South Head/Eastern Suburbs, Sydney |
Cliff and rock platform, South Head |
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.
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.
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, hawkesbury sandstone, West Head Road, KCNP |
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 |
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. |
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. |
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