The coal forest.
During the Carboniferous Period, more than 300 million years ago, the southern area of the Portuguese territory was under the sea, but in the northern region luxuriant swampy forests were abundant, a good environment to preserve organism remains: plants, insects, bivalves, etc, and to generate coal.
This flora was studied by B. Gornes (1965), W. Lima (1888), C. Teixeira (1949) and R. Wagner and J. L. Sousa (1983) and the results published in “Memorias” and “Comunicacoes dos Serv. Geol. Portugal”. The organic matter accumulated in those swamps, whose waters had no oxygen, has suffered carbon enrichment originating important coal measures (anthracite) like S. Pedro da Cova and Pejão, near Oporto.