As a result of the supported pressure caused by the elevation of the Pyrenees and the Iberian Mountain Range: the hollow of the river Ebro caves in and receives the contribution of eroded materials from higher areas since the late Eocene (38 million years ago) until the present.
In the areas nearer to the Pyrenees you can find the most heavy and solid materials like conglomerate, gravel and sandstone. Towards the centre of the hollow, where the slope is falling and the fluvial running is lesser, you can find finer sandstone. In the plainer areas you can find clay, gypsum, limestone and mineral salts.
The exceptional nature of the relief of the Bardenas is due to the quality and the structure of the materials and due to the action of water and wind. The alternation of clay, limestone and sandstone, with predominance of clay, a soft and impermeable material, allows that the erosion of the rain has a fast and severe effect because of its sporadic but torrential character.
For example, the hillocks in the shape of a table are very typical of this landscape. These hillocks are mounds composed of clay and whose upper part is protected by a stratum of sandstone and limestone; only their lower part is eroded.
Another demonstration of the erosion of the rain are the relieves called “lunar”, “elephantskin” or “bad lands”, formed in the plains and hollows where the torrent waters act on the impermeable slime and clay. And also the formation of plateaus and ravines, as well as other phenomenon caused by the absorption of the water like cavities or cavernous spaces.
The wind is the other active agent which contributes to shape the characteristic landscape of the Bardenas tirelessly. The “Cierzo” a north wind with direction NE and NNE dominates over this territory for more than a third part of the year. It can reach a speed of between 20 and 30 km/h.