The Terrain

Sydney’s Geology – A quick introduction

Sydney is mostly Triassic rock, with a few more recent igneous dykes and the odd volcanic neck. The dominant geological member is the Hawkesbury sandstone, some 600 feet (200 metres) thick, with current bedding, shale lenses and fossil riverbeds dotted through it to make the cliffs more interesting.

On isolated ridges, a capping of Wianamatta shale makes richer soil, and below the sandstone, assorted shales, mudstones and other sedimentary layers go all the way down to the Permian coal measures, deep below Sydney.

At some time in the past, the whole Sydney area was worn down to a flat plain, close to sea level. Then the land rose, or the sea fell, and the rivers and streams cut down into the rock. For the most part, the water flowed along a series of joints, planes of weakness in the rock which mainly run north and south. This flow produced a fern-leaf pattern of drainage that cut deeply between the tough sandstone which survived in ridges, high above the water, and in cliffs, where pieces of rock fell away, whenever a shale lens or a softer sandstone bed was eaten out, and a joint in the sandstone was undermined.

In recent times, the sea levels rose again, and the sea came flooding back, filling the river valleys to make what the geographers call a drowned river valley system. The Blue Mountains are the same sandstone, raised up in the past, except for a few ancient volcanic remnants like Mount Wilson, and Mount Tomah, where the Mount Tomah Gardens are located.

The city has been shaped by its geology. Nearly all of the rocks that you see exposed around Sydney will be sandstone. The sand that was to become this sandstone was laid down in the Triassic period, about two hundred million years ago, a time when plants were ferns, reptiles were becoming dinosaurs, and mammals were only just being thought about.

There are some remnants of more recent volcanoes around, but almost everything that you can see is good old-fashioned sedimentary rock, lying in almost horizontal layers. Not quite horizontal: the rocks around Sydney are shaped into a basin (or at least like half a basin) with the bottom of the basin somewhere near Fairfield, to Sydney’s west.

So far as the valley-lines go, it seems that the whole of the Sydney area was a flat coastal plain which was lifted up to a height of some hundreds of metres. Small streams then ran along the rectangular joint-lines, and cut their way down almost to sea level, making a fern-leaf pattern of valleys which are more or less at right angles to each other.

Then the sea-level rose again, “drowning” some of the river valleys and giving us the rich structures of Sydney Harbour, Broken Bay, and Port Hacking. Not Botany Bay, though: it has a shape that depends more on the placement of recent sediment, though the Georges River, which runs into the bay, shows the same fernleaf pattern.

The vegetation of the Sydney region has to thrive in soil miserably poor in essential minerals, sandy soil that quickly drains away most of the moisture. As a result, many of the plants that are native to the area have evolved special survival tricks.

There are only a few interesting fossils to be found in the Sydney area, and most of those are fish. Most of them are found in the Narrabeen shales, although there are a few lenses of shale in the Hawkesbury sandstone that occasionally yield some more interesting fish. The Australian Museum has a good collection of local fossils, including a gruesome looking amphibian, built like a crocodile, only with more teeth and a name to match – Paracyclotosaurus davidi.

They have a good relief-cum-geological map of the Sydney Basin on display in the Planet of Minerals gallery, and there is an even better one in the foyer of the Edgeworth David building at the University of Sydney – named for the same geologist celebrated in the name of the gruesome amphibian in the last paragraph.

West of Sydney, the Blue Mountains were formed by a massive uplift of rocks. At the coast, the beds that make the Blue Mountains are near or even below sea level. The difference between the coast and the mountains is achieved by the Lapstone Monocline, which can be seen as you drive into the mountains when you see the tilted beds near the western edge of the plain that lies to the east of the mountains.

There are a few remnants of more recent volcanoes that have pushed up through the older sedimentary rocks, including one volcanic neck which is right on the coast-line to the north of Bondi Beach , on the North Bondi golf links. There are also many igneous dykes around Sydney. These mostly weather out faster than the surrounding rock, forming kaolin, a clay mineral, but they are hard to spot.

The Hawkesbury sandstone has had a strong influence on transport: the steep-sided drowned river valleys make bridges necessary for road transport. Worse, the sandstone is very hard to carve roads through, or to tunnel through. The chalk of Paris, and the London clay, make underground railways much more feasible in those cities.

The sandstone has also influenced local architecture, since it was readily available as a building material. The beauty of the stone carries a high price: many local buildings of carved sandstone are now beginning to deteriorate. The main buildings of the University of Sydney have hardly been free of stonemasons’ scaffolding in the last thirty years. St Andrew’s Cathedral, near Town Hall, is also suffering the ravages of time.

The shaping of Sydney, a second view

The sandstone that defines Sydney was laid down almost 200 million years ago. The sand was washed from somewhere else, maybe out around Broken Hill, and laid down in a bed that is about 200 metres thick. Currents washed through it, leaching out most of the minerals and leaving a very poor rock that made an insipid soil. They washed out channels in some places, while in others, the currents formed sand banks that show a characteristic current bedding or cross-bedding that can often be seen in cuttings.

Over time, the bed was bent somewhat, so that the Hawkesbury sandstone is now rather like a saucer that has been broken in half. The base of the sandstone is above sea level when you go north of Long Reef or somewhere south of Port Hacking, so that shales and mudstones begin to appear. You will actually see a few small shale lenses in the Hawkesbury sandstone, but these are rare.

At some time in the past, a monocline formed to the west of Sydney. If you don’t know the term, the monocline is a sloping bend that raises the sandstone well above where you would expect to see it, and this is why the whole of the visible top of the Blue Mountains is made of sandstone.

Every rock has its own unique way of responding to the weather and the assaults of nature, but Sydney’s sandstone is better than most, and the process is remarkably quick. In some cuttings, less than thirty years old, the original scars of drilling and blasting are fast disappearing beneath the smoothing effects of nature.

Things to look out for include honeycomb weathering, where softer rock has worn away, leaving a shell of harder sandstone, joints where iron-rich water has washed down, depositing iron in the sandstone and making extremely tough surfaces, and exposures of current bedding (also called cross-bedding), the fossilised remnants of 200 million-year-old sandbanks. Along the seashores, in cliffs and on the rock platforms, in cuttings, and on high ridges, there is a wealth of fine detail added to the rock by the action of wind, water, and slow chemical change.

Most of the Sydney sandstone is not tightly bound together. It may seem hard enough as you walk or sit on it, but that is because most exposed surfaces are steeped in insoluble ferric iron. Over time, groundwater with organic content can reduce this to soluble forms which seep through the rock to some point where the iron settles and oxidises to an insoluble form once more.

In some areas, organic matter trapped in the sandstone has produced concentric shells of insoluble iron as waves of soluble iron have diffused out and then settled back into their oxidised form. Later, when the rock has eroded away, an agate-like appearance shows us where the chemical reaction took place, in some former eon.

Hawkesbury Sandstone

Around Sydney, sandstone is as common as quartz. Most of the city lies on Hawkesbury sandstone, with just a few caps of shale on some of the higher ridges. And even if you dig deeper, down past the Hawkesbury sandstone, there are more sandstone beds in the slightly older Narrabeen Series. The rocks of Sydney form a saucer-like basin, and away from the city, the bottom of the Hawkesbury sandstone is above sea level. In valleys and cliffs, north of Long Reef or south of Port Hacking, this second form of Sydney sandstone starts to join the scenery.

Even going up into the Blue Mountains leaves you on sandstone, for as you rise, the Hawkesbury sandstone rises with you. The Blue Mountains formed when the western side of the Sydney basin was tilted one kilometre up into the sky, and so we stay on the Hawkesbury sandstone, all the way to Woodford and Bilpin. After that, the Narrabeen sandstones start to appear once more, and they run most of the way out to Lithgow.

Sandstone has made the city of Sydney what it is today. All sedimentary rock is full of joints, vertical splits that cleave the large beds into smaller blocks, often running for hundreds of metres, slicing down through the geological millennia. These joints, combined with softer and tougher beds, help shape the scenery in sandstone country.

On a small scale, joints let water into the stone, carrying minerals in, and carrying minerals out. On a large scale, the effects of the joints can be quite breath-taking, for most of our valleys started as trickles of water following a jointing pattern. Sydney Harbour and Broken Bay get their unusual “fern-leaf” pattern from a rising sea invading river valleys that followed the jointing patterns of the sandstone. The cliffs along Sydney’s coastline have been shaped by the joints in the sandstone too.

Sandstone has even had a social effect. The sterile sandy soil around Sydney forced the early settlement to spread out, while the sandstone cliffs of the Blue Mountains hemmed the European settlers in for 25 years. Later, as Sydney grew, the pattern of ridges and cliffs directed the paths followed by roads, trams and railway lines, and that made Sydney spread out in strange loops and whorls, quite unlike certain well-planned and mundane cities in other parts of Australia. Later again, the gaps in the sandstone along the coast gave us a marvellous variety of surf beaches.

Oddly, the sandstone has also left Sydney more at risk from bushfires. People have settled the accessible ridges, leaving the deeper valleys full of bush. Ferry travel is another Sydney special demanded by the sandstone. Ferries gave people a chance to settle near the shore during the 19th century, increasing the chance today of a fire taking off and running up to the ridge tops.

Building bridges over the harbour was made more difficult by the high cliffs in so many places. Tunnelling through the rock, as they do in Paris or London, has proved unprofitable. So most tunnels, like Sydney’s “harbour tunnel” are really just buried tubes, lowered into a trench.

The sand for Sydney’s rocks may have come from Broken Hill originally. The sand quite probably made a stop on the north coast along the way, but it has been around Sydney for 200 million years. There are few fossils to be found, and geologists are still arguing about how the sandstone was laid down, but there are some things we do know for sure.
Diagrams explaining sedimentary rocks show beautiful neat layers of sand, laid out horizontally, but more sandstone is laid down, like Sydney’s, in river deltas where the sand is moved, sorted, shoved and pushed before it is buried. There are few neat horizontal layers in this type of sandstone. Roadside cuttings near Sydney reveal all sorts of sand banks and ‘washouts’ in the ancient delta, where a wandering river has passed through the sand, leaching and sifting and sorting. The sand left behind in the old stream beds is purer than usual, lower in clays and iron.

This gives us a sandstone which is more strongly bonded, with less clay to weaken and give way. A filled river bed of pure sand makes a fine hard rock, smooth on the surface, free of the ironstone contortions that may be seen in rocks close by. The Eora people of the Sydney region knew this good sandstone when they saw it, just as a modern artist recognises a good canvas. They made good use of it for their rock engravings , all over Sydney.

A thin layer of resistant sandstone will stand up to the forces of the weather. Below it, softer beds may fret and wear away, undercutting the resistant bed and leaving a vertical drop for a waterfall. When the decay reaches a joint, the blocks above will come crashing down, leaving vertical cliffs, and fresh rock for the weathering process to start on, all over again.

When there are several long-lasting beds at different levels, each one may act like a small waterfall, producing a tumbling cascade of toughened terraces and spray-covered slopes. In this case, the horizontal toughening has more influence than the vertical weakening of the joints.

Honeycomb weathering used to be blamed on sea spray soaking into rocks. People thought that when the spray dried, salt crystals formed, and sand grains were wedged off, one by one. Yet we find honeycomb weathering many kilometres away from the sea, and the salt spray would be less likely to get into the deepest hollows where the rock is most actively breaking down.

A better explanation sees moisture gathering in the hollows, and ‘drawing’ soluble salts out of the rock, carrying them to the surface inside the hollows, where salt crystals fret the grains away. But however it is caused, honeycomb weathering offers us patterns of delicate stone filigree, dancing over the surface of sandstone under sheltered overhangs, either of durable and resistant iron-rich sandstone, or the equally durable pure-sand form of the stone.

Plants and lichens dig into the surface of even the toughest sandstone, ripping the sand grains away, one by one. Redgum roots infiltrate the joints and burst the stone asunder, tumbling boulders down into gullies where floods can rush over them, wearing the stone back to sand again. Through it all, the silica grains, those tiny rounded pieces of quartz, roll through the eons. The sand grains remain as sand grains, ready to pass through the cycle all over again. They are chemically unchanged and physically constant, shuttling their way between sand and sandstone.

Sooner or later, the sand that has fretted away will settle in water somewhere. If these sand beds are buried deeply enough, the sand may melt and go back to Square One as granite, or it may just form sandstone again. Either way, it ensures that the intelligent beings of the planet earth, a hundred million years from now, will be able to enjoy the same wild sandstone shapes we find today.

This was originally an article written for Geo magazine, and published in the July-August issue of 1995, volume 17(4), 88-98, accompanied by Andrew Smallman’s exquisite photos of the sandstone of Sydney.

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It was last revised on July 20, 1996

It was created by Peter Macinnis — macinnis@ozemail.com.au
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