The Estrecho de la Peña Amarilla (Yellow Rock Defile) lies to the east of the village of Alía and can be reached by the EX-102 road at kilometre point 92.5 where there is a vantage point and parking area on the north side of the road. This defile, constitutes a natural boundary between the Sierra Palomera or western flank of the Guadarranque syncline and the shaley relief of the Ibor-Guadalupe anticline.
The Estrecho de la Peña Amarilla is a fluvial defile that was formed by the Jalihuela Brook on its crossing the strata of the Armorican quartzites that make up this spectacular landscape. These quartzite walls originated in the Lower Ordovician (more than 470 million years ago) and have undergone considerable tectonic movements, which means that although they are very hard and compact rocks they have been highly fractured with numerous perpendicular diaclases (a diaclase is a rock fracture which in contrast to a fault shows no displacement of the resultant blocks). The vertical breaks to the stratification planes are the reason for numerous landslides of blocks to form the striking caves and the steep rock walls of this defile.
The visitor can observe not only the impressive geology of the outcropping of Armorican quartzites from the vantage point, but also fine examples of trilobite trails (Cruziana) in the road cuttings and on some transverse sections of these traces, which makes this geosite an outstanding paleontological location.
Attention should also be paid to the rock plant communities, which include in particular the characteristic bright yellow crustose líchen (Acarospora oxytona) that covers the rocky walls to give the Peña Amarilla its name. Examples of protected, threatened, or relict vegetation can be found (Portuguese laurel, junipers, etc.) and the contrast between the exuberant vegetation of the shady slopes and the more exposed vegetation of the sunny slopes can be appreciated. From this vantage point we can also observe the breeding colonies of griffon vultures together with a few isolated nests of Egyptian vultures and black storks, which make use of the vertical walls as nesting sites.
There is an interesting piece of history connected with this site; in the year 1133 King Alfonso VII of Castile travelled along the Roman road that crosses the Peña Amarilla defile towards Talavera on the way back from Al-Ándalus with his soldiers. Once the army had ended its campaign against the Arabs, it was thus returning via Seville: “Deinde rex movit castra et transivit illum Portum de Amarela et exivit in civitatem suam, quae dicitur Talavera”. (Luis Sánchez Belda, paragraph 42). Since then this old route has been known as the “Seville Road to Talavera” and it was used by the first Christian colonists who entered the district during the Reconquest.