Sphagnum moss plants grow upwards from the tips. As the lower parts of the moss are continuously shaded they die and become peat. These combined factors mean that the bog surface grows upwards to become raised above the surrounding landscape. Wherever Sphagnum cover is well developed, active peat formation is taking place and the bog is growing usually at a rate of 1mm per year.
Sphagnum mosses form the major component of peat.
Vascular plants are less common in the peat and more localised. Typically bogland vegetation where Sphagnum mosses are actively growing develops a system of hummocks and hollows with flat areas or lawns in between. The average difference in elevation between hummock and hollow is about 30cm, but in some bogs the tallest hummock may reach 70-80cm above the mean level of the bog surface and the hollows may attain a depth of 100cm or even lower, below it. The deeper hollows lie below the water table and appear as distinct water-filled pools. Areas where the bog surface is flat and lack such micro-relief are called lawns.