Urban Geology of Lisbon: The Importance of the National Palace of Ajuda (Lisbon, Portugal)

The geological processes that provide building materials as well as the characterization of its lithotypes are commonly overlooked in what concerns the natural and cultural heritage of many historical buildings. The present paper focuses on the use of a particular limestone, the Liós, to build the National Palace of Ajuda located in Lisbon, Portugal. From a mere draft to a Royal residence since 1862, the inner and outer façades of the Portuguese National Palace of Ajuda hold a significant and captivating story about the rocks themselves that add significance to the already rich cultural heritage of the building. The geological nature (fossil assemblages) of the façades of this historical building is here presented, emphasizing the role of the National Palace of Ajuda as a large-scale urban geology patrimony and its importance in terms of geodiversity and geoheritage. Liós is a type of Cretaceous limestone with about 97 million years of age, particularly rich in rudist fossil assemblages, in most cases reflecting paleo-reefs produced by this extinct order of Bivalvia molluscs. The white to pinkish compact microcrystalline Liós limestone was used for centuries to build the city of Lisbon and its surroundings, including Lisbon’s most iconic buildings such as palaces and churches also having an important role during Lisbon’s reconstruction after the 1755 earthquake. For this reason, and since the eighteenth century with the construction of the monumental Palace and Convent of Mafra, as well as the fact as being exported to the former Portuguese colonies, the Liós became known as the ‘royal stone,’ extracted from multiple quarries located inside and outside of the urban perimeter of the city of Lisbon. The National Palace of Ajuda adds further information to the Liós limestone since one of its façades shows a small coral head co-occurring with the rudists. This fossil association has never been reported in the Cretaceous (upper Cenomanian) Bica Formation from which the Liós derives. Furthermore, the importance of the Liós limestone as an ornamental stone with widespread use for construction works in Lisbon and former Portuguese colonies was highlighted when it was recognized as a Global Heritage Stone Resource (GHSR) and its recognition can be strengthened with the future establishment of an International Geodiversity Day.