Geological Source of the Construction Materials of the Gelida Castle (Catalonia, Spain). Historical Implications

Abstract The Gelida Castle was mainly constructed between the 9th and 11th Century as a frontier castle between al-Andalus and the County of Barcelona. The base of the castle wall was constructed with a regularly cut yellowish-orange limestone of unknown origin, while the remaining parts of the walls were constructed with poorly cut local materials, mainly grey Triassic limestone. Finding the geological provenance of the materials used to build the base of the castle walls is the main objective of this study and helps to understand its historical origin. To achieve these aims, samples from the walls of Gelida Castle were compared with samples of outcrops and historical quarries of the nearby Prelitoral Catalan Depression with similar lithological features. These samples were studied from the viewpoint of their mineralogy (XRD and FTIR), palaeontology and petrology. The group of samples with the highest degree of similarity with the castle materials was found ca. 60 km west-southwest of the castle, not far from Tarragona. This together with the technique of rock cutting suggests that the ferruginous limestone materials from the Gelida Castle were most probably transported from this area after plundering the building blocks of an abandoned Roman construction. The historical background of these results suggests that the original castle of Gelida could have been an Andalusi castle. It would have been part of a network of fortifications housing military garrisons before the expansion of the County of Barcelona, which occurred in the first third of the 10th century.