The Penedos de Góis are dominated by schist, formed from sediments that began to be deposited in the Ordovician period, 460 million years ago, in an area that was then a shallow sea.
With the movement of tectonic plates, and high pressures and temperatures over millions of years, these sediments were moved and altered, giving rise to folds and the shale we find today.
The study of folds has become very important to geologists because it allows them to unravel the geological history of a region. Around Pena, you can see several folds of extraordinary beauty in the middle of the Penedos de Góis area.

In the rocks we see today, we find traces (called ichnofossils) left in the rocks by the displacement of trilobites (a dominant animal in the seas at the time) on the seabed.
The first fossils found in the Penedos de Góis were described in 1884 by Nery Delgado and were interpreted at the time as internal moulds of algae.
It took many years to clarify the problem, and there is currently no doubt that these are trilobite tracks left in the quartzite rocks from the Silurian period.
Some of the fossils collected in the Penedos de Góis by Delgado can now be found in the Geological Museum of Lisbon, the National Museum of Natural History and Science and the Science Museum of the University of Coimbra.