Reading sediments to tell the story of Lisbon: geoarchaeology reveals landscapes hidden beneath the city.
What is now concrete, walls and railway lines was once beach, marshland and the lively banks of the Tejo estuary. Beneath contemporary Lisbon lie layers of sand, shells and silt that hold stories dating back thousands of years. It was based on this idea — that sediments are a book waiting to be read — that researcher Ana Maria Costa took the audience on a journey into the city’s past at a seminar held on 28th November 2025 at Campus Tecnológico e Nuclear of Instituto Superior Técnico in Loures.
The session was promoted by the C2TN Academy (Centro de Ciências e Tecnologias Nucleares) as part of the Thematic Line ‘Earth Systems, Radioactivity & Cultural Heritage’ and aimed to raise awareness, in an accessible way, of the role of geoarchaeology in interpreting archaeological records and reconstructing ancient landscapes.
Geoarchaeologist at Laboratório de Arqueociências (LARC), do Património Cultural, I.P, Ana Maria Costa showed how the application of geoscience methods such as stratigraphy, sedimentology, geochronology and geochemistry allows us to understand not only what past societies left behind, but also how these traces were formed, preserved or transformed over time.
“Sediments are like a book. Except that instead of words, they have grains, layers and chemical compositions that tell us the history of landscapes and the relationship between human communities and the environment,” explained the researcher.
Throughout the session, concrete examples of research projects were presented, with emphasis on ‘Lisbon Stories,’ a geoarchaeological study dedicated to the evolution of Lisbon’s estuarine coastline. The project seeks to understand what the banks of the Tejo River were like in different chronological periods and how human occupation shaped and artificialised this boundary between land and water.
Among the results presented, the identification of ancient estuarine beaches on the north bank of the Tejo stands out, some of which were located at the mouth of streams that today cross the city invisibly, channelled under streets and buildings, such as the Alcântara, Arroios and Valverde streams. At the mouth and to the west of the old Alcântara stream, for example, there were beaches that were similar, from a sedimentological point of view, to those still seen today on the south bank of the estuary.
The data collected also shows that Lisbon has literally grown over Tejo, advancing hundreds of metres southwards through successive landfills, some overlapping, since at least the medieval period. This profound transformation has converted a natural riverbank into an anthropogenic one, which is still exposed to extreme events such as floods and storms. This reality raises current questions about coastal resilience.
Another remarkable finding concerns the discovery of traces of a population of marine bivalves at the former mouth of the Alcântara stream, dating back some 6,000 to 5,000 years, indicating the existence of a water column several metres deep in an area that is now densely urbanised.
From a scientific point of view, studies also reveal a very high sedimentation rate on the estuarine margin between Santos and Terreiro do Paço since Roman times, associated with the intensification of human occupation, the control of river channels and the port use of the riverfront. Archaeological remains preserved in the subtidal sediments bear witness to this intimate relationship between the city and the estuary.
The research presented combines contributions from various scientific institutions at Universidade de Lisboa, including Centro de Arqueologia of Faculdade de Letras (UNIARQ), Instituto Dom Luiz of Faculdade de Ciências, and Centro de Ciências e Tecnologias Nucleares of Instituto Superior Técnico, whose collaboration has been particularly important in dating sediments using optically stimulated luminescence. This interdisciplinary approach allows for the construction of robust chronologies and a more detailed interpretation of the environmental and historical evolution of the region.
In addition to its academic interest, geoarchaeology has direct applications in preventive archaeology, cultural heritage management and environmental restoration, helping to anticipate the presence of archaeological sites, assess preservation conditions and support decisions in the context of construction work. At the same time, knowledge of the environmental past of the Tagus estuary contributes to reflection on contemporary challenges, such as changing climatic conditions and the sustainable management of coastal areas.
Ana Maria Costa pointed out that ‘the past is fundamental to thinking about the future,’ emphasising that knowledge about how the estuary responded to changes and extreme events in the past ‘allows us to prepare more informed responses to the challenges ahead.’
The seminar also made a clear appeal: the need for more training, more researchers and more funding for fundamental science, an essential condition for deepening knowledge about the territory and valuing a heritage that often remains invisible beneath our feet.
Because, as became clear throughout the session, many of Lisbon’s stories, from its relationship with the Tagus and the sea, remain buried in the sediments. It is now up to science to read them and return them to the collective memory.