This gorge is in the Montsant mountain range, located on the southern edge of the Ebro basin, near the Prelitoral mountain range. This basin was filled with sediment during the Tertiary period, at the same time as Alpine orogenesis was raising the Pyrenees to the north and the Prelitoral mountain range to the south and southeast. This basin was occupied by the sea, but sediment washed away by rivers from the terrain that was being formed, was deposited on its shores. Montsant was the centre of an alluvial range where hundreds of metres of coarse material was deposited, which later transformed into conglomerates. These coarse sediments pass into the centre of the basin into fine sediments (clays, loams and layers of sandstone).

Montsant mountain, like Montserrat, is not formed by tectonics but by the differential erosion between hard rocks (conglomerates) and softer rocks (clay and sandstone). Its characteristic morphology, which reminds us of Montserrat, is the result of the action of water on the conglomerates, which infiltrates through a thick network of diaclases (fractures) and gives rise to the typical rounded shapes.

Fraguerau Gorge is included in space no. 303 (El Montsant) of the inventory of Spaces of Geological Interest of the Environment Department of the Government of Catalonia.