The wetland beyond this sign is a calcareous fen surrounded by a sedge meadow. The fen is calcareous because it is fed by cold, oxygen-poor groundwater rich in calcium and magnesium which accumulates as a whitish ‘marl’ in pools. As a result of the characteristics of the water, peat (partially decomposed plant material) accumulates and if you look carefully you will see that the entire fen area is elevated by 6-10 feet above the surrounding ground. This fen has many unique and rare plant species. The most abundant plant in this fen is the ‘tussock’ sedge that forms mounds. Sedges look like grasses but the base of a sedge is triangular, not round like a grass. Willow and other shrubs – including hazelnuts near the edge — are invading the fen which is why controlled burns are used to maintain the open nature of the fen. This area represents about 6% of the park but another 20% of the park, near the river is non-calcareous sedge wetland.