This geological formation has aquired its shape due to the erosive action of the rivers on the basalt. It was the effect of two layers of vulcanic lava from eruptions that took place at two distinct and distant times in history, the latest of which took place 192.000 years ago. Actually, in Castellfollit there's the only basalt active quarry in the whole state.
The "old" city of Castellfollit de la Roca is medieval and has squares and narrow and dark streets, which are, for the most part, built from vulcanic rock. At the end of the slope, from where the old cemetery was transfered in 1961, today there's a Josep Pla square. In it, the viewer is guaranteed an amazing view of the region. It's only then you understand the true and incredibly strategic position of the place and the natural safety given to the people of past times.