These are the largest sedimentary formations of continental origin in the Geopark and are always associated with quartzite sierras. They consist of extensive superimposed layers of reddish-yellow clayey and sandy material, with this matrix including numerous rounded boulders of quartzite, sandstone, and sometimes shales. Although geologically the whole is a conglomerate it is neither very compacted nor cemented, which means that it breaks up easily. It is of alluvial origin, i.e. it is the result of material dragged away by rivers and deposited on these plains or sedimentary basins where they are now to be found. The materials were formed by erosion some two and a half million years ago owing to tectonic movements during the last phase of the Alpine orogeny and to the climate (sudden changes in temperature and violent storms) that eroded the most exposed rocks or those raised after these movements.
The thickest materials of the rañas originate from quartzites, the fragmentation of which produces the boulders and blocks, while the shales or lutites break up to form the clayey-sandy matrix. The thickness of this formation varies greatly from 5 to 10 m, and may even exceed 20 metres in the great depression of the River Guadiana. These dispersed materials produced by the fragmentation, transport, and sedimentation of other previous rocks are known as detrital (detritus is the Latin word for pieces). The rañas of Cañamero, Alía, Logrosán, and Valdecaballeros make up as a whole an extensive triangular plain opening towards the south and with the upper slope facing the extensive syncline of the valley of the River Ruecas.
The rañas are considered to date from the Pliocene or Plio-Quaternary periods (between 2 and 3 million years ago), as on occasion they are found to be covering clays deposited in the Miocene (23 to 5 m.y.) near Las Villuercas. They are the subject of an intense multidisciplinary scientific debate because of their great geomorphological, pedological, stratigraphical, and aesthetic interest.
