What is the importance of the "fen" in the book, Fen, Bog and Swamp?

A fen is wetland ecosystem that creates peat and is fed by water from mineral rich soils higher in the hydrological profile. Fens contain the deepest water of peatland biomes and tend to support reeds and marshy grasses. Fens are also the first in the ecological succession processes of wetlands and over vast amounts of time give way to the formation of shallower bogs and swamps.

The author names a litany of critical fen habitats globally, but restricts her study of fens to the regions of Western England. Studies of the region reveal that it was home to rich civilizations of Mesolithic peoples for thousands of years. Because fens preserve artifacts so well, the region has become critical to understanding the evolution of Homo sapiens. Much of the English Fens have been converted to agricultural purposes, and their loss has led to dwindling insect and avian biodiversity.