Wave Rock: What? When? How?
Wave Rock is that famous section of the north side of the Hyden Rock complex that looks like an incoming breaker wave. How did it come to be?
What?
Hyden Rock is an inselberg, an outcrop of the "country rock" that makes up the earth beneath us. Inselbergs are created by the erosion action of water and wind over eons of time. They are usually composed of rock that is more erosion resistant than surrounding rock. This may be because of small differences in composition, grain size. (Fine grained is harder to wear down).
Beneath the surface cover of soil and trees across most of the Hyden district lies a vast sheet of "younger" granitoid rock. Where this is exposed by erosion of the soil, we see this original country rock. There are dozens of these outcrops in the district. Hyden Rock, Camel Peaks, The Humps, King Rock are the largest. The highest of them stand about 50 metres higher than the valley floor which is about 300 metres above sea level. So these rocks are still below the 400-450 metre level of the ancient plateau .
For the geologically minded, Hyden Rock is made up of a "medium and coarse grained porphyritic granite and adamellite". The Humps is of a "variably textured, medium and coarse grained seriate granite and adamellite; locally porphyritic". They are probably slightly different ages. To the rest of us, they both are composed of just granite but with slightly different compositions and grain size. Their survival above the surface tells us they are harder than the country rock beneath them.
When?
This younger granitoid country rock is about 500 million
years old. It flowed as a molten layer over the old granite block for about
500kms to the northwest, across today’s wheatbelt. The old granite block
beneath is exposed to the south and west of the Hyden/Karlgarin area. It is made
up of "reworked" granite: gneiss and recrystallised granite. It makes up most of
the southern corner of the ancient plateau block of WA.
So the Wave is no older than 500 million years even though other country rock in the Hyden district dates back 2,400 million years or more. To get a more exact timing we must examine the "how".
How?
Opinions vary and arguments rage between geologists over
this. Some have suggested that the exposed rock surface of the lip of the Wave
is harder than that below. Hence water eroded the softer rock below into a wave
shape.
Another researcher, C R Twidale believes that the Wave on Hyden Rock was formed by the "weathering" way of granite underneath an old, higher soil surface. This took place while the present river-lake system was being formed perhaps up to 100 million years ago. He describes similar "flared forms" of rock elsewhere on Hyden Rock and in other parts of the world, including the Eyre Peninsula in South Australia. But none are as spectacular as Wave Rock. Weathering is a geological word. It means altering rock where it lies - above or below the surface - so as to break it down, eventually into soil. It takes place by chemical action, helped by physical or mechanical forces ie; heat and cold, frost, and wind blown sand can all play a part. But mostly it needs time - tens of millions of years, in fact.
Twidale firstly notes that the Wave lies at a point on the north side of Hyden Rock between two joints. (Hyden Rock is heavily jointed). The quantity of water going down this particular face is therefore much greater than elsewhere around the Rock. This keeps the sub soil to the north of the Wave wet for much of the year. That moisture gradually dissolves or "rots" the granite underneath the soil layer.
Secondly,
up to 100 million years ago the valley floor of country rock underneath the
north side of Hyden Rock was not yet eroded down by weathering and river action.
Wet soil over a long period of time at the foot of the rock gradually "rotted"
the country rock beneath (see "A" in the diagram at left).
The level of soil and debris was then eroded down a few metres by the old river system, and the process started again (see "B" in the diagram).
Twidale suggests that this finally produced
the undercut wave and bench below. That process would have stopped when the old
river-lake bed to the north was at the bench level, perhaps no later than 20
million years ago. Weathering is probably still going on under the wet soil
layer at the foot of the Wave today (as in "C"). He also notes the processes of
disintegration of Hyden Rock, in layers. This produces water and wind eroded,
round boulders from flat sheets of granite.
A shower of cold rain from a mid summer thunderstorm on rock at over 50 degrees Celsius can produce splitting. About 50 metres of surface granite rock would shrink 2cms in seconds, and split off from the rock below. Or it could form a major joint. Water and wind does the rest.
Twidale suggests that splitting into sheets and the growth of hollows and clefts by weathering eventually reduces granite rocks to debris. The debris becomes granite soil in the hollows and around the base of the rocks.
Source: C R Twidale, Origin of Wave Rock, Hyden, WA, Trans. Roy. Soc. SA (1968),Vol 92, pp 115-124; Clarke & Prider, Elements of Geology, University of WA, 1955.