Sandinista Wall Geology – The Expert’s opinion

In May 2020 Parks Victoria published a series of photos of Sandinista Wall captioned “This image shows the extent of chalk wash and staining at Sandinista Wall, Hollow Mountain” and “Chalk staining is visible in this photograph“.

We doubted the legitimacy of these claims (see our article here) and immediately sought an experts advice on the geology of this particular cliff line. We sent an inquiry to the government department The Geological Survey of Victoria with the following question:

I wish to ask for a geological advice on a particular rock formation in the Grampians National Park. The cliffline in question, located in the northern Grampians near the Hollow Mountain walking track, has what appears to be white streaks that almost look like tears running down large sections of the cliff. They appear to be natural formations but I am baffled as to their provenance. They appear reasonably unique compared to the surrounding area and may have formed through a geological process involving water – but not rain. Since it is almost a desert out there it seems odd.

The location where I have seen these features is an overhanging section of cliffline located at GPS -36.8918, 142.3830. It is located 700m south of the Hollow Mountain Carpark and is right above the walking track that takes you to the summit of Hollow Mountain. It is a nice protected spot, almost a cave, that you can sit and take in the view before scrambling up the left side of the cliff to the top of Hollow Mountain itself.

I have enclosed several photos of this white tear streak phenomenon on the rock. Sometimes it is very prominent – and sometimes just hairline streaks. They seem to come from holes in the rock mostly as if there is something inside the rock coming out and dripping down. At an educated guess some of the larger streaks would be 5m long.

I can see on one section of the rock a prominent white streak is actually bird poo as I can see the splatter on the ground below it (I have enclosed photos of this and circled the area in question so you can eliminate it from your review of the other white streaks). There is a ledge above this streak where the bird obviously sits. There is however no poo splatter below other sections of the cliff with white streaks which is why the rest probably has a geological explanation not biological.

We received the following reply on 13 June 2020 from Ross Cayley, Senior Geologist. In his own words:

Email from the government’s own expert geologist – Ross Cayley

As the senior technical author on a series of recent research reports and associated geological maps, research papers and 3D models that encompass Grampians/Gariwerd geology, I can confirm that I will be able to provide the geological advice you seek and/or be able to direct you to other authoritative sources.

I am highly familiar with the geology of the Hollow Mountain / Mount Stapylton and surrounding areas.

Thanks for the great photos – they show the streaky coloured ‘tear-like’ features you describe quite clearly. Your location is the prominent slightly over-vertical cliff located on the northern flank of Mount Stapylton, close to the walking track to Hollow Mountain. I am familiar with it. This location shows natural colouring and patterning typical of over-vertical cliffs throughout the Grampians Ranges, particularly on cliff overhangs that face easterly, northerly and westerly and are regularly or seasonally exposed to direct sunlight.  

The best way for me to address your query is for me to first describe the general geology, then describe the features illustrated by your photos and, lastly, for me to explain what I consider to be the geological explanations for those features.

General geology

The rock package within the Silurian-aged Grampians Group that forms Mt Stapylton is a quartz arenite-dominated phase of the Kalymna Falls Sandstone. The sandstone at Mt Stapylton is generally medium to coarse grained, but also includes abundant beds of small rounded pebbles.  The relevant geological map that depicts the extent of the unit is this one:

Grampians Group quartz arenite base colour:

The fundamental colour of the quartz arenite rock at Mt Stapylton, when fresh, is nearly pure white. The white colour is fundamentally dictated by the colour of the predominant primary constituent mineral grains, which are quartz (typically ~90 to 95%+ by volume, quartz is translucent), feldspar (up to 5% by volume, the feldspar is typically opaque white) and minor amounts of small polylithic grains and mica flakes, most of which are also white. The pebbles incorporated in the sandstone in places are also made entirely of polycrystalline quartz and are also white. The general rock descriptions are given here:

Selected detailed descriptions of Grampians Group quartz-arenite (not specifically from Mt Stapylton, but the same basic rock type and so still relevant) are given on pages 76-79 in:

There is virtually no carbonate in the quartz-arenite rocks at Mt Stapylton. Carbonate is the base constituent of limestone, and is generally derived from life. While there may once have been some minor carbonate component (ie fossils) in the rocks at Mt Stapylton (occasional shelly fossils and other organics do occur in other parts of the succession elsewhere in Gariwerd), we have never located any carbonate at Mt Stapylton.  

Why and how the base colour of the Mt Stapleton quartz-arenite gets modified:

Hardly any of the in-situ Grampians Ranges quartz-arenite sandstone units appear white to the naked eye. This is because the surface colour has typically been naturally modified into a range of other colours; uniform grey on gentle to moderately-steep terrain; reds, browns, yellows and black, as vertical streaky hues on over-vertical terrain and on cave interiors. All these other colours are secondary, and have been superimposed onto the white base quartz-arenite as thin veneers – generally the superimposed colours only extend a few mm to occasionally a few cm deep into the underlying rock – this is because the base rock is generally relatively impermeable to water, and because some of the colours – primarily the red, yellow and brown colours, are formed through secondary silica growth that is, by its very nature, confined to the rock surface.

Secondary silica growth is controlled primarily by countless episodes of water evaporation from the rock surface, which leaves behind a thin ‘skin’ of coloured secondary silica, formed over the top of the base white quartz-arenite. These ‘skins’ are called ‘desert varnish’.


‘Desert Varnishes’, including those in the Grampians, but also in many other places in Australia and in ancient landscapes on other arid continents, get their striking colours from the thin (fraction of a mm) but multiply-stacked onion-like layers of tiny coloured clay particles that they are made from. (a reference:  Potter RM, Rossman GR (1977) Desert varnish: the importance of clay minerals. Science 196:1446-1448.)

The clay particles in ‘Desert Varnishes’ are derived from dust storms and from soil, gradually accumulated over thousands to millions of years. Australia is an ancient, rusted continent, and dust storms in Australia are relatively homogeneous and are red. This red colour comes from iron oxides (ie the ‘rust’) within the clay particles. This is the origin of the reds, browns and yellows of Grampians Group ‘desert varnishes’, and red desert varnishes that are found across arid Australia. (a reference: Garvie LAJ, Burt DM, Buseck PR (2008) Nanometer-scale complexity, growth, and diagenesis in desert varnish. Geology 36(3), 215-218). Other clay particles contain manganese oxides – in the Grampians, some of these appear to be more locally derived from regolith. Manganese oxides are black, and are typically the origin of occasional streaks of black ‘desert varnish’.. All the different types of clay particles are cemented together by the secondary silica, so that ‘desert varnishes’ are very tough features that are as tough, or tougher, than the base quartz-arenite rock upon which they have accumulated. (a reference: Perry RS, Lynne BY, Sephton MA, Kolb VM, Perry CC, Staley JT (2006) Baking black opal in the desert sun: the importance of silica in desert varnish. Geology 34: 537-540).

The origin of the silica in the ‘secondary silica’ that cements the ‘desert varnish’ at Mt Stapylton is the base quartz-arenite. Quartz is made of silica. Quartz is relatively resistant to weathering and erosion, but can be dissolved by weak acids. Humic and tannic acids, derived from vegetation, have long been recognised as a significant agent that can dissolve small quantities of quartz, liberating silica into solution in water run-off that can then wet cliff faces and other features (a reference: HUANG, W., KELLER, W. Organic Acids as Agents of Chemical Weathering of Silicate Minerals. Nature Physical Science 239, 149–151 (1972). Such acids are implicated as an important agent in the gradual formation of ‘desert varnish’ in the Grampians Ranges. This is because most of the ranges have carried significant vegetation that, for millions of years, has been regularly flushed by rainfall. Water run-off from the Grampians is typically strongly naturally stained by tannic and humic acids.

Where acidified silica-loaded water run-off gets an opportunity to evaporate, the silica in solution will be forced to precipitate out to be left behind as a new layer of ‘varnish’ on the rock surface (a reference: Thiagarajan N. Lee C (2004) Trace element evidence for the origin of desert varnish by direct aqueous atmospheric deposition. Earth and Planetary ScienceLetters.224:,131-141.). Clay dust trapped within the new silica skin gives the varnish its colour. As mentioned above, the most common clay components in SE Australia are iron oxides that, when trapped in the Grampians Group ‘desert varnishes’ locally formed upon over-vertical surfaces and cave interiors, impart the distinctive red brown and yellow colourations. The vertical streaky appearance of these colours reflects the creeping flow of the source water down these surfaces, from which the varnish layers were subsequently able to form through evaporation.

On some over-vertical cliffs and in caves, such ‘desert varnish’ layers can form a type of ‘case hardening’ for softer rock-types. Erosion of such layers can result is a characteristic pockmarked appearance, where the softer base rock-types can be eroded out from behind remnants of the tougher ‘desert varnish’ skin – this is commonly observed at Hollow Mountain, at other places at Mt Stapylton, and elsewhere in the Grampians Ranges and elsewhere.

‘Desert Varnishes’ are only able to form in environments where water evaporation from rock surfaces consistently predominates over water run-off from rock surfaces. A great example of a suitable environment for ‘desert varnish’ formation in Australia are the ‘gibber’ deserts that form in the consistently arid and hot deserts of Central Australia – evaporation is predominant in these deserts, and ‘desert varnish’ on the rock fragments baked in the searing sun is ubiquitous.  In consistently seasonally wet to only semi-arid climates like SE Australia and the Grampians/Gariwerd, the only places where water evaporation can consistently predominate over water run-off to form ‘desert varnish’ are over-vertical cliffs and cave interiors which are protected from washing by the consistent seasonal rainfall. That is why it is only caves and over-vertical cliffs in the Grampians Ranges that show the striking red, yellow and brown colourations of ‘desert varnish’ – the less steep rock surfaces that are exposed to seasonal rainfall are washed clean every year, so that the thin coloured ‘desert varnish’ veneers never get a chance to form.

Instead, these regularly washed areas are generally ‘grey’ in colour. The grey colour is due to the growth of lichens etc – these are most able to grow in places where water run-off predominates over water evaporation. It is the microscopic remains of lichens and other vegetation, lodged in microscopic interstices between the constituent quartz grains of the base quartz-arenite on regularly washed rock, that impart the typical ‘grey’ surface colouration to Grampians Group quartz-arenite. If the Grampians were even wetter than they are, the washed areas of rock would likely show the base white color more clearly – as is seen in the quartzites mountains of wet SW Tasmania (which share a very similar bulk composition to the Grampians quartz arenites).  

(Elsewhere, some black patterning is due to soot from fires – these surface smudges generally look more ‘mottled’ than ‘streaky’, and are not the features illustrated in your photos).

In summary, the different colours seen in the Grampians relate to different surface processes acting on the rock, in different micro-environments. So, your query about the ‘white streaks’ on the steep over-vertical cliffs, such as the one you mention on the north side of Mt Stapylton, isn’t so much about the explaining the origin of the white bits – the white bits are just places where the base colour of the underlying rock is locally showing through to the surface. Your query could instead be reframed as seeking an explanation for why water-streak shaped gaps might exist in the ‘desert varnishes’ which are widely developed on over-vertical cliffs such as this one. The best way to answer this question is to address the specific features on the photos you have supplied.

Other features of Mt Stapylton geology:

The rock on Mount Stapylton, and on Flat Rock nearby, is relatively weakly deformed, so that many original sedimentary structures, features that formed at the time of deposition of the rock as a pebbly sandstone in braided river systems that existed over 400 million years ago, remain well preserved to this day – features such as bedding layers, pebble beds, cross-bedding, and channel scours are all still clearly visible in the rock. In the parts that are weakly deformed, all that has really happened since that time is that the constituent grains have been gradually cemented together by remobilisation of silica to form solid rock, and the rock package has been gently folded and tilted from the horizontal, so that the sedimentary beds now locally dip at 30 to 40 degrees to the northeast. The parts of the quartz arenite sequence that are weakly deformed are relatively monolithic, which explains why Mt Stapylton is a large, upstanding block of rock with high, prominent, well defined cliffs and gullies that generally follow the few persistent fractures that do cut across the rock.  

Just to the north and east of Mt Stapylton, the original quartz-arenite sandstone has been caught up in an ancient fault zone related to the Fyans Fault. This change is illustrated on the geological map. The parts of the rock unit that are contained within this broad, anastomosing fault zone are naturally much more densely fractured, recrystallised, striated and disrupted, due to shearing within the fault zone as different parts of the Grampians Group were moved laterally relative to each other, around 400 million years ago. Some dispersed iron oxides introduced in fluids as part of the faulting process have imparted some of these rocks these more deformed, and more permeable rocks a more generalised yellow colour. The legacy of that ancient deformation is preserved in the rock today as a more fractured rock, upon which a more complex, broken landscape has developed – for example the terrain between Mt Stapylton and Summer Day Valley. As in all fault zones, enclaves of less-deformed rocks also occur. ‘Wall of Fools’ in Summer Day Valley is a good example of an enclave of less-deformed rock that forms a locally monolithic landscape.

Your photos:  


Photo DSC_0424 with the bird excrement dotted across a grey rock, I presume is positioned at the base of the cliff you have mentioned. The grey colour of this rock indicates that this rock is exposed to seasonal rainfall washing – the typical grey colouration can also be seen in the upper, less-steep parts of the background cliffs in photo DSC-0436, and for the same reason.

Presumably the rock illustrated in photo DSC-0424 lies underneath the point on the overhanging cliff illustrated in photos DSC_0430, and  DSC_0436, where you have circled a prominent, large white guano stain that emanates from a high ledge beneath a shaded overhang. This ledge will be regularly used as a perch by birds of prey, hence the local concentration of bird guano. These are common in Gariwerd, but only where cliffs are steep enough to be protected from the seasonal rainfall that otherwise continually washes away the guano. Additional extremely large white guano stains are visible on Taipan Wall, on the western side of Mt Stapylton. Some of these guano stains are giant, extending down from beneath bird perches for distances of 30m or more. Farther afield, bird guano stains are common at Mt Arapiles, and elsewhere in Gariwerd (Grampians).


Photo DSC-0430 and Photo DSC_0436 both look up at the same overhanging cliff and, in addition to the prominent bird-guano streak, show more diffuse white and yellow/brown and black streaky colourations – and I assume that it is the origin of these ‘white’ streaks that you are most interested in. These streaks are not formed from bird guano, but are simply the base colour of the rock showing through gaps in the yellow/brown desert varnish that covers the rest of the cliff. The vertical alignment of all of the desert varnish stains, and the white streaks of base colour that disrupt them, clearly indicates that water run-off down the cliff face is the primary control on their formation.

The geological explanation:

It is apparent in your photos that many of the white ‘streaks’ on the cliff face emanate from, and widen below, pock marks and fractures in the cliff face. Some of the most prominent white ‘streaks’ lead up to pock marks that are also areas where natural ferns and mosses are growing, as evidenced by the small ferns growing in pock marks at the top of Photo DSC-0432.


Ferns and mosses require a consistently wet environment, so their appearance on overhung cliffs that are the driest environments within the Grampians and protected from direct rainfall, are strong evidence for the pock-marks and fractures being sites of periodic but regular water seepage from the cliff face. It such instances, the pock-marks and fractures are acting as natural ‘springs’, draining groundwater that has percolated downwards within the rock interior via a distributed network of joints and fractures.   These points of water seepage sit directly above the points of origin of the various white ‘streaks’, demonstrating a directly association. The yellow/red/brown desert varnish colourations of other parts of this cliff appear to have only formed on the parts of the rock that are not subjected to persistent  water seepage from the pock marks and fractures.


Photo DSC_0433 is a good example of these relationships, since it clearly shows smooth mottled orange/brown coloured ‘desert varnish’ on a part of the cliff face, breached by a complex fracture weakness of white rock patches that crosses the photo from bottom left to top right, below which streaky white bands appear to ‘run’ down the rock .

Where water seeps regularly and persistently from fractures in the cliff face, at rates where water run-off rates exceed water evaporation rates, desert varnishes are unable to form. Thus the persistent water seeps will show as the base white rock colour in a streak-shaped gap in the ‘desert varnish’. This is the origin of most of the white ‘streaks’ depicted in the photos of the cliff that you have supplied. This pattern and mechanism is not confined just to Mt Stapylton – it also occurs widely on other overhung quartz-arenite cliffs throughout the Grampians/Gariwerd and at Djurite/Arapiles – almost all of these cliffs have points of weakness that leak enough groundwater to locally inhibit ‘desert varnish’ formation.

Thus, the majority of the white streaks on the cliff you have mentioned, and elsewhere at Stapylton and on overhung cliffs across  the wider Grampians and at araps, are not ‘stains’– they are just streaks of the native white rock colour showing in-between the reds and yellows of the common ‘desert varnish’ that has formed on adjacent, drier parts of the cliff faces. I hope that answers your query. If you have any further questions, please don’t hesitate to get back in touch.

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