The valley of the Saja River, located in the autonomous community of Cantabria is formed by two natural comarcas.

After that, the geography smooths, with larger lowlands, bordered by the Ibio Ranges and the small pre-coastal hills.

Through the Nansa valley flows the river of the same name, which forms remarkably steep slopes at its sides, due to its torrential and eroding nature, chiefly at the first part of its course.

The municipalities comprehended by the Nansa valley are: Lamasón, Rionansa, Tudanca, Polaciones and the part of Cabuérniga corresponding to the village of Carmona.

The comarca has a great landscape quality, thanks to its high peaks characteristics, large native forests (mostly oaks and beeches), and low population density, mostly dedicated to cattle farming.