During the summer of 2006, the excavations made by the "Bosnian Pyramid of the Sun" Foundation on the two hills of Visocica and Pljesevica have revealed some features (sloping stone slabs, "pavements" and "terraces") that can, for the layman, appear as artificial. But all these features can receive a totally natural explanation, and similar ones have been described by geologists in other places in the world. I do not intend to provide here a real geological and geomorphological study of Visoko region (a terrain work would be needed for that), but rather to show that all the features used by the Foundation as "evidence" of man-made "pyramids" can be explained by the geological history of the place .
The beginning would be about ten million years ago, during the era called Miocene (en) (a part of the Tertiary Era); at this time, when the uplifting of the Dinarides or Dinaric Alps (en) (which form the moutainous skeleton of Bosnia) had already begun, a huge intramountain lake was extending from Sarajevo in the South-East to Zenica in the North-West, between the two ranges of Zvijezda (North-East) and Vranica (South-West).
On the above geological map at 1:300,000 of Bosnia, the location of the former lake of Sarajevo-Zenica appears in yellow, noted "M" for Miocene, between the much older (en) geological layers of the Primary Era (in the South-West, grey, S and D for Silurian and Devonian, with rhyolithic (en) intrusions in red) and of the Secondary one (in the North-East, in purple and green, T, J and K standing for Trias, Jurassic and Cretaceous).
This lake lasted a very long time, in fact most part of the Miocene Era. On the above extract of the geological map, one can see that the layers around Visoko are noted "M2" and "M2,3": that corresponds to Middle and Upper Miocene, roughly from 15 million to 7 million years before present. During all this time (at least during 7 or 8 million years), took place on the bottom of the lake an intense sedimentation, since the authors of the geological map estimate that the total depth of the Miocene sediment in some locations of the Sarajevo-Zenica basin is more than 2,000 meters! The intensity of the sedimentation is related to the fact that all the ranges around the lake were uplifting, alternating phases of swift uplift accompanied by severe erosion thus accumulating a lot of debris at the bottom of the lake, and more quiet phases, of slow uplift, with less and smaller debris. The text accompanying the geological map also mentions some minor geological unconformities (gaps in the regular sequence of geological layers), indications that at times the lake was dried and that the bottom layers had emerged, before the chances of local tectonics brought again the deepening of the lake and sinking of the bottom .
The whole Sarajevo-Zenica basin has so been slowly filled up by thick layers of sediment; and the features of these layers reflect the turbulent history of the Dinarids during the Miocene Era. The stacks of Miocene rocks may call to mind a huge sandwich, comprising hundreds or thousands of different layers of various thickness (from a few millimeters to one meter or more), and each of these layers corresponds to an episode of this long history: layers of thin sediments, lithified (en) in clays (en) or marls (en), deposited during quiet phases; layers composed of more or less fine sands, later cemented in sandstones (en); layers formed of rough debris (centimetric or decimetric), deposited during the most "agitated" periods, later become conglomerates (en) or breccias (en) .
If one looks closer on the sedimentary rocks around Visoko, one will find, from bottom to top, that is, from the oldest to the youngest layers (the more recent sedimentary layers are resting upon the older, deposited first):
lacustrine sediment from the Middle Miocene (M2 on the map), characterized by the predominance of thin sediment (mostly marls and clays) with few sandstone layers, and by a stratification with multiple thin layers, indicating a paleoenvironment of shallow waters with alternate drying and flooding ("lagunal" type but in freshwater). These layers are well represented at the confluence of Bosna and Fojnicka rivers, and they are forming the foundations of Pljesevica hill, where they are visible at the level where most of the excavations are conducted, at the base of the hill:
The fact that the sedimentation of this period happens in shallow waters is responsible for the formation of the "ripple-marks" (en), made by the action, on a sandy bottom, of waves or currents:
the Middle/Upper Miocene sediments (M2,3 on the map) are known by the local geologists by the name "Lasva series"; they are abundant around Visoko (these same sediments form the hill of Visocica and most of the other hills in the vicinity), and comprise two distinct levels:
- at the bottom, layers of sandstone, generally thicker (a few dizains of centimeters) than in the Pljesevica sediment, more rarely interbedded with marls or clays; in these sandstones have been found many fossil imprints of leaves, oak (Quercus), beech (Fagus) or chesnut-tree (Castanea), indicating a temperate paleoenvironment:
The fact that the sediment is a little bit rougher than in the previous period, with less traces of emersion, is probably an indication of a deepening of the lake and of a swifter uplift. These sandstone layers appear in frequent outcrops on the West side of Visocica, along what Mr. Osmanagic calls the "access plateau" of the "pyramid".
- These sandstones are topped by very thick layers of conglomerates, containing rough debris (centimetric to decimetric in size), in a mostly carbonaceous matrix. The debris, coming from the older layers of the surrounding ranges, are various in nature (sedimentary or rhyolithic pebbles) and in shape (some are quite angular, most are more or less rounded, depending of the way and length of their transportation by the rivers to the lake).
This very rough sedimentation of the Upper Miocene indicates a phase of active orogeny (en), with seismic events, swift uplift of surrounding ranges, and very active erosion. These conglomerates form the sloping slabs of the North side of Visocica:
It may be noted that the first hypothesis of M. Osmanagic (being that these huge slabs were cut and transported on the slopes by the "builders" to make the "walls" of the "pyramid") was so difficult to admit that he soon gave hints that these conglomerates, however known and identified by geologists in all Sarajevo-Zenica basin, were in fact an artificial "concrete". At the time I write this text, the Foundation is still pretending that the blocks are concrete "casted" on location , and that it is confirmed by the analysis of the Civil Engineering Institute of Tuzla. However, the single published document from this Institute is a preliminary report (that I have described here), and this report does not mention at all a man-made origin of the conglomerates. Therefore the "concrete" hypothesis is sustained only by sayings from Mr. Osmanagic and the Foundation, when natural conglomerates of the Lasva series are well known and described by local geologists.
The intramountain lake of Sarajevo-Zenica has slowly dried close to the end of the Miocene Era (5 to 7 million years before present); one cannot find around Visoko any layer more recent than Upper Miocene, except the recent fluvial alluvium in the Bosna and Fojnicka valley. The end of the Miocene was the beginning, for this region, of a very agitated "aerial" history, extending, across the Pliocene and Quaternary Era, until our days.
Concerning the structure, the dominant feature of this period is the tectonic constraints on the geological layers: the uplift of the region lasts during all the period, probably with more or less active phases, and exerts on these layers minor and major changes. Here is a list of these changes:
the high tectonic pressures during the uplift have deformed the Miocene layers, in a series of anticlines (en) and synclines (en), each with an axis approximately East/West or ESE/WNW. The above geological map at 1:300,000, or the extract from the more precise map at 1:100,000 hereunder, show that one of these large anticline is present just South of Visocica (which is therefore part of the northern flank of this anticline, hence the important dip (en) of the conglomerate layers):
some other deformations are much more limited, and affect the layers only on a few decimeters or meters, as for instance the small folds that one can find on Pljesevica, where the predominance of plastic layers (marl and clay) facilitates these small localised deformations:
tectonic stresses are also responsible for the numerous little faults one can observe, as for instance here on Pljesevica:
- En rouge les deux failles visibles sur cette photo, avec le sens de soulèvement relatif des blocs ; en bleu ce qui est probablement une couche identique décalée par les failles. - In red, the two faults visible on this picture, with the relative motion of the blocs ; in blue, what appears to be the same layer, displaced by the faults.
as well as for the abundant fractures or joints (en) (with no shifting of the blocks) that one can find in all the layers of the two hills, be it Visocica:
The different aspect of the fractures, as well as the important differences in the size of the blocks "cut" by them, are mostly related with the differences between the layers: the phenomenon is well known to the geologists, more thin the layers, more dense the network of joints; in thick layers, like the conglomerates on Visocica, the joints are rare and distant, when they are much more numerous in the thin sandstone layers of Pljesevica.
The setting of these joints in more or less orthogonal network is related with the fact that there were at least two different tectonic episodes, during which the orientation of the stresses has changed. Numerous other instances of these "jointed bedrocks", looking like real "pavements", are known:
- En Tasmanie
- Tasmania - Source
- En Géorgie
- Georgia - Source
- Dans les Alpes
- French Alps - Source
These natural pavements can easily be mistaken for man-made ones by the layman, as can be seen in the case of this "prehistoric pavement" (en) in Colorado, which has, seemingly, fooled even serious archaeologists during the 1930’s .
The fact that more than one phase of tectonic stress, of distinct age and orientation, took place, is confirmed by the pictures of this kind of blocks:
where one can observe a first series of old joints, filled in with what is probably calcite (en), then a second series, more recent, crossing obliquely the first one.
During the same time that these changes, linked with the tectonic activity, happened, the layers, uplifted by the general orogeny, have been submitted to erosion (en), particularly intense during the Quaternary, and that erosion has given their present shape to the "pyramids". Depending on the climatic phases of the Quaternary (glacial and interglacial periods), different kinds of erosion have been active, and have "attacked" not only the lowest points (fluvial erosion by the watercourses, that creates a characteristic V cross-section of the valleys), but also the slopes and summits (particularly frost erosion, most active during the glacial periods on rocky substrates left without much protection by a scarse vegetal cover of the "tundra" (en) kind, but also more slow processes like surface runoff (en), solifluction (en), carbonate dissolution or chemical alteration (en)).
So, the present shape of the "pyramids" is the result of a mixing of these different factors (en): structural factors (uplift and deformations of the geological layers) and climatic factors (weathering of rocks and removal of the debris). In the case of Visocica, its geomorphological history could be resumed by the sketch hereunder, made by a Bosnian geologist:
1) At the end of the Miocene Era, an anticline with East/West axis is formed; the present Visocica is located on the northern flank of this anticline; the layers forming Visocica, deposited horizontally at the bottom of the Sarajevo-Zenica lake, have thus been tipped, sloping toward the North, but still preserve their original disposition, the more recent conglomerates above the older sandstones. The formation of the anticline has induced an important fracturation of the layers, particularly in the places where the curve is more important, that means at the summit of the anticline.
2) The erosion begins to attack all the anticline, but is much more active on the fractured zones, where the rock is "weakened" by the stresses and where the fractures, even very small ones, ease the passage of percolation water, the most powerfull agent for erosion when combined with the frost. The upper part of the anticline is thus the one where the erosion goes the fastest, even in the hard conglomerates.
3) When the upper layers of conglomerates are "ripped open" by the erosion, the attack of the summit of the anticlinal is accelerated, due to the fact that now are exposed the lower layers composed of sandstone, hard but less thick than the conglomerates, and mixed with much less resistant layers of marls and clays. The eroded place in the heart of the anticline is going to enlarge, and to deepen much faster than the flanks still protected par the hard upper layers.
4) Nowadays, most of the anticline has disappeared, only part of the northern flank is surviving: Visocica is a kind of what geomorphologists call "hogbacks" (en), a fragment, isolated by erosion, of the limb of an anticline.
Thus, the North side of Visocica is a "dip slope" (en): the topography matches exactly the upper layer of conglomerates, the slope of the surface matches the dip of the layer, and that explains the regularity of this slope.
The South and West sides, as well as the slopes of the "access plateau" on the West side, are, contrary to the North side, slopes created by erosion: their shape (much less regular by the way than that of the North side) is the result of fluvial erosion and colluviation (en) ; on these slopes appear the sandstones and marls of the lower Lasva series:
There is still the East side of Visocica: the regular slope, and the presence of conglomerates with a dip identical to the slope, imply that it is again a dip slope like the North side. According to a precise observation of the dips made by a local geologist, the two sides, North and East, would represent a late local deformation of the North limb of the principal anticline, a kind of "secondary" anticline with a North-East/South-West axis, and that would account for the fact that the same conglomerate slabs are found, with a dip toward the North on the North side, and toward the East on the East side.
The case of Pljesevica, the "Pyramid of the Moon", is quite different. First, the geological layers are not the same as in Visocica, as Pljesevica is for the most part composed of older layers (sandstone, marl and clay of the Middle Miocene), thinner and less hard than the Visocica ones ; second, these layers have been much less tipped, as one may observe only a slight dip toward the East or South-East. Pljesevica is thus a hill cut in almost horizontal layers (except for the places with micro-folds as told above), and its slopes do not result from the structure: the North, West and South slopes of Pljesevica have been cut by the drainage network slowly sinking. The single "structural surface" (where the topography matches with a resistant geological layer) is probably the one of the upper "plateau", the flat summit of the hill. Even more than on Visocica, an intense colluviation (due to the mostly plastic and hardly coherent layers) has covered all the slopes with a regular superficial formation, that one can clearly see, above the Miocene layers, on the right of this picture:
This hill, with vaguely triangular sides (but not much more than a bunch of hills in the vicinity), Mr. Osmanagic’s Fondation is slowly giving it the look of the classical "step pyramid" (en), simply by removing part of the plastic layers of marl and clay to expose, on the flanks, the harder sandstone layers: each sandstone layer (and moreover if it is "ornate" with ripple-marks), carefully cleared by a variable width, is limited by a "wall" vertically cut by the workers in the above layers:
- Comment fabriquer une pyramide à degrés - Manufacturing a step-pyramid
- En pointillés, la pente originelle, avant excavation. The dotted line shows the original slope before the excavations.
Naïvety of people totally ignorant of geology? Or attempt to play on the visitors’s naïvety, by making an artificial structure of a simple hill with carefully chosen "arrangements"?