Moors are covered with slow-growing peat mosses. Their lower parts do not rot but carbonize and turn into peat. The layers of peat are then getting slowly but persistently thicker. Until the plants retain their contact with groundwater, this is a fen, but when peat considerably heaps up, the plants can live only on precipitation with an insignificant quantity of nutrients. This is why only the species distinctly adapted to this extreme environment thrive here. In the distant past, the greater part of Ljubljansko barje was covered by raised bog, which was considered the southernmost in Europe.