INTRODUCTION
Geografically, the district of Bages belongs to the Central Catalonian depression which is the eastern sector of the basin of the river Ebre. This low lands region borders on the mountain range of Pyrenees by the north, on the Catalonian Coastal range by the east and on the Iberian range by the south-west. During part of the tertiary period, this basin or depression was filled up by sediments that came from the relieves around it. Therefore, sedimentary stones from that period like conglomerates, sandstones, mudstones, limestones and eventually evaporites are found in this area. Most of the actual stones in the district of Bages were laid in the superior Eocene period or in the beginning of Oligocene. Nevertheless, modern detrital sediments are found as well mainly near the rivers. These quaternary sediments constitute gravel deposits.
As a rule, the layers of rocks in this area do not show intense deformations. In most of the area, the layers only slant softly toward the north-east, but strongly folded layers are found in the north of the region.
The main shapes of this land are due to the differential erosion, thus, to the specific resistance to the erosion of every kind of rock. The district of Bages includes the following three basic types of land morphologies: Montserrat type, tabular, folded areas and plain of Bages.
MONTSERRAT TYPE
The particular shapes of Montserrat massif set the so-called Montserrat relief type. Undoubtedly, the slim monoliths or needles are its most spectacular and characteristic morphology. The current relief of Montserrat comes from the interaction of three elements: the rock material, the structure and the erosion.
The material is the conglomerate, a rock made of a different nature stones that are glued in a matrix of sand, clay and calcareous concrete. As a whole, conglomerate rocks are very homogeneous and resistant to the erosion.
The structure is a deep deposit of almost flat layers of conglomerate rock with a net of vertical fractures that isolates columns.
The erosion by water and ice which is reponsible for sculpting and rounding the columnar structures.
Thin layers of easy to erode clay alternate with the conglomerate rock. The vegetation grows on these clay layers. Often they are related to the presence of caves.
In the near area of Sant Llorenç del Munt i Serra de l’Obac natural park, fractures are more distant than in Monserrat. That’s why the erosive model produces there rounder summits like, for instance, Montcau.
The tabular morphology of slopes that culminate in almost flat surfaces like a table comes from the erosion in non-folded areas. The tabular morphology is originated by the alternation of soft rock layers in the slopes (with mudstones predominance) and hard layers in the summits (with conglomerates, sandstones, and limestones eventually). The best tabular morphologies cases are located in the south-west area of Bages (Cogulló de Cal Torre, Pòpia de Montgròs, Collbaix,…).
The plain of Bages comprises the wide plains with little hills in the middle of the district. This area is the most in depth carved by the erosion in the region. These plains are often covered by quaternary, younger than one million year sediments. Some of them are fluvial terraces, so sediments that the rivers laid down when they were flowing at higher than present level. These sediments are gravels, sands and silts.
The warps of the strata are responsible for the relief of the folded areas. The tectonic forces of Pyrenees which were pressing from the north together with the plastic behaviour of the salt of the geological formation Cardona beneath folded the rock layers in the north of Bages district. Violent anticline structures follow the direction SW-NE in parallel to the Pyrenees. The most abrupt anticlines are cut in faults often with long displacements. The oldest rocks, like salt in Cardona and gypsum in Súria and Santa Maria d’Oló, appear in the eroded open centers of these anticlines. From the NW corner of the district to the SE, the sequence of large anticline structures is as follows:
– Anticline of Pinós and dome of Cardona
– Anticline north of Súria
– Fault of Migmón (Súria) and anticline of Balsareny
– Antciline of Súria and fault of El Tordell (Súria)
– Anticline of de Santa Maria d’Oló
– Detachment of El Guix (Sallent)
The water streams have opened natural caves in three types of rock in Bages: in conglomerate in the mountain areas of St. Llorenç del Munt i l’Obac and Montserrat, in marine limestones in El Toll (Moià) and in salt in Cardona and beneath Sallent. In addition there are mine galleries for salt or potassium explotation below Cardona, Súria, Sallent and Balsareny.
In the two mountain areas of conglomerate, Sant Llorenç del Munt i serra de l’Obac and Montserrat, the previous vertical crevaces network and the softer layers of clay drive the water circulation beneath the surface. After that, the dissolution of the carbonated concrete by karst process generates cavities, whether caves, vertical shafts or just rock roofs. The highest number of cavities in Bages belongs to the conglomerate rock type.
In the larger area of Sant Llorenç del Munt i l’Obac massif there are many caves. Amongst them, the cave of Mura stands out by the beauty of its internal formation. The cave of Simanya, near to Mountcau peak, is the most visited because of its convenient access. In L’Obac range there are the shafts of El Llest and Castellsapera, both with big entrances, and those of Sant Jaume de la Mata and L’Espluga which is the dippest one with -127 m. The Puig de la Balma (Mura) is a building from XII century that takes advantge of the natural rock roof.
The biggest and the most beautiful cave in Montserrat is certainly that of Salnitre. It’s located in the south slope, in Collbató (Baix Llobregat), together with caves Freda and Gran. The cave of Salnitre shows an ancient water drainage of the massif. The Mentirosa spring of Monistrol is an usually dry cavity close to the road to the monastery. However, from time to time after heavy rains, there is a big, powerful water spring from Mentirosa cavity. Main shaft in Montserrat is Els Pouetons, -123 m dip. It’s located in the area that’s called Agulles , so the neddles or pillars.
In Moià, there is the complex of caves of El Toll into marine limestones. They were originated by a plenty of water stream. Then, mud laid on the caves galleries. In addition to the sediments, the entrance to the main gallery of cave of El Toll was completely covered by a landslide some thousands of years ago. The careful dig of these sediments provided a lot of bones and fossil remains of the living in the area animals ranging since the period of first Würm glaciation (100.000 year ago) to nowadays. Some of these animals became extincted. Some of them were also really big and frigthening, like the lion (Panthera spelaea), the cave bear (Ursus spelaeus) and the rhinoceros of Merck (Stephanorhinus mercki). In addition, the complex of caves of El Toll has provided archeological rests of the human presence in the middle Paleolithic (50.000 years) and mainly in the Neolithic. The findings from the excavation campaigns in the caves of El Toll are presented in the Museum of Archeology and Paleontology of Moià.
Furthermore, there is the rare carst in salt rock in Bages district. Only the salt cave Forat Micó in one side of the salt mountain of Cardona is known for a long time. It’s 640 m length and some of its walls are decorated with delicate crystalls of halita. But the mining in second half of century XX changed the scenario of Cardona. Galleries were mined into the salt mountain and salt residues were thrown over the valley. The hydrologic balance was disturbed. As a result, the water suddenly opened new channels, a new endo-karst process into the salt environment broke out. The common salt is much more soluble than any other so-called soluble rocks that are able to sustain a karst process, like limestone or conglomerate that also provide caves in separated areas of Bages district. One liter of water solves 357 g of salt to become saturated brine. In just several years, water flowing through abandoned galleries has reshaped them and has opened new cavities into the bottom of the salt mountain of Cardona. Recent cavities are the long cave of Meandres de Sal (salt meanders) 4300 m length, the cave of El Riu (the river), a salt water lake beneath the surface and the cave of Rierol Salat (salt stream), the last one into the old salt residues dumpfill. The sudden carst process in Cardona salt was growing with chaotic collapses on the surface. It was threatening the feasibility of a salt mine which to some extent was responsible for the out of control water- and even the chance of pulling the whole river Cardener and sending it to the deep mine. The uncertain, chaotic circumstances lead to the deviation of river Cardener through a tunnel in order to move it away from the salt area. The research of Espeleo Club de Gràcia has reported and outlined the new born endo-carst into the salt mountain in Cardona.
There is a big cave beneath L’Estació (Sallent) which is in contact with the old potassium salt mine Enrique. Finally, other endo-carst into salt environment may be found or can develop into the salt dumpfills, specifically in the abandoned ones like in La Botjosa (Sallent).
The caves of El Toll (Moià) and Salnitre (Collbató) as well as some mine galleries into the mountain of salt of Cardona are open to tourism.
[Oriol Oms, Florenci Vallès, Josep Biosca and Jordi Badia]