Grotte e abissi nell'unità geologica Alt Freser

The Alt Freser is a unit of the Pirineu Axial, in the upper valley of the Freser in the Ripollès, with Pyrenean limestone karst among crystalline reliefs. It brings together 45 cavities: 12 pits and 33 caves, with a total length of 3099 m and a maximum depth of 67 m. The lithologies present are limestone (31), marble (8), granite (4), slate (1) and schist (1).

Among the longest cavities are the Cova de les Encantades, 865 m, on the south slope of the Puigmal, known since time immemorial and explored in 1960; the Sistema dels Forats Nº3 I Nº4 de l'Embut, 542 m, at Núria, known since remote times and explored speleologically since 1953; and the Cova de Rialb, at Queralbs, 501 m in length and 38 m of drop (+12/-26), known since time immemorial and studied by speleologists since 1910.

As for depth, the Sistema dels Forats Nº3 I Nº4 de l'Embut reaches 67 m; the Avenc del Migdia, at Toses, 49 m, with shafts of -18, -11 and -15 m; the Cova de Rialb, 38 m; and the Avenc del Tronc, in the area of the riu de Tosa (Queralbs), 37 m, with an entrance in a wall of crags near Roc Pinós.