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Landscapes of South-west Western Australia

Jim Barrow

Two obvious things can be said about the south-west of Western Australia: it's very sandy and it's very flat. Actually there are some important and interesting exceptions. But let us look at the obvious characteristics first. Both of them arise from the fact that the land surface is very old indeed.

When the earth was young, it took a long time to organize the constituents that had come together from interplanetary dust. The heavy (and meltable) material had to find its way to the centre and the rather small proportion that we walk on and call the continents had to come to the surface as a "scum". Relative to the rest of the earth, this material was rather light. It contained a good deal of silica - the oxide of silicon. Geologists refer to material with a lot of silica as "acidic". Beneath this acid scum was (and still is) a layer of rock with less silica, more calcium and more phosphate - a more "basic" rock.

Fortunately for us, this scum wasn't smeared uniformly across the surface. If it were, the whole of the earth would be level and would therefore be covered by water. It seems that convection currents in the semi-molten rock gave rise to lines of scum that came together to form rafts. Weathering was pretty fierce of course and there was a lot of churning so that there was some mixing of the scum with the basic rock underneath. This is a very simplified description of the processes that gave rise to the eastern section of the south west - the eastern goldfields. The greenstone belts are the remnants of this early churning and they tend to run in more-or-less north-south lines.

Once these early rafts started to form, they seemed to serve as zones alongside which there were further accumulations of silica-rich rock. Building of this area lasted until about 2 billion years ago with the western section rather younger than the eastern. A huge proportion of the south west is underlain by this granite (coloured red in the diagram). And the bits not directly formed from the granite are mostly from sediments that originally came from the granite.

The rocks cooled from a molten state very slowly. This meant that there was plenty of time for decent-sized crystals to grow. In very simplified terms, three different kinds of crystals formed in the rocks. One kind was largish, glass-like crystals of silica, which we call quartz. The second kind was semi-rectangular crystals of a potassium aluminium silicate called feldspar. And the third was black mica, looking like a stack of thin leaves. Have a look at a lump of granite one day with a hand lens. You can easily see these separate minerals. Mind you, that is a pretty gross oversimplification. For example, the term "feldspar" covers a suite of minerals with various amounts of potassium, calcium, magnesium and sodium. And there can be some other black minerals.

Granite doesn't breakdown all that quickly. That is why it is often used in monuments. But we have had a very, very long time. Most of the area has been very stable. There has been almost no volcanic activity, no major uplift and faulting, and no glaciation at least for the last 250 million years or so (unlike northern Europe that was scraped clean little more than 10 000 years ago.)  The mica goes first and in the process gives rise to some clay. Somewhat slower is the feldspar and it too produces some clay. The clay also continues to weather and when it is very old most of it has turned into the white clay we call kaolin. It tends to move down through the soil to lower levels leaving the quartz particles, the sand, behind. Just how this happens has been the subject of much debate. One idea is that it is a similar process to that which enables walnuts to come to the top of a mixture of nuts when they are shaken. As the bigger particles jiggle, the smaller ones can slip past them. Whatever the cause, we finished up with large areas with sandy surface and with clay underneath. You see lots of evidence of this clay in road cuttings and in the white clay beside farm dams.

This gives the broad picture. But it is much more complex, and interesting, than this. Look at the map. The first thing you see is the Darling Scarp running as a North-South line for almost 1000 km. We often say "Darling Range" but it is not a "range" - it doesn't go up and then down again but rather up then along. As you go towards the western edge of this low plateau, the streams run in steeper and deeper valleys. The are obviously younger here - or more accurately rejuvenated.

Next you might notice the "chin" the bottom left hand corner between Augusta and Leeuwin that sticks out in a chin-like fashion. Then you might notice the southern ranges: the Stirlings, the Porongurups, and the Mt Barrens. Finally you might notice that some of the soils in the south don't follow the pattern of being very sandy: the "Karri loams". And they grow a rather different vegetation. All these have a common explanation. In one word: Gondwana; in two words: continental drift.

The continents that seem so stable are actually racing around the globe. Australia is sprinting northwards - at about 5 cm a year, slightly faster than your fingernails grow. Five cm a year is 1 m in 20 years; 1 km in 20 000 years; 1000 km in 20 million years; and about 3000 km since Gondwana broke up about 60 million years ago. So it adds up.

Sometimes as the continents skitter about the globe they bang into each other. Just the other day India banged into Asia. What must happen then is that one of the continental crusts slides under the other. If it is really big collision, as for India, enormous mountains are thrown up. We call them the Himalayas. An essential part of this collision is that with the tremendous forces involved, the basic rocks that underlie the granites are forced upwards and to a certain extent are mixed into the granites. The rocks show the evidence of this squeezing in the formation of parallel bands of minerals. We then call these rocks "Gneiss" (pronounced "nice"). With long-term weathering, these bands of rocks may become exposed as the "seam" showing where two continents once joined.

This is the origin of the "chin". It is the seam showing the join with what is now part of Tibet. It is no coincidence that the chin lines up with the hills near Northampton. These are the other end of the seam. Similar forces gave rise to the Porongurups. These too are part of a seam, but this time of the join to Antarctica. Because of the injection of some material from the basic rocks under the granite, the soils here are more fertile. The "Karri" soils at Manjimup and Pemberton also have had an injection of this basic material. They are not as sandy as much of Western Australia and they are more fertile.

Karri Karri has a tiny seed, smaller than Jarrah (E.marginata) , and much smaller than Marri (E.calophylla). To compensate, it can have a higher relative growth rate - a higher rate of compound interest. But to achieve this it must have a bit of phosphate. It can only get this on soils with a fair injection of this basic material - and it normally also needs the phosphate and the lack of competition obtained when growing on an ash bed. That is why Karri is confined to specific regions. Select the thumbnail image or highlighted name for a higher resolution image (57k). Photo: Lyn Barrow.

Continents can also split. Often this involves zigzagging straight lines. A series of such lines split south America from Africa. The split continued to open, giving rise to the Atlantic Ocean. The initial stages of these splits is a "linear sea" and the obvious one of them at the moment is the Red Sea. If you could have visited the south west about 250 million years ago, there would have been a linear sea off shore instead of the Indian Ocean. Somewhat earlier, more than 400 million years ago, there was a similar split a bit further north as another bit of Gondwana sailed off. Once the earth has opened up in this manner, the molten rock from below can sneak out. We have all seen wildlife programs about the volcanic activities in Kenya, where the earth is currently deciding whether to split or not. We had only the merest trace of such activity in the south-west. You can see it on the beach at Bunbury where there is a small exposure of basalt dating from the time of the split.

Once these splits occur, the edges of the splits tend to slump in a series of parallel faults. One of these faults is the Darling Scarp. Many of us who live in Perth use it daily for quick orientation. And many are convinced there is nothing quite like it in the world. Well there is!

There is a series of these faults to the west of the scarp, of course well buried in sediments. Beneath Perth, there is about 4 km of sediments dating back to the Permian - about 250 million years ago. It is even deeper beneath Rottnest. As these sediments filled up the basin caused by the split, there was a chance that organic materials might have accumulated and given rise to oil deposits. That is why there was a lot of off shore drilling. The result is good knowledge of the sediments - but no oil.

The more-northerly split also filled with sediments and these are now the red sandstones we know at Kalbarri.

It is of course a bit more complex. Sometime between 2 and 5 million years ago, there was an uplift along the Darling Scarp. The streams that had wandered across a flat plain were then presented with a sharp drop. So they started cutting back with deeper valleys. The streams were rejuvenated. Some of these valleys are relatively fertile, suggesting what Western Australia might have been like had the geology been more active. The cutting back of the streams has reached a line that runs near Merriden. East of that line we still have the very broad valleys that are so flat that the streams don't know which way to flow.

Very much later, about 60 million years ago, there was another split as Australia broke free of Antarctica and started our northward journey. The trailing edge tended to sink a bit and this lower region also received sediments both from the rivers that now flowed south and from sponges that grew in the shallow seas. Separating from Antartica wasn't completely smooth and at one stage, our continent twisted so that the accumulation of sediments were jammed between Antarctica to the south and the mass of granite to the north. This pushed up the Stirlings and the Barrens. You can easily see how the sediments were pushed up and jumbled. So the neighbouring ranges of the Porongurups and the Stirlings have quite different origins: the Porongurups were formed when Gondwana came together; the Stirlings when it came apart.

We started off by noting that much of the South West is very sandy. That is true but there is another important characteristic. There is also a lot of iron-stone gravel. Often the gravel adds to your photos of wildflowers providing a contrasting background as in this photo of a Dampiera. Often the iron is cemented into massive horizons we call break-aways.

Iron metal loves oxygen. That is why your cars keep rusting. Once iron has grabbed enough oxygen, it is very stable. That is why soils are usually reddish brown. The colour is largely due to iron oxides and these are so stable that they get left behind as other materials weather away. But iron can become mobile under some conditions. One of these is if you can pinch some of the oxygen from the iron metal - that is you can reduce the iron a bit. You can do this under waterlogged conditions as the decomposition of organic matter uses up the oxygen. The iron can then move around with the water and if the water comes back to the surface the iron will get re-oxidised to the stable form. This is probably the origin of the iron-stone soils on the coastal plain.

If you break up some of the iron-stone gravel and look at with a lens you can see that the round particles consist of concentric layers and that these particles are often cemented together by more material. The standard explanation is that this was due to alternating periods of wet and dry. During the wet periods, the iron was reduced and moved around. During the dry periods it was re-oxidised and stabilized. By this argument, the whole of the south west was at one stage covered by a blanket of this material and by implication the climate must have been pretty extreme at that time with alternating wet and dry seasons. The break-aways are then supposed to be the remnants of this material left behind as the rejuvenated streams cut back.

Diagrammatic slice through a break away. The question asked is: if the break away is the residue of a previous blanket, why are they always asymetric?

	Above: Dryandras and eucalypts on the "back" slope of a break away. Left: Looking down the steeper face of a "break away". The gravelly slope is beneath the photographer's feet. The trees are Eucalyptus astringens - brown mallet. It commonly grows just below the break away.

The widely-accepted picture of a previous very wet phase has recently been questioned. There is another way iron could become mobile.

Many of our native plants secrete large amounts of citrate from their roots. Dryandras and banksias are very good at this. The citrate dissolves iron oxides and this releases some of the phosphate locked up by the oxides - one reason why these plants can grow in such low-P soils. Citrate doesn't last very long in soils. It gets eaten by the soil bugs. So you have another mechanism by which soil iron can have alternating cycles of mobility and stability: mobile while the citrate is there, fixed when the bugs have eaten it. According to this theory, the break-aways are not remnants of a previous blanket of iron oxides but faces that are exposed to air so the bugs can thrive and oxidise the citrate there. They are not remnants but are forming still. They too exist because of Gondwana. The upheavals associated with its formation injected lines of more-basic rocks (called dykes) into the granite. These dykes are richer in iron. It is on them that the break-aways form. We have always argued that banksias and dryandras grow on these soils because they can survive they pretty tough conditions. That is still true, but the extra dimension is that maybe the plants cause the tough conditions.

This article is adapted from Western Wildlife Vol. 5 No. 3, originating from a presentation to the Perth Branch of the Wildflower Society of Western Australia.

Dr Jim Barrow, Ph D., D. Ag. Sc. is a "semi-retired" soil scientist who spent his research career in CSIRO, retiring as a Chief Research Scientist. He is the author of almost 200 research papers. He is a past president of the Perth Branch of the Wildflower Society, and a member of the WA Management Committee.

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Australian Plants online - June 2001
Association of Societies for Growing Australian Plants