Parts of the province of León have a granite nucleus that once spread into Zamora and Galicia, primary rock of the Cambrian period to be found in the Babia and Luna valleys and in the municipalities of Adrados, Corniero, Crémenes, La Vecilla, La Valcueva, Valdoré, Vozmediano, Aleje and Alejico, where fossils abound. Pedro Alba wrote, in the mid 20th century, that “in antediluvian times, the Cantabrian Sea reached Boñar”, on the basis of the numbers of marine crustacean fossils found in the district at Colle, La Losilla and Boñar itself.
More than half the area of the province is made up of Devonian and Carboniferous rock, where there are coalfields. Coal was discovered in the province in the second half of the 18th century by the French engineer Le Maru, when he was at Torre del Bierzo building the road from Madrid to Corunna; and in the first half of the 19th century, Casiano de Prado described the Valderrueda coalfield and immediately began to exploit it around Cistierna and Matallana. The Devonian lands in León are rich in fossils of plants and small insects in coal, especially at La Magdalena and Ciñera and in all El Bierzo. The Hullera Vasco-Leonesa mining company has a petrified tree from a mine outside its offices in La Robla.
There are tertiary areas around León, Valencia de Don Juan, Sahagún, and in the Moorland and Tierra de Campos. Clay, limestone and marl soils are planted with cereals and vines, and this area of León may be considered the northern subplane. In the district of La Sobarriba, near the city of León, and in the districts of La Mata del Páramo and Los Oteros, fossilized mastodon bones have been found.
The water meadows of León are quaternary, where the silt deposits have given rise to the main agricultural areas. Quaternary glaciations produced the alluvium which supports most of the province’s agriculture, and there is much more evidence of their passing, such as valleys like that of the Cáscaro stream, shaped like a flat-bottomed trough; moraines and gullies to the south of Mt Arcos del Agua (2,058 metres). There is also evidence of glaciation in the Montes de León, like the many lakes and ponds mentioned elsewhere. The Cantabrian range, which runs along the northern part of the province, is one of the youngest ranges in Spain, as is borne out by some of its peaks, of around 2,000 metres.