These are cut by rivers flowing through limestone. The vertical walls are carved by the eroding force of rivers as they find their courses over millions of years, giving rise to these spectacular formations so characteristic of the Pre-Pyrenean basins.
Foces

Inside these ravines, locally known as “Foces” (sing. “Foz”), the elements join forces to create special environments: vertical walls, shelves, slopes, stones, riverbanks.
Shelter from the winds of winter, shade in summer and humidity from the river itself are just some of the factors which condition the appearance of the wide range of interesting vegetation peculiar to ravines: kermes oak, strawberry trees, maple, service berries, beech, cherry, lime, walnut trees, elm, ash and willow. Together, they provide a spectrum of undeniable beauty which varies from season to season.

The shelves and walls remain inaccessible, protecting endemic flower varieties which have found refuge in the ravines for millions of years. There is also a wide range of wild animals. Of particular note are the birds of prey and nesting species which live on the rock face: lammergeiers, vultures, Egyptian vultures, golden eagles, peregrine falcons, eagle owls, stock doves, choughs and swifts.
Due to their natural wealth, the ravines are protected areas and those visiting should respect them as they deserve. The roads which line them, the Arbayun viewpoint or the path through the Ravine of Lumbier are all ideal ways of coming into contact with these wonders of nature. The Ravine Nature Interpretation Centre in Lumbier should not be missed.

There are several ravines in the Pre-Pyrenean basins: Iñarbe, Gaztelu, Txintxurrenea, Arbayún, Lumbier and Ugarrón. They are all legally protected due to their interest in terms of nature and landscape.
Ravine of Lumbier
Cut through the massif of the Sierra de Leyre by the River Irati, this ravine is the doorway to the Pre-Pyrenean basins. With vertical walls measuring more than 130 m, it is an excellent refuge for different plant and animal species.
The Ravine of Lumbier is a special place and has provided an outstanding place of passage for thousands of years.
First the birds and animals that lived there, then those that hunted them. Later, shepherds leading their flocks to new pastures and then the Romans. Pilgrims on their way to Santiago de Compostela from Jaca also admired the “Foz” from the roadside.
And then the Irati. The first electric train on the Iberian Peninsula, covering the Pamplona-Sangüesa route, at the beginning of last century. The tunnels which breach the threshold of the “Foz” were bored for the railway. Nowadays, thousands of visitors come to enjoy the outstanding beauty of this unique spot.
Ravine of Arbayun
The River Salazar, cutting its way through the limestone of the Sierra de Leyre, carved this 6-km ravine. Its shelves and cliffs are home to rare species of flowers. Of its fauna, the lammergeier is worthy of special mention, as is the largest colony of griffon vultures found in Navarre (more than 290 pairs), which lives on its walls. The ravine connects up with the woodland and crags of the Sierra de Leyre and the Sierra de Lumbier, making it all the more special.
Ravine of Benasa
A nature reserve with outstanding vegetation and habitat of the griffon vulture. For more than 6 km, the River Salazar carves and bores cliffs measuring more than 300 metres. There is a viewpoint on the Iso pass from which it is possible to take in this natural wonder.
Ravine of Ugarron
Cut by the River Areta as it flows through the limestone of the Sierra de Ugarra, to the north of Murillo, in the Valley of Urraúl Bajo. Its 104 hectares are a protected nature spot. Birds of prey abound in the ravine: griffon vultures, peregrine falcons, eagle owls. It is also home to the lammergeier.
Other ravines of interest include those of Aspurz and Burgui.
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