Life
Page 9
If you think about it you will see that this imagining reveals a great biological truth—that the ecosystems of the land and sea in the Sydney region are utter opposites, organised as mirror images of each other. The land forms a food pyramid whose broad base is made of plants. Feeding on these are fewer herbivores, and feeding on them in turn are even fewer carnivores. That’s why you will get snagged land-fishing long before anything takes your bait. The seas are different because their food pyramid stands on a tiny base of plant life, which supports carnivores in huge numbers. Thus there is relatively little phytoplankton, algae and kelp existing at any one time. Balanced on this pinprick of plant life is a moderate number of marine herbivores, many of which are microscopic, though a few such as oysters and blackfish reach an edible size. On top of these herbivores in the theoretical food pyramid is balanced a vast number of carnivores. These include most of the fish recreational fishermen are familiar with—from jewfish to flathead and bream. Were it otherwise, fishing as we know it simply would not exist.
Sydney’s sandstone region is an extreme kind of land environment, for it supports a plethora of plant species—indeed it stands in the top dozen or so environments on the planet for plant biodiversity—yet it supports fewer animals than most. Thus its food web structure is as different from the sea as any land ecosystem gets. Its soil is so poor that even the miserly koala has a hard time making a living, for most of the eucalypts growing on the sandstone produce leaves that are not nutritious enough to sustain it. Sydney’s harbours and bays, in contrast, are relatively rich, for there fresh and salt waters meet, and rocky refuges abound. This difference between land and sea has meant that for as long as people have lived in the sandstone region they have looked to the sea for sustenance. The people of Sydney are and always have been a maritime people who do not fear to go to sea in their craft. A very strange stone indeed lies in Sydney basements. The story of its origin and properties is an intriguing one. Imagine standing on a vast floodplain, bigger than any you’ve ever seen before. From horizon to horizon stretch meandering channels filled with ripples up to a metre high, testimony to the vast volume of water that sometimes flows here. The date is about 230 million years ago. The place—Bennelong Point, where Sydney’s Opera House now stands. The significance? We are looking at the Hawkesbury sandstone in the making. It’s the rock that will in turn make a city.
No city has been as profoundly influenced by its rocky foundation as Sydney, for its sandstone has given form and colour to its finest buildings, shaped its economy, guided its spread and protected its natural jewels—the rainforest gullies, coves and beaches made inaccessible to builders by its steep bluffs.
Sydney lies atop six kilometres of sandstone and shale, and all of it was laid down at a time when the world’s first dinosaurs, mammals, ginkgos and pine trees were coming into existence. It was a temperate, wet world, a time when leafy swamps flourished. One day their debris would give the Sydney basin its coal mines.
Two hundred and thirty million years ago the Sydney area was hundreds of kilometres inland—as far from the coast as Broken Hill is today. It then lay in a vast valley, while to the east the highlands of what are now New Zealand and New Caledonia rose out of a prototypical Pacific Ocean. The entire continent lay well south of its present position and was firmly attached to Antarctica.
One of the enduring mysteries of the Sydney sandstone is just where the tiny grains of sand that constitute it came from. Geologists employ a handy trick in determining in which direction ancient rivers flowed (and thus from where they brought their sediment). They look for the remains of ancient ripple marks. These marks are very distinctive and are readily seen almost anywhere in the Sydney sandstone. They look like closely spaced lines running through the rock at an angle, something like this: \\\. These marks are left behind when the ripples move forward, just as waves do in water. Each ripple has a gentle slope (which faces upstream) and a steep side (downstream). The sand grains are pushed up the gentle slope and then fall down the steep side one by one. The lines in the rock are the steep faces, each covered by succeeding falls of sand.
Once you understand this you can never get lost in Sydney as long as you can see the rock. That’s because the highest part of the lines you’ll see always face approximately south, and the steeper the lines are the closer they are to facing true south. Even underground these ripples of the ancient river will guide you.
The ripples tell geologists that Sydney’s sandstone must have originated in the south, but just how far south no one quite realised until a sophisticated means of determining the ages and origins of sand grains became available. Dr Keith Sircombe, a geologist working at the Australian National University, has examined hundreds of grains from the Sydney sandstone using a technique called SHRIMP (Sensitive High Resolution Ion MicroProbe). Sircombe has discovered that most of the grains are derived from rocks that formed between 500 and 700 million years ago, far to the south of Australia in what is now the eastern Antarctic.
We can only imagine the river that brought these grains to rest, for it is long vanished. Its vast fossilised floodplain, however, indicates that it was the size of the Ganges or larger and its headwaters lay in the high mountains of Antarctica. As it flowed north along what is now the east coast of Australia it lost velocity. By the time it reached the Sydney area it was too feeble to transport sand grains more than a few millimetres in diameter, so the stone is composed of remarkably uniform grains of about that size.
David Roots, a geologist, explained to me that parts of the sandstone are such pure silica that were it not for iron stains it would be virtually clear. Imagine being able to see from the Harbour Bridge to Parramatta through crystal-clear rock. Several hundred million years ago the sands were buried deep in the earth’s crust, where they were compressed and heated until they formed the solid stone we see today.
By 150 million years ago the great Antarctic river had stopped flowing past Sydney and the region was watered by streams whose headwaters lay in what is now New Zealand and New Caledonia. As they flowed past the Sydney area towards Australia’s great inland sea (which then occupied the continent’s heart) these ancient rivers cut into the sandstone to form channels, some of which are probably still occupied by waterways today.
These west-flowing rivers were also fated to be interrupted, for 90 million years ago the Pacific Ocean would finally come to Sydney as New Zealand and New Caledonia were torn from eastern Australia. Continents are broken up by a process called rifting. Heat from deep within the Earth boils up along the line of the rift, causing a ribbon-like bulge in the land. Then the bulge collapses at its centre, forming a series of vast, rocky steps leading down to a central valley. As the land on either side pulls apart, this valley is eventually filled by the sea. In the Sydney area the remains of the steps formed during this process can still be seen today, along the Lapstone escarpment where the Blue Mountains jump up from the Cumberland Plain, and along the coast itself.
This process of bulging and collapse reversed the flow of the region’s rivers (which now flowed east towards the newly created Tasman Sea), and cracked the sandstone in ways that dictated the position of harbours, coves, ridges and creeks. In essence, it laid Sydney out on a primitive, natural grid system that was profoundly to affect the city’s development.
The Hawkesbury River, about forty-five kilometres from the harbour itself, is a most curious waterway, for its course describes a large semicircle that encloses the Sydney region. It follows this peculiar path in part because the direction of flow of the river has been in places reversed. Some of its headwaters still run westward, but its lower section now drains to the east, probably in a valley cut by west-flowing rivers over 90 million years ago.
The peculiar course of the Hawkesbury has deprived Port Hacking, Botany Bay and Sydney Harbour of significant catchments, for all are hemmed between the sea and the narrow arc of the Hawkesbury’s flow. Because of this, very little silt flows into the harbour and it re
mains remarkably clear and deep, even close to shore. It was a feature that was important to Aboriginal fishermen, who speared fish in the clear water, and it also attracted the attention of the first European settlers, who could anchor their ships metres from the land.
One other exceedingly peculiar characteristic of Sydney Harbour is that as one goes further downstream the cliffs become higher and the topography more rugged. Thus the land around Parramatta is formed of relatively gentle and rounded hills, while North Head forms a startling precipice. This is exactly the reverse of the common pattern for waterways, which usually originate in rugged mountains and terminate on plains. This peculiar characteristic of Sydney Harbour is probably due to the ancient tilting of blocks of the continent as they subsided during the rifting process.
Sydney Harbour’s principal catchment is the insignificant Parramatta River, and geologists have long wondered how this tiny stream could have cut such a vast harbour out of the solid sandstone. The answer is time, for the stream has been on the job for tens of millions of years, removing the sandstone grain by grain until a huge chasm was created. Parts of the harbour are quite deep, and as streams can only cut into the rock at sea level or above, some of the cutting must have been done when the oceans were much lower, such as during the last ice age.
From this it is clear that Sydney Harbour has not always held seawater. The last time it was dry was just 15,000 years ago when so much water was frozen into ice at the poles. Then the ocean was 140 metres lower than at present and the sea lay thirty kilometres to the east of the heads. The harbour would have looked like a valley in the Blue Mountains or the wetlands of Kakadu. By then Aboriginal people had already occupied Australia for 30,000 years or more and they doubtless hunted on the grassy flats as the sea withdrew, then fished over them as it flooded back in again.
We owe the construction of Sydney’s Harbour Bridge, at least in part, to ignorance of this ice-age history. In 1890 the commissioners charged with examining the options for linking the north and south shores rejected a tunnel because ‘so little is known as to what the waters of the harbour hide from view’. Likewise they rejected the option of placing piers in the water to support a series of shorter and lower spans with a swing bridge in the middle, because they lacked geological data on the nature of the seabed. What worried the commissioners in both cases was the depth and distribution of the ice-age sediments that filled the old valley cut by the Parramatta River. And so they set about the seemingly impossible task of constructing a single span bridge tall enough to allow a ship with a sixty-metre mast to pass underneath.
The Sydney Harbour Bridge appeared in the mind’s eye long before it was made a reality. Erasmus Darwin, Charles’ grandfather, was so moved by the potential of Port Jackson that in 1789 he wrote a poem eulogising the future bridge to adorn the future city. Darwin (who, unlike his grandson, never visited Australia) prophesied of the infant Sydney Cove:
There, rayed from cities o’er the cultured land,
Shall bright canals and solid roads expand.
There the proud arch, Colossus-like, bestride
Yon glittering streams, and bound the chafing tide;
Embellished villas crown the landscape scene,
Farms wave with gold, and orchards blush between.1
It was not until 1923 that work commenced on the gargantuan task of construction. The arch was finally closed on 30 August 1930 and the bridge opened for traffic on 19 March 1932, in the midst of the deepest economic depression Australia has ever known. In 1961 the structure was floodlit, and today Sydney is unimaginable without it. Why did the harbour it spans form where it did, and not a few kilometres to the north or south? To answer this puzzle we must study cracks. Look at any flat, weathered surface of Sydney sandstone and you’ll notice a series of narrow fissures in it. One curious feature of these hairlines is that they run predominantly in two directions; one lot paralleling the coast and running roughly northeast–southwest, the other crossing these at ninety degrees. These cracks sometimes form a pavement full of little squares, like a mosaic, a fine example of which can be seen below The Gap at South Head. This pattern is also repeated at a gigantic scale, and it is these very large cracks that have guided the flow of rivers and creeks. Warragamba Dam, west of Sydney, occupies one great coast-paralleling crack while its many tributaries, which meet it at 90-degree angles, fill the other set of fissures. The watercourses that followed such cracks eventually dug the harbour and its tributaries, giving the waterway the complexity that even twentieth-century development is forced to follow.
The vegetation the early Europeans found growing on the Sydney sandstone both delighted and appalled them. In 1770 Joseph Banks was amazed by its diversity, and James Cook changed the name of his new discovery from Stingray Bay to Botany Bay to celebrate the discoveries made there. Eighteen years later, however, when the First Fleet arrived, the hungry settlers realised in despair that this magnificent vegetation offered little sustenance. They found no significant fruits, roots or berries growing amidst the botanical profusion, and they never learned to suck the honey-filled flowers as did the Aborigines. To the First Fleeters the sandstone flora seemed to gratify all the senses but taste. It was a wet desert that left a man starving in a visual garden of Eden. Sandstone was even to figure in the vocabulary of these first European inhabitants, as the term was applied to convicts who could not endure their treatment in this harsh and weird environment. Sydney gets about a metre of rain per year, yet the soils of its sandstone are often parched, for the water drains away almost as soon as it falls to ground. Where a layer of humus builds up the runoff is retarded, but here another factor comes into play. Rock beats water, but so does fire, for fire burns humus. For millions of years the infertile, rapidly draining sandstone has promoted the evolution of a hardy flora, which comprises one of the most intriguing botanic realms on the planet. There are 1500 species of plants growing within a 150-kilometre radius of the city, including the brilliant red waratah and gymea lily, whose blooms have been the pride of the bush since Aboriginal times. It’s a region full of biological mysteries. Why, for example, should the gymea lily be absent from the area bounded by the harbour’s north shore and the Hawkesbury, while it flourishes elsewhere? How did the wollemi pine survive its five-million-year seclusion, hidden in a single canyon in the region’s northwest, and why do waratahs grow as patchily as they do? Tragically, given the present rate of development, changes in burning and the effect of introduced species, much of Sydney’s flora will be dramatically altered before it becomes well studied.
The region’s floral diversity and spectacular blooms have been nurtured by the sandstone’s curious chemistry, for the soil it produces is so poor that it cannot support rapidly growing, dominant species. Instead, myriad specialists co-exist. Some grow only on ridges, some in slopes, some in wet gullies and some only on shale lenses. Some grow for only a few years after a fire, while others will disappear if a hot fire comes more than once a decade. In short, the flora is adapted to exploit a thousand ecological opportunities, each partitioned by time or space.
The Sydney sandstone is the heartland of those most characteristic of Australian trees, the eucalypts. One of the strongest arguments for the recent World Heritage nomination of the Blue Mountains area is the fact that over 140 species of eucalypt occur in the Sydney region, and they include representatives of all the major divisions of the genus. Some botanists take this as evidence that the sandstone was the cradle of this most emblematic group of Australian plants.
Where nutrients are scarce, plants can’t afford to lose leaves to herbivores. As a result they defend their foliage with a deadly cocktail of toxins and it’s these toxins that give the bush its distinctive smell—the antiseptic aroma of the eucalypts and the pungent scent of the mint bush. When the leaves of such plants fall to the ground the decomposers in the soil often find it difficult to digest them, for they are still laden with poisons. The dead leaves thus lie on the rapidly draining sand until
a very hot spell. Then, fanned by searing north winds, there is fire.
Although fire is the one great natural terror the city must face today, it has not always been so. In 1790 the First Fleeters experienced the kind of summer that strikes fear into the heart of twentieth-century Australians. Temperatures rose into the forties and the wind blew from the north-west as if out of an oven. The heat was so extreme that birds fell dead into the streets and the Europeans succumbed to heat prostration. At one stage a great mob of flying foxes passed by, dropping from the air as they died. For all this, there is not one mention in the early journals of the threat of fire. The reason seems to be that the Aborigines’ firestick farming, where they regularly burnt the bush to create pasture lands for the animals they hunted, had kept fuel loads down. Despite the tinder-dry conditions there was little to burn. Without these burning practices, there is every chance that the infant Sydney would have perished in flames. Given the difficulties with starvation and sickness the inhabitants of the settlement were experiencing it is unlikely that another attempt would have been made to settle Port Jackson for a long time.
Sydney has been repeatedly threatened by far less extreme conditions in the twentieth century. The most recent major fire occurred in January 1994 when hundreds of houses burned, principally in the southern suburbs of Como and Jannali. The risk has been made all the greater by appalling town planning. Many suburbs are laid out along the ridges, while the gullies are densely forested. Given current management of these gullies, it’s probably best to think of the houses perched above them as temporary structures. Sooner or later they will find themselves sitting atop thousands of tonnes of fuel, with a fire raging their way. No one has found a solution to this problem. No one knows how to implement Aboriginal fire policy any more, and some botanists fear that frequent burning will lead to a decline in biodiversity.