|Lough Fea||Londonderry, Tyrone|
|Site Type: ||Inland exposure, Pit|
|Site Status: |
|Grid Reference: ||H27993879, H27483855|
|Rock Age: ||Quaternary|
|Rock Type: ||Gravel, Sand|
|Other interest: ||No data, Glacio-fluvial sediment, Glaciolacustrine deposit, esker, kame terrace, kettle-hole, moraine ridge, raised bog|
Summary of site:
This area is richly endowed with a wide variety of features created in the long and complicated retreat of an ice sheet to the south at the close of the last important Irish glaciation, the Midlandian. In this area 15 distinct inter-related and sequential features are recognised and their interpretation has proved controversial, different experts proposing very different accounts of events. This account will list the chief sites providing evidence and will follow the explanation outlined in the full report.
The amount and variety of evidence present in the Lough Fea region is exceptional, ranging from bedrock morphology, through ice contact ridges, moraines, kame terraces, eskers, deltas, lake level indicators, outwash sediments, kettle holes and a range of sediments, many clearly exposed, showing the conditions of their formation and the origin of the rock fragments they contain indicators of the flow direction of the ice sheet.
The main features are
the Crockandun Terrace, a flat topped feature between 310 and 320 m above modern sea level.
Corick Terrace, north east of Crockandun Terrace, at 280 m above modern sea level, extensively quarried revealing cross-bedded sands and cobble beds.
Carnbane Ridge and terrace, north west of Corick Terrace at 250 m above modern sea level. The sediments are sands and cobble gravels lying in channels and they thicken to the north east. The cobbles are a mixture of granites, basalt and limestone.
Brackagh plain extends north from Lough Fea for over 3 km. Its surface is flat, peat covered and punctuated by flooded hollows. A discontinuous ridge extends onto the plain about half a kilometre from the north east corner of the Lough Fea platform. The surface of the plain lies almost exactly at 240 m above modern sea level.
Slaught Ridge extends for half a kilometre beyond the northern slopes of Fir Mountain trending slightly east of north. It is composed of disrupted sands and cobble gravels.
Stone Hill Ridge forms a long streamlined feature in the Brackagh plain. It trends north east/south west and is flanked by boulders.
Mill Lough Ridge is a feature at the north east rim of Teal Lough plain which is an extension of the Brackagh plain to the south west. The ridge consists of segments forming a curved structure 1.5 km long and about 250 m wide. An exposure at Mill Lough reveals cross-bedded sands, gravels and cobbles with an indication of northward flow.
Ted Lough plain extends westwards and northwards from Slaught Ridge to the margin of the platform defining the southern slopes of the Moyola alley. It is peat covered with frequent pool-filled depressions.
West of Beleevnamore Mountain there is a series of branching ridges, quite narrow at 30 m across, extending up to the 210 m contour. The largest, arcuate in form, is oriented north west/south east and defines the edge of the Teal Lough plain. These ridges continue southwards along the flank of Beleevnamore Mountain where, at Dunnamore, on the northern slopes of the Ballinderry valley, their tops are at the 150 m contour. They appear to consist of deep boulder gravels with a fabric suggesting northerly flow.
At the northern end of the Lough Fea depression lies Black Lough Ridge. Trending north east/southwest, it is composed of sands and gravels in channels with flat-lying beds of mixed sands and boulders. Flow indicators point to a northern trend.
Ballybriest Ridge is the extensively quarried feature linking the south west flank of Glenarudda Mountain and Fir Mountain. Remnants of a flat top at about 253 m above sea level still survive. The workings expose 4 m of massive boulders that probably extend far below the working floor of the quarry. 25 m of cross-bedded cobble gravels top the boulders and bedded silts and sands also occur on the site.
There is a series of depression between the western edge of Ballybriest Ridge and an area of flat ground. The main one is occupied by Lough Fea and further east are the Black Loughs and Fly Lough.
Grouse Lodge Ridge is about half a kilometre further south. It is short, about 200 m long, dissected, with its crest 230 to 250 m above modern sea level. It consists of a pasty mixture of gravel, sand and clay (diamict) with sands and gravels. It was deposited by water flowing north and north east.
Near the weir at Claggan Bridge on the Lissan Water, a series of ridges cross the river and reach up the valley sides to between the 120 and 190 m contours. An exposure here showed a variety of diamicts and boulder gravels draped with clay, topped by more boulders and, on top, alternations of silt and sand.
North of all these features lies the Moyola River valley and on both sides there are extensive, now dissected, flat-topped terraces with two distinct levels, one at the 150 m contour, the other on the 120. They extend to the Lough Fea platform in the south reaching the mouth of a rock-cut channel. These levels coincide with valley-fill terraces at Disert, 6 km north of Lough Fea. The White Water flowing north west from Slieve Gallion has eroded through them south east of Disert creating two unequal facets. The sediments of these terraces are nowhere exposed.
Taken together, these landforms provide a narrative of events as the ice sheets began their final retreat towards the end of the last glaciation.
The main ice mass originated from the south west and covered most of the area at least to the height of 320 m (Crockandun Hills) but earlier had probably completely enveloped the summits south of the Moyola Valley (evidenced by Tyrone granites and basic rocks). The Crockandun terrace formed in a small lake on the margin of the ice sheet at 310 to 320 m above modern sea level, where it met the rocky flank of Slieve Gallion. The ice sheet was relatively large and probably occupied all the lower ground to the north west and west, now Brackagh and Teal Lough plains. The ice then retreated further, its surface deflating as it melted and at Carick in a second, lower lake with its surface at 280 m above sea level a delta formed. The next development was the formation of the fan of sediment in an even lower ice-dammed lake at between 250 and 260 m above sea level (Carnabane) with the supply route of sediment through the ice marked by the prominent ridge. More general melting created the outwash plain at Brackagh, at first confined in channels but later spreading more widely to create the plains. The ridge segments at the edge of the outwash to the south and north west rim probably mark major channels at the edge of the ice sheet during melting and retreat. The ridges on Teal Lough plain and the Davagh esker ridge could be caused by local pushing in short-lived re-advances of the ice margin. Masses of stagnant ice were rapidly buried in outwash deposits and, when they eventually melted, the cavities became the water-filled kettle holes.
In this general retreat, the ice sheet progressively thinned to the point where the rock foundations of the landscape began to exert an influence, splitting the ice into two lobes, one retreating west and south (the Davagh lobe), the other (the Lissan lobe) retreating through the Lough Fea gap. The linear ridges of Davagh are the remains of sediment fills in high pressure meltwater conduits in the ice, still draining northwards at this time. The same pressure system probably provided the Lough Fea deposits but the conduit is now almost certainly buried by later deposits formed in quieter water. The ice retreated through the gap leaving ridges in wide fans of sediment. Here, in the gap, huge masses of stranded ice were enveloped by the sediments pouring northwards and it was their final melting that created the kettle hole lakes. This same ice formed temporary dams creating transient lakes and the fan deltas at Ballybriest and Grouse Lodge in the form of residual ridges that are their only surviving landscape features. As the ice lobe retreated further into the Lissan valley, the cross-valley moraines, possibly submerged, formed from the flushing sediments.
The Black Water and White Water river channels were carved through the unconsolidated sediments by the final surges of the meltwater and the terraces in the Moyola valley were created in a water body ponded at the 150 m, and later the 120 m contours.
There is so much evidence here that many interpretations of event are possible and future research should clarify the history of glacial retreat still further. This is a rich landscape already scarred by sand extraction and some features are in danger of becoming so degraded that their scientific value could be lost. The balance between exploitation of a commercially important resource and conservation of a scientifically important landscape is starkly enacted here. A management plan to enable working to continue, while defining features that should be preserved, is an achievable and worthwhile objective.