In a secluded valley in the Serra da Sao Mamede, Alto Alentejo, Portugal.

Serra de Sao Mamede - Geology

Map of the geology of Parque Natural da Serra de Sao Mamede, Portugal

Alentejo has a reputation for being made up of widespread undulating plains of grain and cork oak with smaller areas given to production of wine which is deservedly gaining a good reputation. So easy, then, to ignore the small unique series of folds at the top right of Alentejo which became a crumple zone for colliding tectonic plates when the ancient supercontinents of Pangaea and Gondwana were splitting up and moving around.

The Parque Natural da Serra de Sao Mamede at the southern border of the Central Iberian Zone is a ripple on the surface of the earth's crust which occurred possibly 180 million years ago. Granite that metamorphosed under the extreme pressure of this collision is 500 million years old from the Primary Era.

The tectonic plate boundaries that exist today are on the other side of Portugal some way off in the Atlantic ocean and Sao Mamede has been at rest since those ancient times.

The orogenetic, mountain building forces produced a group of folds which are called synclinal because the fold in the strata is turned upwards. As a group of folds this is called a synclinorium. Primary Era granite which was formed from magma being squeezed upwards into weak areas of softer rock was then subjected to extreme compression causing metamorphism such that in places it has the layered appearance of schist. This is called Portalegre tectonic granite.

To the north, just beyond the Sao Mamede mountain range there are huge plutons of a more modern granite which were not compressed and are visible as huge rounded mounds where the softer rock around them has been eroded away. At the northern end of the range the synclinorium abruptly ends in the dramatic hills of Marvao, Fria which borders Spain and the protruding tectonic granite fault next to the hilltop village of Castelo de Vide.

The folds dip such that at the south-east end of the range they descend gracefully into the undulating Alentejo plain beyond the hill-top village of Alegrete and villages of Mosteiros towards the town of Arronches and hills of pinkish quartz around Esperança.

The peaks of the folds of Sao Mamede are from the Ordovician Period of the Primary Era, and descending into the valleys we find the more recent Silurian Period and in the lower parts the younger Devonian Period.

Apart from those places where tectonic granite protrudes, the peaks are made up of hard rocks especially quartzite which form ridges with a predominately NW-SE direction.

Down from the peaks we find schists, some with fossils and other rocks of the Silurian Period.

In the Devonian valleys beyond the schists there can be found dolomitic limestone as quarried at Escusa between Castelo de Vide and Portagem which is below the hill-top village of Marvao.

500 million year old Portalegre tectonic granite 500 million year old Portalegre tectonic granite