Associació d'amics del camí de Pallerols de Rialb a Andorra
 
Geology Section 3: Gully of La Ribalera - Les Masies de Nargˇ

Octavio Rico recounts geological observations while he travels the Road to Andorra



Route 3: Gully of La Ribalera - Les Masies of Nargˇ

The surprises in store for us this day, in addition to the placement of a Nativity by the house of Aubenš, accompanied, of course, by the traditional Christmas celebration, are mainly of the landscape and botanical type.


The geology of the massif of Aubenš is of great interest, of course, but the greatest highlight of the excursion is probably, the impressive views that can be seen south from the summit of Aubenš. And again while going down the north face in the direction of Les Masies de Nargˇ, we were blown away by the vistas, one after another until they link up with the high peaks of the Pyrenees of Lleida and Andorra. See Fig 1. Panoramic of the Pyrenees from Mount Aubenš

From a geological point of view, the Aubenš is a sedimentary massif of predominantly solid limestone composition, that is, rocks formed by chemical and biochemical origin of the limestone group. It simply means, that those rocks were formed by processes of precipitation of the salts that were dissolved in the water that long ago invaded the "sea of the Pyrenees". All this occurred in the Cretaceous geological period, in the final time of the Mesozoic Era, meaning a little less time than 100 million years ago. It was such a long time ago, of course, that not even our ancestors remember any of those events.

The truth is that at the end of the Cretaceous period, in that Pyrenean basin there had accumulated a huge amount of sediment, many of them in marine environments; others in mixed sedimentary environments (deltas), being dragged by the rivers from the Ebro massif (which no longer exists) and the new Pyrenean reliefs. Over time, these sediments eventually became the group of strata that today outline the cliffs of this mountain and which we will travel throughout the day. See Fig 2: Overview of Aubenš

Among the main rocks of the mountain, geologists have identified mostly nodular limestones and calcareous marl, which are rocks formed in marine shelf environments. We can see these rocks especially while going up through the channel of La Jaša, up to the summit of Aubenš. It is easy to find fossils between the strata, as in that inland sea, like in the current ones, there was a great diversity of living organisms. With time and patience, we can find, for example, remains of mollusks, such as ammonites or hipurites (a kind of bivalve). There are also abundant pectinidae, shells very similar to Pecten jacobaeus (or shell of St. James), which is the typical shell worn round the neck or in the hat by pilgrims making the Way of St. James. The difference is that pectinidae that can be found in the slopes of Aubenš lived in this area almost a hundred million years ago. In Figure 3 you can see a little walker showing a Pecten -marine mollusk from the Upper Cretaceous- found in the Channel of La Jaša

In the pass of Aubenš, and later,  on the descent to Les Masies de Nargˇ we will see new types of rock: bioclastic limestone, sandstone and bioconstructions. Below the house of Aubas, marls, clayey rocks and sandstone will predominate in the terrain. These last materials, the sands, are the ones fully predominating up around Les Masies de Nargˇ. We often see cross-stratification in them, which is a fairly typical sedimentary structure of rocks from sediments deposited by rivers. In Figure 4 we can see the Cretaceous sandstones with cross stratification

The route is only about 12 miles long, but in terms of time, progress is on a much larger scale: no less than 20 million years. If the starting point, near the gully of La Ribalera was about 85 million years, the arrival point of the third stage, at Les Masies de Nargˇ, "catapults" us twenty million years later, to the end of the Cretaceous (65 million years ago).

One of the traits of the mountain range of Aubenš we must highlight is its geographical orientation. The watershed of the massif (the ridge of the mountain) is oriented East-West. This orientation explains, in part, the sharp contrast that can be observed between the sun-exposed (south side of the mountain), and shady side (found on the north side). In addition to geographic orientation, we must highlight the strong asymmetry observed in this mountain range, meaning the differences in slope between its northern and southern mountainsides. Thus, while the northern slope gently descends towards Les Masies de Nargˇ, the south side shows steep slopes which are cut in many places almost vertical. This asymmetry has to do with the geological history of the massif, which as we saw in the previous excursion, relates to the landscape shifting to the south (thrusting and layers of landslides) that occurred during the formation of the Pyrenees. It is a long and complex history which we will not fully explore now (we would stay up all night....) Suffice it to say that the processes of deformation (folds, fractures, etc.), were followed later by the effects of erosion (continuing today), and are the cause of the last remodeling that we see today and admire - along with the birds characteristic of the area like the common vulture, the common partridge or the eagle owl. See Figure 5: Common vulture, 6: Eagle Owl and Fig. 7: Common partridge

From the summit of Aubenš, if we look to the south, we can see what is surely one of the most beautiful and spectacular panoramas that can be admired from this area. One can see perfectly, the Central Depression (with Oliana Basin in the foreground), and the mountain range of the pre-Pyrenees, among which are the majestic Mountains of Sant Honorat and the Rock of Corb. Further to the right, can be seen the forests of the Barony of Rialb. On clear days, one can glimpse in the distance the mountain of Montserrat, but during winter another picture is common because of the thick and leaden clouds that typically fill the valleys and depressions. Many people are ecstatic at the sight for the first time in their life of a "sea of fog" from above. And it is true that it certainly is a great show. It is something magical and difficult to explain. You have to see it with your own eyes! To get an idea, see Figure 8

The climb up the south face of Aubenš is demanding, not only because of difficult terrain, but also because of the abundant thorny vegetation, predominantly shrub, dominating the field during the entire climb. The thorny vegetation is compounded by another problem: the large amount of loose rock (talus slope) lining the road. These gravels, rather angular, slightly worn, are the scree (pans) formed as a result of the stress experienced by rocks by the combined action of freeze-thaw.  Eventually, these multiple freeze-thaws completely disintegrate the rocks, which are thus reduced to gravel. This phenomenon of weathering or disintegration of rocks, is what geologists call frost weathering.

For the reasons stated, and the peculiar climate of the area (semi-humid Mediterranean climate, which here combines with typical characteristics of mountain climate), it is normal to find very specialized plants such as juniper, cypress and kermes oak on this southern slope.

Many species, such as chamomile and thyme, are aromatic. Almost at the summit, the low shrubby bushes catch your attention, with their pillow shapes, it seems as if someone had scattered a few cushions on the floor. The problem with these peculiar "cushions" is that they are extremely thorny. They are known in Catalonia as "coixinets", "erišo" or "cadireta de pastor", see Fig 9. It is a species adapted to high altitude and cold, prepared to live with snow and biting winds that often dominate the ridge of this mountain range.

Above our heads, we may still see vultures, with their distinctive and majestic flight in circles, looking for ascending thermals. Forests of pine, beech and oak follow one another, as we descend towards Les Masies de Nargˇ. Unlike the beeches, that at this time are naked, oaks still have their typical copper toned leaves, although they are about to fall. The careful observer can enjoy watching some beautiful scenes of mixed forest of pine and oak, with their typical color contrasts, as shown in Figure 10.


The climatic conditions of the last months of the year tend to create more or less prolonged periods of thermal inversion in the region of Lleida. When these conditions happen -which generally coincide with anticyclones (stable and sunny weather in Catalonia) fogs also often appear, which fill the valleys and depressions, leading to the formation of so-called "seas of fog". When weather conditions are such, you may see images as suggestive and spectacular as those seen on this tour near the dam of Oliana. See Fig 11 for the reservoir of Oliana and the hermitage of CastellĚllebre poking through the fog.


Link to the corresponding News of the Walk

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