Parque Natural de las Bardenas Reales, Castel de Tierra by Traveller_mike

It is formed by Tertiry and Quarterney soils that were raised by pressure associated with the creation of the Pyrenees and the mountains of Central Spain that caused the collapse of the Ebro basin forming an inland sea enclosed by the Cordillera Catalana Costal. Sediments laid down were then eroded from the Eocene onwards. These formed gravel and sandstone that stayed at the top, fine sand, clay and limestone occupy the lower slopes. Clay remained in the centre and limestone and gypsum formed at the edges. The gravels, sandstones and clay were all assoiciated with alluvial deposits. Mixtures of sandstone and clay formed two mountain ranges in between the centre and perimeter. The sediments may have been 4 metres thick. Ten millions years ago the basin opened to the Mediterranean Sea and drained, leaving the Ebro which began water erosion along the southern perimeter. This left the remaining land relatively flat. The folding effect of erosion is caused by the alternate soft and hard materials. If the layers are horizontal then the Cabezas become isolated. Wiki Jan2013