GEOARCHAEOLOGICAL INVESTIGATION OF CITY WALL CONSTRUCTION TECHNOLOGY IN OYO ILE, OYO STATE, NIGERIA

Abstract

Oyo-Ile, the capital of the Old Oyo Empire located on the extreme northwest of presentday Yorubaland, Nigeria was an important polity in West Africa between the 16th and 19th centuries A.D. The city walls, in their current conditions, are in need of restoration towards conservation of the wall system. Previous archaeological investigations on the palace walls had focused on their circumference, the areas covered, description of the types of walls, and elucidation of their functions. However, limited attention has been paid to construction techniques, the composition of materials used, and the mode of emplacement of the artefacts found in the walls. The aim of the study was to investigate the construction techniques, the composition of the sediment used, and the mode of emplacement of the potsherds in the wall. A cross-section excavation was conducted on the palace wall remains which enabled the examination of the stratigraphy of the excavated units and the techniques used to build the wall and its foundation, and the collection of artefacts, and sediment samples. The sediment samples collected were subjected to granulometric analysis to determine their grain size. The pH, exchangeable cations of the sediments were determined by standard methods and minerals of the clay were determined from thin section. The mode of incorporation of the potsherds into the wall was determined using pottery analysis. Stratigraphy of the excavation showed the composition of the soil to be lateritic clay comprising a very fine-grained gravely sand thoroughly kneaded with plant fibres. The construction technique of the palace wall was no more traceable because the wall has been reduced to a massive hillock. The colour characteristics of the sediments ranged from reddish gray, yellowish red to reddish brown. The pH of the sediments ranged from 5.5- 7.5, and was consistent with the pH of the “brownearths” soil type characteristic of mixed deciduous woodland zone within which Oyo-Ile is located. The sediment was stable and resistant to erosion by the nature of its mineral assemblages (Quartz, Garnet, Tourmaline, Rutile and Rock fragment) and exchange cations (K+, Na+, Ca2+, and Mg2+). The potsherds were crude and poorly fired with thickness of 0.5-1.2 cm. The sherds were mainly undecorated and their emplacement in the wall remains was evenly spread across the stratigraphic layers. The features of the potsherds were not consistent with those used for domestic, ritual or industrial purposes; rather the sherds were intentionally added to the palace wall during construction The palace wall at Oyo-Ile was built directly on a lateritic ground as its foundation. However, the specific technique used to construct the palace wall could not be established. Potsherds were deliberately mixed with the sediments to strengthen the wall.