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Achill Peatlands EarthCache

Hidden : 09/01/2015
2 out of 5
1.5 out of 5

Size: Size:   other (other)

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Geocache Description:

87% of Achill Island is made up of bog. As a result, the island is only largely suitable for sheep farming, which is highly evident, with sheep seemingly everywhere! 

There are two types of bog in Ireland: blanket bog and raised bog. 

Blanket Bog

Blanket bog is typically found in areas of high rainfall and also mountaineous areas. They are so called as they appear to fit tightly over the landscape, much like a blanket. Blanket bogs are much more common in Ireland than raised bogs. 

Despite popular belief, blanket bogs are fundamentally a man-made feature, if unintentionally and aided to a degree by climate.

After the end of the Ice Age, Ireland was gradually colonised by deciduous and pine forests. By 4,000BC, almost the entire surface of the island was thickly forested. Even the higher land was forested, if not as thickly as the lowlands.

During the Neolithic Age, Ireland’s first farmers began to clear land to build their farms. They preferred to clear the upland areas with thinner forested areas. The highest areas were used as pasture, and the hillsides were for cultivation. Devoid of their trees, the soils became vulnerable to the washing away of nutrients (leaching) by rainwater. In turn, the soil became more acidic. In addition, the leached minerals were deposited in a ‘hard pan’ at lower depths, hindering drainage and waterlogging the land.

By the end of the Bronze Age, the farmers had been forced to clear the lower levels as the uplands became no longer usable. Heathers and rushes grew on the acidic soils, but their debris didn’t decompose and a layer of peat built up.

In modern times, many of the blanket bogs have been changed by human actions. The cutting of peat (which is called 'turf' when cut) for fuel began in the 17th century and continued at an increasing rate until the mid 20th century. Turf from blanket bogs is not as good quality as that from raised bogs, so blanket bogs have escaped the wholesale destruction inflicted on raised bogs.

Raised Bogs

Raised bogs are found almost exclusively in central Ireland, mostly in the Shannon basin. In contrast to blanket bogs, they were formed naturally. When the land surface in Ireland was new, the glacial morraine left behind created a hummocky and chaotic plain which consequently had poor drainage. These depressions filled with water, creating thousands of tiny lakes. It was these tiny lakes that, over the 10 intervening millennia, have turned in to the raised bogs.

Around 7,000BC, one of the many hollows left by the glacial moraine was filled with water to form a small lake. The landscape surrounding it was wooded with hazel trees and pine trees. Mesolithic hunters would have fished the shores of the lake. At the lake edges, communities of reeds were developing.

The reed communities extended in to the lake, depositing poorly decomposed vegetable matter (peat) 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 significance to the nearby Bronze Age farmers. The water table rose until no more 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 500 BC, the lake was entirely filled in, becoming a raised bog. By 500 AD, the climate had turned wetter and the bogs became marshy once again. With no decomposition of waste, the bog’s dome continued to rise, becoming much higher than the surrounding landscape. Hence the name ‘raised’ bog.

Starting in the 1700s, the raised bogs of Ireland were exploited as a source of cheap fuel. Half of Ireland's raised bogs were destroyed (at a rate of 800,000 tons per year) between 1814 and 1946. Most of the remaining raised bog will be exhausted by the middle of the coming century.

To log this earthcache as a find, please email me the answers to the following questions:

1. Describe the landscape you can see in front of you to the south.

2. Channels appear in the bog here – WHY?

3. Is this a blanket bog or a raised bog? WHY?

4. What is peat primarily composed of?

Please log your find as soon as you have sent me the answers. I will contact you if there are any issues.



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