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Blanket Bogs are expansive, generally formed in wet or upland areas.
Raised Bogs are smaller, generally formed in lowland areas.

Blanket Bogs

Blanket bogs are found wherever there is high rainfall, which is typically in western Ireland and also in mountainous areas. They are called blanket bogs because of their appearance - from a distance they appear homogeneous and they hug the topography like a blanket. With almost 1 million hectares of Ireland covered by blanket bog, it is far more common than the smaller-scale raised bogs. Contrary to popular belief, blanket bogs are essentially a man-made feature, if inadvertent and aided somewhat by the climate. The graphic below is a cross-section of a hypothetical mountainside in Ireland showing how it evolved into a blanket bog.

After the Ice Age ended, Ireland was slowly colonised by deciduous and pine forest. By 4000BC, Ireland was thickly forested across almost all of its surface. Even the highland areas were forested, if in a thinner manner.

In the Neolithic Age, the first farmers began to clear land on which to build farms. They chose to clear the upland areas as the forests there were not so thick. The highest upland areas they used as pasture, while the hillsides were used for cultivation. By 2500BC this process was well under way. However, devoid of their trees, the soil became vulnerable to leaching (washing away of nutrients) by the rain. This caused the soil to become more acidic. Additionally, the leached minerals were deposited in a 'hard pan' at a lower depth, impeding drainage and causing the land to become waterlogged.

By the end of the Bronze Age, around 500BC, the farmers had been forced to clear lower level land as the upland areas were no longer usable. Heathers and rushes grew on the acidic leached soils, but their debris did not decompose and a layer of peat began to build up. Any remaining trees were choked by the peat. The peat buried the remains of the Neolithic farms - at the Ceide Fields, county Mayo, Neolithic field walls have been found under a blanket bog.

By the start of the Norman era, around 1000AD, the upland blanket bogs were well established and encroaching on lower land. The lowlands were by now almost totally devoid of forests as population continued to increase. By the time a blanket bog reaches maturity (ie before human action begins), they are approximately 3 metres (10 feet) thick. There is no universal blanket-bog ecosystem, as it depends a lot on altitude and the direction of the prevailing wind. The picture below shows the blanket bog on the Antrim Plateau which covers miles and miles of hummocky terrain. Note the small lakes, these are in fact small raised bogs within the vast blanket bog. Image by R Tomlinson.

In modern times, many blanket bogs have been modified by human action. The cutting of peat (called 'turf' when cut) for fuel began in the 17th century and continued at an increasing rate until the mid 20th century. About 15% of blanket bog in the Irish Republic, and 50% in Northern Ireland, has been destroyed by cutting. In fact, turf cutting on blanket bogs has actually increased since 1980. However, the turf from blanket bogs is not as good quality as that from raised bogs, and thus blanket bogs have escaped the wholesale destruction inflicted on the latter. Another use of these bogs has been to drain and convert them into grassland for grazing. More common in recent years has been to use the land for the planting of conifer plantations. Ireland has the lowest percentage of forested land of any country in the European Union, and the large scale development of conifer plantations has been one way of trying to resolve this discrepancy. Reduced government grants have helped to reduce the reclaimation of blanket bog areas for both agriculture and forestry, but private turf cutters continue to utilise them. While not in acute danger, the blanket bogs are nonetheless threatened and clear policies are needed if important sites are to be preserved for future generations.

Raised Bogs

Raised bogs are found almost exclusively in central Ireland, chiefly in the Shannon basin. There are a few examples in Ulster and Munster, but the vast majority are to found in the midlands. Unlike blanket bogs, they were formed naturally. To understand raised bogs, one has to realise that Ireland's vegetation landscape is very young - it began to form only at the end of the Ice Age about 10,000 years ago. When the land surface was new, the glacial morraine left behind created a hummocky and chaotic plain which had poor drainage as a consequence. The depressions filled with water, creating thousands of tiny lakes. It was these tiny lakes that, over the 10 intervening millennia, have become the raised bogs. The graphic below is a cross-section of a hypothetical lake in central Ireland showing how it evolved into a Raised Bog.

Around 7000BC, not long after the Ice Age ended, one of the many hollows left by glacial morraine has been filled with water to form a small lake. The surrounding landscape was wooded with hazel and pine trees. Stone-age (Mesolithic) hunters would have fished around the shores of the lake. At the edges of the lake, communities of reeds were developing.

These reed communities extended into the lake, depositing peat (poorly decomposed vegetable matter) on the lake bed. Over many years, this peat built up and up, choking the lake, until it began to emerge above the surface of the lake. Only small areas of open water remained by 1500BC, and these would have had a special, probably religious, significance to the Bronze Age farmers nearby.  The water table rose until no water was running into the lake from surrounding land, and it stagnated. The peat and water thus became acidic, further preventing decomposition. The oak, ash and elm which now grew were partly cleared by farmers.

By 500BC, the lake had been completely filled in, becoming a raised bog. The dome extended higher than the edges of the bog, carrying the water table upwards with it. Thick cushions of bog-moss would have covered the mass of peat. For a period around this time, the climate was drier than today, and large trees were able to grow on the bog itself. These have been found buried in bogs today, along with grasses that could not survive there now. Farmers would have used the drier bogs for grazing in summer.

By 500AD, in the Celtic period, the climate had turned wetter and the bogs became more marshy again. Their vegetation cover reverted to thick mats of bog-moss. At the edges of the bog, the wetter conditions allowed areas of stagnant water to develop, which allowed the bog to encroach upon, and bury, surrounding areas of woodland. With no decomposition of waste, the bog's dome continued to rise, becoming much higher than  the surrounding landscape. This is where raised bogs get their name. For humans, the water at the edges of bogs provided places to find iron- rich ochre (which seeped out of the bog), helping to usher in the Iron Age in Ireland. As the bogs were now too wet to walk upon, let alone graze animals on, some Iron Age farmers built wooden walkways over them.

It is uncertain how raised bogs would have evolved after this stage in their development, because significant human activity started to influence them to a great degree around 300 years ago. Most intact raised bogs vary between 3 to 12 metres in thickness, with a mean of 7 metres (23 feet). The image on the left shows an intact part of Ardee raised bog, county Louth, which was partially drained in the 1700s. Image by Leo Swan.

Starting in the 1700s, the raised bogs of Ireland were exploited as a source of cheap fuel. Most of this was cut by hand, and laid in the sun to dry before being burned. At the time of the famine, peat (called 'turf' when cut) was often the only source of fuel available. In 1934, the Irish Free State (the name of the Republic of Ireland after independence in 1921) set up the Turf Development Board, which bought land under compulsory-purchase orders and cut turf. Half of Ireland's raised bogs were destroyed (at a rate of 800,000 tons per year) between 1814 and 1946. After World War 2, the government set up Bord na Móna to cut peat by mechanical means and this simply accelerated the process. In 1969, there were just 100,000 hectares of raised-bog left in Ireland, of which Bord na Móna owned 45,000 hectares. Most of this will be exhausted by the middle of the coming century.

In recent years, there has been increased awareness of the importance of raised bogs to science. In the Republic of Ireland, there are plans to set aside 10,000 hectares of raised bog for conservation purposes. In Northern Ireland, which has less raised bog to begin with, almost all raised bogs are being preserved as Areas of Special Scientific Interest. It has been noted that the removal of large areas of bog is leaving behind a new landscape for which some use will have to be found. Most of the cut-over areas are being carefully restored to blend in with the local environment. Given that the European Union is already over-producing food, it seems unlikely that the land will become agricultural.

Author: Patrick Abbot.
Source: Feehan, J; McIlveen, S; writing in "The Atlas of the Irish Rural Landscape", Cork University Press, 1997.