In the westernmost part of Sierra Morena, north of the province of Huelva, is a range of mountains of moderate elevation. The visitor, observing the scenery, would be correct in concluding that this is a very ancient landscape.
Approximately 570 million years ago, most of the world's landmass was in the form of one super continent known as Gondwana. Gondwana was in the process of splitting up and the tectonic plates that made up the continent were moving away from each other. This stretching produced a depression, that became a marine basin. On the floor of this depression, material carried down by rivers from the surrounding mountains, settled into sedimentary layers that later became a part of the Sierras de Aracena and Picos de Aroche. At the same time, the stretching caused volcanoes to erupt through the thinner crust. The volcanoes contributed granite to the sedimentary mix, along with mineral deposits that, millions of years later, would be exploited by man.
About 450 million years ago, the tectonic plates had dispersed over the surface of the world. Their continued motion started to bring them together again. The marine depression disappeared and the Sierras de Aracena and Picos de Aroche was pushed into the air.
As soon as the rock emerged from the waters, erosion started to take place, dissolving and transporting the softer materials and softening the edges of the rugged granite scenery. As a result the landscape is one of rolling hills with eighteen steep side valleys. In places where the granite is exposed, such as in the Picos de Aroche and in Santa Olalla, the granite formations provide a sharp contrast.