The Northern slate producing region of Spain has unique geological characteristics that are evident in the natural aesthetics and durability of Spanish roof slate. Its complex geological development contributed to the high quality slate resources found in the region today.

These slate deposits were formed over 500 million years ago, during the Palaeozoic era, specifically within the Cambrian, Ordovician and Silurian periods.

Slate is created when shale, which consists of clay minerals, is put under pressure with high temperatures. The clay begins to revert to the mica minerals from which it formed causing the rock to grow hard, with a pronounced cleavage direction, which enables it to break along flat, even planes.

The type of slate created depends on a number of factors, the most important of which is the tectonic environment, and where sedimentary materials came from, as well as the physical and chemical conditions prevalent during the sedimentation process.

Once sediments were deposited, they began to be compacted and decreased in porosity as the water was squeezed out, turning clay minerals into mica and forming solid mudstone. Over the next few million years, the mudstone was further compacted and metamorphosed during mountainbuilding processes, such as those that created the Alps and the Pyrenees. This created an even stronger rearrangement and planar orientation of the minerals, leading to a well-developed cleavage seen in Galician slate.