The Serra del Montsec extends almost 40 kilometres between the regions of La Noguera, Pallars Jussà and Baixa Ribagorça and its history begins with the formation of the rocks that compose it, now more than 200 million years ago (Ma).
The current forms and relief have not always been the same, in fact, a large part of the sedimentary rocks that make up this mountain range are of marine origin, since where we now find the Pyrenees there was a sedimentary basin, a depression covered by the sea.
This basin originated by the fragmentation process of the supercontinent Pangaea during the Triassic (about 250 Ma ago) which gave rise to the European and Iberian tectonic plates, which were separated along a scar from a previous fracture called the North Pyrenean Fault. This process of détente led to an increasingly depressed topography that was gradually invaded by the sea.
The sea of the Pyrenees
The ancient Pyrenean Sea was bordered to the north by the mountains of the Aquitaine Massif and to the south by those of the Ebro Massif, both of which have now disappeared due to erosion. The rivers that drained and eroded these massifs transported the sediments generated to the Pyrenean Sea, in the form of gravel, sand, silt and clay that were deposited at the bottom.
These materials, over time, were transformed into rock through the lithification process, forming conglomerates, sandstones or shale.
On the other hand, rocks of biological and chemical origin were also formed in this sea. The accumulation of skeletal remains of carbonate composition (such as corals or shells) in areas with abundant marine fauna gives rise to limestone rocks and dolomites. These rocks can also be produced by chemical precipitation or by accumulation of calcareous mud. The calcium carbonate or magnesium rocks tend to have greyish hues and are very well observed on the long southern slope of Montsec.
On the other hand, in the shallower and calmer parts of this basin invaded by the sea, evaporitic rocks were also formed. These are produced by the evaporation of water and the consequent precipitation of salts, giving rise to deposits of gypsum and salts, among others.
Growth of Montsec
About 85 million years ago, the dynamics of the Pyrenean Sea changed. The tectonic processes are modified and we go from a regime of distension to one of compression: from this moment on, the Iberian and European plates begin to come closer and collide. As a result, the area where the old North-Pyrenean Falla remains in this basin, begins to emerge and rise. The rocks, which were just beginning to form at the bottom of the sea, still poorly consolidated, deformed and fractured, and the relief of the Pyrenees was born.
This transition from a moment of distension (the plates separate) to a moment of compression (the plates approach and collide) causes changes in the configuration of the basin, which is restricted in the face of the emerging relief and is called the foreland basin. Foreland basins are formed by the collision between plates where one of them is located on top of the other, giving rise to compressive fractures and thrusts. The weight of one mass of rock on top of the other deforms the lower materials and generates a depressed area in front of the mountain range that is being formed, and in this case a basin south of the Pyrenees. Compressive movements and uplift generate sliding mantles, gigantic masses of rock that move southwards, deform and mount on top of each other, overlap. The Serra del Montsec forms the front part of one of these mantles.
During the Cretaceous, the Bóixols-Sant Corneli mantle (to the north) overlaps the Montsec mantle. Later, during the Paleogene, the processes of deformation continue and, like a shock wave, the Montsec begins to rise, overlap and deform the terrain to the south, contributing to the subsequent formation of the Marginal Mountains. The tectonic forces of the Bóixols-Sant Corneli mantle on the Montsec conditioned the concave shape of the latter, generating the Conca de Tremp, behind the Serra del Montsec.
At the end of the Oligocene, approximately 20 million years ago, the Pyrenees stopped growing. The rise of the mountain range stopped and erosion began to sculpt the current relief, generating the impressive cliffs of Montsec as they can be seen today. For a more visual understanding of the formation of the Pre-Pyrenees, it is advisable to look carefully at the following figure, which shows a step-by-step diagram of the formation of the mountain range. This interpretation is the result of recent studies; although there is still a lot to learn from Montsec.
The geological record of Montsec
Once the genesis of the current relief has been described, it would be necessary to know what geological record is written on the walls of Montsec. The record of geological times is read through stratigraphy and palaeontology, through the type of rocks present, their age and the fossils they contain.
It should be mentioned before, however, that sedimentation is not constant and over time there are stages in which material is sedimented (information is recorded) and stages in which it is eroded (information is erased) or there is no sedimentation at all. In Montsec we find recorded only the periods in which sediments were deposited and, therefore, not all geological times are recorded.
The oldest sediments in Montsec date from the late Triassic period (K, Fig. 4). They are gypsums, clays and loams with volcanic rocks (ophites). Gypsum and clay behave plastically when subjected to pressure and are considered to have been the sediments that acted as a lubricant and allowed the materials of Montsec to move southwards to where they are today. For this reason, these rocks are very damaged and are only preserved in specific points.
Above these sediments lie the Jurassic rocks that are composed in the lower part of limestone with balls of calcium carbonate (oolites) (Fig. 4, L1) typical of shallow water coastal environments. Above, there is a level of very fossiliferous loams (Marges of the Conill farmhouse) that contains remains of brachiopods, crinoids, cephalopods (such as ammonites, belemnites, etc.) and bivalves (Fig. 4, L2) that correspond to environments of a deeper sea than the previous one. The rocks of the Jurassic culminate in a level of dolomites, carbonate rocks that are mainly composed of magnesium carbonate and that also correspond to a marine environment. In Montsec it has grey tones and is the rock on which the hermitage of La Pedra rests (Fig. 4, D).
During the Lower Cretaceous, the separation between the European and Iberian plates provided the ideal conditions for the formation of transitional environments, i.e. coastal areas, marshes and freshwater lakes, with a record of limestone and loams containing fossils of a very specific type of algae, the charophytes (Fig. 4, Ci). It is worth mentioning the freshwater lake areas, since the rocks that form in these lakes of calm waters and of a certain depth are gray limestones of very fine grain In fact, the size of the grain is so fine that classically they have been used for the production of plates for the art of lithography and therefore they are called lithographic limestones. They contain the fossils in an exceptional state of preservation from the sites of La Pedrera de Meià and La Cabroa that we present on this portal. Among these fossils are fish, birds, amphibians, insects and a large amount of plant matter such as montsequies, the first flowering plants in the history of life (with the toponym of Montsec integrated into their name).
During the Upper Cretaceous, the erosion of the Ebro massif provided the material that was sedimenting at the bottom of the Pyrenean Sea (Fig. 1). The rocks of Montsec record the most coastal part of this system. In chronological order we first find the Cave Limestones (Fig. 4, Cs1), a set of levels of micritical limestone with microscopic fossils (foraminifera) that lived in shallow waters. The lower level of these limestones is the most continuous and forms the picons limestones that can be seen in the Finestrelles ravine. The upper levels are more discontinuous and contain fossils of rudists, cone-shaped bivalves and reef builders.
On top of these limestones there is a loamy-nodulous level known as the cultivated “Montsecs” loams (Fig. 4, Cs2). This unit changes between Montsec de Meià and L’Estall; in the first, sandstones and conglomerates dominate and in the second, these sandstones are found at the base and ceiling of a loamy package with a large amount of fossils. These sediments correspond to the formation of a delta in the Pyrenean Sea with contributions from the Ebro massif located to the south (Fig. 1). Among the fossils are colonial and rudist corals, but there are also solitary corals, brachiopods and foraminifera.
When the contributions of sediments by the rivers came to an end, a new sedimentary dynamic began where the calcium carbonate fixing organisms began to form calcareous rocks that today form the highest cliff of Montsec (Fig. 4, Cs3).
Above these limestones are the sandstones of the northern end of the Pas de Terradets or Isona (Fig. 4, Cs4), which correspond to a new cycle of erosion of the Ebro massif that contributed the sediments to the Pyrenean Sea and deposited them in the form of deltas. These sandstones are the well-known sandstones of the Areny Formation that also emerge in the north of the Conca de Tremp and correspond to coastal environments where numerous dinosaur remains are found. Thus, sedimentation in these seas is conditioned by moments of erosion on the continent, generating rivers that transport “dirty” water with a lot of sediment, which is deposited in the form of sandstones and other materials, alternating with moments of calm, where the contribution of rivers decreases, the waters are “cleaner” and allow the calcium carbonate fixing organisms that form limestone to develop.
At the end of the Cretaceous, the Pyrenean Sea was filled with sediment and the marine environments gradually passed to continental environments. This replenishment, combined with a drop in sea level and the rise of the Pyrenees, causes the coast to migrate westwards, marking the end of the Cretaceous and its era, the Mesozoic, with reddish sediments. The sediments of this period represented in Montsec are conglomerates, sandstones, reddish and limestone pellets with stromatolites at the top (a type of green algae that generate very characteristic rocky structures) (Fig. 4, Ga). These sediments, called Garumnian facies, mark the Maastrichtian, the final age of the Cretaceous from which the dinosaur fossils extracted on both sides of the Pyrenees date.
The next era, the Cenozoic, begins with a rise in sea level that opens to the west and floods the Meià and Àger, to the south of Montsec, and the Conca de Tremp and Dellà to the north. At this point, the “Montsec” already sticks its head above the sea and separates the Pyrenean Sea into two individualized basins. The Montsec will erode and generate sediments that will be deposited in the most depressed areas. On Passarel·la there are some limestones known as the Àger Group, which are characterized by being formed by alveolins (foraminifera) (Fig. 4, Ei1). Interspersed with them are levels of conglomerates, the result of the erosion of the “Montsec”.
Deltaic sediments were deposited above the limestone with alveoli. The oldest and most spectacular example of these sediments is found in La Règola, in the Àger, in the form of exceptional tidal stratification structures (Fig. 4, Ei2). This coastal environment, however, was soon flooded again, due to a new rise in sea level, as shown by the levels of clays observed above (Fig. 4, Ei3): when sea level rises, the erosive power and contribution of river sediments decreases.
Then the sea level dropped again, generating a new delta on the aforementioned silts and clays. This deltaic system is where the village of Àger (Ei4) is located today. It is made up of large sandstone slabs interspersed with blue-toned silts that have been interpreted as bars of sand that laterally transform into coastal ridges. Above it we find clay levels with oysters and saltwater gastropods that mark the death of the delta.
A new drop in sea level, 52 million years ago, causes the formation of new deltas in the Àger (Fig. 4, Ei5).
At the end of the Eocene (about 38 million years ago) the uplift of the axial Pyrenees increased dramatically, causing the establishment of torrents with a lot of slope and energy that provided large quantities of pebbles, gravel, sand and silt that received the most depressed areas located to the south, and which today we find in the form of conglomerates. In Montsec there is a record of these deposits at the E-W ends (Fig. 4, O).
To finish with the sedimentary history, during the last million years, it must be said that the valleys located at the foot of Montsec received the last brushstrokes with the deposition of angular gravel from landslides and landslides later deposited in the form of slopes, where almond plantations are currently located. It is also worth mentioning the deposits of calcareous adobe (locally called pumice stone) associated with groundwater upwellings along the Fred river in Vilanova de Meià (Fig. 4, Q).
The result of the inevitable interaction of sedimentary dynamics linked to the tectonics that raises mountains and the erosive processes that are generated is what has been sculpting the spectacular landscape that this area enjoys.