Category Archives: Granite Outcrops

Harvesting Water

The numerous granite outcrops throughout Western Australia’s agricultural regions are a valuable source of water if the runoff can be captured or ‘harvested’.

Many of these granite outcrops, particularly eastward towards Kalgoorlie and beyond, have had low walls, typically 300-500mm high, erected on a selected contour of the outcrop that channels water flow into another channel or a culvert and ultimately into a reservoir or tank located lower down the rock. Many of these reservoirs were roofed to slow down evaporation.

The harvest walls that aggregate water from the rock and the channels that then direct the precious liquid into a tank or dam are invariably fine examples of stonemasonary.

Wave Rock (more correctly Hyden Rock) is probably the most famous rock to be harvested. Clever photographers avoid including the wall at the top of the wave in their photographs.

Water harvesting and collection in tanks was often for community water supplies but the most common use was for the boilers on steam trains.

The amount of water harvested from a granite outcrop depends on the size, steepness and shape of the rock. Estimates of runoff at, for example, Karalee Rock are that a 25mm/hour downfall would have a runoff of 6200L per acre.

Reference
Laing, I.A.F. and Hauck, E.J., Water harvesting from granite outcrops in Western Australia, Journal of the Royal Society of Western Australia, 80: 181-184, 1997.

 

© Kim Epton 2019-2024
292 words, two photographs.

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Gnammas

Don’t say gnamma hole

Although often used, the term ‘gnamma-hole’ is a tautology; the concept of ‘hole’ is already incorporated in the word ‘gnamma’.

The word ‘gnamma’ is from the Western Desert aboriginal language. The Nyungar word ‘nama’ has the same meaning.

What are they?

Gnammas are a peculiar type of hole that occur in rock outcrops, particularly granite. They usually have a small opening on the surface of the rock with a larger bowl shaped cavity beneath. They hold anything from a few litres to a thousand litres or more, although the larger holes are rare.

Gnammas are formed by eons of chemical weathering by water. It is suggested that some aboriginal groups started or enhanced gnammas by lighting fires on suitable rocks.

Gnammas are not a soak – they fill after rain. Aboriginals often covered them with sticks to prevent access by native animals. This had the bonus of minimising evaporation. Gnammas were vital to explorers and prospectors and are an important water source for all kinds of fauna.

A more recent practice is the insertion of a stick into the water that allows small animals and/or insects to exit the gnamma should they fall in.

In Darkest West Australia: A Guide To Out-back Travellers, H.G. Mason wrote:

In desert spinifex, gnamma holes and soaks may be found in patches of scrub mulga, which occur here and there throughout the interior, generally low-lying formations of granite and desert sandstone, clothed with weeds, silver grass and scattered small narrow-leafed salt bush with occasional quondong and kurrajong trees…

 

© Kim Epton 2015-2024
326 words, four photographs.

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Mystery of a Painting of a Ship at Walga Rock

Walga Rock (Walgahna) is the second largest monolith (single rock) in Australia after Uluru, although that is sometimes disputed. It is a 1.8 kilometre long, granite ‘whaleback’ about 50 kilometres south-west of Cue.

There is an extensive gallery of Indigenous art at Walga Rock.

A painting of what may appear at first glance to be a sailing ship is superimposed over some of the earlier works. Underneath the painting are lines of writing that resemble Cyrillic or Arabic script, however, they have not been accurately identified.

There has been a great deal of speculation about the painting, especially considering it is located 325 kilometres from the coast. It has been postulated that it was drawn by survivors of the heavily armed three-masted Dutch East India (VOC) ships Batavia or Zuytdorp; or that it represents a ‘contact painting’ by indigenous Australians who saw a ship on the coast and then moved inland.

While there are many examples of Indigenous art depicting vessels on the Western Australian coast, including others showing what appears to be the SS Xantho and possibly another steamer at Inthanoona Station east of Cossack, the Walga Rock painting is one of the most inland examples.

A visit to the rock created further discussion of the possibilities.

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