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Thursday, 28 September 2017

Sandstone facts

Hawkesbury sandstone, Fairlight Sydney, showing bedding,
cross-bedding and vertical joints.
The city of Sydney is the child of its geology.  That is the first lesson you must learn if you are to understand our scenery.  Nearly all of the rock you see exposed around Sydney will be sandstone.

The sand that would become this sandstone was laid down in a time when plants were ferns, reptiles were becoming dinosaurs, and mammals were only just being thought about.  True, 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.

The sandstone has also influenced local architecture, since it was readily available as a building material, but the beauty of the stone carries a high price.  Many local buildings of carved sandstone are now beginning to deteriorate as the stone frets and falls apart, returning to sand again.  I don't think I have once seen the 19th century buildings of the University of Sydney free of stonemasons' scaffolding in over thirty years, and the Anglican St Andrew's Cathedral, near Town Hall, is also suffering the ravages of time.

At the Heads, in the cliffs at the harbour's mouth, there is sandstone from sea level and below, 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.  If you look down, you will see a few lenses of shale scattered through the cliff face, almost lost among huge layers of sandstone.

The heat of the lava 'baked' the sandstone, changing it to
quartzite. This is called contact metamorphism.
There are a few remnants of more recent volcanoes which have pushed up through the older sedimentary rocks, including one volcanic neck right on the coast north of Bondi
, but the shale lenses and volcanic rocks are very much the exception, and Sydney is Sandstone City.

North and south, the bottom of the basin starts to curve up.  This curving brings the buried shale beds up to show at sea level near Narrabeen in the north and at the Royal National Park to the south.  To the west, the surface of the land rises as the rocks curve up, so right up into the Blue Mountains, a kilometre in the air, the surface rocks are still Triassic sandstone of the Narrabeen series that lies under the Hawkesbury sandstone on the coast.  Beneath that again, there are beds of Permian coal which are nominally about twenty five million years older, but these lie hidden under a ponderous overburden of sandstone.

These lurking coal beds bob up above sea level when you go further away, at Wollongong in the south, and Newcastle in the north, and they break the surface at Lithgow in the west, once you drop down off the mountain tops.  Around 1900, coal was actually mined in Balmain, a Sydney suburb, thousands of feet below sea level, with the coal starting at about 3000 feet (900 metres) from sea-level, with the bottom of the coal lying between there and some 4500 feet (1250 metres).

A cap of younger Wianamatta shale tops off some of the higher sandstone ridges, giving patches of rich soil.  The early development of Sydney was partly determined by people moving out along these ridges, either for farming, or for brick-making clay.  The roads, the farms, and the railways all followed the ridges.

But almost the whole of Sydney's vegetation and the topography of the harbour and the north has been determined by the Hawkesbury sandstone, and it is there that we must concentrate our attention.  The plants of the Sydney region have 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.  Our plants either live on the smell of an oily rag, or they use guile and cunning to gather what they need.

Sundew, Drosera spatulata.
Some of them have allies, fungi and bacteria, living in their roots, able to make nitrates for them out of the nitrogen in the air.  Sundews in the swamps trap passing insects on sticky leaves, and carefully digest them with enzymes, extracting valuable phosphorus and nitrogen.  Grow these plants in good soil, and they will stop making the sticky traps.  When they die, these plants put their hoard of minerals back into the common pool for all the other plants to share.

Usually, cliffs wear away fastest at the top, forming gentler and gentler slopes.  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-line vertical, like the cliffs of Sydney's headlands, and some of the inland cliffs of the Blue Mountains.

The whole of the Sydney area was once a flat coastal plain that was lifted up to a height of some hundreds of metres — either that, or the sea level fell, but the effect was much the same.  Small streams on the new plateau ran along the rectangular joint-lines, and cut down almost to the new sea level, making a fern-leaf pattern of valleys at right angles to each other.  Then the sea rose, drowning the deeper river valleys and giving us the rich structures of Sydney Harbour, Broken Bay, and Port Hacking.  Botany Bay has a shape that depends more on the placement of recent sediment, though the Georges River, which runs into the bay, shows the same basic pattern.

By controlling the road and railway routes, the Hawkesbury sandstone determined where people would live and build their homes.  The deep sandstone valleys were left largely alone, for roads were too hard to build, and the ground too precipitous for easy building.  Now, when bushfires come in the summer, they spread through the bush in the valleys and rush up the hills to sear the houses above.

The sandstone has also had a strong influence on public works.  Steep-sided drowned river valleys demand bridges for road transport.  Worse, the sandstone is very hard to carve or tunnel through.  The chalk of Paris and the London clay, make underground railways much more feasible in those cities, but only the very centre of the city sees our trains going beneath the surface.  Sydney is a city of bridges, with just the occasional tunnel, all ruled by the sandstone that lies beneath our feet.

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