The study of the rocks that emerge to the surface of Bages district reveals their conditions and dates of origin. In geological views, the rocks in Bages are not too ancient. Most of them belong to the Tertiary period and some to the Quaternary. At the beginning of the sedimentation process that led to the present rocks of the region the dinosaurs were already extinguished since 23 million of years before, but it still lasted for 37 million-years for the outbreak of the hominids.
The explanation of the geological history of the Bages district starts when the materials of the oldest rising rocks of the region were laid.
1. From 42 to 37 million-years (middle Eocene, basically) a narrow sea spread to the area of Bages. This sea was connected to a big gulf of the Atlantic ocean. In this shallow sea, the muddy and sandy sediments which in the future would be the components of the Bages subsoil were accumulated.
The coast was on the south of the region. The places where nowadays the massifs of Montserrat and Sant Llorenç del Munt are found were, in that epoch, deltas that were made up by the sediments coming from a mountain range of the Catalonian massif, a continental area that was located in the current Mediterranean sea.
Coral reefs with a really diverse fauna grew on coastal areas with water temperature similar to that of the current Caribbean sea.
2. Around 35 million-years (upper Eocene) the sea steadily reduced until it dried off. Therefore, evaporites (halite, potash and gypsum) precipitated in the most internal part of the basin. The thickness of the evaporites stratum is quite noticeable.
The gravel sedimentation persisted in Montserrat and Sant Llorenç del Munt.
3. From 35 million-years to Oligocene (*), Bages was a continental basin with rivers that loaded it with sediments and lakes at which bottom limestones can be originated. This continental sedimentation resulted in the reddish detrital rocks (mainly mudstones and sandstones ) of the west, north and center of the district. These materials cover the previously deposited evaporites and the materials of marine origin.
The gravel sedimentation was kept in Montserrat and Sant Llorenç del Munt.
(*) The age of the most modern rocks of the Bages Oligocene is not accurately known and there isn’t any proof of further sedimentation during Tertiary.
4. From the end of sedimentation of Paleogen materials to mid Miocene (16,4 to 11,2 million years), the growth of the Pyrenees folded the north half of district of Bages in a few parallel, narrow anticlines that often become fault folds welded with large and smooth synclines in between.
At the time when the anticlines were created, evaporites beneath spread into their nucleus in a diapirism process. As a result, the saline geological formation, that was originally several hundreds-meter depth, nowadays emerges to the surface in Cardona.
5. For 13 million-years (since mid Eocene), a heavy erosive process have emptied the basin of Ebre. This process is the main responsible for the current relief of Bages. In the beginning of this period, the former fluvial network that was flowing towards the Ebre’s basin was replaced by the present one as a result of the sinking of the Catalonian massif and the simultaneous rise of the Ebre’s basin.
Differences amongst the resistancy of the rocks to be eroded allows the modeling of prominent morphologies like those of Montserrat in places with hard rocks in opposition to the depressed areas with soft materials like in Sant Vicenç de Castellet. The karstic process of dissolution (the process that originates the caves) in mainly conglomerates but also marine limestones and evaporites contributes to the present land shapes.
The rivers Llobregat and Cardener are nowadays the outlet towards the Mediterranean sea of the eroded materials. From time to time, episodes of local sedimentation are registered in the quaternary fluvial terraces (not displayed in the scheme). These quaternary sediments makes the gravel deposits that are often exploited.
[Oriol Oms, Florenci Vallès and Josep Biosca]