McDermid Rock

McDermid Rock is very very old - the granite from which it was formed dates from more than 2.6 billion years ago.
The same is true for Disappointment Rock, Bank Rock, Wave Rock and many other similar outcrops in the Yilgarn Block, the geological feature that underlies much of southern Western Australia.
The shape you see today is the result of countless earthquakes, tremors and upheavals - and more than a little weathering - through that vast span of time.

Like miniature gardens, "islands of vegetation" are scattered across the rock face, supporting a surprisingly rich array of life forms.
They develop wherever soil can gather in pockets, depressions or fissures in the rock.
As soon as the soil build-up is adequate they are colonised by plants that are often unique to the particular rock on which they are found.

In this case Calothamnus tuberosus is present, as is the well known "resurrection bush" (Borya constricta), but there are literally dozens more species in this one small "rockery".
Such is the opportunism of these plants that they will seize upon the slightest prospect of life and - in a wink of time - a natural garden is born.

In Australia a rock basin is known as a gnamma, an Aboriginal word meaning "rock hole".
They are generally about a metre in diameter and roughly 15 cm deep - though they can be several metres across and quite deep.
So important were they in traditional lifestyles that Aboriginal people are known to have visited certain sites regularly, to keep favoured gnammas clean.

Some gnammas - especially larger ones - hold water for a long time after a good rain.
These long lasting gnammas become focal points for life (both human and animal) in times of heat and drought.

They form through the weathering action of water trapped in small depressions in the rock surface.
This water gradually breaks down and rots the rock that it is in contact with, enlarging the basin as it does so.
They often form on joints in the rock, where water can more readily penetrate the surface.

While pan "gnammas" are relatively common on McDermid Rock this pan is unusual in that it is in effect two pans, a "pan-in-a-pan", a secondary pan that has eroded in the floor of the larger basin.
This is most likely the consequence of differing layers in the rock strata.

The top layer is known as "laminated granite" - many thin sheets stacked upon each other. This has eroded out to form the larger outlying pan, the base of which is grey granite.

The inner, hemispherical pan has formed in this "substrate", despite its significant hardness.

When full of water these pans burst into life, hosting various crustaceans, frogs and other creatures, all adapted to the short life cycle demanded by the temporary pool.

On the high shoulder of McDermid Rock one can see a balancing boulder perched up there, like a flea on an elephant.
How did it get there, and why does it not tumble off?
Isolated boulders such as this are most likely the result of the break up of larger sheets into blocks, and the subsequent weathering and rounding of the edges. This theory may be supported by the jumble of broken blocks and slabs around the base of the main rock.

The balancing boulder must have been either harder or more firmly "planted" than all its neighbours, to have remained alone on the face. Whatever the reason, it has clung to its perch to become the signature image of McDermid Rock.
Let's just hope it doesn't choose today to fall!

This scattering of loose boulders may well be the "siblings" of the balancing rock up there on the rim.
It is likely that sometime in the distant past - perhaps many millions of years ago - McDermid Rock wore another layer (or sheet) of granite over what is now the surface.
Fracturing, weathering and then a good seismic shaking might have tumbled these rocks down the slope.

Notice the surface that almost looks like a series of chickenpox scars?
At least some of these indentations are plunge pools, caused by water tumbling down the face in miniature waterfalls.
Where the stream strikes the rock it gradually gouges a pock-mark - which, when well developed, is known as a plunge pool.

Most of the "shaping" of the various elements of McDermid Rock are the result of some form of weathering. Water is a major cause, whether liquid or frozen, flowing or static.
Chemicals and biological organisms also contribute to weathering, usually in tandem with water and wind.

This massive split rock is probably the result of what started as no more than a thin fracture. This split perhaps formed along a weakness in the basic rock structure or may have been the result of seismic activity way back when this boulder was still part of a much larger slab.
More than likely the crack was then prised open by the immensely powerful action of ice forming and then expanding in the split on cold nights.
Eventually - crack! - the whole thing broke asunder, just like the two halves of a dropped watermelon.

Sometimes the terminology of geology can be daunting indeed!
This line of large ragged crystals is known as a pegmatite dike (dyke).
Pegmatite is a coarse grained igneous rock with exceptionally big crystals, formed from a magma that contains a high proportion of water.
And a dike is a vein injected across the base structure of the host rock. So this stripe of chunky crystals running up the rock face is a pegmatite dike.


Some of the earliest Europeans to pass through this country were the sandalwood cutters. Following hard on the heels of gold prospectors and explorers they began combing the arid inland way back in the mid 1800s.

It was trees such as the lone specimen of Santalum spicatum nestling in the cleft in the rocks that they were looking for.

The delightful aromatic timber from the sandalwood tree is highly sought after in Asia, where it is made into joss sticks used in religious ceremonies.

During the 1840s up to 45% of the fledgling Swan River Colony's export income came from sandalwood. It is still cut and exported today, but under tight control by the Department of Conservation & Land Management.
Fortunately for us (and for the tree) this is the one that got away ...




No liability for timeliness, integrity and correctness of this document is accepted.
Last updated: Friday, 04.06.2010 12:47 PM