Cuevas en arcilla en el barranco Norte de La Nasa (Bardena Negra).

New examples of chasms and caves in clay discovered in a sector of the Black Bardena called North gully of La Nasa are described. The region is arid to semi-desert and is located in the south of Navarra. Important caves-systems are developed in clays of Tudela Formation, of Miocene age (Tertiary). The cavities are essentially formed by processes of piping in dispersive clays rich in sodium salts. To a lesser extent dissolution and precipitation of secondary minerals in the form of speleothems occur. The examples found in this region are the largest cave systems in clay hitherto known worldwide. The work provides new geomorphological and hydrological data of interest, while discussing the genesis and probable evolution of the caves explored.