The source of the Tirón at Pozo Negro Nature · Fresneda de la Sierra Tirón, Montes de Oca, Burgos Image of Fresneda de la Sierra Tirón, not of the place itself. The Tirón rises at 1,825 metres of altitude in the spot of Pozo Negro, within the Sierra de la Demanda and in the municipal district of Fresneda. The path that climbs to the spring crosses beech, ash and hazel woods. Photo: Juanje 2712 (CC BY-SA 4.0) What to see and do in Fresneda de la Sierra Tirón