Arrens-Marsous results from the merger of the two villages in 1973. At the bottom of the Val d'Azun, at an altitude of 2,870 ft (875 m), it extends near the Spanish border at the Col de la Peyre-Saint-Martin. A promontory known as Mont de la Source shelters an old sanctuary and its Pouey-Laün chapel. The ancient glacial valley stretches at the foot of the Pic du Balaïtous (10,315 ft/ 3,144 m) in the Pyrénées National Park. The mountains of Hautes-Pyrénées like the Col du Soulor link the Arrens valley to that of Ouzom.