How were the famous limestone plateaus (loras) of the Las Loras UNESCO Global Geopark formed? What processes have shaped this unique landscape located between Burgos and Palencia? These are some of the questions that a team of researchers from the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH) has addressed in an exhaustive study that has reconstructed the titanic struggle between plate tectonics and erosion processes; that is, between the internal and the external processes of our planet over millions of years.

The study, published in the journal Cuaternario y Geomorfología, combines field mapping and digital data analysis (aerial images, LiDAR data and drones), to identify a series of relict erosional plains formed by denudation millions of years ago. “In geology, denudation is the erosion of the relief that occurs over such long periods of time that it flattens and lowers the relief, razing and dismantling the mountains, until it forms almost flat erosional plains, also called erosion surfaces,” explains the first author of the study, Alfonso Benito Calvo, a researcher at CENIEH.

This study has identified up to eight levels of erosion surfaces (S1-S8) whose formation occurred between 55 million years ago, just after the extinction of the dinosaurs, and 3 million years ago, when the Douro basin was opened to the Atlantic Ocean and the rivers began to empty it slowly but inexorably, forming the current valleys.

At the beginning of this period, continental drift caused the Iberian Peninsula to collide with the Eurasian tectonic plate. This impressive impact caused the rocks formed in the ancient Mesozoic seas to break and fold, rising little by little until they began to form the Cantabrian Mountains and the Pyrenees. What were once oceans were transformed into mountains.

As soon as the rocks emerged and were exposed to external erosive agents (weathering, rivers, glaciers, coasts, etc.), the struggle between tectonic uplift and erosional processes began, the first pushing the rocks upwards and the second trying to cut off what protruded. In this struggle for equilibrium, various erosional surfaces were carved out, sculpted in the rocks over millions of years, and recorded at the top of Las Loras.

The denudation gave rise to plains created in warm and rainy environments such as, for example, the highlands of Peña Amaya and Peña Castro, formed during the Paleogene-Eocene about 55 million years ago; or plains generated in colder, arid environments between 12 and 3 million years ago, like the flats of Sargentes de la Lora and Úrbel del Castillo, when the Douro basin was finished filling up and opening to the Atlantic Ocean. "The upper plains of Peña Amaya, Peña Ulaña, Valdivia, etc., represent these relict plains, whose eroded materials were transported and accumulated towards the Douro basin, forming the sedimentary units that we currently see on the slopes of the plateaus," says Benito Calvo.

The identification of these erosional surfaces has provided new data and points of geological interest on the Cenozoic evolution of the landscape in the Las Loras UNESCO Geopark, providing unprecedented information on the denudation cycles in this southern edge of the Cantabrian Mountains.

This project was funded by the 2022-2023 Research Grant awarded by Galletas Gullón and the Association for the Geological Reserve of the Las Loras UNESCO Geopark.