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BallykellyLondonderry
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Summary Full report
Site Type: Inland exposure, River bank
Site Status: PASSI
District: Limavady Borough Council
Grid Reference: C630224
Rocks
Rock Age: Quaternary (Pleistocene)
Rock Type: Gravel, Sand
Interest
Fossil Groups: Foraminifera
Other interest: No data, Glaciomarine deposit, hummocky moraine, delta

Summary of site:

From the early days of the geological section of the Ordnance Survey, when Captain J.E. Portlock and his team mapped the area, there has been speculation about landforms around Ballykelly. Recent resurvey has arrived at a modern synthesis giving the deposits national status.

Three distinct Quaternary events are now recognised. The earliest is represented by the sediments best exposed near Sistrakeel. Here, from the base upwards, there is a bed of mixed grain size from clay to gravel (also including sandy lenses with whole and fragmented bivalves), grading upwards into a bed of gravely cobbles which in turn gives way to a massive mud containing whole shells of Arctica islandica (a bivalve). Passing upwards, thinly-bedded silts with beds of mixed grainsize are topped by massive deposits similar to the beds immediately below, except they are unstratified. The fossils do not yield carbon isotope dates, which means they are older than 45,000 years. Age estimates, based on their amino-acid content, suggest deposition early in the Midlandian, before the onset of the last major glaciation (perhaps 60,000-70,000 years ago). This pile of undisturbed sediments is thought to have formed where it now stands; it tells a story of interaction between glacial processes and the sea. Mass flows of mixed debris from the land were interspersed with periods of fast flowing water capable of transporting a cobble load. This powerful discharge could quickly relocate as its channel became blocked, leading to the formation of muds in the quieter water (disturbed only by the occasional larger stones, which were rafted in on melting ice). The thick final bed of mixed debris includes erratics of basalt and Cretaceous chalk thought to be derived from the extensive glaciers of Scottish ice eroding the rocks to the north. The height of the top of these sediments (52m) suggests deep depression of the land surface beneath an earlier massive load of ice now near equilibrium in its unloaded state. On top of these early Midlandian sediments is a series of moraine ridges, till hummocks and ridges of fluvioglacial debris. The strongest moraine feature is a pronounced arcuate ridge extending south from Clagan to Drumraighland, from where it veers east to Moys. This feature, clearly exposed in the sand and gravel workings at Moys, is predominantly composed of coarse gravelly sediment. East of Limavady it continues as hummocks of boulder clay in the low ground below Keady Mountain. North of the main moraine, between Ballykelly and Limavady, is a series of glacial sediments derived from the north. The interpretation of these deposits, which form the second major event, suggests a major glacial advance into the lower Roe valley from the north, creating the arcuate moraine which formed around the ice lobe at the furthest push of the Scottish ice. As the lobe shrank back to the north with improving conditions, meltwaters surging from beneath the ice discharged their mass of sediments along the Ballykelly to Limavady line, as a series of deltas extending into a lake to the south. The final feature is the large delta extending from Limavady to Fruitfield, now extensively dissected by the Roe, Castle and Curley rivers. These deposits are clear evidence of the third and final phase in the area and are explained as a late glacial event, following the retreat of the Scottish ice and the release of water from melting ice further south (probably in the Dungiven area). This mass of water swept vast quantities of the unconsolidated muds, sands and gravels that draped the barren post-glacial landscape into the sea, forming the delta which is now stranded and degrading rapidly. Clearly the landmass was still adjusting itself, following the depression by regional ice sheets, and has risen 27m to its present position. The glacial deposits in the Ballykelly area provide unique evidence of the key events during the Midlandian glaciation and are of national importance. The Sistrakeel exposures, however, give some cause for concern because extensive dumping is masking key elements of the outcrop. More active management than the ‘No Dumping’ sign is warranted for this important area and removal of existing rubbish is essential.


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