As there are lowland and upland moors, so there are lowland and upland bogs or tracts of fen. But the greater portion of the fen formation, which is made up of fen and carr (or wooded fen), is lowland, being developed chiefly in East Anglia. There the fen is very little above sea-level along the coast, and in fact may be continuous with salt marshes at the margin.

The plants that are most distinctive of the fen are Grasses and Sedges, such as Reed, Prickly Twig Rush, and Juncus obtusiflorus; Purple Moor Grass also occurs on a wide scale, and Sphagnum and Cotton Grass, whilst other plants are Sweet Gale, Bog Orchis, Sundew, Winter Green, and elsewhere Glyceria aquatica, Reed Grass, Rough Meadow Grass, Ragged Robin, etc.

Fen associations pass into moorland, and where the peat rises above alkaline waters, moor with acid water may occur. The upland fen tracts are probably akin to lowland moors, and consist of Cotton Grass, Sedges, Rushes, &.c, with Ragged Robin, (Enanthe Lachenalii, Orchis incarnata, Scirpus rufus, etc. These have been so far little investigated, and a field lies open for their special study.