Localisation of the ruins of ancient megalithic structures near water sources – a case study at Porto Santo Island, Portugal

Porto Santo Island  is the northernmost and easternmost island of the archipelago of Madeira, located in the Atlantic Ocean west of Europe and Africa. The island is rare populated as result of historical terrace farming decline because of island slopes erosion and shortage of water resources. Modern days these areas are under afforestation activities.

Our research carried out in summer 2022 indicates that Porto Santo Island was inhabited before it was discovered by the Portuguese. The ruins of megalithic structures priory discovered on the island of Madeira, led to assumption that such ruins could also be found on Porto Santo. When the island, some stone blocks processed with stone tools were found. Additionally, the characteristic shapes given to the stones of ancient megalithic cultures have been determined, and the very simplified treatment of their surface with stone tools has been documented. Fragments of ancient marks and sings are still traceable on the surfaces of some stones, which are being studied. In some parts of the island the concentration of megaliths is higher, and the ruins of building structures can be mostly found in the eastern and northern parts of the island.

Typically, such ruins are not concentrated at the coastal area, but are located on an elevated relief surface at a height of about 130-150 meters. At this level, where the terrain is crossed by ravines with periodically flowing streams, there are places where the ruins of megalithic structures are most often found. In this area the ravine streams are still constant throughout the year, but towards the ocean they completely disappear and resume their flow only during the rainy season.

Most likely, the connection directly to water sources, which is a critical resource for life on the island, has determined the location of the monuments of the megalithic culture. Presumably, such a connection has also existed in the settlements of the inhabitants in unknown antiquity.

The limited number of monumental ruins on the island indicates the relative temporality of the existence of this ancient culture, as well as the small size of its population. When due to climatic changes, the rainy season is short and the amount of precipitation is insufficient for their infiltration, and such a phenomenon lasts for decades, the sources of drinking water dry up irretrievably and people must leave the island. In the case of Porto Santo, it is likely that the ancient inhabitants migrated to the nearby island of Madeira, which is richer in water resources.

Key words: ancient cultures, prehistory, stone processing, stone marks

The study has been founded by Iceland, Liechtenstein and Norway through the EEA and Norway Grants Fund for Regional Cooperation project No.2018-1-0137 “EU-WATERRES: EU-integrated management system of cross-border groundwater resources and anthropogenic hazards”.