Geoscientist Online

Did the pharaohs of Egypt make their tombs in solid rock on the Theban Plateau? Peter Cobbold, John Watkinson and John Cosgrove* have their doubts.

Geoscientist 18.6 June 2008

The Theban Necropolis, on the banks of the River Nile near Luxor (Fig. 1), is perhaps the foremost archaeological site on Earth. The river winds its way along a flat fertile valley, between pink cliffs of weathered Eocene limestone1. North of Luxor, it does a sharp U-turn around the arid Theban Highlands and the cliffs rise to 500m (Fig. 2). Here, the pharaohs of the New Kingdom (1570 to 1070 BC), including the famous boy-king, Tutankhamun, built their tombs in the hidden Valley of Kings.

Nearer the Nile, amongst low rolling hills, their consorts had another burial ground, the Valley of Queens, while officers of the realm occupied the Tombs of the Nobles. Finally, at the edge of the Nile floodplain, the pharaohs built immense mortuary temples. Well-known examples are the Ramesseum, and the temple of Queen Hatshepsut at the base of the Theban Cliffs (Fig. 3). Why did the rulers choose these resting places? Did they have mystical significance - or were the pharaohs looking for somewhere solid that would be safe for all time?

Safety and stability were clearly of major concern for the builders of these tombs. They were looking for sites that were easy to guard and also to hide - unlike the pyramids of the Gizeh plateau, which attracted tomb robbers from all around. However, the more we look at the geological structure of the Theban Hills, the less we feel confident about their long-term stability.

True, the ancient Egyptian tomb builders came across massive, tabular, flat-lying beds of Eocene limestone (Fig. 3).

These were solid enough to support galleries, shafts, and the roofs of burial chambers, while the underlying Esna Shale was easier to work. Also, the Valley of Kings was remote and easy to guard. It was deeply incised, providing good opportunities for horizontal tunnelling into the hillsides (Fig. 2).

However, there were - and still are - two major problems. First, the Esna shale is prone to swelling when it gets damp, and this damages the surrounding rock. Second, there are some large geological faults, which could be seismically active. The Valley of Queens and Tombs of the Nobles are even less stable. At some time in the past, there were large landslides of Eocene limestone over Esna shale. As a result, the limestone beds now dip steeply. The tomb builders found that they had to plaster the walls of the tombs, before they could decorate them properly. Today, some of the plaster is peeling off and the tombs themselves may be unstable.

Necropolis

The Theban Necropolis is a world-famous site, and has been for centuries. Greek, Roman and European travellers came to describe it in detail, while more recently, archaeologists from many countries have organised digs, and have cleaned, described, and restored its ruins. The discovery of Tutankhamun’s tomb in 1922 by Howard Carter did much to popularise the culture of the ancient Egyptians. Nowadays, discoveries continue, under the dynamic leadership of Dr Zahi Hawass, charismatic Secretary General of the Supreme Council of Antiquities, who runs his own website: www.zahihawass.com. Only this year Hawass announced that his staff had identified the mummy of Queen Hatshepsut herself.

Consequently, the Theban Necropolis receives many visitors (over 7000 a day in 2005). For many, such a visit is the highlight of a trip to Egypt, while for the country, it is a very welcome source of revenue. However, the tombs are suffering – and mainly from heavy breathing. The damp exhaled air causes the Esna shale to swell, the walls to effloresce, the plaster to peel, and the marvellous paintings to fade.

 

Theban Mapping Project

For many years, Kent R Weeks, Professor of Egyptology at the American University of Cairo, has been running the Theban Mapping Project: www.thebanmappingproject.com. This has resulted in a collection of plans, photographs, and articles about the Valley of Kings, including some information about the rocks. Engineering geologists and architects, such as Raphael Wüst and James McLane, have investigated the mechanical properties of the rocks and their susceptibility to wetting².

However, it is more difficult to find information about the geological structure of the region. Has it remained stable since the Eocene, when the limestone accumulated, or has it been subject to tectonic activity and deformation? Did the opening of the Red Sea and the collision of Africa and Eurasia have significant effects, this far from the plate boundaries? Dr A Badawy and colleagues, of the National Research Institute of Astronomy and Geophysics in Cairo, think that they did3. They have evidence that middle Egypt is seismically active today and prone to thrust faulting, as well as strike-slip faulting.

 

Faults and landslides

In November 2007, we spent a week at the Theban Necropolis. Like most other visitors, we paid to enter the tombs and other monuments, but were free to walk around the area. We were able to take measurements and photographs on the ground. We also viewed the landscape from a hot-air balloon. For a geologist, nothing can replace the privilege of being aloft in a slowly moving balloon, while the early morning light is oblique, and the air wondrously clear.

In the Valley of Kings, we found good evidence for faulting (Fig. 4). The Theban Mapping Project refers to a Valley of Kings Fault that runs N-S. Indeed there is a zone of faulting in the SW corner of the valley - but it would appear to be composite. We found a number of faults, cutting the Eocene limestone. Typically, the fault walls have separated during sliding, and veins of crystalline calcite have grown in the intervening spaces. The calcite is fibrous and forms overstepping bundles, which give the direction and sense of slip (Fig. 4a).

Normal faults, indicating horizontal extension in a NE-SW direction, are abundant. However, one large fault (P1, Fig. 4a), on the NW side of the valley, is dominantly strike-slip (and left-lateral), whereas others are oblique-slip (left-normal, or right-normal). Although the five faults that we measured are not enough to be statistically significant, they are mutually compatible (Fig. 4b). For the bulk deformation, the principal extension is NE-SW, the principal shortening is vertical, and the intermediate axis is NW-SE. This deformation could have accumulated under a uniform state of stress, where the greatest compression was NW-SE and the least compression was NE-SW - provided some of the faults had formed previously. Such a state of stress is similar to the one that is causing earthquakes today, and may indeed reflect collision between Africa and Eurasia, as well as opening of the Red Sea.

Of more interest perhaps to the archaeologist, it would seem that the ancient Egyptians recognised and even appreciated the striated fault surfaces. In the burial chamber of Tomb KV9 (Ramesses VI), the builders did not destroy a prominent sloping calcite vein that lines an oblique-slip fault (Fig. 4a; fault P2, Fig. 4b). Instead, they integrated the vein into the design of the chamber and even cut an arch through it. The vein is fragile and the builders must have taken quite some care not to destroy it completely. Moreover, an unknown ancient hand painted characters onto the upper surface of the vein, following the lines of the oblique fibres. This could be one of the earliest known recognitions of tectonic structures!

The online atlas of the Theban Mapping Project describes the vein, but does not mention the fault, the oblique striations, or the painted characters. Could there be some scope here for detective work? A more flat-lying fault (P3, Fig. 4b), carrying striations that point ENE, now forms part of the ceiling to the burial chamber (J2) of tomb KV47 (Siptah). Again, the builders integrated the fault surface into the design of the tomb, and may even have adjusted the height of the ceiling accordingly. Finally, another fault crops out along the northern wall of the unfinished and undecorated corridor (K2) at the end of tomb KV14 (Tausert-Setnakht). There is no obvious sign here that the builders took the fault into account when building the tomb. The fault surface strikes almost E-W and carries down-dip striations. It should be a thrust fault - if it is to be compatible with our five measured faults and their bulk deformation. Indeed, the northern wall appears to be offset in reverse sense across the fault. If the offset is real, it indicates that the fault has moved in historical times - and that would be of great interest to historians, architects, and geologists alike. Alternatively, if the offset is apparent, it might be due to differential rock spalling. We were not able to decide, because a barrier prevented us from approaching the fault surface.

Listric faults

For the low-lying hills between the Theban Cliffs and the Nile floodplain, the geological context is very different (Fig. 5). Here the bedding in the Eocene limestone is no longer flat lying, but dips generally northward at 45° or more. The Theban lowlands would appear to be an area of large landslides. The arcuate form of the Theban Cliffs, concave toward the Nile, is typical of an upper landslide scar. On the southern side of the cliffs, normal faults appear, southward dipping. The faults flatten at depth - in other words, they are listric (smoothly curving). They also link into a flat-lying slip surface (detachment) near the top of the Esna shale. Motion on the listric faults has caused blocks in their hanging walls to tilt, accounting for the steep bedding. As one approaches the Nile valley, the foothills become lower and normal faults are less visible. Instead, one finds a few low-angle thrust faults and folds, indicating horizontal shortening. Such structures are typical of the lower end of a landslide.

The Valley of Queens lies within a large rotated fault block, where beds of Eocene limestone dip at about 48° to the NW. The tomb builders, as they tunnelled horizontally, encountered successive beds of different properties. This made for uneven walls. The builders’ solution was to cover the walls with plaster and decorate them with painted figures. Unfortunately, the plaster tends to come unstuck, when it gets damp. That is why the beautiful tomb of Queen Nefertari is now closed to the public once again, after having been restored and opened a few years ago. We do not know if sliding is still going on in the area. We saw no evidence for it. The Tombs of the Nobles are in a separate hill, in another area of landslides. Again, the beds dip at about 45° in a northerly direction. The hill is a warren of tombs and other ruins, but we saw no evidence for recent sliding.

Why did sliding occur in the past on the Esna shale? We may have found a clue to that. The shale contains abundant horizontal veins of crystalline gypsum. The crystals are fibrous and close to vertical. Because of their aspect, such veins go under the traditional name of “beef”4. The fibres show that the veins have opened vertically, against the force of gravity. The probable reason for that is fluid overpressure - in other words, a pressure in the pore spaces of the rock that exceeds the hydrostatic pressure (which is due to the weight of a free column of fluid). Such an overpressure will take the weight off a flat fault surface, allowing it to slide against little frictional resistance. The overpressure in the Esna shale may have come from a rising water table or from chemical reactions. In any case, the presence of gypsum indicates that the veins formed at low temperatures (probably less than 60°C).

Conclusions

We have described evidence for regional faulting and land sliding around the Theban Necropolis. In the Theban Hills, the Eocene limestone dips at a few degrees to the north. That is why the cliffs are on the southern side. Regional tectonics may be responsible for tilting of a large basement block beneath the hills, against a basement fault beneath the cliffs. If so, we would infer reverse motion on the fault, as for current earthquake motions in the area. The growth of an escarpment, in a context of undercutting by the Nile and overpressured shale at depth, might have led to slope instability and landsliding.

However, more work is needed to check these ideas and solve outstanding questions. Big unknowns are the ages of regional faulting and land sliding. They must have happened since the Eocene - but that was 50 million years ago! Another unknown is current stability. Judging by earthquake data, regional faulting may still be going on. Is land sliding active?
Were the pharaohs right in choosing their last resting places in the Theban Hills, or did they overestimate their apparent stability? The colossi of Memnon (Fig. 6) may have the last word!

References

1. Egyptian Geological Survey. 1981. Geological map of Egypt. Scale 1:2,000,000, 1 sheet.
2. Wüst, R.A.J. and McLane, J. 2000. Rock deterioration in the Royal Tomb of Seti I, Valley of the Kings, Luxor, Egypt. Engineering Geology, 58, 163-190.
3. Badawy, A., Abdel-Monem, S.M., Sakr, K. and Ali, Sh.M. 2006. Seismicity and kinematic evolution of middle Egypt. Journal of Geodynamics, 42, 28-37.
4. Cobbold, P.R. and Rodrigues, N. 2007. Seepage forces, important factors in the formation of horizontal hydraulic fractures and bedding-parallel fibrous veins (“beef” and “cone-in-cone”). Geofluids, 7, 313-332.

* University of Rennes, France; Washington State University, USA; Imperial College, London.