PR-S72 Cotero La Marruya starts on PR-S75, in the direction of the Aján valley and Yera riversides. This is an area of scattered cottages, where the small groups of buildings represent medium Pas settlements.
Once in Cañao de Aján, we enjoy a special river, strong, brief and fast, fitting into the high hillsides of the area, somber and with a big flow, with Hormaza, Brinía and La Marruya as its affluents. Here we can see most of the habitats of the LIC of the Pas river.
Aside from Alpine ecosystems, we also see humid heathers, poor pastures and vegetation formations. The beech forests of Ilex and Taxus, and the oak groves of Carpinion betuli should be pointed out.
From the northern entrance to the tunnel of La Engaña, we access a beech forest through which we ascend until the cottage neighbourhood of Brinías. This is the most difficult stage of the route, since it has an altitude difference of more than 600 metres to Collado de La Marruya.
During the first climbing stage, we can see some Pas cottages. We then reach the division line of Atlantic waters at Cotero de La Marruya. We leave the town of Merindad de Valdeporres, in Burgos to the south.
The cottage neighbourhood of La Marruya has some differences from other cottages in the region, specially their protection systems against snow. From here, we can clearly see all of the Pas affluents.
We now must take the path between Aján and Bustalvain. We will cross the cottage neighbourhoods of Lamesalcarro, Coterón, La Estoja and Falacuesta, always following the division line between both rivers.
From here, we return to Vega de Pas following GR-74, therefore crossing Rellano and El Cuadro, direction to La Torre and La Frente, to descend towards Candolías.