Excavating in Breccia: New Methods Developed at the Benzu Rockshelter
Dominguez-Bella, S., Ramos, J., Bernal, D., Vijande, E., Cantillo, J. J., Cabral, A., Perez, M., Barrena, A., Antiquity
Breccia is a densely consolidated material consisting of angular fragments of rock cemented in a matrix (Loucks 1999; Loucks & Mescher 2001). Sedimentary breccias typically form within and on the talus of limestone caves as a result of water flow, and may be regarded as lithified colluvium. They can continue to form after human occupation, and thus may contain artefacts. The problem of excavating in breccia is basically one of defining an archaeological sequence in a deposit that resembles concrete.
Breccia sites are relatively abundant in the world (Latham 1999), occurring in karst environments associated with caves and rockshelters. Well known examples include Limeworks Cave, Makapansgat, South Africa (Hill & Forti 1997), Zhoudoudian, China (Wu & Dong 1985) and Red Barns, Portchester, Hampshire, UK (Wenban-Smith et al. 2000). At Grotta di Porto Infreschi, Camerota (Italy), a series of marine conglomerates containing Spondylus (probably of the Tyrrhenian geological stage) was succeeded by alternating levels of red clay, breccias, pyroclasts and stalagmitic crusts, sealed at the top by colluvial deposits and rock falls with a Mousterian industry (Bachechi 1989-1990). Examples from the Iberian peninsula include Gibraltar-Beefsteak Cave (Giles et al. 2007), Rosia Bay breccia (White 1913; Finlayson et al. 2006), Tajo de Dona Ana, Alfarnatejo (Ramos et al. 1995-1996) and Cueva del Angel, Lucena, Cordoba (Barroso et al. 2011), in Andalucla, and Figueira Brava Cave (Portugal), a rockshelter of the Middle Palaeolithic (Upper Mousterian), dated to between 30 and 31 ka (Antunes 1990-91; Antunes & Cardoso 1992).
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Excavators have been obliged to work round the densely cemented breccias, or attack them with heavy machinery and even with low-power explosives. In such cases, the recovery of archaeological material has naturally been poor. In this paper, we describe a new approach to the problem, developed during research at the rockshelter of Benzu.
The Benzu rockshelter
The archaeological site of Benzu is situated on the coast of North Africa, on the south side of the Strait of Gibraltar in the autonomous Spanish city of Ceuta. It is located about 230m from the present shoreline (Figure 1). The Palaeolithic rockshelter, discovered in 2001 (Bernal 2001), contains an archaeological deposit mostly consisting of limestone breccia, of a high degree of compactness and hardness. The shelter was formed through erosion before Isotope Stage 9 and any human occupation (Abad et al. 2007). It may have lost its overhanging cornice during the Quaternary (Duran 2003; Ramos & Bernal 2006; Ramos et al. 2008). Adhering to the rock wall of Triassic dolomite is a deposit of highly consolidated calcareous breccias. This was defined in plan and section (Figure 2), and subsequently resolved into ten stratified levels (Table 1). Geological studies of the sedimentation (Duran 2003) show that stratigraphic levels 1 to 7 contain evidence for early human occupation. This took the form of a Mode III-Mousterian knapped stone industry, with chronologies ranging from 70-250 ka (Duran 2004; Ramos et al. 2008), and include indications of marine resource exploitation (Ramos et al. 2011). Lithics, animal bone and shells (malacofauna) all survive well within the breccia (Figure 3).
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The rockshelter measured 15.52 x 6.20m, and the top surface of the breccia deposit occupied an area of 61.2[m.sup.2] in plan (Figure 2C). Our methodology was developed over six seasons between 2002 and 2008. In 2002, the upper level of the deposit (Stratum 7) was defined (Ramos et al. 2003), and a 25cm grid laid out on its upper surface. From this surface we first attempted vertical excavation, using chisels and hammers. However, these tools also fractured the siliceous minerals and rocks, such as flint, radiolarites and sandstones that constituted the artefacts (Chamorro et al. …