Sandstone thoughts

South Head/Eastern Suburbs, Sydney

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-rich sandstone 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.

Cliff and rock platform, South Head

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.

Bedding, Lion Island, Broken Bay

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.

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 .

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.

Washout, hawkesbury sandstone, West Head Road, KCNP

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.

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.

Inner North Head and Old Man's Hat: the joints make the vertical surfaces

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 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.

Cross-bedding and honeycomb weathering.

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.  The roots of gum trees and the related Angophora trees 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.

Walking from Bondi to Watson's Bay.

Looking north on the outside of North Head, early morning.

Looking north on the outside of North Head, later in the day.

The Royal Botanic Gardens and climate change

We had to get to the Australian Museum a few weeks back, but the night was warm and we had some time to spare, so we took the scenic route.  From the ferry, we strolled around the harbour's edge to the Opera House, and on through into the Royal Botanic Gardens.  It took us a little longer, but we knew in advance that it would be worthwhile.  It always is, and not just because my wife and I both took our first degree in botany.

It is suffering right now, from one of the effects of climate change, but I will come to that in a moment.

Sydney's Gardens were founded in 1816.  They aren't the oldest in the world, for Padua, Paris, Edinburgh, Cambridge, Rio de Janeiro and Munich are all older, but few gardens could have such a fine setting.  Certainly the greatest of them all, Kew, has less to offer in the way of vistas.

It was probably fitting that Australia should establish Botanic Gardens so early, since so much of the history of white exploration and settlement was tied up with botanists and naturalists.  Not only was there Sir Joseph Banks, who sailed with Cook to Botany Bay, but Robert Brown (who told physicists about what we now call ‘Brownian motion’, and who also told biologists about the cell nucleus), Allan Cunningham, and any number of other botanists who spent time in the young colony: you can find traces of some of them in the gardens.  The tradition was later to lead Charles Darwin, T. H. Huxley to our shores, and the naturalists are still coming, even today.

At first, the gardens were seen as little more than a glorified and official vegetable garden, but slowly the plant collection developed.  The name of the water near the gardens is a reminder of this earlier role, as it is still called Farm Cove.  Six months after the first settlement, there were ‘nine acres in corn’ at the first farm, but the soil was poor, and the crop was a failure, so agriculture moved elsewhere, and the area became a centre for acclimatising new plants from overseas.

If you visit the Gardens and find a poor and unkempt looking garden bed, you are looking at a reconstruction of this first ‘farm’, but that was soon displaced by other activities.  People who wanted to grow oaks to remind them of ‘home’ could obtain acorns there, and those with damp patches on their farms could get bamboo plants.  By 1816, it had become more of a plant collection, and by 1825, there were more than 3000 plant species in the collection.  Many of our worst modern plant pests came into the country this way.

Even so, Allan Cunningham, whose tomb is to be found in the gardens, still referred to it slightingly in 1838 as the ‘Government cabbage-garden’.  The science of botany had come to take second place to horticulture, with convicts being trained in practical farming there.  The gardens declined until 1848, when Charles Moore, a trained botanist, started a 48-year reign as Director of the Gardens, and from then on, they were to be truly Botanic Gardens, as well as being the home of the National Herbarium.  This is a research collection of dried plant specimens, used in the identification of unknown species.

Moore's successor, Joseph Maiden, was also a botanist, and it showed: one of Maiden's daughters was actually named Acacia!  This is the scientific name of what we plebs call a ‘wattle’, and I have this theory about the family sitting around, asking each other, ‘What'll we call her, what'll we call her?’  Well, it's only a working theory . . .

Botanic or Botanical Gardens?  Most Sydneysiders call them the ‘Botanical Gardens’, so I asked a friend who works there, and I was told that ‘botanic’ is the old-fashioned adjective which is part of their official name.  The gardens are indeed ‘Botanical Gardens’, but not in formal matters, it seems.  Still, Sydney people aren't all that formal at the best of times.  Even so, to most people there is only the one ‘Botanical Gardens’, and that is the one at Farm Cove, just to the east of the city.  As you will read in the next paragraph, we actually have three official ‘Botanical Gardens’.  I will say more about the other two branches of ‘The Gardens’ some other time.

Entry to the gardens is free to all.  There may be an entry fee for one or two spectacular temporary displays, but it is well worth paying to see the contents of these.  On the other hand, if you are in a hurry or on a tight budget, there is a great deal more to be seen for free, out of doors, including more than a hundred bird species: you can hope to see at least twenty on any given day.

Sadly, you will also see evidence of climate change there, and two days ago, I went there to record a tidal incursion. The highest "king tide" of the year comes when the planet is closest to the Sun, which happens on January 2. On January 3 this year, there was a high tide in Sydney of 2.07 metres, and the following day, when I was there, it was 2.05 metres.

Along Farm Cove, there is a sandstone wall to stop waves breaking in, and at the bottom, there are drain holes to let rain water out, but if the tide is high enough, the sea comes in. The wall is 86 cm: remember that number.

Here is what I found:

Now just to show how bad it might have been, here is some wave action at the western end, near the Sydney Opera House (or for Sydneysiders, near Man o' War Steps).

The wall is much higher there, but it was easily overtopped. Given the right wind, this would have happened all along. Now here is what I photographed the day before, when the tide was 2.07 metres: this was at North Harbour Reserve, where I have played for 70 years, and I have asked old people, when I was young, about its history.

In the 1930s, the local civic fathers saw fit to "reclaim" a former bay to make North Harbour Reserve. They built a seawall, higher than the highest tide, and pumped in mud to make a level playing field. Some years ago, as the sea started running in, they raised the sea wall and topped it with a path. Well, that defence is failing.

Our local champion of scientific idiocy, Tony Abbott, trumpeted overseas that he had looked at local historical photos of the area, and could find no evidence of climate change.

I have lived and played around North Harbour for 70 years, and I rather think this counts as evidence. That's a king tide that could easily have been 15 cm higher, if there had been a sou'easter blowing: the old men told me all about that happening, and floating one boatshed away. We got off lucky, this time, because the weather was mild.

In 1957, when I was a teenager, a super-high tide at our standard spot for the harbour (Fort Denison) was 6 feet 1 inch, or 185 cm: now, 60 years on, it is 22 cm higher. Two days ago, the CSIRO published estimates that by the end of this century, within the likely lifetime of my grandchildren, tides will be between 45 cm and 88 cm higher than now.

Those tides will be unstoppable, all around the world, and if the upper level is reached, that increase will overtop the highest point of the Gardens' stone wall by 2 cm. Seawater will extend well into the Gardens, killing and ruining our heritage.

One wonders if our do-nothing, look-at-me polly will seek funding to raise both the seawall and the park surface, or if he will just scream "fake news!".

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