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KilreaAntrim, Londonderry
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Summary Full report
Site Type: Inland exposure, Pit
Site Status:
District:
Grid Reference: D29494259, D29484037
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacial streamlining, Glaciolacustrine deposit, Subaqueous outwash, anastomoses, esker, gravel ridge

Summary of site:

In the central Bann valley between Vow in the north and Clady in the south there is a thick coating of glacial deposits. They were washed out by meltwater from a rapidly retreating ice sheet at the close of the last phase of the Ice Age in Ireland (called the Midlandian) and they smothered the floor of the valley in a linear zone about 2 km wide and over 15 km long. The deposits take various forms including outwash, flat-topped features, water laid sediments deposited below lake ice, regularly repeated sequences of sediments, ridges (some branching) and many more.

There are four subdivisions of this complex, recognised by their differing landforms. The first is centred on Vow where flat-topped ridges trend east north east/west south west and ridges with periodic mounds (described as beaded) trend north north east/south south west. The sediments forming the ridges can be seen in two gravel workings at Vow and nearby Artiloman. At Vow, in a long section through a flat-topped ridge, boulder gravels occur at the base, followed by channelled sands with mud drapes, lenses of cobble sands and finely bedded silty sands (some with ripple marks), all topped-off by 8 m of regularly alternating finely laminated sands and clays dotted with isolated cobbles and pebbles. At Artiloman, on a beaded ridge, rhythmic sediments, like those at the top of the Vow section, are draped over cobble and pebble gravels. The second subdivision extends from Movanagher to Kilrea and is dominated by both sharp-topped and flat-topped ridges. It includes Tully Hill and Kilrea town itself. Immediately south of the town is the third subdivision where the landscape is hummocky and pitted by a series of small lakes in the Washings Lough and Kathleen Lough area. The hummocks are formed of stratified glacial drift, here boulders in a clay matrix. The fourth subdivision extends from the lakes to about 1.5 km west of Clady, some 7 km, where steep-sided ridges and mounds, trending north north east/south south west and between 20 and 40 m in height, predominate. At the northern limit of this area at Moneygran a good exposure shows slumped and convoluted sands, pebbles and cobbles topped by finely bedded and laminated sands passing progressively into silts. A sequence of events can be deduced from these landscapes. The first and most northerly subdivision is of deposits washed out from a rapidly retreating glacier by massive and violently surging torrents. They were swept into temporary lakes with a constantly changing pattern of distribution where finer sediment settled as the ice margin receded. There are no deltas in this area confirming such a rapid retreat of the glacier that there was insufficient stability and time for the sediments to accumulate and reach lake surface level. Glacial retreat slowed sufficiently for deltas to form in the second zone to the south. Here flat-topped features with a common height of 40 m above sea level indicate a stable lake level with an initial torrential fill of cobbles and gravels succeeded by finer sediments. The beaded ridges are eskers, sediment-filled tunnels within the ice which marked the main feeders into the lake. The third subdivision is essentially part of the same system but differs because large masses of static ice were buried in the lake deposits. When they melted, the side walls of the resulting cavities subsided under gravity but the depressions were large enough to retain their identity. Their flooding created the lakes of this area and features formed in this way have a special name, kettle holes. The train of steep sided ridges of the final subdivision were formed in gushing channels under the ice surface where deposits packed against the ice. Many of these ridges are coated in later sediments suggesting that the lake in the Kilrea area persisted and extended for some time after the ice finally retreated. These events are believed to date from around 17,000 years ago and later. They confirm a southerly ice retreat in the area, sometimes rapid but also with periods of relative standstill allowing substantial deltaic deposits to form that survive as recognisable features in the modern landscape.


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