The change in the evaporation system (from forced to natural) in Valle Salado took place in the first century B.C. (about two thousand years ago), when this area in the north of Spain was taken over by the Roman Empire.

The increasing need for salt of a society that was so advanced compared to the local tribes, not only led to significant changes in the habitat, but also in the production and distribution systems. Consequently, Valle Salado adopted a natural evaporation system that, although it implied higher construction costs, multiplied salt production exponentially.

Archaeological investigations being carried out within Valle Salado are certifying the change of the evaporation system. More specifically, the first salt-pans have appeared built over the remains of the old fire-based salt factory. They were built of compacted clay with raised edges to seal the surface.

The changes not only led to radical changes to the production system, but also to a complete transformation of the settlement. The inhabitants of Valle Salado and of the surrounding area gradually moved to a large Roman site known as "Las Ermitas" (located six miles from Añana) that can be associated with the town of Salionca cited during the second century by the famous Roman geographer, Ptolemy, in the territory of the Autrigones tribe.

The importance of this civitas lay not only in controlling the production of salt, but also in the convenience of its location for distributing the product. We must not forget that it was located on a branch of the XXXIV road that linked Astorga (León) and Bordeaux (France) and also linked Deobriga (Miranda de Ebro) with Flaviobriga (Castrourdiales), becoming one of the main lines of communication between the centre of Spain and the Bay of Biscay.

Regarding the socio-spatial organisation of Roman Salionca, the settlement and the work place were physically separated. The individuals dedicated to supervising salt production, storage and marketing resided in the city, as did the salt workers. We must keep in mind that salt was obtained through evaporation only during the summer months and, therefore, it is reasonable to assume that workers only went to Valle Salado during the period when the salt-pans were in use.