Fenor Bog began to grow 10,000 years ago in a lake basin at the bottom of Ballyscanlon Hill. Over time, anaerobic conditions (absence of oxygen) allowed partially decayed plant material or peat to infill the lake. The accumulating peat eventually formed a thick dome called a raised bog. Turf was cut by local people going back many centuries, but for the past 80 years or so the bog has regenerated itself into the wet-fen habitat it is today.
Dr David Bellamy provides more detailed indication of bogland that is fen. It relates to two of the many stages of peat formation in the succession of change associated with a bogland. One sees growth of large perennial plants which slow the water flow through the bog. These with the peat form a barrier so water flow is no longer continuous to all parts of the basin.
At the next stage, lighter peats are dominated by smaller sedges and brown mosses, and groundwater is directed beneath "a floating mat of vegetation". Peats formed during these two stages are known as fen peats, and the areas in which they form are called fens. Their vegetation is rich in plant species as the fen is continuously fed by spring water rich in minerals, especially lime. This means the peat substrate on which the plants grow is alkaline. More acidic conditions exist during other phases.