The Serra del Montsec is a limestone mountain system that is more than 40 km long and covers a total area of 18,696 hectares, spreading across the comarques (local districts) of La Noguera and El Pallars Jussà. The thrust and erosive force of the rivers Noguera Pallaresa and Noguera Ribagorçana have left their mark on the landscape in the form of the spectacular gorges of Mont Rebei and Terradets, which now divide the sierra into three separate parts. From west to east, these are: Montsec d'Estall (which is in Aragon), Montsec de Rúbies and Montsec d'Ares.