Sci: Sandstone matters

Sandstone cliff, near Wentworth Falls, Darwin Walk.

Sydney is a sandstone city. We have cappings of shale here and there, and if you dig deep enough, you will find coal below the sandstone, but looking at the surface, looking at the hills and the headlands, Sydney is undoubtedly a quartz-richsandstone city. Quartz is silica, silicon dioxide to the chemists. Silica is a very common mineral, dirt common, even. By itself or in combination, silica makes up more than 50% of the planet earth. Sand is silica, flint is silica, even granite is mainly silica or silicates.

Sand grains from North Head, almost pure quartz.

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 quartz grains 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 any further pounding. After that, they just roll along, getting a little bit rounder, 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 is still quartz, 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. In time, the sand becomes sandstone, waiting deep within the earth for erosion to uncover it once more.

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.

Sandstone with quartz pebbles.

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. 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. 

Liesegang weathering, Dee-Rub-Bin (Hawkesbury River).

Hand specimen of sandstone, with Liesegang markings.

In some simple chemistry, the rust is reduced to soluble ferrous iron which drifts 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 make fantastic patterns in the stone, for the spheres will eventually be revealed as complex rings and ovals of tougher rock, etched and ridged and sculpted into the surface of the stone.

Diagrams explaining sedimentary rocks show beautiful neat layers of sand, laid out horizontally, but more sandstone is laid down in river deltas where the sand is moved, sorted, shoved and pushed before it is buried, and there are few neat horizontal layers. Sometimes, there is even cross-bedding. We will come back to this in another blog entry.

Cross-bedding, Sandy Bay.

The sand may have come from Broken Hill originally, it may well have made a stop or two along the way, but it has been around Sydney for 200 million years. Roadside cuttings around Sydney reveal all sorts of sand banks and washouts in the ancient deltas, 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.

Washout, exposed in a cutting. Ku-ring-gai Chase National Park.

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.

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.

A thin layer of resistant sandstone stands 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 gentle spray-covered slopes. In this case, the horizontal toughening has more influence than the vertical weakening of the joints.

Honeycomb weathering, Bondi.

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.

Three lichens on sandstone.

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

Sooner or later, those sand grains that have fretted away will settle in water somewhere. If these sand beds are buried deeply enough, the sand may melt and form granite, or it may 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.

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