The rock outcrop on the col (where the walkers are resting) is of Ediacaran metapelites (metamorphosed sediments more than 500 million years old). Here, and as is often found in this zone of the Eastern Pyrenees, the rock has a ferruginous sheen and a schist-like appearance. (The crag just above and slightly to the left is on an outcrop of slightly younger conglomerate.)

The col lies on the crest of the Pyrenees mountain chain. Mountains on either side of the col rise to over 2800 metres. The col is on a relative low point because it lies on the line of the Finestrelles fault, which extends from here for several kilometres to the north-west, along the line of the Segre valley, to Llo. The Finestrelles fault (a normal fault) is of Neogene age - i.e., of relatively recent geological age. Movement along the fault has crushed the beds along its line, thus rendering the rocks along the fault line more vulnerable to weathering and erosion than they otherwise would be.

Topographical map on Géoportail.

See:
- Sentier géologique de Llo (English version). Parc naturel régional des Pyrénées catalanes.
- B. Laumonier et al., Notice explicative de la feuille Prats-de-Mollo-La-preste (1099) à 1/50 000, BRGM Éditions, Orléans, 2015, especially pages 26 et 27, ficheinfoterre.brgm.fr.
And also:
- Vallée de Llo - Sègre.jpg.