Approaching Sydney from the east by sea or by air offers an experience rare for those entering a great city. Perhaps only San Francisco has comparable cliffs at its harbour gates. The majestic North and South Heads flank the entrance to the grand harbour called Port Jackson. These cliffs extend north towards Manly embayment and south to Bondi. It is this southern stretch that I want to reflect upon here.
I grew up in Rose Bay on the harbourside of the eastern peninsula of Sydney. Up the hill from where I lived is the suburb of Dover Heights. The word Dover is quite apt as like its English namesake, the sea has carved sheer cliffs by constant wave attack. For many years this was part of my playground where friends lived and where one could explore; freedom to roam had its risks but parents did not need to know! The area was a mix of homes built before WW2, part evacuated during the raids of 1942, then further developed and “mansionised” in subsequent decades. Fortunately, not all clifftop land was lost to houses with protection of old military sites and narrow strips of Crown land. Some of this land decades before, believe it or not, had been an ostrich farm!
Today we live not far from what I am calling the Dover Heights cliffs. Waverley and Woollahra councils have built excellent walking tracks along with historic markers of shipwrecks such as at The Gap, and information on biota. Even during these socially distant times they are well used. Views to seaward can be quite dramatic such as we experienced during the June 2016 storm, or during calmer days with slicks amidst low swell on the water surface; then there are whales and birds, some of which hide amongst shrubs on the cliff edge. At times you can spot gulls hovering around the sewer outfalls at Christison Park and Diamond Bay where local effluent flows into the ocean without treatment just below sea level—a legacy of disposal soon to be diverted to the main Bondi treatment works more than 100 years after the sewer tunnels were built.
The towering cliffs are carved into Hawkesbury Sandstone, a rock sequence of tremendous importance to the architecture of Sydney. In 2010, I co-authored a paper on the origin of these cliffs published in a book in honour of John Chappell and Martin Williams (“East Australian marine abrasion surface”, Geol. Soc. London, Special Pub., 346). This paper traced cliff formation back to the early Tertiary involving rising sea levels over a subsiding continental margin. At Dover Heights, the Hawkesbury Sandstone is displayed in all its glory; horizontal layers of quartz sand metres thick showing units with distinct cross bedding while others are quite massive. Locally there are mudstone units. All this is well described in a paper by Rust and Jones in Journal of Sedimentary Research (1987, 222-233). These rocks were laid down as part of a braided river system around 200m in thickness during the Triassic period 200-220 m yrs ago. What created the conditions under which these sandstones were deposited? The river system must have been huge and fast flowing and of sufficient length and duration to have allowed for the concentration of quartz and the breakdown of less resistant minerals like feldspar. Standing on the cliff edge I ponder its origins.
Three other features of the cliff rock formations fascinate me (besides the Pleistocene cliff-top dunes which have been discussed elsewhere). They are all secondary to the deposition by this huge river in the Triassic. I’ve added a few photographs taken to illustrate some of these features.
First is their cementation. The basic cement is also composed of quartz which binds the grains together producing a differentially hardened rock. Iron bearing solutions have also permeated the rock and once precipitated form regular banded patterns often cutting across the primary bedding of river deposits. These patterns are called Liesegang bands and create some amazingly picturesque forms which are very hard to explain. Because the iron bands are bonded more than the quartz cement, they sometimes stand out as “case-hardened” features—miniature ridges surrounding hollows or overhangs.
The second feature is the joints or cracks in the sandstone, especially in the massive units where cross bedding formed by migrating sand waves is absent. These joints are usually vertical and divide the sandstone into blocks up to container size. Where they appear at the base of a cliff as rock fall, they are a reminder of how the cliff face can dramatically retreat; a house located on the edge is always at risk if sited within or marginal to such a joint-defined block. I once upset some people when quoted correctly in the press as saying this was a nightmare for me and future cliff edge development needs to be constrained. Recent drone images near the Diamond Bay sewer outfall show undercutting of softer mudstone and sandstone units below the massive sandstone on the cliff top on which houses are located.
Finally there are the dykes. In the Tertiary, igneous (basalt) intrusions penetrated the jointed sandstone along the Sydney coastline. They were first mapped by Rev. J. Milne Curran in his 1898 book Geology of Sydney., and discussed in that wonderful book by Griffith Taylor Sydneyside Scenery (1958, see Fig. 14). Heat from the intrusions helped hardened rocks into columnar structures at North Bondi, near the golf course, a site of numerous geology excursions (see Figure 73 in Curran). But the igneous rocks weather to form what Curran called fissures (Fig.13). These narrow, vertical sided slots in the sandstone are one of the most interesting features of the Dover Heights cliffs best seen from the sea. They are referred to by different names in the literature besides fissure: gryke (grike), slot, slit, cleft, although gryke which I like is mostly used for solution weathering in limestone.
No doubt there are others like me who are captivated by cliffs such as those at Dover Heights and elsewhere. They offer a history hard to reconstruct, sights that are ever changing, drama both sad and thrilling, and danger. I feel privileged to have them in my backyard and continue to look forward to enjoying them.
Words by Prof Bruce Thom. Please respect the author's thoughts and reference appropriately: (c) ACS, 2020, for correspondence about this blog post please email firstname.lastname@example.org