What is a fen?
A fen is a wetland rising, not from soil, but from peat that is kept moist by cold, calcium-rich groundwater. The mineral qualities of a fen make it home to rare, highly specialized plant and animal species.
Fens are varied and unusual, and their form and exact composition can depend on their topographical situation (see below for the details of Kishwaukee Fen’s geology). In order to understand how they form, we need to explore some geologic history. The following paragraphs include some details that are specific to northeastern Illinois, but the principles of fen development are similar across the Northern Hemisphere.
As the last glacier retreated from the Chicago region some 13,000 years ago, its meltwaters cut into the landscape, carving out valleys and river beds. Along the path of glacial outwash, deposits of calcareous (“limey”) sand and gravel accumulated to form the morainal slopes that today characterize the Fox River Valley region. Gravelly soil is highly porous, and when it rains or snows in such areas, rather than moving along the surface of the ground, much of the precipitation is able to percolate swiftly through the gravel. As it does this, it takes on an alkaline quality. You may hear this referred to as fen water “recharge”, the process of surface water changing its chemical properties through exposure to underground limestone.
The now alkaline and mineralized water continues moving underground until it finds a place further downhill to seep out back onto the ground’s surface. In such places, fens form.
This groundwater flows continually, and constantly saturates the soil at the place of discharge. Most plants and microorganisms have a hard time growing in such extremely basic (alkaline or calcareous) environments. Specialist “fen plants” adapted to such places thrive here with the benefit of genes that in the future may help food plants adapt to changing climatic conditions. Decomposition of plant litter is slow in fens, and over centuries peat builds up. Peatlands are said to store an immense amount of carbon, more even than tropical rainforests – so conserving and better understanding fens is crucial for many reasons in our warming world.
Kishwaukee Fen Nature Preserve is home to two hanging fens and two raised fens.
A) A hanging fen forms along a hillside, as cold, mineralized groundwater seeps out of the side of the hill and peat accumulates along its path. At Kish, both hanging fens are a combination of peaty terrain and shallow calcareous pools that form terraces along a slope. Owing to the harsh mineral conditions and rich diversity of species growing here, the vegetation in and around the hanging fens' calcareous pools is particularly short and delicate. Also growing here is tufa, a crystalline rock that forms when the cold fen water meets the air, warms up, and the dissolved minerals precipitate out.
B) A raised or "domed" fen forms when groundwater wells up onto a relatively flat ground surface. The resulting peat accumulates in the shape of a mound. From a distance, the raised fens may resemble a wet prairie, but upon closer inspection, trademark fen species stand out: Kalm's lobelia, Ohio goldenrod, grass of Parnassus, to name a few.
C) Sedge meadows compose the majority of Kishwaukee Fen Nature Preserve. Although not a fen, a sedge meadow may be seen as a kindred spirit, as it, too, is a type of peat-forming wetland. Sedge meadows are not as extreme in their chemical composition. As their name suggests, they are dominated by sedges, around whose bases peat forms small hummocks.