In a stagnant, acidic swamp, partially decayed plant matter will accumulate forming a peat layer. The nature of peat varies from a recognizable mass of leaves, roots and woody tissue at the surface to a middle partially decayed muck layer to an almost unrecognizable gel-like material after burial to a depth of 20 or 30 feet. The volume of the peat is reduced by half or more as it goes through the burial and decay stages.

Peat has a high water and ash content and a relatively low BTU. It is used today as a fuel only where other energy sources are unavailable. Here is peat cut and drying from a peat bog in Scotland.