The sierra between two river basins Nature · Ibeas de Juarros, Burgos (Alfoz), Burgos Image of Ibeas de Juarros, not of the place itself. The sierra of Atapuerca is a modest limestone hill that separates the basin of the Arlanzón from that of the Ubierna. That natural corridor between the meseta and the valley of the Ebro explains why it was an obligatory passage for fauna and for humans for a million years. Photo: Mario Modesto Mata (CC BY-SA 3.0) What to see and do in Ibeas de Juarros