The Upper Pleistocene glacier of the Aragón Subordán Valley, Central-western Spanish Pyrenees, is studied. At the moment of the maximum extent of the ice the Aragón Subordán glacier was 23 km in length, with the melting area at 820 m a.s.l., a height similar or even lower than that of other neighbouring valleys (for instance, Aragón and Gállego valleys). Nevertheless, though the height of the peaks at the headwater is remarkably lower, it was compensated by more abundant snowfalls in the Aragón Subordán Valley, due to its western position, open to the Atlantic fronts. There are many morainic deposits in the ablation area and in the lateral valley walls, as well as several glacio-lacustrine deposits. Lateral and terminal moraines are the basis to distinguish several stages of glacial advance, coinciding with those identified in other Pyrenean valleys. As in other Pyrenean valleys, three lateral morainic ridges can be distinguished around the moment of the maximum ice extent, above all in the confluence with the Agüerri and Acherito valleys. By that moment, the Aragón Subordán glacier was 400 m thick before it arrived to the Hecho terminal basin, and the main tributary glaciers were connected to the main glacier, particularly during the development of the first two lateral moraines. In a second stage (probably related to the re-advance of 20,000 yr B.P., coinciding with the global LGM) most of the tributaries were already disconnected, thus explaining the reduction in length and thickness of the main glacier. Then, the front of the glacier would reach the Oza Forest. Later, other minor re-advances have been recognized within the tributary valleys, with several frontal moraines in the Acherito Valley. The absence of local dates of the glacial stages obligates to establish comparisons with other Pyrenean valleys.
