Reverse pseudo-gours: a new sub-type of folia observed in the Nerja Cave (SE Spain)

A new sub-type of folia named “reverse pseudo-gour” has been observed and described in the Nerja Cave, southern Spain. It consists of fairly vertical, thin barriers (about 5 mm high and 2 mm thick) that develop on the underside of a sub-horizontal surface (shelfstone) and grow in the opposite direction to normal gours (rimstone dams), generating sinuous shapes. Their mineral composition is essentially calcium carbonate, although globular aggregates composed of clay and phosphate minerals have also been identified. The genesis and evolution of these reverse pseudo-gours occur just at the air-water interface and are controlled by (1) the sub-horizontality of the surface on which they develop, (2) scarce to very scarce calcite supersaturation, and (3) progressive and slow lowering of the water level. Thus, reverse pseudo-gours represent the shape limit to which a folia reaches when its development occurs just below a nearly horizontal ceiling and the feeding water saturation is extremely low. Although this new sub-type of folia has only been observed in the Nerja Cave, it is highly probable that it exists in several other caves around the world because the factors controlling its development are not very strict.