Octavio Rico explains his geological observations while touring the Road to Andorra

Itinerary: Peramola-Gully of la Ribalera

Unlike the previous stage, when we traveled all day through the foothills of the Pyrenees, bordering the Basin of Oliana but not yet in it, this second stage introduces us firmly to what is known as “the hall of the Pyrenees”, in the outer ranges of the mountain chain.
This land, still very modern (Cenozoic), of Peramola and the Rock of Corb, abruptly gives way to the first mountains of the Pre-Pyrenees, older than the previous ones. The last point of this journey, in the gully of la Ribalera offers the opportunity to gaze, to the north, at one of the most incredible sights of this tour: the impressive limestone rocks of the upper Cretaceous (last period of the Mesozoic) that form the southern slope of the mountain range of Aubenç. To the south, as can be seen in Fig 1.

Standing out with personality, is the oligocene conglomerate (Cenozoic) of the mountain range of Sant Honorat, which is located at the Rock of Corb.
Climbing gently but progressively between Peramola and the Rock of Corb is perhaps the clearest evidence that something begins to change in the ground beneath our feet. And, in fact, we start crossing the threshold of the Pyrenees. The Rock of Corb, like a guardian of the mountain chain, is like a foretaste of the mountains that we will find from now on in the upcoming walks. In Fig 2, we can see a panoramic of the Mountain range of Aubenç (left) and the Mountain range of Corb (right), and to one side, the House of La Mora.
The Rock of Corb consists predominantly of conglomerate rock. Between the thick conglomerate banks, we can easily discern some levels of sandstone and lutite (clay rocks) that are quite compact. Thanks to these last levels, which erode more easily than the conglomerate, the House of Corb can be accessed with relative ease. Indeed, the softer and therefore more easily eroded levels of lutite and sandstone, have led to the formation of the peculiar modeling of the Rock of Corb. The cornice that surrounds the Rock of Corb is the clearest sign of differential erosion that occurs in these monolithic formations, as shown in Fig 3.
The resemblance of the mountain of Corb to Montserrat is no coincidence. These sedimentary formations originated at a time, during the Cenozoic (in the Eocene and Oligocene periods), as a result of intense erosion and sediment that took place as the Pyrenees were waking up from their slumber. We are talking about the times and geological upheaval that lead not only to the Pyrenees, but also to other large mountain ranges like the Alps and the Himalayas. We are speaking in short, about what we geologists call the alpine orogeny. In Fig 4, from the Rock of Corb, looking north, we can see the majestic peripheral mountains of the South Pyrenees, among which the mountain range of Aubenç stands out with its highest point, the Coscollet (1,611 m), highlighting the horizon.

The peripheral mountains of the South Pyrenees include a set of geologic units formed by a powerful series of strata, among which the Mesozoic sedimentary materials stand out. Almost all of them have the same marine origin, hence the abundance of fossils in them.
The area between the Rock of Corb and the Mountain range of Aubenç is, in fact, a series of geological structures formed as the Pyrenees were appearing. Predominating the materials thrust southward, as if the process of elevation of the Pyrenees in the area had caused a huge “wave” of rocks and sediments to the Basin of Oliana, The calcareous rocks of Aubenç are the spitting image of the result of these tectonic movements.
The point of arrival of our itinerary, the head of the Gully of La Ribalera, offers an extraordinary example (perfect example to write a book about without any doubt) of the folding structures of this area. Specifically, you can see perfectly a syncline, that is, a concave deformation of the strata of limestone, whose axis coincides exactly with the place where Mass is celebrated regularly in remembrance of the Mass celebrated by St. Josemaria Escriva, on November 28th, 1937. In Fig 5 one can see the Gully of La Ribalera and a syncline in Cretaceous limestone.
Above us, the fabulous limestone cliffs of clay (the clay fraction is the cause of the characteristic red hue of these rocks), seems to be about to collapse at any moment. In fact, it is a place where rocks frequently do fall because of the phenomenon known as frost weathering. The rainwater is filtered through the cracks and joints of the rocks. As it freezes, it creates tensions between minerals and, eventually, the rocks disintegrate. Nevertheless, these rocky walls are popular places for climbers. They can be seen climbing up to the top very often. Fig. 6 shows clearly the Cretaceous limestone (western flank of the syncline) in the Gully of la Ribalera.
Returning by car on the same road, we can see a country house that seems to be crowning a Jurassic limestone promontory: It is the country house of Juncàs, where the fugitives used to stop and were assisted in the gully of La Ribalera. Beyond Can Juncàs, to the southeast, we see again the depression in the terrain that is the Basin of Oliana, as shown in Fig 7.
The next stage will also be very exciting, with a few more geological surprises. Although the Pre-Christmas culinary surprises will likely be the ones most anticipated at that time. Anyway, good nougat candy can be as good and hard as the conglomerate, … see you soon!
Link to the corresponding News of the Walk