The Environment of Sydney

Australia, Island Continent

Australia is unique, as the world's largest island, and at the same time, the world's smallest continent and the only continent which is a single nation. In land area, with 7.6 million km2, it is the world's sixth biggest nation, after Russia, Canada, China, the USA and Brazil. The Australian population, at less than twenty million, is remarkably small, in world terms.

Australia consists of just a single tectonic plate, slowly moving north, and rumpling up the islands to the north as it goes slowly up into the topics. New Guinea and Indonesia have earthquakes, volcanoes, rich soil formed from new rocks, and high rainfall, Australia gets almost none of that. There has been minor volcanic activity in Australia in the past, as seen in the Glasshouse Mountains of Queensland, the Warrumbungles in NSW, and even at Bondi, close to Sydney.

Much of Australia's volcanic activity seems to be a result of a single hot spot, now somewhere under Bass Strait, the sea between the main continent and Tasmania, so there may be some action there in the future - but then again, there may not.

With no volcanoes to speak of and little earthquake activity, Australia is a ground-down plate, a flat continent where Mount Kosciuszko, at 2228 metres (7300 feet) is the highest point in an old, low land, where rivers wind slowly, evaporating as they go, so that much of the inland is dotted with salt lakes, usually dry, like lake Eyre, which covers more than 9000 km2 when it is full.

Australia was part of Gondwana, the great southern continent where marsupials came to rule, and may well have been the last haunt of the warm-blooded dinosaurs, because some of Australia's dinosaurs lived in Victoria at a time when it was very close to the South Pole.

The land and the soils are old, missing many of the minerals that will be found in an area scraped clean by glaciers, or replenished by lava flows. If you want to understand Australia's animals and plants, the geology explains it all. If you want to understand Australia's shape and climate, the geology explains it all.

Sydney rests on the Hawkesbury sandstone. Have you ever wandered a strange city, wondering why it is shaped the way it is, why transport is the way it is? Mostly, it comes down to two things, the water (whether it is river or sea) and the geology. Sydney is largely shaped by its geology, because that determined how the harbour would be shaped.

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.

North Head at the entrance to Sydney Harbour. Surf below North Head, looking north. Sandstone in the cliff, North Head.
Pictures above, from left to right:

At the Heads, there is sandstone at sea level and below, and all the way up to the top of the cliffs. If you visit North Head, you can look back along the near coast-line to the north. As you do, you will see a few lenses of shale scattered through the cliff face, in among huge layers of sandstone.

As you go north or south, the basin starts to curve up. Because of this curving, the underlying shale beds start to show at sea level near Narrabeen and in the Royal National Park. To the west, the surface of the land rises as the rocks do, so that right up into the Blue Mountains, the surface rocks are still Triassic sandstone of the Narrabeen series, discussed below.

As well, the sandstone of the Blue Mountains has been pushed skyward. If you are travelling up into the mountains by road, look for the tiltes rocks of the Lapstone Monocline, just where you start heading up off the plain.

Below the Hawkesbury sandstone (as geologists call it), we find the Narrabeen series, a mixture of shales, sandstones and conglomerates, and then, beneath that, beds of Permian coal that are nominally about twenty five million years older. These coal beds are above sea level at Wollongong in the south, at Newcastle in the north, and they come to the surface at Lithgow in the west.

At the turn of the century, coal was actually mined, thousands of feet below sea level, beneath Balmain, a Sydney suburb. The Sydney Coal Measures lie between about 880 metres and 900 metres (2880 feet and 2937 feet) from sea level, while the Illawarra Coal Measures lying between there and some 1250 metres (4500 feet). It is easier to access at the margins, at Wollongong in the south, where it reaches the surface, and again at Lithgow in the west and in the Hunter Valley to the north, and the mine has since been filled in.

There is a small amount of younger Wianamatta shale lying on top of some of the higher sandstone ridges, giving patches of richer soil. The early development of Sydney was partly determined by people moving out along these ridges, either for farming, or for brick-making clay. Brickfield Hill in the City, just south of the Sydney Town Hall, was one of the earliest sites for exploiting this shale. These days, all you can see there is buildings.

But almost the whole of Sydney's vegetation and the topography of the harbour has been determined by the Hawkesbury sandstone, and it is there that we must concentrate our attention. The first thing to say about the Hawkesbury sandstone is that it is full of joints, vertical planes of weakness which show up in the cliffs and the valley-lines. That may sound a bit peculiar to the non-geologist so here is a quick layperson's explanation of joints.

Usually, cliffs wear away fastest at the top, forming gentler and gentler slopes as time goes by. But when a cliff of jointed rock is undercut by waves or running water, the rock above the cut falls down, breaking off along the joint-plane. This keeps the cliff vertical, like those you can see in Sydney's headlands, and the same effect can be seen in some of the inland cliffs of the Blue Mountains.

When a small shale lens (or a softer sandstone bed) in the vertical sequence is exposed and weathers away faster than the sandstone above it, the same sort of undercutting occurs, and the same sorts of cliffs are formed in this way as well.

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.

Shaping Sydney Harbour
Around Sydney, though, the whole area was ground down by erosion to a flat plain, but then either the sea level fell, or the land rose, and erosion could start again. Most large beds of rock have vertical planes of weakness running through them, and in the Hawkesbury sandstone, there are two sets at right angles, running more or less north-south and east-west.

Under erosive forces, the joints tended to take the water, and cut down faster, so when water began running off the newly-raised plateau, water found its way off through and along the joints, and created a fern-leaf pattern of gullies and valleys.

Then the sea level rose again, flooding Broken Bay, Port Jackson (Sydney Harbour), and Port Hacking, all of which show the characteristic pattern of drowned river valleys.

This has made for a disastrous transport situation: roads have to go up and down hills or over bridges, and tunnels are very hard to cut. Worse, when it comes to putting in railways, it is almost impossible to get reasonable gradients. As Sydney was settled, transport determined where people would build.

This is why Manly got its start, because people could get there by Manly ferry, but there was a different geological effect along the 'North Shore', where settlements spread along the ridge, catching the good soil that derived from a thin capping of Wianamatta shale, the bed that overlies the Hawkesbury sandstone. Because people were there, a train line started near Milson's Point, and worked its way to the far end, while in the inner west, it was tram lines that determined where developments would happen.

Later on, many of the gaps were filled in, but there are still many places where the shores were hard to access, so they were left fringed with bush, which is now able to catch fire in summer and carry fire to the houses above.

The other thing that shapes Sydney is the desire to get a water view. You haven't really arrived until you have a water view. As you travel around Sydney, you will see evidence of this wherever you go.

Geological sculptures

Wind-sculpted sandstone, North Head. Current bedding on Bell's Line of Road, Blue Mountains. The Three Sisters, Katoomba. Pictures: Left: Wind-sculpted sandstone, North Head, centre: Current bedding on Bell's Line of Road, Blue Mountains, right: The Three Sisters, Katoomba.

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.

The environments of Australia

The Soil wherever we saw it consisted of either swamps or light sandy soil on which grew very few species of trees, one which was large yeilding a gum much like sanguis draconis, but every place was coverd with vast quantities of grass.

- Sir Joseph Banks, The Endeavour Journal, May 1, 1770.

The secret of Australia's vegetation, we have since realised, is that it has a specialised ability to recover from fire. In fact, it generally needs regular fire to maintain itself. Where well-meaning do-gooders have prevented all fires, the habitat has suffered by a loss of species. Then, after the inevitable major fire, weeds are able to colonise the niches which have now been left vacant.

If you simply wish to see as many of Australia's plants as possible in a short period of time, give some serious thought to driving to the Mount Annan Gardens branch of the Royal Botanic Gardens, south-west of Sydney, near Campbelltown. This has pleasant picnic spots, and is well worth a full day's visit.

Australia has just about every vegetation type in the world, with the possible exception of tundra (and we get close to that in alpine regions). Here, we will just deal with the main ones you may encounter.

Desert
Dunes
Heath
Dry Sclerophyll
Wet Sclerophyll
Rainforest

Desert

It is a serious mistake to picture the Foreign Legion, trekking over sandhills, camels, mirages, oases, and all the other clichés which make up a Hollywood desert. The Australian deserts are places where people can and do survive quite well, so long as they know what they are doing.

Briefly, the Australian desert is a place where there is life, where there are tough and hardy plants, where the animals are specially adapted, and humans should not wander unless they know what they are doing.

We note that the Australian desert has a strong fascination for some people from overseas, apparently because of Marlo Morgan's "Mutant Message Down Under", which is regarded as offensive fiction by the desert communities of aboriginal people. For an aboriginal perspective, see the links which follow from http://dumbartung.org.au/

Useful reference for young people: The Desert, Peter Macinnis and Jane Bowring, illustrated by Kim Gamble, Puffin Originals, 1997, ISBN 0-14-056123-3

You may also like to see my Flickr collection of central Australian photos. These will show you what a "desert" (arid zone region) is like in Australia.

Dunes

As sand blows up our beaches, it is trapped by the sand dunes lying behind the beaches. Near the shore, you will find pennywort and Spinifex grass, while further back you will find stunted wattles and banksias, all holding the sand down. In the past few years, the authorities have started serious dune stabilisation work, so that many of the dunes are now fenced off, to protect them from the carelessness of people.

The best dunes to examine and to explore carefully are those near Kurnell, where the walk to Cape Baily, while well supplied with snakes in summer, is a pleasant one.

The dune systems along the coast between the Myall Lakes and the ocean, just north of Port Stephens are also impressive. The inland dunes along the Myall Lakes are especially worth visiting.

Heath

The Dobroyd crossroads, Manly-Spit walk. Picture on right: Heathland on Dobroyd Head.

Myself in the afternoon ashore on the NW side of the bay, where we went a good way into the countrey which in this place is very sandy and resembles something our Moors in England, as no trees grow upon it but every thing is coverd with a thin brush of plants about as high as the knees.
- Sir Joseph Banks, The Endeavour Journal, May 4, 1770.

Sir Joseph was, without a doubt, walking on the heath, close to the shores of Botany Bay. Heath is found in the harshest environments, often overlooking the sea, where the soil is shallow and sandy, the water supply is poor, the wind is strong, and where there is often salt spray blowing across as well. Heath is usually no more than two metres high, although given enough time between fires, it can reach three or four metres, at which time most of the species have been choked out. Keep in mind that the stuff known as heath in Europe is quite a lot lower, and that many heath species can also be seen in dry sclerophyll.

Mature heath is mainly a level of shrubs, with an under-storey of smaller plants, mostly herbs and grasses. You can see good examples on top of almost any sandstone ridge in any national park, but especially on the "moors" in the Royal National Park, and on North Head (part of the Sydney Harbour National Park), or on the road to Wollongong, south of Waterfall. Even though it is a harsh environment, the heath is worth a visit at any time - there are always a few species in flower, even in winter, but between August and October, the heath is a mass of flowers, and a must for a visit.

Dry Sclerophyll

The track near Mount Kuring-gai. Picture on right: Dry sclerophyll -- in this case, with a track running through it.

Dry sclerophyll is like heath, but taller, with an added upper layer of stunted and gnarled trees, mostly gums, but with a range of other species as well. The shrubs are now more scattered, but they are generally the same or similar species to those found on the heath. With three levels of vegetation, the vertebrate life is much more diverse.

Like the heath, dry sclerophyll needs fire to maintain its openness and diversity of species. After a fire, most of the species will drop seed into the warm ashes, or they will immediately begin sending up new shoots from inside, where the living plants have survived the tremendous heat of the fire. Like the heath, dry sclerophyll is found mainly on the sandy soils derived from the Hawkesbury sandstone. Dry sclerophyll is what most people think of when they say "the bush", and there are good examples in all of the national parks around Sydney.

Wet Sclerophyll

When the soil is formed from shale or volcanic material, and where the water supply is good enough, wet sclerophyll develops. In lay terms, the easy way to recognise this sort of vegetation is to look for the tall straight timbers that would make good logs. If you see those, you are on good soil. And by the way, that is probably a leech, crawling across your shoe, if you are in wet sclerophyll. Watch out!

The best examples are probably to be seen in the Royal National Park, when you turn into the park at Waterfall and stop, a little distance below the waterfall which gives the town its name, where some of the better areas verge on turning into rain forest. (To find the waterfall, drive slowly, with your windows open, and listen - or stop when the road starts to drop faster.

Rainforest

In the Royal National Park, and in one or two valleys along the northern beaches, there are some remnants of rain forest, Here you will find good water supply, coupled with rich soil, and magnificent trees. Early settlers logged most of these areas, but some of them have now been protected for a hundred years or more, and they look quite healthy. In rain forest, you have the "typical jungle" of the "Tarzan" movies. Tall trees, dangling vines, ferns growing high up in the trees, and the tree canopy overhead is largely closed. For visitors with the time to make the trip, Minnamurra Falls National Park near Kiama has perhaps the best-interpreted example of rain forest within reach of Sydney, although the Forest of Tranquillity, near Gosford, is also quite impressive.

More impressive rain forest will be found in areas along the coast in northern NSW and also in Queensland.

Useful reference for young people: The Rain Forest, Peter Macinnis and Jane Bowring, illustrated by Kim Gamble, Puffin Originals, 1999, ISBN 0-14-037855-3 August 1999. Yes, there ought to be a declaration oif interest there, but there isn't.

One feature of Sydney (and of Australia) in summer is bushfires. This link will fill you in until I do something more. You can also read more about Sydney bush environments here.

External links

Peter Adderley is a mate who does amazing stuff. He lives north of Sydney, and has produced an excellent account of coastal features on the central coast.
This file is http://members.ozemail.com.au/~macinnis/syd/environment.htm, first created on February 28, 2006. Last recorded revision (well I get lazy and forget sometimes!) was on October 17, 2006.

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