Previous SiteNext Site

Fruitfield Delta, LimavadyLondonderry
No picture
Summary Full report
Site Type: Inland exposure, Pit
Site Status:
District:
Grid Reference: C26474220, C26774228, C26784284
Rocks
Rock Age: Quaternary
Rock Type: Sand
Interest
Other interest: No data, Glacio-fluvial sediment, Late-glacial deposit, deltas, delta

Summary of site:

The Fruitfield delta is significant for the evidence it provides of a higher sea level at the close of the last Irish glaciation, the Midlandian.

The delta is shaped like an irregular wedge with its base around 3km wide between Tamlaght and the centre of Limavady and its point around 7km to the north at Shanvey. It is the largest single, onshore, late-glacial structure in Northern Ireland. It is dissected by the valleys of the Roe, Castle and Curley rivers which are broad and now floored by modern alluvium, but their steep walls probably date to the period of maximum incision by meltwaters derived from the south and east. The valleys effectively carve the delta into four areas: the Dowland in the north, mostly east of the River Roe; the Drenagh to the east between the Curley and the Castle; the Limavady between the Castle and the Roe; and the Lisnakilly in the west and south west between the Roe and the western fringe of the delta. The delta surface is remarkably flat and in the Dowland sector forms a striking contrast with the rocky southern slopes of Binevenagh behind. The Lisnakilly flat can be seen to weather over the Ballykelly moraine to the south, firmly establishing its later date. The Dowland, Drenagh and Limavady fragments have a consistent height of 18m above sea level and the Lisnakilly is around 2m lower. There is a smooth seaward-sloping surface everywhere. Late glacial and more recent sea levels have eroded pronounced notches in the margin of the delta. There is good exposure of the delta deposits in an overgrown pit at Fruitfield where they consist of five ‘units’. The lowest is a cross-bedded, pebbly sand inclined less than five degrees slightly east of north. The second, third and fifth are sands of similar grain sizes, cross-bedded and inclined to the north, while the fourth is another well sorted (of matching grain size) sand without cross-bedding and with fine laminations suggesting that these sands settled evenly through relatively still water. In total, the five ‘units’ achieve a thickness of around 2m and, in general terms, grain size reduces from the base of the section to the top. Although no longer visible, an ice wedge was recorded here in 1989, establishing a sustained cold period after the delta was formed and sea level had fallen. Ice wedges develop in deep cracks that open on frozen ground. They fill with water that quickly freezes and later, when the ice melts, sand washes into the natural mould to preserve the shape of the ice wedge (thus creating a pseudomorph). The delta was formed towards the very end of the glacial melting, with the retreating ice front some distance to the south east. Pebbles in ‘unit’ one and the sand beds and drapes above are all derived from schists (ubiquitous in this area) and granitic rocks, suggesting derivation from the Sperrins, probably carried along the Roe. The delta overlies the Ballykelly moraine that marked the southern limit of the Scottish ice The delta shows all the signs of building from the elevated mouth of the Roe River after all ice in the immediate area had melted. From the Fruitfield exposure, most deposition seems to have been underwater but there is the possibility that the delta surface reached sea level at times. The delta surface level matches the raised beaches of similar age along Ireland’s north coast. The late glacial notching seen on the delta fringes marks progressively lower sea levels but so far it has not proved possible to date these events (although somewhere between 15,000 and 16,000 years ago has been tentatively suggested). The importance of the delta lies in its confirmation of a higher sea level towards the close of the Midlandian and in the structure itself which formed well clear of the ice margins of the period. It records rapid environmental change during the final stages of ice retreat and, as the largest structure of its kind in Northern Ireland, is of national importance.


Previous SiteNext Site

|Home|