Aghabrack ASSI

The Aghabrack area is of importance in understanding the recent glacial history of Northern Ireland. The landscape of this area has been defined by the action of water and ice that occurred towards the end of the last Ice Age, between 17,000 and 13,000 years ago, a period of gradual climatic warming. Sand and gravel were laid down in front the ice as it was retreating south toward the main Sperrin Mountains. In Aghabrack, a hummocky ridge of this material, called moraine, was deposited when the ice briefly stopped it’s retreat.

The majority of the site is composed of a strikingly flat outwash plain surface to the west of the Burn Dennet. This formed as meltwater deposited and smoothed out sand and gravel transported from the glacier. It lies behind, and is separated from, a hummocky moraine ridge by the meltwater-eroded trench of the river valley.

The site also contains part of an esker ridge, formed when a water channel under the ice becomes blocked up by sand and gravel as the flow of water declines. It is these ice channels that feed sediment from under the glacier to it’s front. The esker, an intermittent sinuous ridge deposited along the course of a subglacial stream, extends north eastwards along the eastern side of the Camus Burn. The esker ridge increases in altitude towards the northeast and therefore records upslope meltwater flow along the subglacial surface and is evidence of high hydraulic pressure beneath the ice mass. The water was actually flowing up hill because of the pressure from the ice to the south. The esker terminates near the ice distal slope of a hummocky moraine ridge that occupies the lower slopes of a bedrock high to the east of the main Burn Dennet channel.

The moraine marks an ice front standstill position during overall retreat southwards towards the Sperrins. The Burn Dennet meltwater channel separates the moraine from an extensive flat surface, a pristine example of an outwash plain, lying at around 170m OD. The course of the meltwater flow has clearly been directed by the position of the moraine. The main area of the outwash plain is defined by the meltwater channels in which the underfit Inver Burn and Burn Dennet currently flow. Outwash plains indicate a pro-glacial environment and are formed by the transport and deposition of sediments by normal water flow from beneath the glacier. Subsequent meltwater erosion has cut through a part of the outwash feature producing the precipitous slopes down to the Burn Dennett.

Together, the outwash plain and esker ridge alongside a minor tributary of the Burn Dennet these features are an excellent example of a deglacial landform assemblage.
This landform assemblage shows clearly the temporal and spatial relationships between the landforms associated with the three distinct, sequential episodes of esker, moraine and outwash plain emplacement.

The site displays the classic association between glacial landscape features and the post glacial accumulation of peat. Since the end of the ice age, peat has built up on the outwash deposits to form a raised bog. This habitat supports unique raised bog plant communities with vegetation such as bog mosses, heather and bog cotton. It is the build up of bog mosses that eventually form peat over thousands of years.
The landforms found at Aghabrack are fossil and once damaged or destroyed cannot be replaced since the processes that formed them are no longer active. The raised bog habitat at Aghabrack has taken many years to develop due to the complex vegetation communities present.

Site Related Documents

Site map (.PDF 3.07Kb) and Combined Citation and Views about Management document (.PDF 317Kb). Aghabrack Colour Leaflet (.PDF 1.64Mb)