The Fluvial Nature Reserve of the upper Tirón Nature · Fresneda de la Sierra Tirón, Montes de Oca, Burgos Image of Fresneda de la Sierra Tirón, not of the place itself. The stretch that runs from the source to the village figures in the Catálogo Nacional de Reservas Hidrológicas as a representative example of a siliceous Mediterranean mountain river, with the channel practically free of alterations. Photo: Maintenance script (CC BY-SA 3.0) What to see and do in Fresneda de la Sierra Tirón