ABOUT 10,000 years ago, as; our climate warmed at the end of the last Ice Age and the great ice sheets retreated towards the pole, large shallow lakes formed over much of the Irish midlands where the gravel ridges, left by the glaciers, impeded proper drainage. Pond-weed and water-lilies floated on the surface of these shallow lakes and reeds grew prolifically around their outer edges.
It was a luxuriant growth, because the water - mainly groundwater - was extremely rich in nutrients, and moreover, the dearth of oxygen in this submerged environment delayed decomposition. Over thousands of years, succeeding generations of dead vegetation built up, layer on layer, upon the lake-bed; with time this "peat" reduced the area of open water and then filled up the lake entirely to produce a
Growth on the surface of a fen was rather different in character. The roots of plants were now no longer in direct contact with the groundwater and its mineral richness; they acquired their moisture from the rain, but rain is low in nutrients and the original vegetation died away to be replaced by plants that could tolerate these new conditions.
The most dominant one was sphagnum moss, over the millennia it also accumulated layer by layer without significant decay, creating what we now refer to as a "bog" several tens of feet above the original fenland - the classic "raised bog" of our Irish midlands.
Bogs are the product of a cool and wet environment. Since their continued growth depends on saturation, they are found only where rainfall is greater than the amount of water lost through drainage and evaporation. Drainage is related to the topography, but the need for low evaporation means that bogs also flourish best in regions where the average temperature is relatively low. They are found on all the continents except Antarctica, but are most common in the northern latitudes where the desirable combination of high rainfall, low evaporation and poor drainage is most easily found.
Under certain conditions, especially after deforestation, the grass-like sedges and sphagnum of the peatlands may begin to invade higher ground. Such peat thrives on a poorly-drained landscape where persistent rainfall may have leached minerals from the soil to provide a subterranean impermeable layer. In this waterlogged environment fed constantly by the Atlantic rains, generation after generation of poorly decomposed vegetation gradually grew into the second classic type of Irish bogland - the extensive "blanket" bogs common in the mountainous regions along our western oceanic coasts.