The use of a cultural overlay in defining Landscape Character Areas (LCAs) means that they frequently subdivide natural physiographic units. It is common therefore for significant geomorphological features to run across more than one LCA. It is also possible in turn, to group physiographic units into a smaller number of natural regions. These regions invariably reflect underlying geological, topographic and, often, visual continuities between their component physiographic units, and have generally formed the basis for defining landscape areas such as AONBs. It is essential therefore, that in considering the 'Geodiversity' of an individual LCA, regard should be given to adjacent LCAs and to the larger regions within which they sit. In the original Land Utilisation Survey of Northern Ireland, Symons (1962) identified twelve such natural regions.
This LCA lies within the region described as the Lough Foyle Lowland. The chief morphological features of this lowland region are a series of postglacial raised beaches, the nineteenth century enclosure of large areas of saltmarsh and slobland beside the Foyle estuary and the sand and shingle ridges and blown sand of the Magilligan foreland. Inland from the coastal zone, the lowland continues along the floodplains of the Roe and Faughan rivers into areas that are underlain by till and glaciofluvial deposits, especially an important moraine complex between Limavady and Ballykelly. The Levees along these rivers testify to their previous tendency to flood.
The Lough Foyle Alluvial Plain stretches along the shores of Loyle Foyle from Enagh, on the eastern fringes of Londonderry, to the mouth of the River Roe. The sands and gravels of the alluvial plain originated primarily as glacial outwash along the lower course of the River Faughan. Postglacial isostatic uplift has created raised beaches and the alluvial plain is backed by a steep, low bench. Parts of the coastal plain are reclaimed land, protected by steep embankments. Large arable fields are found on these areas of reclaimed land, to the north of Eglinton and towards the mouth of the River Roe. The fields are bordered with straight drainage ditches and often have no hedgerows or trees. The landscape in the west of the LCA is influenced by glaciofluvial deposits of the Faughan and Dungiven basins complex that is of high scientific interest due to the presence of extensive glaciolacustrine and glaciofluvial deposits consisting of deltas, moraines, eskers and outwash plains occurring in close field associations. In terms of coastal development, Orford (in Whalley et al. 1985) has described Lough Foyle as a zone of low wave environment, the entrance to which has been progressively narrowed during the Holocene by the growth of Magilligan Foreland. Extensive coastal terracing, reflecting late-glacial shoreline positions, occurs on the eastern shore of the lough. As do shell ridges that are driven onshore and over the coastal marsh by storm surges that impinge into the lough. Much of the southeastern shoreline has, however, been reclaimed from the intertidal flats. More detail of this section of coastline can be found in Knight (2002).
Pre-Quaternary (Solid) GeologyThe stratigraphy of this area is made up of the mapped formations in the table, the youngest of which usually overlie the oldest. The older formations can be upside down (tectonically inverted).
Stratigraphic Table (youngest rocks at the top of the table)Triassic - from 240 to 210 million years old |
Mercia Mudstone Group |
Sherwood Sandstone Group |
Carboniferous - about 350 million years old |
Barony Glen |
Dalradian (Neoproterozoic) - about 550 million years old |
Londonderry |
Ballykelly - Southern Highland Group (undivided) |
This LCA comprises a mix of Dalradian, Carboniferous and Triassic rocks. The southwest is dominated by Dalradian (Neoproterozoic) metamorphosed sedimentary, igneous and volcanic rocks. Carboniferous overlies unconformably; Triassic in northwest.
Two major tectonic phases have affected the area: the Caledonian (Ordovician - Silurian) and Variscan (end Carboniferous). Caledonian deformation is very apparent throughout the Dalradian succession. Minor structures are evident in the Triassic.
Quaternary (Drift) GeologyNorthern Ireland has experienced repeated glaciations during the Pleistocene period that produced vast amounts of debris to form the glacigenic deposits that cover more than 90% of the landscape. Their present morphology was shaped principally during the last glacial cycle (the Midlandian), with subsequent modification throughout the post-glacial Holocene period. The Late Midlandian, the last main phases of ice sheet flow, occurred between 23 and 13ka B.P. from dispersion centres in the Lough Neagh Basin, the Omagh Basin and Lower Lough Erne/Donegal. The clearest imprint of these ice flows are flow transverse rogen moraines and flow parallel drumlin swarms which developed across thick covers of till, mostly below 150m O.D. during a period that referred to as the Drumlin Readvance. At the very end of the Midlandian, Scottish ice moved southwards and overrode parts of the north coast. Evidence for deglaciation of the landscape is found in features formed between the glacial maximum to the onset of the present warm stage from 17 and 13ka B.P. - a period of gradual climatic improvement. Most commonly these are of glaciofluvial and glaciolacustrine origin and include: eskers, outwash mounds and spreads, proglacial lacustrine deposits, kame terraces, kettle holes and meltwater channels (McCarron et al. 2002). During the Holocene, marine, fluvial, aeolian and mass movement processes, combined with human activities and climate and sea-level fluctuations, have modified the appearance of the landscape. The landforms and associated deposits derived from all of these processes are essentially fossil. Once damaged or destroyed they cannot be replaced since the processes or process combinations that created them no longer exist. They therefore represent a finite scientific and economic resource and are a notable determinant of landscape character.
The drift geology map for this LCA illustrates the importance of this late- and postglacial environmental change as well as human actions in shaping the landscape. The margins of Lough Foyle are fronted by a coastal zone comprising reclaimed marine deposits. These are found in front of raised beach deposits that are indicative of a Holocene high sea level. Behind these are elements of two significant deglacial complexes that are of considerable scientific importance.
The Faughan/Dungiven Basins Complex consists of glaciofluvial deposits that are primarily deltaic in origin and are situated along structural lows in the upper Faughan and upper Roe river's drainage basins. The area is of high scientific interest due to the presence of extensive glaciolacustrine and glaciofluvial deposits consisting of deltas, moraines, eskers and outwash plains occurring in close field associations. The complex occupies 4.2km2 in the southwest of the LCA. Most of this Complex lies in LCA 30, smaller areas occur in LCAs 27, 29, 31, 34 and 37.
The Ballykelly Moraine Complex occupies a small area of 1.2km2 in the northeast of the LCA. Most of the Complex is in LCA 37, with a very minor element in LCA 34. Ridges and spreads of morainic sand and gravel extend as a broad arc to the south and west of Limavady, immediately to the north of the lower slopes of the Sperrins covering an area of about 20 km2. Landforms and sediments record the extent of a surge of Scottish ice onto the Lough Foyle lowlands late in the last deglacial cycle. A prominent ridge marks much of the furthest extent of this ice readvance and fronts an area of discontinuous lower ridges and sediment spreads. Exposures within the stratified deposits show folded ice-push structures typical of a dynamic ice front environment.
In the southwest corner of the LCA is a small area of Late Midlandian till resulting from ice that moved northeastwards down the Foyle Valley. The direction of ice flow is recorded by a limited number of streamlined drumlins that are mapped by McCabe (in Knight 2002) along the lower course of the river.
Key Elements ASSI051 Lough foyle
Contemporary coastal processes especially chenier ridge development. Exposure through the southern (earliest) portion of the Magilligan foreland complex.
Deglacial Complexesthe Faughan/Dungiven Basins Complex
Deltaic deposits are preserved at seven principal locations and are grouped within three main altitudinal levels, representing the control of former ice-dammed lake levels on their formation. They are of special scientific interest, as their widespread extent and relationship to proglacial water levels implies that substantial, deep lakes were impounded along the Faughan and upper Roe valleys as Irish ice masses retreated southwards and Scottish ice advanced southwestwards into the lower Roe valley. The upper Roe (Dungiven) and middle to upper Faughan valley basins have been used for mineral aggregate production in the northwest of the province for approximately twenty years.
BALLYKELLY MORAINE COMPLEX
The Ballykelly morainic complex is considered to be of national and international importance in understanding the complexity of deglacial processes. The relationship of the moraine to the mid-Ulster drumlin field, to the Fruitfield deltaic deposits to the north, and to late glacial raised beaches permits the construction of a relative framework of events towards the end of the last glaciation. The identification of surge conditions is important for a reconstruction of climatic change in the North Atlantic and suggests correlation with similar events elsewhere in the British Isles.