Abstract

A non-invasive method is trialled for date ordering certain petroglyph elements on a rock panel at Cochineros, on the river Mala, south-central Peru.

The petroglyphs have been engraved into a dark rock varnish coating to give a strong contrast between the white crystalline powder created by pecking or scratching, and the black background of the rock surface. The brightest image is a modern heart with an inscribed date of 1968.

At least five different levels of brightness can be observed by the naked eye, ranging from the bright white heart to images that are almost the colour of the unmarked rock, and can only be discerned by their rougher texture.

Several superpositions of lighter elements over darker support the theory that the images have darkened with age.

The average pixel intensity of the elements was measured from images taken with a digital camera using the software package FiJi. Compressed jpgs gave RGB values from 220/256 for the brightest images down to 140/256 for the darkest. The measurements were taken from photographs taken at different angles to the rock. Uncompressed RAW digital image files were created using a Canon digital camera the following year and the brightness was measured on a scale from 0 to 64,000.

The measurements were found to be robust and consistent, within fairly large error margins of around 3%, for images taken on different days and at different times.

It is proposed that the change in brightness is due to the growth of a layer of rock varnish over the images. If the varnish layer grows thicker with time, then the brightness measurements will allow the elements to be placed in age order of age, from darkest to brightest.

If the rate of growth of the varnish is constant over time, or otherwise predictable, and also consistent across the face of the panel, then we might be able to derive quantitative results by calibrating the measurements.

The panel offers two calibration points. The first is a heart with a date of 1968. A second is deduced from Peruvian history.

The upheaval following the invasion of Peru by the Spanish in 1532, the subsequent pandemics (80% of the native population died within 40 years of the invasion), relocations of people (the reducciones, around 1573), and attacks on indigenous religion and culture (the extirpaciones, from 1608 onwards) makes it unlikely that normal activities around the petroglyph site continued. I therefore assume the brightest element other than the heart was completed around 1580CE.

There is no reason to assume that the rate of growth of varnish is constant, and several studies suggest it is not. These however have considered its growth over scales of tens or hundreds of thousands of years, through periods of climate change and wet and dry periods. It may be that over a scale of one or two thousand years before the present, constant growth is a reasonable approximation at this particular site. In any case, it is the simplest assumption,

It is readily apparent at the Cochineros site that the rate of growth of rock varnish varies from panel to panel, dependent on orientation and inclination. South and eastern facing panels, and panels closer to the vertical, are in general darker and glossier. Panels facing north and closer to the horizontal have a greyer appearance, and the difference in brightness between images is less.

In some cases, changes in appearance suggest that the growth of rock varnish changes across one panel. There are images which have a clearly delineated change in brightness down the middle of the image, or apparent image groups, such as a pair of llamas, where one is darker than the other.

However, the panel chosen is so thickly covered with drawn elements that any sudden variation across the panel would be evident. In addition, it has a line of dots which runs from the foot of the panel almost to the top, and another which runs from left to right across the foot of the panel. The consistency of brightness of these elements suggests that the rate of varnish formation does not change across the area of the panel being studied. There are adjacent areas, with folds or shoulders of rock, where the varnish seems to be darker and glossier, and these areas have not been used for measurement.

The approach is only suitable for pecked or abraded elements, where the dark surface layer of the rock is fully removed rather than simply scratched. Similarly, filled designs are preferable to outlines, as they offer a greater area for measurement. The method has therefore been trialled on selected elements on one panel of the site.

Despite the limitations above, the results allow some grouping of images on the basis of their measured brightness, their style and their subject matter. This in turn suggests a long history of engraving at the site, with a succession of people using it in different ways. A narrative is proposed for the several stages of use of the site, ranging from very dark, barely visible abstract motifs, such as lines or curves, to lines of dots, followed by a group of elements pointing towards or clustered around the peak of the rock. This group, according to the dating, would have been created around 1000BP, a time when the Paria Caca cult was brought to the coast by Tutay Quiri, according to the Huarochiri Manuscript. I propose elsewhere that the peak of this rock, and these elements in particular, are relating the story of Paria Caca for an audience close to the coast.

Subsequently a large number of images which could represent tumis were added to one giant boulder in particular, and also to every other boulder and almost every panel across the site. The dating from this panel suggests this was one of the last additions, and other authors have suggested it relates to the coming of the Inca to the central coast.

Finally, a vertical dot track which may relate to the Paria Caca theme was redrawn and figures added that could represent people carrying llamas up to Paria Caca to sacrifice, a practice outlined in the Huarochiri Manuscript. The late stage of this could suggest an attempt to re-establish the cult in the face of Spanish repression.

The proposed dates for the pre-hispanic petroglyphs vary from 440 to approximately 2000 BP. An assessment of images at the site as a whole shows that many relate to the late intermediate period on the basic of similarities to icons seen on ceramics and textiles.

Darker images, largely abstract, are proposed to date from the Middle Horizon and Early Intermediate.

There are some panels at the site, that have a wealth of imagery but are difficult to access. One particular panel with an area of around 15 square metres displays largely dark, abstract images. It might be possible to extend the dating by using stylistic similarities between images on the panel analysed here, and other panels, together with measurements of brightness, by using drones or otherwise accessing these panels.

Given the scarcity of alternative dating methods, especially those that do not require destructive sampling, even the relatively rough age estimates obtained here are a valuable contribution to the archaeological study of rock art.

Many

Introduction

The Cochineros petroglyph site in central Peru comprises a group of giant boulders mostly on terraces above the river Mala, on a line between Huarochiri and where the river Mala enters the sea, 30 kilometres from the coast and 45 kilometres from Huarochiri. There are ten abandoned settlements in the valley from fifteen kilometres downstream of the site to fifteen kilometres upstream, spaced three to four kilometres apart, indicating that it was much more heavily populated at one time. All the settlements are thought to date the Late Intermediate (1000CE to 1473CE ) or Late Horizon (1473CE to 1532CE).

In total there are at least twenty stones with images at Cochineros, seventeen stones of a black diorite and three small stones which are reddish-brown rather than black. There are nine particularly large boulders, with panels several square metres in area. Three boulders have engraved areas totalling more than twenty square metres. Five boulders appear to be dominant, in terms of the quantity and quality of engraved images. Three boulders nearest the river are now on the river bed to where they appear to have fallen as the river cut away the terracing.

The river above the site runs north to south, and turns as it passes the stones on the right, western bank to run 35 degrees west of south.

The first investigation of the engravings at Cochineros was undertaken by the Cuban scientist and adventurer Antonio Nuñez Jimenez in or around 1976. He published 36 pages of tracings and photographs in his “Petroglifos del Peru”[1]. He did not speculate on the age of the site and what he records is meticulous and almost wholly accurate, although incomplete. His line drawings cover about 75% of the petroglyphs at the site.

Henry Tantalean described the site in 2010 and began to tabulate and categorise the images [1]. He proposed that the many tumis at the site indicate that the majority of images date to the Inca period.

Photographs of images and panels are included in a photographic record of petroglyph sites by Guffroy in Imágenes y Paisajes Rupestres del Perú, 2009, together with a brief summary of the site [1].

Guffroy suggested that the majority of the engravings at Cochineros and Calango dated to the Late Intermediate and the Inca periods, from 1000 to 1534 CE, though he recognised there might be elements from earlier. Huancor and Checta, by contrast, he proposed went back through the Early Intermediate and into the Early Horizon, 200 to 400 BCE. Guffroy published photographs of a couple of deeply marked dark images low down on Piedra 12. “With the exception of a small group of figures which are engraved with a certain depth, which could represent an early stage, the figures are mostly lightly drawn,” he wrote. He suggested that the site could be linked to the nearby settlement at Huancani, a kilometre upriver, which Tantalean proposed was Inca.

Some researchers have looked at archaeological sites nearby to help estimate the age of the site. The valley is filled with the remains of villages, roads, and irrigation systems spread over several thousand years, as well as modern settlements, so this approach is problematic.

Measurement

On the south facing panel of Piedra 11 (Figure 1) it is easy to see that there are brighter, whiter drawings and duller greyer drawings.

Where one image has been drawn over another image, the newer image is brighter. Such superpositions can be seen with the heart, which is drawn over a spiral and other figures (Figure 2), and with the three shawl pins (tupus), on the upper left area of the panel, drawn over a four legged camelid (Figure 3).

Figure 4 shows the north facing, upstream facing panel of the same rock. There is some variation in brightness, but it is much less clear. This pattern of clearly visible petroglyphs on the panels facing downstream or facing the river, and less distinct petroglyphs on the panels facing upstream or facing the sky, is seen throughout the site. Possible explanations for this will be explored later.

To see if it was possible to get a consistent quantitative measure of the different apparent brightnesses, I chose to investigate this south facing panel of Piedra 11. The panel is a broad flat panel with clearly differentiated bright and dark images (Figure 5), two metres high and four metres wide at the base, rising to an apex. The panel is at an angle of about 60 degrees to the horizontal, and faces south.

The petroglyph creators were varied in their execution techniques. Some simply scratched the surface, removing only a portion of the varnish, but enough to leave a visible mark. Other elements are drawn deeply and uniformly. The rounded end of the tupus (Figure 3) is an example of the latter, and the vague tumis (sacrificial knives) to the right of the tupus are examples of the former.

Comparing the present brightness of images as an indication of age will only be useful if the manner in which the petroglyphs were made gave the same initial brightness. As I will be measuring the average brightness over an area, it also helps if the elements a large area. In practice this means choosing elements where the rock surface has been pecked rather than scratched, and the varnish completely removed over an area. Measurement on thin lines is inconsistent.

The elements selected were therefore those well delineated and infilled, pecked rather than scratched, with a large surface area where the varnish has been completely removed. There are several such suitable images on the chosen panel which I named, without any claim to interpret their meaning, as the “Old Llama”, the “Three Tupus” which overlay this, the upper and lower “Swallows”, the left, centre and right “Lake Deities”, and an “Old Spiral” (Figure 5). This last is not ideal due to its relatively thin lines but it is an important central motif. The “Dot Track” and the “Heart” provide calibration.

To see if the brightness could be consistently measured, I took photographs of the rock panel at different angles, one early afternoon in August, and measured the average pixel intensity of the engravings as seen on the digital images. Further photographs were taken in subsequent years, at different times of day and year. The photos were taken at angles chosen so as to avoid direct reflection of sunlight off the rock face.

Brightness is a subjective term meaning how the elements appear to the eye and brain. Photographs provide a more objective measure of the elements. Observing the petroglyphs under the bright sunlight of coastal Peru is difficult, and many more elements are revealed on the photographs than can be clearly seen on the site.

The first sets of Jpegs were taken with a simple digital camera. Average pixel intensity was measured using FiJi, a version of the ImageJ programme developed at the National Institutes of Health, which can be freely downloaded.

The software offers many options for measuring average pixel intensity. After trial and error, two methods were chosen for final measurement. An area of the image was selected either manually by defining an oval or a rectangle within the image, or by using a wand (Figures 6, 7). The analyse/measure option then measures a definable range of parameters. I chose area, mean, median, mode, max and min and SD.

Defining an oval or a rectangle appears to give more consistent results for large area elements, but it is difficult to apply to thinner elements such as the heart and date. The wand was therefore mostly chosen for consistency. For the darkest images, such as the central Lake Deity and the Old Llama, the wand could not differentiate between the element and the surrounding rock panel and so area measurement was used. Typically, four or more areas were selected and the readings averaged. More measurements were taken on the two calibrators, the Dot Track and the Heart.

The wand automatically selects an area of the same brightness around the point where the wand is pointed. The tolerance was adjusted, typically to between 10 and 20, in the Legacy mode, so that the wand selected as large an area as possible within the element, without going outside its boundaries. Lower tolerance might mean that only a tiny area of the element is selected, whilst higher tolerance can lead the wand to select large areas of the panel.

Trial measurements were made with both wand and area tools and plotted for comparison. The results were similar.

Mean, mode, median, min and max values were all measured together with Standard Deviation. Trials suggested that mean and median both gave similar consistent results but mode, min and max did not show much variation from one element to another, nor did they appear to reflect apparent brightness. The mean was used in subsequent measurements.

The first measurements were made on image 215.jpg, taken at 13.19 on 22 August 2016. Subsequently measurements were made on image 4602.jpg, taken on 30 June 2018 at 11.57, and 5067.jpg, taken 21 July 2018 at 13.45. The intention was to see if the results were robust and independent of, for example, the angle of the sun.

These jpeg images are compressed and adjusted to match the non-linear performance of the human eye. So I repeated the measurement with a RAW unprocessed photograph 5067.CR2 converted to a TIFF, to give the pixel intensities without correction.

The grayscale value or pixel intensity was given as a number between 0 and 255 for jpegs, and 0 to 1 for the TIFF.

Results

The results of the series of measurements made on all four photographs are shown in Figure 8. For clarity, the error bars are only shown on one series as an indicator of the range of measurement.

The absolute values of the average pixel intensity of a petroglyph element varies from one photograph to another. However the relationship between the average pixel intensity of different elements is consistent between digital photographs. In particular, the ordering of the images from brightest to darkest is 93% consistent, that is there are only two inconsistencies out of 27 measurements from the three jpegs.

This tells us that when taken with care a digital camera image can consistently record the relative brightness of different elements of a petroglyph panel. The measurements could be a useful tool to arrange the elements chronologically, like strata.

Those that are buried deeper under rock varnish can be assumed to be older. But in the same way, we can only apply this method in an area where we have reason to believe that the levels have built up consistently over time and have not been disturbed.

It would help if we can establish a plausible link between darkness and age. Specifically, we would like to be sure that the darkening is uni-directional. That is, the images become darker consistently, though the rate of darkening may change. We already have evidence for that in the overlay of images. A possible mechanism for darkening would provide additional support.

Rock Varnish

The appearance of the rock surfaces here suggests film formation – some panels have a darker, shinier surface. Such a coating has been described on rock surfaces in desert environments all over the world. The scientist and traveller Alexander Humboldt was the first to study it, after he observed a dark black film coating rocks on the banks of the Orinoco [1]. The rocks, were “smooth, black, and as if coated with plumbago” he wrote. A chemist in Paris analysed samples and told him it contained oxides of iron and manganese. Humboldt realised that the manganese did not come from the granite rocks, and so must be air or waterborne.

This film is now known as rock varnish, and it has been widely studied. It consists of a dull to lustrous, black to bluish-black coating of iron and manganese oxides, clays and trace elements that forms on a variety of rock surfaces.

Its formation is thought to be dependent on inclination and orientation of rock panels, type of substrate, pH conditions, wind, moisture and sunshine, amongst others. Despite these many variables, there have been several attempts to use aspects of rock varnish to date the petroglyphs that have been created by pecking or scratching through the varnish [2].

A recent study using DNA analysis of varnishes from a range of sites across the Western USA suggested that the manganese is collected and deposited primarily by the photosynthetic Chroccodidiopsis genus of Cyanobacteria [2]. The same organism was found in an analysis of rock varnish from Yungay in the Atacama Desert in northern Chile.

The study indicated a mechanism for varnish formation that involved cyanobacteria with access to sunlight and water.

“Thus, we propose a hypothesis for varnish formation. These Cyanobacteria grow on sunlit rock surfaces with intermittent access to water …. When they die, the residue from their biomass provides an enriched manganese source that is ultimately oxidized to form the oxide mineral cements that comprise varnish. Since varnish forms over timescales of millennia, a well-developed varnish sample represents the time integrated manganese accumulation of many, many generations of cells, which are sparsely distributed at any given time.”

Researchers have measured the thickness of rock varnishes on rock surfaces with an age of up to 250 ka, at sites across Western USA [L]. They found that there was no correlation between age of a rock surface and varnish thickness. Dividing the latter by the former they presented an “assumed growth rate” of between 1 and 40 micrometres per thousand years. The measured thickness varied from 1 micrometre to 200 micrometres, regardless of age.

One coat of paint with a dry film thickness 40 microns – 2000 years.

Whether the measurements at Cochineros can be a useful tool to aid chronological ordering of petroglyphs does not depend on whether rock varnish growth rates are consistent over millennia, or over the American South West, or both. We do not even require that growth rates are consistent across the Cochineros site, which clearly they are not. We merely require that growth is consistent, that is unidirectional, across some panels over the timescales of the engravings, perhaps one to two thousand years.

Secondly, we must be able to identify which panels, or parts of panels, show this consistent growth.

It is easy to identify some areas where the growth of rock varnish is locally enhanced. One of the most prominent is a dark band of varnish running down the river-facing panel of Piedra 1 (Figure 8a, 8b, 8c).

Above this band there is a sloping upper rock surface which leads towards a concave lip on the edge (Figure 8b). Dew or moisture collecting on the upper surface of around two square metres would flow down towards the band.

It is also possible that rituals here involved pouring or diverting water over the rock. Nine tupus pecked into the upper surface of rock indicate the direction of flow, and there are fifteen serpentine motifs drawn in the direction of flow, on the upper surface and the vertical surface (Figure 9).

The band crosses a white band along the foot of the panel. A band like this is seen on other rocks at the site, including Piedra 11. It seems likely that the historic ground level was higher here and the white band is on a part of the panel which was below the soil surface, so that rock varnish did not form. The white band runs continuously, except where it is crossed by the dark band. This suggests that rock varnish formed rapidly over band, but only where there was increased moisture.

Similar darker areas are seen where a flow channel leads down from the upper surface of the rock on Piedra 3 or where there are folds in the rock surface on Piedra 4 ( Figure 10a, 10b, 10c). On the almost vertical faces of Piedra 1 and Piedra 4 there are concave dark channels and protruding clay-brown areas, clearly showing localised varnish formation.

Where rock panels are covered with engraved elements close together, which is the case with manz panels on the most significant five rocks at Cochineros, the change in brightness due to areas of greater varnish growth is easy to identify. Equally, this allows us to choose areas which are free of such variation.

We can not assume that varnish formation is consistent from panel to panel. We would have to calibrate each panel, or part of panel, independently. However, because the Cochineros site has nine panels with areas of from 2 square metres to four square metres, there may be scope for extension of the calibration across the site. Some elements which are repeated on several panels are so similar that it would be reasonable to assume that they are made by the same artist at the same time, opening the possibility of cross-calibration between panels.

Dew formation

It almost never rains on the western slopes of the central andes below about 1500 metres. However heavy dew and humid fog drift in from the cold Antartic current from July to October, up to a height of about 800 metres. Cochineros is too low for rainwall, but low enough for seasonal fog.

The localised formation of darker areas, and thicker rock varnish, appears to be linked to sources of moisture.

If Type I and II manganese rich rock varnishes are produced by cyanobacteria then the key ingredients would be sunlight and water. The presence of these in sufficient quantities necessarily depends on inclination and orientation of the rock panels, and prevailing wind.

It is clear that moisture gathers on depressions in the rocks at Cochineros, indicated by the dusty ring marks that can be seen there (Figure 11).

In arid regions, moisture will condense on cool surfaces in the night or early morning. Man-made dew condensers to exploit this behaviour have been built and trialled in desert regions around the world. They can collect half a litre of water per square metre of surface, each night and morning.

Designers have studied the efficiency of these dew condensers to optimise their ability to collect water [D]. They must heat up minimally during the day and cool rapidly in the night. There must be sufficient wind to move moist air over the surface, but not so much that the surface is heated too rapidly by the wind. Wind also speeds up evaporation during the day. To collect and retain the maximum dew, designers suggest that the face be angled at 30 degrees [c].

These design points for dew condensers neatly match the different panels on the Cochineros rocks. The panels heat up during the day from air and sunlight. They then cool overnight, particularly if the skies are clear. Those panels which are almost vertical will receive only glancing sunlight, so they will not heat up so much during the day. However they will still radiate heat at night. The near vertical panels are therefore more likely to create dew, or will create more dew. The six darkest panels seen at Cochineros, on Piedra 12, Piedra 11, Piedra 6 and 6a, Piedra 1 and Piedra 4, are at an angle of around 60 degrees (12, 11 and 6), or near vertical (1, 4, 6a), and facing east or south.

Similarly, the horizontal surfaces may heat up too much if the wind is high. The six panels above are also somewhat sheltered from the wind because of their inclination.

These differences in rock varnish can be seen from panel to panel, and they can also be seen where the orientation or inclination of a panel changes.

All the rocks at Cochineros have a similar profile, with a gently rounded upstream side and a more abrupt fall on the downstream end (Figure 12a,b, Figure 13a, b). This is consistent with embedded boulders being worn away by flowing water, which would have been occasionally exacerbated by extreme floods and the grinding action of gravel and pebbles swept downstream. As a result, the northern, upstream panels receive more sunlight during the day. This may explain their generally grey appearance compared to the darker south facing panels.

The downstream and river facing, or south and east panels on the rocks at Cochineros present a dark glossy surface against which the engravings stand out most clearly. On north and west facing panels the variation in image brightness is small. This suggests the varnish formation is most rapid on these south and east facing panels.

Concave areas in the panels, or channels which lead from the upper surface, are often darker. Raised areas such as ribs of rock or protruberant areas or rock are usually a clayey brown colour. Smooth northern, upstream facing panels or near horizontal panels are usually a dull grey. These observations give us some clues about differentiations in varnish formation. Across twenty rocks and boulders, only six panels appear to offer a varnish formation which is consistent over broad enough areas to allow some useful conclusions from brightness meaurements.

Calibration

If some petroglyph elements are of known date this would enable us to tentatively put numbers to our chronologically ordered sequence of elements.

If we can plausibly assume that the change was for example linear with time, then we would have a method of dating. (Figure 4 a, b, c).

Here I have a powerful aid, in the otherwise deplorable graffiti on the panel, the heart which is dated 1964.

There is a minor complication here: it seems unlikely that “1964” is the date of the drawing, since it is not shown on Antonio Nuñez’ Fig 1488 sketch of the rock surface.

Nuñez was Cultural Ambassador to Peru 1972-1977, and presumably visited the sites and made his sketches during this time. His four volume work on Petroglyphs of Peru was published in 1986. So the heart would appear to have been drawn after 1972 at the earliest, unless Nuñez simply omitted it. This would not seem to match his punctilious style. The heart is seen on a photograph by Tantalean which was published in 2010, though it could have been taken some years earlier, as he wrote about the site as early as 2005.

It could be that the writer met his partner or got married in 1964, and the engraving was made twenty or thirty years later. For the purposes of calibration, these uncertainties in date are not significant, compared to the uncertainty in pixel intensity values. I have taken 1989, or 30 years Before Present, plus or minus 20 years, as the calibration date for the heart.

A second calibration point is provided if we assume that drawings on the rocks did not continue long after the Spanish invasion. The Spanish reached Pachacamac in 1533. They founded Lima in 1535. In 1571 the Reducciones began, and Peruvians were forced to move to new settlements under Spanish control, such as Calango. By 1586 Corregidor Davila Briceño had moved the people 0f 200 Yauyos villages into 39 Spanish centres. In 1608 the Extirpacion de los Idolatrias began its visits throughout the Huarochiri region, destroying sacred sites, and intimidating those who practised traditional worship and rituals. In this context, it seems unlikely that people would have continued to visit their holy places and leave their marks upon the rocks, after say, 1579, or 440 BP, the present being 2019. So I assume the brightest element, the dot track, can be dated to 440 BP.

An alternative possibility is that the Dot Track and associated figures relate to a rededication of the site to Paria Caca – an Inca re-branding, which would include the addition of around 100 tumi elements across almost all rocks of the site. This would date to around 1450CE, or around 569 BP.

The calibration is not particularly sensitive to +-10% variation in this date.

Assuming a linear relationship, I then simply draw a calibration line, using the two images, the heart and the dot track, of known age.

The pixel intensity of other images, whether measured from two jpeg photographs taken in 2017 or four RAW images taken weeks apart in 2018, fit to this line consistently. The brightness scale as determined from photographs is robust for other images on the panel.

Using a linear fit gives age estimates for different images that span around 1500 years.

This fit takes us back from the bright heart, around 1989, past the colonial dot track of 1579, to the tupus, around 1250CE. Older still are the lake figures at the top of the panel, holding up the mountain, dating to around 1000-1200CE. And then we can see before the Late Intermediate Period. The two swallows flying up the side of the panel may be 900CE but the spiral on the centre of the panel – it is commonly proposed that central images are the earliest – is closer to 600CE, while the Old Llama, the four legged camelid over which the three Tupus were drawn, may be 500CE.

The uncertainty in the brightness measurements means that each of the dates quoted is plus or minus several hundred years. Nevertheless, they provide a narrative for the evolution of the panel which is revealing.

From data compiled by Liu and Broecker (2000), the measured thickness varies from 1 micrometre to 200 micrometres, regardless of age. It has been suggested that spalling, or shedding of outer layers, occurrs when the varnish reaches a certain thickness. Even at its fastest, such growth amounts to a coat of paint (25-50 microns) every thousand years. So we might expect it to take two to three coats of paint, or two or three thousand years; to cover over the whiteness of drawings at Cochineros.

The reduction in pixel intensity of 10% over 400 years is compatible with the growth of a

There are large motifs on the panel which provide a check on consistency in space, that is from left to right across the panel and from top to bottom.

The dot track runs for a length of almost two metres from close to the top to close to the bottom of the panel, and the dots have a consistent brightness (with a large variation) until the lowest seven dots, which are also smaller. Similarly the heart extends across 60 cm horizontally and vertically, and appears consistently bright. The white band at the foot of the panel is unbroken, which suggests there are no vertical areas with differential film formation. 

On the other hand, the top and shoulder of the rock, and the upstream facing panels, seem to offer different conditions for rock varnish formation as can be seen from their patina.

The linear fit simply assumes that if the image brightness falls, for example, by 20, from 220 to 200, in 400 years, then it will fall by five times as much, from 220 to 120, in 2000 years. In fact if the darkening is caused by a film which grows at a constant rate of say four microns every hundred years, the relationship would be exponential. That means if it falls by 10%, from 220 to 200, in 400 years, it will fall by 10% plus 10% plus 10% plus 10% in 2000 years. So the brightness will fade to 90% to 81% to 74% to 62.5% (or to 137 pixel value) rather than 50%. This makes little difference to the ages of the brighter images such as tupus, but suggests the darkest figures could be a few hundred years older. As an example, the exponential plot for one of the images 4606.cr2, is shown below. The Spiral behind the 1964 still dates to 1400 BP or 600 AD, but the dark figures may be pushed back to 100-200 AD.

If the film forms at a constant rate with time then the fit would be exponential. Rock varnish formation is thought to depend on climatic conditions – temperature, humidity, and wind. These conditions would not have changed greatly over just a few thousand years, prior to the spanish invasion and the destruction of the environment that followed.

Late Intermediate imagery

Several drawings on other panels recall Chancay-style textile or Late Horizon images.

There are engravings of sea-birds which reflect the engravings at Lunahuana and are similar to Chancay or Chimu textile imagery found at Pachacamac, from 1000 to 1470 CE.

A pair of felines close to ground level on Piedra 9 resembles Chancay woven textile designs.

Birds flying over the shoulder of Piedra 1 recall Ychsma (1100 to 1470 CE) ceramics.

These drawings are not on the dated panel, and so the comparison is only qualitative, but they are clearly visible bright images, and they can be assigned stylistically to the Late Intermediate or 1000 to 500 years BP.

Another confirmation of the dating comes from the main tableau of the panel which shows several lake figures reaching up to peak outlined with jagged lines, and the dot track and related figures climbing upwards. If these motifs represent Paria Caca and related rituals such as annual pilgrimage (the dot track) and llama sacrifice, then their dates should agree with what we know of that.

A clue here is the alleged age of Tutay Quiri, founding father of Huarochiri and the Checa, whose mummified remains were taken to Lima in 1611 to be publicly burnt as a gesture of destructive contempt for everything Peruvian. Jesuit Acosta was told by the Peruvians that he was more than 800 years old (another source says 600 years old). If this date is reliable, then Paria Caca’s presence in the mountains and valley may date from 800 CE through to, say, the extirpation of the idolatries around 1620.

The proposed dates from the brightness measurements put the lake deities at around 1000 CE, whilst the dot track and associated figures that may be holding llamas date to the early years of the Spanish invasion. I feel that there was an older original dot track, contemporary with the horizontal tracks, which was refurbished, with new figures added, in Inca or spanish times.

But at the further end of the brightness and time scale, there are the dark images – those that do not appear as white marks on the black stone surface. On this panel, these include a trumpet player, a complex dark figure, and other marks. They do not appear to relate to the Paria Caca imagery, and they are mostly purely abstract.

These dark images are almost invisible for someone walking amongst the stones under a bright midday sun (Figure 10). But when I position myself to see the sun close to the June solstice reflecting off the slanting southern face of Piedra 12, the hidden engravings stand out dark and clear against the shining rock surface (Figure 11). The motifs show up even more clearly when the image is stretched to account for perspective (Figure 12).

The sloping faces of Piedra 12 have at least thirty dark images. A few bright images – llamas, foxes – have been drawn over them. These dark images are also drawn on other rocks at Cochineros. But without the fortuitous angles of the late June sunlight slanting off the rocks, they are very difficult to see.

The majority of these images are abstract and geometrical, including parallel lines, curves and dots. There is little or no repetition. They are deeply inscribed, and evident from the surface depression or change of texture rather than colour change. On the estimated time scales above, they may date to 1500 to 2000 years BP or earlier.

These dates would include the period of Wari presence on the central coast, but there is nothing in the motifs on the rock that suggests Wari influence. However, the abstract geometric images do reflect a similar change in ceramic decoration over time. The Lima people built and occupied many of the city pyramids including Huaca Pucclana in Miraflores and Huaca San Marcos, behind the zoo, from 1700 t0 1200 BP. They created the earliest constructions at Pachacamac. Lima culture ceramic styles were also geometric, in contrast with the brighter Late Intermediate Chimu/Chancay/Ychsma figurative motifs of camelids, birds and felines.

This site has been used as a drawing board for close to two thousand years. Complex abstract imagery dating to between 1000 and 2000 BP has been overwritten by more recent figurative images of birds, llamas, fish, tupus and tumis from 1000 to 500 BP. Several phases of drawing are evident, and there is scope for much more detailed study.

Go back to 151b – The Rock of Paria Caca 2

Go forward to 152c – Conclusions and Ruminations 2

Macholdt, Dorothea & Jochum, Klaus & Pöhlker, C. & Arangio, Andrea & Förster, Jan-David & Stoll, B. & Weis, Ulrike & Weber, Bettina & Müller, M. & Kappl, Michael & Shiraiwa, Manabu & Kilcoyne, David & Weigand, M. & Scholz, D. & Haug, G.H. & Al-Amri, A. & Andreae, Meinrat. (2017). Characterization and differentiation of rock varnish types from different environments by microanalytical techniques. Chemical Geology. 459. 10.1016/j.chemgeo.2017.04.009.

Geochemical studies on rock varnish and petroglyphs in the Owens and Rose Valleys, California

• Meinrat O. Andreae, Abdullah Al-Amri, Tracey W. Andreae, Alan Garfinkel, Gerald Haug, Klaus Peter Jochum, Brigitte Stoll, Ulrike Weis

TY – JOUR
AU – Liu, Tanzhuo
AU – Broecker, Wallace
PY – 2000/02/01
SP –
T1 – How fast does rock varnish grow?
VL – 28
DO – 10.1130/0091-7613(2000)28<183:HFDRVG>2.0.CO;2
JO – Geology
ER –

Anthro@email.arizona.edu

Possible to extend data using fabulous birds (Chimu, relocated after the Inca conquest 1470?)

Lake deity also on Piedra 11, fabulous bird on 11, 6, 6b, dogs/foxes on 11 and 6a,

Method

Fiji, sampling,

The book does not record perhaps 25 percent of the artwork at the site, including several major panels. Nuñez is photographed sitting on top of one such panel, so he clearly saw and probably recorded some of these images, but did not include them in the published book. His partial recording of images on some difficult to access panels suggests that he did not see some darker images which only become apparent under the right lighting conditions. That is, he was probably looking down on the panels from above, which means the dark images are all but invisible.

Whilst there are many tumis, they represent less than 10% of the images.

What is necessary to use rock varnish growth

Stable macro-climate over the period under consideration

Stable micro-climate – eg increased humidity from change or course of a river could affect

A panel with consistent growth, ie consistent orientation to sun, wind,

in simple terms, this probably menas flat panel.

However, eany individual site is likely to have panels with a range of orientations and a site assessment may enable choices to be made about what meausrements might be valid.

elements drawn using the same technique, or producing the same results that is an element with large area, and fully pecked or abraded to reveal a new surface within that area.

when the sun was behind the panel and its direct light was glancing off the surface. By standing to the west of the panel, any direct reflection of light from the sun could be avoided. The ideal would be that the panel receives illumination of equal intensity from all directions, and in practice the bright sky provides a good approximation to this.

↵

1. [2] A. von Humboldt

, Personal Narrative of Travels to the Quinoctial Regions of America during the Years 1799–1804 by Alexander von Humboldt and Aime Bonpland (Bell, London, 1812).

↵

1. [2] A. Watchman, A review of the history of dating rock varnishes. Earth Sci. Rev. 49, 261–277 (2000).

[L] T. Liu, W. S. Broecker, How fast does rock varnish grow? Geology 28, 183 (2000).Abstract/FREE Full TextGoogle Scholar

[2] An ecophysiological explanation for manganese enrichment in rock varnish

Usha F. Lingappa, Chris M. Yeager, Ajay Sharma, Nina L. Lanza, Demosthenes P. Morales, Gary Xie, Ashley D. Atencio, Grayson L. Chadwick, Danielle R. Monteverde, John S. Magyar, Samuel M. Webb, Joan Selverstone Valentine, Brian M. Hoffman, and  Woodward W. Fischer

PNAS June 22, 2021 118 (25) e2025188118; https://doi.org/10.1073/pnas.2025188118

[2]  D. S. Macholdt et al., Characterization and differentiation of rock varnish types from different environments by microanalytical techniques. Chem. Geol. 459, 91–118 (2017).

[D] A review: dew water collection from radiative passive collectors
to recent developments of active collectors
B. Khalil1,2 • J. Adamowski1 • A. Shabbir1 • C. Jang1 • M. Rojas1 • K. Reilly1 •
Bogdan Ozga-Zielinski3