The Foz de Lumbier is a small gorge located in the Navarrese Pyrenees. The defile is cut by the Irati river through the limestone rocks of Sierra Leyre, near the municipality of Lumbier. The surrounding area is covered in a scarlet oak grove, whereas the interior of the gorge is home to an interesting vegetal community, including species of rock, anual and ligneous plants.
Access to the interior of the defile is through two tunnels, hollowed out of the rock face. There are two routes available: the shorter one goes from the entrance to the first tunnel to the exit from the second one.
It covers an area of approximately 1Km along the old railway. The longer route starts from the path near the recreational area situated at the entrance to the gorge and ends at the starting point. First, it goes up to the Nature Reserve of Acantilados de la Piedra and San Martín, and then goes down to the river banks before reaching the path along the old railway. This route is approximately 5 Km long.
The vegetation indigenous to the area includes willows near the river, and poplars, willows, black poplars and elms at the entrance and exit from the gorge. In the cracks of rocks grow some interesting species such as the "Saxifraga cuneata", "Erinus alpinus" and "Petrocoptis pyrenaica".
The fauna of the area includes an important community of vultures, made up of fifty couples. "Corvus spp.", Egyptian vultures, swifts and owls can also be found. The river is home to otters.