The Geology of Vega Huerta
All the limestones of the Frontal Nappe Unit are potentially cavernous although the Vega Huerta-Carbanal Series which has a significant shale content, contains only collapsed shafts at the surface with no negotiable entrances found to date. Erosion of this formation has produced 'windows' into the underlying Picos de Europa Limestone Formation within which the caves I8 and I34 are developed.
Several major post-Variscan faults cut the Frontal Nappe Unit. These trend east-west and southeast-northwest and three come together just to the south of the entrance of M2. There are also many less persistent fractures, but which are clearly important controls on cave development.