The peat bogs in the central part of the Šumava are also called fens. The Šumava formed ideal conditions for the formation of peat bogs: shallow basins, saddles and moderate slopes on the plateau of the Šumava plains and wide shallow valleys, extremely cool and humid climate, profusion of stream springs and little permeable bedrock underneath the weathered mantle. The peat bogs were formed at the turn of the last ice age and post-glacial period (about 9000-10 000 years ago), when the extremely chilly but dry climate warmed up and became more humid.
The basis of the Šumava peat bogs formation were the mellow basin lakes originally overgrown with reed and various types of sedge. Later on, the peat moss predominated in vegetation of the lakes that was attaching best to their open surface. The centre of the future peat bog was therefore rising above its surroundings, which is why their profile is called raised peat bogs.
Lower layers of the moss die away and settle, while the upper layers keep overgrowing. Slow carbonization of the dead parts of the plants due to prevented access of oxygen, low temperatures and extreme acidity of the environment give birth to the peat. The other plants contributing significantly to the formation of peat are cottongrass and rannoch-rush. The peat bogs reached the peat heights of over seven metres in course of the past centuries.
At certain stages of development there were insufficient water sources (underground springs, precipitation) to saturate the peat bogs growing to heights with water. The lakes vanished and the dwarf mountain pine, spruce and birch took up.The low vegetation of the peat bogs is usually formed of the bog pine and dwarf birch, in the herbal layer there are mountain crowberry, alpine cotton-grass, bog-rosemary, oxycoccus palustris and one carnivorous plant - round-leaved sundew. There are two accessible peat bogs accessible in vicinity of Modrava - Tříjezerní peat bog (Three Lake Fen) and Cikánská peat bog (Gypsy Fen).The visitors of the Šumava unfortunately cannot see many other interesting peat bogs due to the enclosure of the territory of the National Park.