laminates weather more rapidly and are recessive cliff faces.

 4.20-4.50 Rounded shrubs on the upper bench with an orange brown colour are female Allocasuarina distyla (shrubby she-oak) which has a limited distribution on the central coast and is tolerant of salt spray.  The lowest slope is a Themeda australis grassland growing in scree. This species also likes sea cliffs and headlands and is an endangered ecological community.

 5.25-5.31 Brown vertical stains on the lower cliff are caused by deposits of iron oxide carried by soil water and precipitated when the water reaches the atmosphere as it seeps out of the bench and onto the cliff.

 5.45-5.48 Sandstone beds within the shallow overhang are contorted as a result of slumping and folding of saturated sediment when they were deposited. Heavy black staining on the sandstone is blue-green algae.

 7.50 The same staining creates a dark line on the boulders just above the breaking waves on this aspect of the headland, also visible at 8.57. At about the same point there is a marked colour shift in the vegetation on the scree where the dull grey green of the Themeda changes to something else.

 7.57-8.01 At sea level and just to the left of the lighthouse a short section without boulders is occupied by a dark grey rock. This the first point where the Barrenjoey dyke can be seen.

The Barrenjoey dyke is probably Sydney’s largest. It trends west for at least 16km, is perhaps 9-12m wide, composed of dolerite, and has been dated to the middle Jurassic between 171 and 173 million years ago. These properties are similar to those of the ‘Great Sydney Dyke’ which extends from Glebe Island Container Terminal 10km offshore from Waverley Cemetery. The Great Sydney Dyke has caused numerous engineering headaches and the only differences with Barrenjoey is that it is about 10 million years younger and much better studied. Perhaps we have been a bit hasty labelling one as ‘Great’ without having paid much attention to any other.The origin of Sydney’s dykes have often been suggested as a consequence of the opening of the Tasman Sea 84 million years ago. But for these particular dykes their orientation is wrong and they are too old, therefore their structural origin remains a mystery.

9.57- 10.20 Dyke outcrop.

12.19-12.31 Dyke again with clear vertical jointing indicating that it dips at 90 degrees. The edges are not visible here but the dyke is at least 7m wide and will have a contact metamorphic zone on either wall. Finding fresh rocks in local dykes is unusual, was this place a source for Aboriginal edge ground axes?

12.42 Weathered dolerite in the dyke.

13.17-13.54 Moving around into Pittwater a strong water seepage or spring is found and joint spacing on the shore platform is much closer – probably on or close to the dyke margin.

14.05-14.10 Looking back across the isthmus it is hard to believe that there is a river channel 76m below the sand. A marked difference in beach width and gradient between Palm Beach (left) and Barrenjoey beach is also apparent.

14.18-14.48 Another length of sheared jointing linked to the dyke and perhaps a fault line. On this alignment the dyke can be projected across to West Head.

14.50-17.17 The seals in this recently established colony are completely unaware of the geological significance of their basking site.

17.24 Remains of the WWII West Head Battery and its haulway are very close to the location of the dyke under the West Head Lookout.

 

Long Reef – Special by any measure (Video 6)

The title says it all. The Long Reef headland and shore platform is one of the most visited and most interesting on the Sydney coast. It has been an Aquatic Reserve for 41 years and explored by thousands of people viewing the wonderful biology and marine ecology described by Prof Dakin, Isobel Bennett, and Elizabeth Pope in numerous editions of their Australian Seashores.

Not only does it have a wealth of inter-tidal life forms but the geology of the headland has intrigued observers since the earliest days of settlement. Prior to that Aboriginal people used the area as a supermarket and have left their trace. No doubt they had stories about this major promontory but regrettably these have been lost in time.

In this 35 minute video we travel right around the headland from Dee Why Lagoon to Fishermans Beach with Dr Peter Mitchell who relates the geologic story and in passing, challenges a number of conventional interpretations about the strata we see. Naturalist Phil Colman has haunted the reef for decades and contributes insights into the dynamics of life in the inter-tidal zone. Our stories are not complete. Quite likely a number of our points are wrong. But that is one of the attractions of a place like this where all previous tales can be challenged and anyone, including you, can make new observations that will test traditional knowledge. You should hurry, as it will not be so easy to visit in a few decades thanks to sea level rise. We hope you enjoy the journey, and look forward to the next generation telling a better tale.