Habitats range from swamps, carr (fen), water meadows and tall reed beds to waterlogged scrub consisting mainly of willow, where wetter areas merge with woodland.
The site provides an important refuge for a range of wetland birds like the bittern, water rail, sedge and reed warblers, bearded tit and grey heron, which can often be seen or heard there.
Crymlyn Bog is an historic boundary between 'South' and 'West Wales', between the lordships of Glamorgan and Gower, and between the medieval and modern (i.e. until 1923) dioceses of St. David's and Llandaff.
Crymlyn Bog was largely owned by the CEGB who used the fen to dump waste PFA and in this same area Swansea City Council used it as a landfill site.
On an eastern limb of the bog is Pant-y-Sais fen, where there is a memorial to Mr Lees, which carries a quotation from him: "At some point I had to stand up and be counted.