The first section of this stage of the GR 262 between Peñarroyas and Obón is listed as a Site of Geological Interest in Aragón. Peñarroyas is located on the boundary between the Carboniferous geological strata (more than 300 million years old), easily identifiable by their brownish and grayish tones at the entrance of the locality that contrast with the Triassic sandstones of the Buntsandtein of a marked reddish color. Several points of geological interest with thematic interpretive feet (faults, polygonal drying traces, ripples…) give shape to a paved route from the town to reach the viewpoint of Portillo, a pass that opens at the top of the Peñas Royas.
The viewpoint offers a beautiful view of the surroundings, highlighting the polychrome of the landscape, especially the contrast between the red color of the sandstones, the grayish of the Carboniferous or the lighter of the Triassic and Jurassic limestone packages that frame the path to Obón, with the varied note of color provided by the abundant pine forest, the poplars of the river and the diversity of plantations in the orchard.
From the top of the Portillo viewpoint the trail begins its descent. Another detour brings us in about 10 minutes to a viewpoint over the canyons of the rocky canyons that give meandering shape to the Martín River, where the reproduction of some ichnites of an Arcosaurus (a reptile of the Triassic) that appeared in this spectacular environment is located.
The GR continues descending, passing through the castellones, whimsical geological forms that resemble a castle as the name suggests and that erosion carved on whitish dolomites still from the Triassic, until you reach a footbridge anchored to the rocks thus saving the passage through the water. On the other bank of the river the ruins or remains of the hermitage of San Quiteria of Romanesque tradition. In the event that this walkway is not operational, it is feasible to use a variant of the trail, known as vuelta a los palomares, which surrounds this area of canyons to the ravine of the Val, also configuring a hiking loop.
After a small steep slope, we will flatten out through some rocky cliffs very close to the area of the olive groves and cross the ravine of the Val. Again the trail descends to the river itself, entering again by some canyons that force us to use some handrail ropes and staples or steps anchored in the rock that will allow us, not without some balance, to overcome the riverbed without having to take off our shoes, something that should not be ruled out because everything depends on the course of the river and if any recent flood has changed it.
After overcoming these gorges, the trail ascends winding up to the Rocha pass to flatten the slope, observing the gorges and cliffs that the water has been carving in the Jurassic limestone through which the Martín river made its way.
Again we descend looking for the riverbed. Here are two areas where there are two shelters with cave paintings declared World Heritage in 1998. Protective fences prevent access to the interior unless accompanied by an authorized guide. The Cerrao paintings are clearly visible from outside the trellis. Not so those of the Hocino de Chornas, which are accessed by an iron ladder anchored in the rock up to the grille that closes the cave, and whose black paintings are very blurred with the blackened color of the rocky support of the cave, being barely perceptible from outside the grille.
On the left bank of the river, sheltered by the limestone canyons and abundant riparian vegetation, we will reach the surroundings of the Fuente del Batán, equipped with picnic tables, where we can rest and have a snack. Then we will cross the river again twice, by footbridges if they are in place and have not been displaced by river floods, and in just 30 minutes we will arrive at Obón.