Associació d'amics del camí de Pallerols de Rialb a Andorra
Geology section 4: from Les Masies de Nargó to Valle de Cabó
Reflections of an itinerant geologist 

The walk between the farmhouses of Nargó and the valley of Cabó, through Fenollet, passes mostly by the municipality of Coll de Nargó. I daresay it's awesome, just like in the movies. And this is because this territory is one of the most important dinosaur sites in the world. Around Coll de Nargó there are, in fact, numerous traces of the presence of these large reptiles, which Michael Crichton described so masterfully in his book "Jurassic Park." A bit later on and based on this novel, Spielberg was commissioned to immortalize these disturbing monsters in the movies. In short, it is an excursion like you see in the movies, as I said before.  

The dinosaur sites of Coll de Nargó, which are particularly rich in eggs and nests, help us understand a little how the world was in the late Cretaceous period, or about 65 or 70 million years ago. Paleontologists think that the last dinosaurs became extinct at the end of the Cretaceous. Many other species on both continents and in oceans died out as well. Much to our relief, these enormous reptiles disappeared in the blink of an eye, and with them also disappeared many other species, such as ammonites, which were the ultimate marine mollusks. The first specimens of ammonites found, in England, led their discoverers to think that they were snakes coiled and petrified, but soon they concluded that these invertebrates were actually a kind of cephalopod mollusc, like squid, but with an outer shell like snails. 

Shortly after leaving the farmhouses of Nargó, we find the sedimentary (sandstones) of the Cretaceous. Then we descend to the bottom of the valley carved by the river Sallent  See Picture 1. Panoramic between the farmhouses of Nargó - on the left of the picture- and Nargó Mountain. 

Here the fluvial terraces on both sides of the river become very evident, forming a carpet made of the most recent sediments in the area. Soon we step again on the sands of the Cretaceous. Shortly after, now going up, you can see clearly the blue marl of the Cretaceous, which appears intensely eroded by surface runoff. Gullies and ravines form a distinctive landscape that geologists know as the bad-lands. See Photo 2: Gullies and ravines in the valley of the river Sallent 

Along this climb we can see also the spectacular Jurassic limestone peaks that rise toward Montanisell. Some of these projections peek from the marls, recalling a disturbing little shark fin. Soon we will pass between two of the fins, using a channel similar to the one that allowed us to climb the mountain Aubenç. See Photo 3: Jurassic limestone Crest 

After about an hour's climb from the river, we find ourselves faced with limestone cornices with their characteristic colors from grayish-blue to red. They are like the teeth of the mountain! ... They are marine rocks (mainly limestones, some with heavy fossil content) of the Jurassic. That means we have gone back a leap back in time. Although the mountain ahead seems insurmountable, there are small gorges or canyons that allow us to pass through smoothly. Along the way up we often see areas of limestone with sharp grooves and ridges: they are the result of chemical weathering on these rocks caused by rain water over time. This form of weathering is what geologists call limestone pavement. See Photo 4: Mountain range of limestone from the Jurassic period, and Photo 5: Limestone pavement of the Jurassic period

The rise, zig-zagging between rocky debris of all sizes, brings us after a while near Nargó Mountain, located about halfway between Coll de Nargó and Organyŕ. We walk again on Cretaceous material, but with the peculiarity that this limestone beneath our feet now is rich in fossils characteristic of tropical marine environments, such as corals, algae and other organisms. Among them, abounds a very special kind of small coin-shaped fossil called orbitolinid. They were foraminifera(a type of protozoa) of between 1 and 2.5 cm in diameter, which abounded in the waters of the seas of the Mesozoic. See Photo 6: Orbitolinid Cretaceous 

From the pass between Montanisell and Nargó Mountain, we see clearly the northeast valley where the farmhouse of Fenollet is located. Behind, one can make out very well the profile of the mountain range of Sant Joan, at whose eastern end stands the mountain of Santa Fe, which marks the northern boundary of the municipality of Coll de Nargó. Farther to the east, we can see on the horizon the peaks of the mountain range of Cadi. See Figure 7: Valley descending from the mountain range of Sant Joan, through Montanisell and Fenollet, and in search of Segre. On the left, the mountain of Santa Fe, in the background, the mountain range of Cadi 

The forests of european black pine and red pine on the south side of the mountain were rather scarce, but they dominate now, as we descend the north side towards Fenollet. See Photo 8: pine and oak forest 

The materials of the valley that we find when approaching Fenollet are clearly mostly calcareous, with a predominance of marls and marly. These rocks are less resistant to erosion, and that has facilitated the erosive work of the geological agents (such as rivers). Almost all materials exposed in this valley are from the Lower Cretaceous, which takes us back in time to ages ranging from 100 and 120 million years ago. Very fossiliferous, indeed. 

The climb to the mountain of Santa Fe starts at the highest part of Fenollet traveling over limestone, moving on to pass over marls and dolomites of ocher colors. See Figure 9: Mountain Fenollet and Santa Fe. Just before reaching the pass that is below the hermitage of Santa Fe, there is abundant limestone and marl of the Upper Cretaceous, in which there are abundant fossilized sea urchins that are preserved in fairly good condition. No doubt, the one Leticia found is the best of all that we collected. See Figure 10: sea urchin fossil of genus Micraster Upper Cretaceous 

From the mountains of Santa Fe you can see an extraordinary view of the mountain range of Ares, and at its feet Organyŕ basin. This mountain range is highly regarded by geologists, as the material of the lower Cretaceous that forms it has attained its greatest thickness at this spot in the Pyrenees (over 1,000 meters of depth in some places). However, that does not stop us from appreciating perfectly the erosive effect of water from the river Segre on the limestone of these mountains. The action of erosion, slowly but inexorably, is particularly evident further north of the town of Organyŕ, where the river has broken through the spectacular gorge of Tres Ponts, where it also passes the road from Lleida to Andorra. View Photo 11: Gorge of Tresponts 

... But we will see this story on the next trip. 

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Associació d'Amics del Camí de Pallerols de Rialb a Andorra.   Av. Diagonal, 620, 1er. 2a, 08021 - Barcelona (Spain) . Tel.: (+34) 629 910 612