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Moyola Valley ComplexLondonderry
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Summary Full report
Site Type: Inland exposure, Pit
Site Status:
District:
Grid Reference: X27524972, X28324968, X28543922
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacio-fluvial sediment, cross-valley moraine, deltas, ice-dammed lake, moraine ridge, proglacial lake

Summary of site:

This area of the Moyola valley contains a set of landforms linked to a complex series of events towards the close of the last major glaciation of Northern Ireland (the Midlandian). They were all created as the ice sheet that previously completely covered the area began to break up, melt and retreat. The deposits from which this general picture has been deduced are described in groups with similar morphologies, namely dissected flat-topped ridges, flat-topped ridges, cross-valley ridges and streamlined hills on lowland flats.

There are three areas of dissected flat-topped ridges. The largest is the Nutgrove ridge, north east of Crocknamohil in the area defined by Belmont Hill, Gortahurk, Mormeal and Nutgrove Wood. There is a 10 square kilometre spread of sand and gravel here seen most clearly at a working sand pit about 1 km south of Nutgrove Wood on the Iniscarn road. Two distinctive facies (associations of sediments) can be examined. A lower deposit of 50 cm of imbricated (overlapping in a similar fashion to fish scales) cobble conglomerates grades upwards into 70 cm of cross-bedded cobble and pebble conglomerates. Flow direction was east of north and almost half of the cobles are of granite. A sharp contact is crossed into the second facies which commences with a very thick sequence, up to 25 m, of ripple-marked sand and silt alternations entirely made up of channel fills. Channels are over 20 m wide and 6 m deep with beds up to 1 m thick. The silt to sand contacts are sharp and again the direction of flow was east of north. The second dissected flat-topped ridge is at Belmont Hill, north of Nutgrove Wood, where a poor exposure shows less than a metre of cross-bedded sands with cobble to boulder sized clasts, again predominantly of granite. The third is at Spring Grove, 1 km north west of Belmont Hill, where cross-bedded, medium grained sands are present with cobble conglomerates. The beds are inclined by 15 degrees, approximately towards the north west and the cobbles are roughly evenly divided between limestones and granites. There is only one undissected flat-topped ridge in the area, at Wood Hill, between the Nutgrove sites and Tobermore. About 1.5 square kilometres of land here has a flat top at 80 m above sea level, defined by steep slopes, especially on the north side. The eastern slope is the side of a large channel with a west south west/east north east trend, now occupied by a small stream. The north slope descends to the modern Moyola flood plain north of Tobermore. There are closed depressions near its southern end. Near Tamnyaskey, on its east side, there is an exposure showing three distinct facies. The first starts with 6 m of stacked channels, criss-crossing each other, up to 20 m wide and 4 m deep, filled with alternations of cross-bedded, ripple-marked sand and silty sand with flow directions between north east and east. These grade into 6 m of sharply defined sand/silt alternations with beds varying between 2.5 and 40 cm in thickness. The sands are ripple-marked. A 1 cm thick silt bed forms a sharp division between facies 1 and 2. Facies 2 starts with 2.3 m of grits, coarsening upwards into gravels in cross-bedded units. Flow was to the east. These pass upwards into 3 m of bedded cobble conglomerates progressively fining into medium grained sands. These are spectacularly cut by 20 to 25 m of cross-bedded sands with pebble gravels which can be seen to fine into silts to the north. Facies 3 commences with 1 m of crudely bedded, imbricated cobble conglomerates with a flow direction somewhat east of north, followed by 2 m of almost vertically bedded sands which top the section. There are two cross-valley ridges, one at The Black Hill between Draperstown and Tobermore, the other west of Moneyneany in the upper, western section of the catchment. The ridge, terminating just north of The Black Hill, is about 2.5 km long and 250 m wide and forms a strong linear feature trending north west/south east almost at right angles to the valley axis. Its south eastern limit is near the church at Spring Grove. The central portion is flat-topped, near the 90 m contour, about 35 m above the surrounding valley floor. Smaller sub-parallel extensions to the ridge are lower in height but have sharper crests. One small exposure reveals alternation of sand and pebble conglomerates but is too limited to provide reliable evidence. There is a small area of hummocks west of the ridge. In the upper reaches of the Douglas River west of Moneyneany There are 3 remnants of a ridge formerly 500 m long by 100 m wide on the lower slopes of Crockmore. They are flat-topped with crests at the 180 and 140 m contours. It has a north east/south west trend. The steep sided breaches are caused by minor tributaries of the Douglas River, now occupied by streams too small to have cut them. There is one exposure showing 3 facies, the first cross-bedded, medium grained sands with pebble clusters (flow to the west), the second, above a sharp contact, 4 m of cobble conglomerates becoming finer-grained upwards, forming pebble beds, followed by sandy beds becoming coarser towards the top (flow to the east), the third 2.5 m of convoluted diamict consisting of cobbles in a sandy, pebble matrix. The Nutgrove sites are interpreted as deposits on the bed of an ice-dammed lake with its surface 150 m above modern sea level. The ice margin retreated southwards supplying meltwater and sediment as it went. As the lake level fell and revealed the deposits, they were subjected to river erosion. The flat-topped ridges reflect the effects of this river erosion. Northerly flowing meltwater in the Spring Grove channel with its load of eroded sediment, poured into the contracting lake forming a simple (Gilbertian) delta at Wood Hill. Large ice blocks trapped in the delta surface deposits later melted to create the kettle holes (the water-filled depressions). The Black Hill ridge was a creation of a lobe of Bann Valley ice that retreated eastwards towards the Lough Neagh Basin. The lobe acted as a dam across the Moyola valley and the ridge was created when the ice front stabilised, probably due to a temporary period of cold conditions. The sediments released during wasting of the lobe poured west into the lake with its surface then at the 90 m contour (evidenced by the flat facets on the ridge). The Moneyneany ridges have a different origin, thought to be a consequence of erosion of deposits formed in a temporary lake with its surface at the 180 m contour. The streamlined hills in the Moyola flood plain are believed to be late remnant features shaped by catastrophic release of meltwaters as ice barriers failed and lakes drained. Sequencing this series of events is difficult but what follows is the simplest explanations of the landforms of the Moyola Valley. At the height of the Midlandian glaciation the Moyola Valley was filled by an extension of the Bann Valley ice. As glacial melting took hold, the Bann Valley ice retreated southwards towards the Lough Neagh Basin and the lobe in the Moyola Valley began to retreat from its most western upper reaches. In the earliest stages, the ice furthest west around Moneyneany melted, forming a lake at the 180 m contour, probably also fed by meltwater from the Lough Fea area 10 km to the south. The sediment released at the Moneyneany ridges was swept into this lake. The ice then began a rapid and steady retreat eastwards with an expanding lake behind it until it reached The Black Hill area where the retreat was arrested for a time. During this temporary cold phase, the concentration of sediment released along the ice front accumulated to form the ridge in the lake to its west. At one stage the lake stood at the modern 150 m contour and a delta formed from northerly flowing meltwater. During this period the ice on the Lough Fea Platform (site 447) retreated south, feeding its huge volume of meltwater northwards into the Moyola lake. As conditions warmed again and the ice surface melted and released more water, the lake level fell and exposed the lake bed deposits which were then eroded. The level again stabilised briefly at the 80 m contour but for long enough to allow the Hill Wood delta to accumulate. The final torrential release of meltwater lead to the erosion and streamlining of the outwash deposits around The Island, 3 km east south east of Tobermore. Lack of exposure (and consequently of clear evidence) could compromise this deglacial history but its major components are likely to stand.


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