(Photomicrograph of a feldspathic wacke [sandstone] [Michael C. Rygel; CC BY-SA 3.0])
Greywacke, Siltstone, and Metasedimentary equivalents (also schist, mudstone, slate, basalt, black basalt, green basalt, grey basalt, basanite)
(Last modified January 27, 2003)
In the past, some Egyptologists have use the term “schist” in describing certain ancient Egyptian artifacts. In almost all cases the term “schist” is not being used in a modern geologic context (i.e. a medium- to coarse-grained foliated metamorphic rock), but rather it was being used to describe two types of metasedimentary rock called metagreywacke and metasiltstone. These rocks are essentially the sedimentary rocks greywacke and siltstone that have been very weakly metamorphosed, and are homogeneous and have no visible schistosity. The metamorphism of greywacke and siltstone increases the cohesiveness of the mineral grains and its rock hardness, making these rocks less susceptible to fracture during carving. This allows for fine detail and intricate shapes to be carved in objects such as in palettes and vessels (Fig. 1 and 2). Metasiltstone is also misidentified as slate, and both metagreywacke and metasiltstone have been misidentified in some cases as basalt (Nicholson & Shaw 2000, Penny 1993). There are, however, examples of the correct use of the term schist such as in the description of mica schist that was used by the ancient Egyptians for vessels (Aston 1994).
Greywacke is a term that refers to an immature sandstone, generally indurated, dark grey, and consisting of poorly sorted angular to subangular, sand-sized grains. These grains generally consist of quartz, feldspar, and a variety of rock and mafic mineral fragments embedded in a compact clay matrix, which makes up at least 10% of the rock, having the general composition of shale. Siltstones are sedimentary rocks ranging in their modal % of silt, which dominated over a clay matrix similar in compositions to the clayey matrix of greywackes.
Metagreywackes and metasiltstones are found in Egypt in several localities in the Eastern Desert, where they are associated with Precambrian basement rocks and are often found together in the same locality. The main source of these rocks are the immense quarries at Wadi Hammamat (Fig. 3). These quarries were worked from the Predynastic period to the Roman period. The typical grain-size of metagreywackes and metasiltstones at this location are very fine- to fine-grained (0.06-0.2 mm) and silt sized (0.01 to 0.02 mm), respectively (Nicholson & Shaw 2000). The metagreywackes are occasionally pebbly and dark greenish-gray to mainly grayish-green in colour. Metasiltstones are dark purplish- and greenish-gray to mainly grayish-green in colour. The green colour is associated with a high content of the mineral chlorite, formed during weak metamorphism due to deep burial in the Earth’s crust (chlorite zone). Photos of polished rock slabs obtained from various metagreywacke quarry sites used by the ancient Egyptians can be seen at the Ancient Egyptian Quarries website section #28.
| The metagreywacke of the Wadi Hammamat quarries, as well as rocks of similar appearance, were called bhn (bekhen) by the ancient Egyptians (Harris 1961). A number of inscriptions associate the rock bhn with the Hammamat quarry sites, where metagreywacke was the main rock type. Some stone artifacts were also inscribed with this name bhn and are made of metagreywacke or similar looking sedimentary rocks. The darker grey varieties of this rock are also often mistaken for basalt during archeological identification by Egyptologists. The Greek and Roman translation of the ancient Egyptian word bhn (bekhen) are basanites lithos and the lapis basanites, respectively. The origin of the modern word basalt is from these words for greywacke (Harrell 1995). Basalt in a modern geologic context is a fine-grained extrusive igneous rock consisting of calc-plagioclase and pyroxene. Metasiltstones were used during the Predynastic period for palettes, vases, bowls, cups, spoons, ornamental objects, and bracelets. Stone vessels and statues of metasiltstones are also known from the Early Dynastic period onwards. As well, starting in the Old Kingdom, and through the rest of ancient Egyptian history, both metasiltstone and metagreywacke were used for large items such as sarcophagi, stelai, naoi, and statues. References Aston, B.G. (1994) Ancient Egyptian stone vessels: materials and forms. Heidelberger Orientverlag, Heidelberg, 196 p. Emery, W.B. (1949) Great Tombs of the First Dynasty: Excavations at Saqqara v.1. Goverment Press, Cairo, 157 p. Harrell, J.A. (1995) Ancient Egyptian origins of some common rock names. Journal of Geological Education, 43, 30-34. Harris, J.R. (1961) Lexicographical Studies in Ancient Egyptian Minerals. Akademie Verlag, Berlin, 262 p. Lucas, A. & Harris, J.R. (1962) Ancient Egyptian materials and industries. E. Arnold, London, 523 p. Nicholson, P.T. & Shaw, I. (2000) Ancient Egyptian materials and techniques. Cambridge University Press, New York, 702 p. Penny, N. (1993) The materials of sculpture. Yale University Press, New Haven, 318 p. Links to examples of metasedimentary rock usage a) Bowls Early Dynastic period (1st Dynasty) metasiltstone ornamental “plate” (image #1, image #2, image #3) from the tomb of Prince Sabu (Tomb 3111), Saqqara (height: 10 cm, diameter: 60 cm; Egyptian Museum, Cairo)Early Dynastic period (1st Dynasty) metasiltstone libation dish (height: 17.5 cm, width: 14.5 cm; Metropolitan Museum of Art, New York). b) Vases Early Dynastic period metasiltstone cup with pink limestone base, from the tomb of Queen Her-nit (height: 18 cm, diameter: 8 cm; Egyptian Museum, Cairo). c) Statues Old Kingdom period (4th Dynasty) metagreywacke statue of King Khafre seated (height: 120 cm; Egyptian Museum, Cairo). Old Kingdom period (4th Dynasty) metagreywacke statue triad of King Menkaure (height: 96 cm; Egyptian Museum, Cairo). Old Kingdom period (4th Dynasty) metagreywacke triad of King Menkaure with goddess Hathor and the deified Hare nome, from Giza, Valley Temple of Mycerinus (height: 83.5 cm: Boston Museum of Fine Arts, Boston). Old Kingdom period (4th Dynasty) painted metasiltstone statue of King Menkaure and a Queen (height: 139 cm, width: 57 cm, depth: 54 cm; Boston Museum of Fine Arts, Boston). Old Kingdom period (4th Dynasty) metagreywacke statue of King Menkaure and his queen (possibly Kha-merer-nebty II) found at Giza, Valley Temple of Mycerinus (height: 139.5 cm: Boston Museum of Fine Arts, Boston.) Old Kingdom period (6th Dynasty) metagreywacke statuette of Prince Tjau seated on the ground, during the reign of Merenre I or later (height: 34.5 cm; Egyptian Museum, Cairo). Old Kingdom period (6th Dynasty) metagreywacke statuette of Pepi I in kneeling position (height: 15.2 cm, width: 4.6 cm, depth: 9 cm; Brooklyn Museum of Art, New York). Note: inlaid eyes of alabaster and obsidian mounted in copper cells. Middle Kingdom period (12th Dynasty) statuette of King Amenemhat III (height: 21.4 cm, width: 10 cm; Musée du Louvre, Paris). Late Dynastic period (27th Dynasty) metagreywacke head of an antelope with agate and travertine inlaid eyes (height: 9 cm: Metropolitan Museum of Art, New York). Late Dynastic period (28th Dynasty) fragment of metagreywacke bust of an old man from Memphis (height: 25.2 cm, width: 18.5 cm; Musée du Louvre, Paris). Late Dynastic period (30th Dynasty) metagreywacke statue of the god Horus protecting Nectanebo II (height: 71.1 cm, width: 20.3 cm, depth: 45.7 cm; Metropolitan Museum of Art, New York). Late Dynastic period (30th Dynasty) metagreywacke portrait sculpture head of a priest from Saqqara (height: 10.2 cm: Boston Museum of Fine Arts, Boston.) Late Dynastic period metagreywacke bust of a man with a shaved head (height: 12.9 cm: Musée du Louvre, Paris). d) Stela Late Dynastic period (30th Dynasty) metagreywacke magical stela (height: 83.5 cm: Metropolitan Museum of Art, New York). e) Palettes Predynastic period (Naqada I, 4000-3600 BC) metasiltstone palette in the form of a hippopotamus EA29416 (The British Museum, London. Photograph by Jon Bodsworth The Egypt Archive). Predynastic period (Naqada I, 4000-3600 BC) metasiltstone palette in the form of a fish (The British Museum, London. Photograph by Jon Bodsworth The Egypt Archive). Predynastic period (Naqada I, 4000-3600 BC) metasiltstone palette in the form of an ox (The British Museum, London. Photograph by Jon Bodsworth The Egypt Archive). Predynastic period (Late Nagada II, 3600-3250 BC) bas-relief metasiltstone palette of grasslands E 11052 (height: 32 cm, Musée du Louvre, Paris). Late Predynastic period (Naqada III 3250-3100 BC) lower half of a metasiltstone ceremonial palette “The Battlefield Palette” (The British Museum, London. Photograph by Jon Bodsworth The Egypt Archive ). Links Ancient Egyptian Stone Technology Archaeological Geology Research Group The Artchive Boston Museum of Fine Arts Brooklyn Museum of Art The Egypt Archive Jon Bodsworth’s image archive of ancient Egyptian antiquities Metropolitan Museum of Art Musée du Louvre, The Petrie Museum |
