Peat is the result of the accumulation of vegetable matter in such a state that it has preserved its original character to a very great extent. It is relatively dry at the surface, and owing to oxidation browner, whilst below it is much wetter and black. The lowest layers may be amorphous, and exhibit no traces of the original types of plants. Thus it is similar in character to coal, but much younger, and therefore physically different Peal is formed in stagnant water, and is charged with humous acids, which tend to preserve the constituent plants. There is very little foreign matter (3 per cent) in it.

Generally speaking there is upland peat and lowland peat, or hill peat and bog or marsh peat. The plants that respectively make up each layer, form distinctive types of peat, and from sections it is possible to determine the origin of the different peat beds, and to denote the changes in the flora.

Thus in the Lake District one may find the following succession, above lacustrine ooze and shell marl, viz.: amorphous peat; Hypnum peat, made up of moss stems; lower sedge and reed peat, with drifted tree stumps occasionally; upper sedge and reed peat; wood peat, with Birch; a mixed brown peat, with Cotton Grass, Sedges, and bog moss (Sphagnum); and finally an upper grey spongy Sphagnum peat.

Peat beds vary from a few feet to as much as 30 ft. or more. In Ireland peat is cut into turves and used for fuel, paper-making, etc.