The geology of the massif of Aubenç 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 Aubenç. And again while going down the north face in the direction of Les Masies de Nargó, 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 Aubenç

From a geological point of view, the Aubenç 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 Aubenç

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 Jaça, up to the summit of Aubenç. 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 Aubenç 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 Jaça

In the pass of Aubenç, and later,  on the descent to Les Masies de Nargó 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 Nargó. 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 Nargó, "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 Aubenç 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 Nargó, 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

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 Castell·llebre poking through the fog.

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