Sandstone thoughts

Quartz is silica, silicon dioxide to the chemists. Silica is a very common mineral, dirt common, you might say. By itself or in combination, silicon dioxide makes up more than 50% of the planet earth. Sand is silica, flint is silica, even granite is mainly silica or silicates. Silica, or quartz, is a very common mineral indeed.

Yet quartz is also a most uncommon mineral. You can scratch the toughest steel with a piece of ordinary glass, you can scratch the toughest glass with quartz, but very few things will scratch quartz. Quartz is uncommonly hard.

This combination of the common and the uncommon explains why sandstone is so easy to find, all over the world. As sun, wind, frost, snow and water all worry away at the rocks, some minerals break down to clays and salts, but quartz grains stay as they are. As the other minerals fall apart under the stress of the weather, quartz grains just fall to the ground and join the soil.

As the soils blow around, softer minerals are crushed to dust, but silica grains just get less angular as they rattle against each other. Acids in the soil destroy other minerals, but the bits of quartz just lie there, inert and uncaring. When a flood washes soil in a raging torrent down to the sea, the quartz grains may crack until they are too small to be affected by further pounding, but after that, they just roll along, getting a little bit rounder. They are still silicon dioxide, unchanged and unchanging.

Sand travels till it reaches the sea, where some grains wash up on the seashore. They get rounder still, as they roll and pound along our beaches, but still they do not change. Chemically the sand remains as silica, uncommonly tough, and incredibly common.

Bury these sand grains under a kilometre of sediment, and they will settle a bit closer together. Give the buried sand long enough, and a few atoms will wander and fall into a new place, linking two grains together, bridging the gaps. If water soaks through the sand, crystals of dissolved minerals may form in the crevices, locking grains together even more. In time, the sand becomes sandstone, waiting deep within the earth for erosion to uncover it again.

But sandstone is not as tough as pure quartz. The fragile bonds between the grains can be broken, the minerals in between can dissolve out, and the grains can be wedged off, one by one, just as soon as the weather reaches the stone, within a few metres of the surface. In time, weathering will shape the sandstone into new and marvellous shapes.

Pure silicon means big money to the techno-whizz kids in Silicon Valley. They "dope" their pure silicon with a few impurities, tiny amounts of other elements, just enough to make their valuable pure silicon truly wonderful.

Traces of impurities are just as important in the sandstone in all the silica valleys of the world. A few bits of plant stems or leaves, or a tiny dead animal, are all the impurities you need to get started, along with a bit of rust.

In some simple chemistry known to any HSC student, the rust is reduced to soluble ferrous iron which seeps slowly through the rock until it is oxidised again to ferric iron. This chemistry makes spheres of tough iron-rich stone, waiting deep inside the sandstone. Uncovered, they will make fantastic patterns in the stone, for the spheres are eventually revealed as complex rings and ovals of tougher, more resistant rock, etched and ridged and sculpted into the surface of the stone.

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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