Found on all continents excerpt Antarctica, bogs are most common in northern latitudes where retreating glaciers left moist, depressed land with poor drainage. Precipitation is their only water source. If fed by other waters, usually springs, bogs are called fens. Climate and terrain determine bog type. Waterlogging first lowers oxygen levels and slows plant decay.
Dead vegetation settles and becomes peat, a precursor of coal.
Swamps and some marshes also produce peat. But accumulation is most dramatic in bogs, where sphagnum mosses excrete antibiotics and raise water acidity. In some parts of new England, glacier-melt Kettle Holes may simply fill in with peat and vegetation. In areas such as central Ireland, they may climax as Raised Bogs. Blanket Bogs spread across poorly drained landscapes, most often in northern coastal regions.