The drawings on the stones of Huancani stand out so clearly because they are cut through a shiny black surface patina on the stones. The fifteen stones that are decorated with glyphs are all the same type of rock, with rounded profiles and surfaces or panels that are, in places remarkable smooth and dark. There are other giant boulders scattered across the river bank, but only the “blue” rocks, as they are called in Lunahuana, the next valley to the south, are used as a canvas here.
Alexander von Humboldt was the first to observe dark coatings on riverside rocks. By cataracts high up the Orinoco River in Venezuela he found granite boulders that appeared “smooth, black, and as if coated with plumbago.” Plumbago here is most likely to mean graphite or “black lead”, used to add a deep black polish to cast iron fireplaces in the nineteenth century.
Von Humboldt was one of the greatest South American explorers of the nineteenth century. In the tradition of the times he travelled extensively and observed, questioned, analysed and hypothesised wherever he went. Camped on an island in the river below the cataracts, he speculates on how European visitors to the coast were told of the cataracts, and indians around the cataracts told vague tales of a giant salt water downstream. “Three hundred leagues from the coast, in the centre of South America, among nations whose excursions do not extend to three days’ journey, we find an idea of the ocean,… In the infant state of society, the exchange of ideas precedes, to a certain point, the exchange of productions.”
A league was an hour´s travel on foot; three hundred leagues would be thirty or forty days journey. Humboldt himself had left the coast on March 1st and travelled overland from Puerto Cabello, to Aragua, close to Caracas on the coast, to Fernando de Apure and in forty days to reach the Orinoco.
En route he found the Cow Tree “This fine tree rises like the broad-leaved star-apple. Its oblong and pointed leaves, rough and alternate, are marked by lateral ribs, prominent at the lower surface, and parallel. Some of them are ten inches long….When incisions are made in the trunk of this tree, it yields abundance of a glutinous milk, tolerably thick, devoid of all acridity, and of an agreeable and balmy smell. It was offered to us in the shell of a calabash. We drank considerable quantities of it in the evening before we went to bed, and very early in the morning, without feeling the least injurious effect.”
In addition to drinking the “considerable quantities” of sap he tried mixing it with water, tested it with nitric acid and preserved it with carbonate of soda, sending two bottles to Paris for analysis.
He also searched out electric eels. “I was impatient, from the time of my arrival at Cumana, to procure electrical eels. We had been promised them often, but our hopes had always been disappointed.”
He found them in stagnant ponds in the plains, where the local people caught them for him by “fishing with horses” – rounding up thirty wild horses and mules and driving them into the water. The fish, disturbed from their burial beneath the mud, attack the horses in a frenzy of electric shocks until both horses and eels are worn out. Von Humboldt then analysed the eels minutely, testing their electrical power on himself and his colleagues.
“I placed my hand…within a very small distance from the electric organs; yet the strata of water transmitted no shock, while M. Bonpland irritated the animal strongly by an immediate contact, and received some very violent shocks….after having made experiments during four hours successively with gymnoti, M. Bonpland and myself felt, till the next day, a debility in the muscles, a pain in the joints, and a general uneasiness.”
Von Humboldt was a scientist traveller. He investigated at the cotton, cocoa and indigo plantations in the plains around Caracas, he wrote about the behaviour of the troupes of howler monkeys as he climbed out of the plains towards the town of Ortiz, the entrance to the steppes. He studied the thermal waters of San Juan “The waters are impregnated with sulphuretted hydrogen … the thermometer rose only to 31.3°”.
But von Humboldt was above all a geologist, and he found plenty to satisfy his curiosity in the hills above Caracas. “On the southern slope of the mountains of the coast, four different formations of rock cover the gneiss. We shall first give the description of the different strata…”
And so he does, in considerable detail, for the next five pages.
“On the south of the Cerro de Chacao, between the ravine of Tucutunemo and Piedras Negras, the gneiss is concealed beneath a formation of serpentine, of which the composition varies in the different superimposed strata. Sometimes it is very pure, very homogeneous, of a dusky olive-green, and of a conchoidal fracture: sometimes it is veined, mixed with bluish steatite, of an unequal fracture, and containing spangles of mica. In both these states I could not discover in it either garnets, hornblende, or diallage.”
He speculates on the nature of deserts: he could give 35 terms which which Arabian authors express their ideas of desert, he tells the reader, but contents himself with ten. He takes measurements wherever he goes – the latitude by observation of the stars, altitudes by barometric pressure, the angular displacement of mirages.
His journey is interspersed with observations of his surroundings, the vegetation, pre-conquest roads and tumuli, farming techniques, indigenous and colonial peoples, albino deer, the price of jaguar skins, the height and diameter of palm tree trunks, and how herd animals and the rise of pastoralism leads to nineteenth century government.
The economy of what was then coastal Venezuala, the unwooded savannah or pampa along the coast around present day Caracas, consisted, by von Humboldt´s observations, of a hundred landowners owning up to 15,000 cattle. Lllaneros, plainsmen, some freed men and some slaves, riding on horseback naked to the waist and carrying lances rounded up and branded the cattle.
“The proprietors of the great hatos are entirely ignorant of the number of the cattle they possess. They only know that of the young cattle, which are branded every year with a letter or mark peculiar to each herd. The richest proprietors mark as many as 14,000 head every year; and sell to the number of five or six thousand.”
The meat was sold locally, the skins were exported to Europe. Buenos Aires, further south in Argentina, had a similar economic base, but four times the size.
These observations were as von Humboldt travelled inland, through the plains and then into the first range of hills behind the coast. But there was more to come.
“Boas are killed, and immersed in the streams, to obtain, by means of putrefaction, the tendinous parts of the dorsal muscles” observes von Humboldt, in one final observation in the town where they were staying, “of which excellent guitar-strings are made at Calabozo, preferable to those furnished by the intestines of the alouate monkeys,” before he left on March 24th, to head towards the river Apure. This was the age before plastics when the natural world provided raw materials for manufacture and travelling through a new continent was to be a guest in an Aladdin’s Cave of previously unknown treasures. The New World, of course, had been in contact with Europe for three hundred years, but Spain and the Spanish Crown had guarded their assets closely, and it was only with the Bourbon Reforms that they began to rethink their colonial assets. This was the context that had enabled von Humboldt to get permission to visit the Spanish Americas, under his own funding.
Three days later, after nights spent in hammocks fending off giant bats, they reached a Capuchin mission centre.
The river here was several hundred metres wide, but only a minor branch of the Orinoco, which flowed west and then north before being joined by the Apure. There were dolphins in the water, over 600 kilometres from the coast. He took measurements of humidity and temperature as storm clouds formed overhead and the sky turned from blue to grey, with thunder rolling around. Rain began to fall heavily, and von Humboldt stood on the banks of the river, drenched in a tropical storm, holding an electrometer aloft to measure the electricity in the air.
From this, and his previous observations as rains changed to heat during the course of the year, from Columbia and the Caribbean islands down to Venezuela, the breezes and calms, and the condensation and the clouds, stars twinkling low on the horizon, the electric tensions in the air, he proposed in a few paragraphs how wind and airborne moisture, driven by the warming of the land and ocean surfaces, gave rise to global weather patterns.
Whilst he was in the region he also noted the presence of rock art. “A few leagues from Encaramada, a rock, called Tepu-mereme, or the painted rock, rises in the midst of the savannah. Upon it are traced representations of animals, and symbolic figures resembling those we saw in going down the Orinoco, at a small distance below Encaramada, near the town Caycara. Between the banks of the Cassiquiare and the Orinoco, between Encaramada, the Capuchino, and Caycara, these hieroglyphic figures are often seen at great heights, on rocky cliffs which could be accessible only by constructing very lofty scaffolds.”
A week later, after boating downstream on the Apure, they reached the main body of the Orinoco, four hundred metres wide at the time, the waters at their lowest. It was now early April. Travelling upstream they visited a famed nesting site for turtles, an island in the middle of the river, where several different peoples of the forest had gathered, as they did every year, to dig up the turtle eggs from the sand.
“On the 15th of April, we left the island of Panumana at four in the morning, two hours before sunrise. We did not arrive till very late at the foot of the Great Cataract, in a bay called the lower harbour (puerto de abaxo); and we followed, not without difficulty, in a dark night, the narrow path that leads to the Mission of Atures, a league distant from the river.”
Von Humboldt observed that among the cataracts, and wherever the Orinoco, between the Missions of Carichana and of Santa Barbara, periodically washed the granitic rocks, they became smooth and black. The colouring matter did not penetrate the granite, which was coarse-grained with large crystals of feldspar, quartz and hornblende. The black crust he estimated as being is 0.3 of a line (a line being about two millimetres) in thickness. On breaking the stone with a hammer, he found the inside white, without signs of decomposition. The shiny black surface was seen on stones both in the bed of the Orinoco, and several hundred metres from its present shore, on heights which the waters would never reach.
“What is this brownish black crust,” he asked, “which gives these rocks, when they have a globular form, the appearance of meteoric stones? What idea can we form of the action of the water, which produces a deposit, or a change of colour, so extraordinary?”
He observed that similar black rock coatings had been seen by the cataracts of Syene in Egypt, and in the rapids that obstruct the river Congo or Zaire. The Indians of the Orinoco told him that the rocks were black only where the waters were white.
He sent samples of the Orinoco rocks to a Parisian chemist who found that the black crust was composed of oxides of iron and manganese.
The big questions were asked by Von Humboldt as he camped on a strip of sand besides the Orinoco and pondered on the shiny dark coating he found on the rocks around. He saw a dark black layer largely composed of manganese oxide, built up on rocks around the falls. And he asked where did the manganese come from.
If the black crusts were formed by decomposition of the granitic rock which they covered, von Humboldt reasoned, how is it that they are spread so uniformly over the whole surface of the stony masses, and are not more abundant round a crystal of mica or hornblende, which contain up to 20% manganese and iron oxides, than on the feldspar and milky quartz which contain virtually none? And why do the same rocks, but further from the Orinoco, or bathed by the neighbouring Rio Negro, not develop the same coatings?
He thought it more likely that the oxides were brought to the rock surface by the river. As the varnish did not contain grains of sand or spangles of mica, he thought that it must come not from particles suspended in the water, but from minerals in solution.
Nile mud did not contain manganese, so could not be the source of the crusts there. These three sites shared constant high temperature, periodic rises in the water level, and granite, gneiss or hornblende rocks.
A long residence at these cataracts, he concluded, would be necessary to examine the phenomenon and solve the problem.
In contrast to von Humbolt´s simple visual observations and open-minded inquiry on the banks of the Orinoco, a battery of high powered, high cost instrumentation was used to analyse rock varnish throughout the 1980s and 1990s.
Scanning Electron Microscopy (SEM) revealed that the varnish was laid down in layers. X-ray diffraction (XRD) showed that these contained clay and quartz as well as manganese oxides and hydroxides.
The resultant coating forms a clear layer distinct from the underlying rock, and with a different chemical or mineralogical composition. It typically contains micrometer scale alternating layers with pocket of clay, feldspar and haematite.
The varnish, it was found, is primarily composed of particles of clay along with iron and manganese oxides. There is also a host of trace elements and almost always some organic matter. The colour of the varnish varies from shades of brown to black. It is observed only on physically stable rock surfaces that are no longer subject to frequent rainfall, fracturing or wind abrasion.
It may be that the clay is carried in the air. Once on the rock surface, it catches additional substances that chemically react together in the presence of moisture.
An important characteristic of black desert varnish is that it has an unusually high concentration of manganese, 50 to 60 times more abundant than in the atmosphere or the local environment.
Time estimates for the formation of desert varnish range from 25 years to 300,000 years. Archaeological evidence suggests an age of 2,000 years for a discernible coating and 10,000 years for a thick coating.
The Orinoco varnish found by von Humboldt, or the rock varnish of Huancani, formed on rock along stream banks, may have a different compositions and formation processes to “desert varnish”.
This desert varnish or rock varnish has been found in the Sonoran and Mojave deserts of the southwestern United States, California and Arizona, in northern Mexico, the Negev Desert in the Middle East, the Gibson and Great Victoria Deserts of western Australia, and the Gobi Desert of China, as well as the dry Pacific coast of South America, from Chile to Peru.
It can be a thin reddish brown coating, rich in iron. This is the surface on which the petroglyphs of Checta and of Toro Muerto are drawn, as well as those of Checas, the town on a elevated river terrace a few hours upriver from Huancani. Or it can contain high levels of manganese – a hundred times higher than the rocks on which it forms – giving it a deep black colour. These are the coatings on rocks at Huancani.
Field observations made in the arid and snow-free Dry Valleys in southern Victoria Land, Antartica in the 1970s, and published in 1981 made it clear clear that its presence and thickness depended on several local conditions.
The reddish brown coatings – largely iron oxides – developed best, they found, on north-facing dolerites in areas with a prevailing southerly wind. Thinner varnishes were observed on sandstones and granites. Altitude and orientation to the sun were also observed to be factors. Coarse-grained dolerites grew thicker coatings.
As the varnish coating in the Antartica depended on wind, sun, altitude and type of rock, it varied from one face to another of the same rock. Most commonly, it was seen to form only on one or two faces of the rock, those faces protected from the prevailing wind. For similar protected faces in different valleys, north facing rocks had a thicker coating than those facing East.
One explanation is that it is caused by manganese-oxidizing microbes which are common in environments poor in organic nutrients. The stratification of layers seemed to reflect changes in the proportions of iron and manganese, which might relate to changes in the atmosphere or local environment at the time. If the pH is above 7.5 the manganese-concentrating microbes can not flourish. Then the red and orange varnishes develop, rich in iron.
One hypothesis for Mn/Fe fluctuation considers Mn-rich and Fe-rich varnishes to be related to humid and arid climates. Where rock varnishes show alternating orange and black layers, these could relate to dry and wet periods, or high and low pH periods. In either case, if rock varnish has grown slowly over tens or hundreds of thousands of years, these layers could provide a record of local climate change.
The surface of the black rocks is glossy and the designs such as the bird in the upper right are cut through this to the rock beneath.
The formation of rock varnish catalysed by bacteria suggested another direction. Microorganisms were found to be common within varnishes on Earth. DNA analysis of rock varnish in California found 100 million bacteria per gram in rock varnish samples. Several bacteria were involved, a multi-ethnic community building their homes on the surface of the rock.
As the layers appeared to be formed from compounds reaching the rock from airborne dust, processes where microbes gathered the manganese from the environment to form a protective coating from damaging solar radiation were proposed. Bacteria were the collectors of material from rock, air and water that formed the surface.
Designs are cut through a surface coating, here an orange varnish with a largely iron component. Of the petroglyphs so far identified at Huancaini, only three are on rust-red rocks, with sixteen or more on shiny black varnish.
The study of varnishes on earth suddenly became more fashionable with a view to the proper design of experiments in coming decades for detection of life on other planets. Mars, and any life forms it harbours, faces similar conditions to earthly deserts – dryness, intense solar radiation, extremes of temperature.
It has been suggested that varnish or varnish-like materials may exist on Mars. If so, it could be where we might find extraterrestrial life forms such as bacteria. If there is life on Mars, it could be in the form of micro-organisms sheltering in rock varnish.
Rock varnish in the Atacama Desert in northern Chile has attracted interest because the environment has been seen as a model for Mars. Virtually no organic life has been found in the surface soil in the Atacama –but only a few micrometers of varnish has been found to provide a shelter for many different microorganisms.
We can see, walking around the rocks of Huancani, that there is a variation in formation of varnish. The southern, downstream faces, relatively steeply angled, have a shiny black patina. flatter surfaces or those facing north have a more grey appearance. The almost vertical faces towards the east, the river, on several rocks have a smooth dark appearance. In some rocks, there are patches, streaks of varnish, running vertically down some rock panels. There are even drawings on the rocks where the brightness is selectively reduced, where a streak of varnish appears to have grown across part of the image.
It seems that the patina on the rocks of Huancani is due to rock varnish, and the slow growth of the varnish leads to the loss of brightness of our petroglyph designs. This would provide a more convincing mechanism for the change of brightness, than the previous idea that a layer of broken white crystal is being eroded. Understanding the varnish growth may provide a better route to dating the drawings on the stones, or at least providing a stratigraphy.