Cliffs in the Narrabeen Group, Sydney

Main Image

Cliffs form a magnificent front door for Sydney. A previous blog discussed those cut into Hawkesbury Sandstone to the south of the entrance to Sydney Harbour. Here we describe those carved into an older set of Triassic age rocks, the Narrabeen Group.

During the Permian and into the Triassic some 200-300myrs ago, a subsiding sedimentary basin was filled with a vast array of sediments from different sources. This saucer-shaped basin allowed for older rocks to outcrop along todays coast to the north and south of metropolitan Sydney. Rocks forming the Narrabeen Group outcrop at sea level from Long Reef in the north and south of Garie in the Royal National Park. The group reaches a maximum thickness of 700m at the centre of the basin and constitute a much more diverse sequence of sediments than the “purer” quartz-dominated Hawkesbury Formation that overlies them. It is this diversity that creates the instability and coastal management problems facing local councils and property owners.

Geologists have recognised 21 distinct formations in the Narrabeen Group. This number reflects different locations where distinct types of rocks occur within the group. There is an extensive literature on these rocks, including the writings of Dave Branagan in his overview of the geology of the Sydney region (1985, Pells, P. ed., Engineering Geology of the Sydney Region, A.A. Balkema; see also work by Chris Herbert and colleagues  for the Geological Survey of NSW, e.g. Bulletin, 26, 1980). Pells, Branagan and Emerson have also published on the engineering properties of these rocks. There also specific papers on aspects of the mineralogy of rocks that occur on the coast such as the “red beds” at Long Reef (Loughnan et al. 1964, J. Geol. Soc. Aust.,11, 65-77).

All these studies highlight the complex nature of the Narrabeen sequence containing beds of sandstone of different lithologies, together with layers of siltstones, claystones, shales, and even conglomerates, sourced from a variety of provenances, including volcanic. All the rocks were deposited under non-marine conditions with the basal and top sections formed in river and deltaic environments. The middle division was a huge lake rich in plant life and freshwater fish. Curran in his 1899 description of what was then termed the “Narrabeen Shales” vividly explained the setting:

“We must picture the area now occupied by the Narrabeen Shales as a land-locked lake of considerable extent. Rivers from the surrounding highlands swept into this lake. A dense vegetation fringed the waters, and the hill sides were covered by the straight-stemmed and graceful Phyllotheca. The sediments brought down by the rivers and deposited layer above layer we now recognise as the Narrabeen Shales” (J.M. Curran, 1899, Geology of Sydney, p.184). He goes on to quote Professor Edgeworth David on the input of “contemporaneous volcanic eruptions” which added ash layers that weathered to produce the characteristic red colour of the shales seen at Long Reef. As Curran puts it: “the rich, warm hues of the chocolate shales, the soft yellows of the sandstone, and the rich reds of the ironstone bands are excellent examples of the variety and life given to rocks by iron oxide-Nature’s universal dye”.

It is this variety and variable strengths of layers within the Narrabeen Group that creates its fragility when exposed as a cliff face. But this fragility is added to by intrusions of basalt in the form of dykes as we saw in the Hawkesbury Sandstone. Perhaps the best example is at Avalon. Professor Griffith Taylor in his book Sydneyside Scenery, illustrates this with his account of St Michael’s Cave (Fig.14, p.54). Here intrusive rocks have entered the Narrabeen and have weathered to form a cavern 70m long. Sadly the roof collapsed possibly triggered by building construction above during the 1970s.

There are many examples of instability along cliffs in this rock sequence that has been of concern, and remains a concern, to coastal engineers in Wollongong and Pittwater councils (now Northern Beaches). In one case a development application had to be stopped by Ministerial intervention at Stanwell Tops when it became clear that the house was going to sit on top of potential slide. In 1974 a cliff collapse at Whale Beach resulted in a property losing its entire back yard. At Avalon cliff collapses have been a feature of both the southern and northern headland. The northern headland, in recent times known as the Indians Head suffered a significant collapse by which the “Indian” lost his nose. But before this there are drawings in the mid-1800s showing an arch at the end of the headland which by the late 1800s had collapsed leaving a column on the rock shelf and the “Indian Head” of which the nose was the remainder of the arch. Eventually the arch collapsed. We include two figures with exact dates to be verified. They highlight the dynamic nature of the cliffs in these rocks.

There have been progressive collapses at Bilgola, where the headland had to be “netted” because falling rocks were injuring people swimming in the pool. Rock falls also have been noted at south Newport, where houses are close to the edge, and at Bungan Beach which has been potentially compromised by a volcanic dyke. In August 2017, a small earthquake triggered a large chunk of the North Avalon headland to collapse. See images for Avalon Headland. This was reported in the Manly Daily on 14 August. Angus Gordon was quoted as saying such a collapse was “symptomatic of the fragility of the cliffs along the northern beaches and a warning to anyone who walks around them”. Critical to his concern is the fact that the headland is comprised of shales overlain by “blocky sandstone”. As is the case with the Hawkesbury Formation this blocky sandstone unit is jointed and these “cracks” are prone to rockfall.

The story at Narrabeen has many dimensions. Although the shale erodes relatively easily the cliffs are relatively low and any housing is well set back. Its rocks have been eroded over millions of years by rivers and the sea. We can observe offshore the drowned river valley of the Narrabeen River. In the 1970s Andy Short mapped the Collaroy/Narrabeen embayment using an echo sounder.  During the 1980s Angus Gordon and his team used overlapping side scan and a precision echo sounder along with detailed bed sampling to prepare detailed maps of the seabed of the embayment. At the same time Alberto Albani covered the area with seismic studies and thereby charted the rock valleys under the sediments. Interestingly there are two main flow paths which come together approximately 2 km offshore. Between these there is a large reef in the Narrabeen rocks which would have been a hill at a lower sea level. This reef acts like a lens, focussing waves onto the shore. Long Reef rock platform also causes changes to the wave pattern and both these combine to alter the wave directions and amplitudes at the beach. Depending on the approach direction of storm waves, different areas of the embayment suffer more damage than the rest.

An interesting side-line is that Cook departed Stingray Harbour (Botany Bay) on the 6th May 1770 having spent about 8 days collecting water, timber and fish and in light winds proceeded north past Port Jackson where he was 2 or 3 nm offshore and, contrary to popular belief actually noted the entrance to the Harbour, without entering it, but continued north. What comes next is interesting and depends on interpretation, but it would seem he was about 7 nm offshore when, late in the afternoon he noted “some broken land, like a bay”. By dawn the next morning he noted Cape Three Points, but therein is the issue as it would seem that he passed what we know as Broken Bay at night and well offshore. It was Flinders who recognised that what we know as Broken Bay was well north of Cooks “broken land”. Taking into account the relatively light winds and slow progress of the Endeavour (Log shows 1 to 2 kts), along with Flinders later charting, it would seem Cook’s “broken land” (aka Broken Bay) was actually the Collaroy/Narrabeen embayment with its prominent rock headlands as observed from well offshore and slightly to the south. Ray Parkin, in his book H.M. Bark Endeavour provides an interesting account of the trip up the east coast including excerpts from the Log, Cook’s, Banks’s and other diaries.

The rocky headlands and cliffs of the Sydney region are remnants of a long geologic history. They tell us many stories, some of which fascinate natural scientists of all persuasions. But they are also relevant to our human history. Into the sandstone have been carved representations of country by our First Nations. The headlands and their associated bays form part of early European exploration and navigation. Today their presence provides opportunities for recreation, sadly at cost of life to some through rock fishing, but to great enjoyment of surfers impacted in the pursuit of the perfect wave influenced by rocky reefs. But it is the coastal manager that must always be alert to the risk of development near cliff edges. Along the northern beaches we have records of cliff rock fall. Whether it be in the Hawkesbury Sandstone of North Head, and the undercutting in that same rock at Queenscliff, to the fragility of the different rock types within the Narrabeen Group around Avalon and elsewhere, it is clear all our cliffs are in a potentially precarious state.

Bruce Thom and Angus Gordon

 

Words by Prof Bruce Thom and Angus Gordon. Please respect the authors' thoughts and reference appropriately: (c) ACS, 2020, for correspondence about this blog post please email austcoastsoc@gmail.com

#165