Roe Basin Geodiversity Profile

Outline Geomorphology and Landscape Setting

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 broad, saucer-shaped basin of the River Roe lies to the west of the basalt escarpment and to the north of the Sperrins. It is dominated by the dramatic landforms of these highlands, and in particular by the striking silhouette of the basalt cliffs. The alluvial floodplain of the River Roe, towards the centre of the valley, and the lower slopes of the surrounding highlands are fringed with rounded ridges of glacial moraine. Numerous branching streams flow from the foot of the surrounding slopes towards the River Roe, which winds between grassy embankments. The character of the Roe changes from an alluvial floodplain to a deep narrow gorge (at the Roe Valley Country Park) as it flows north across a band of more resistant metamorphic rock. Sand and gravel quarries are a significant pressure in areas with ridges of glacial moraine towards the outer margins of the vale. The basin as a whole is overlooked by views from the surrounding uplands and the landscape pattern is therefore sensitive to change. The restoration of quarries will provide opportunities for habitat creation, new farmland, recreational access or, perhaps, new development.

The landscape in the north of the LCA is strongly influenced by extensive late-glacial fluvioglacial deposits in the lower reaches of the Roe Valley. These include the Ballykelly moraines and the Fruitfield delta. The moraine forms a topographic mix of linear ridges and hummocky drift occurs east of the lower Roe channel in the triangle between Limavady, Moys and Ballykelly. In comparison, the delta deposits are an extensive, flat-topped area (7 km2) of sands and gravels in the lower reaches of the Roe, Castle and Curly meltwater channels, which are cut by late and post glacial beach notches. In the south of the LCA, the upper reaches of the Roe valley contains extensive glaciolacustrine and glaciofluvial deposits consisting of deltas, moraines, eskers and outwash plains occurring in close field associations. The high relief range allows pleasant views both from the basin bottoms and from the Sperrin valleys.

Pre-Quaternary (Solid) Geology

The 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)

Tertiary - Upper Basalt Formation, about 55 million years old
Cretaceous - Ulster White Limestone & Hibernian Greensand, about 100 million years old
Jurassic - Waterloo Mudstone - about 200 million years old
Triassic - Penarth Group & Mercia Mudstone - between 220 and 205 million years old
Triassic - Sherwood Sandstone Group, about 240 million years old
Carboniferous - about 350 million years old
Barony Glen
Dalradian (Neoproterozoic) - about 550 million years old
Un-named metabasites
Ballykelly
Claudy
Dungiven

This LCA comprises two thirds Carboniferous sedimentary rocks of the Londonderry succession, the remainder being Mesozoic - Tertiary and Dalradian (metamorphosed sediments and volcanic "green beds".

Quaternary (Drift) Geology

Northern Ireland has experienced repeated glaciations during the Pleistocene period that produced vast amounts of debris to form the glacigenic deposits that cover >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 shows that the wider Roe valley is largely underlain by Late Midlandian drift that Davies and Stephens (1978) attributed to 'Sperrin/Tyrone ice' that flowed northwestwards over the Sperin uplands and down into what is now Lough Foyle. Although the important site at Bovevagh contains glaciomarine deposits of Early Mindlandian age. Beyond the Bovevagh site, the Quaternary features that are of greatest geomorphological and geological significance are located within areas of deglacial sand and gravel. This LCA contains significant elements of three such deglacial complexes that are important scientifically and for their sand and gravel resources.

The Ballykelly Moraine Complex occupies a large triangular area (28km2) in the northwest of the LCA, with a very minor element in LCAs 33 and 34 to the south. 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.

The Fruitfield Delta Complex covers 10.9km2 of the northern tip of the LCA.

Extensive topographic flats, separated by the valleys of the River Roe and its tributaries, are bounded by sharply defined to gently sloping margins and overlook the flat coastal plain that abuts Lough Foyle. To the south, the flat terrain onlaps onto the Ballykelly moraine, and to the east lies against the bedrock high of Binevenagh Mountain. The delta formed as the result of northward flow of meltwater and progradation of associated sediments into a Late Glacial sea at some distance from the ice front. Deltaic deposition postdated moraine formation and it records ice mass decay in the Irish interior following the retreat of Scottish ice.

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 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. In this LCA deposits are restricted to areas deltaic sediments (2.9km2) in the headwaters of the Roe. Other areas of the Complex occur in LCAs 27, 29, 30, 31, 33 and 34.

The drift map also highlights the alluvial deposits associated with the present-day river Roe.

Key Elements

ASSIs

092 BOVEVAGH

Bovevagh is an Early Midlandian prograded glaciomarine delta including a fossiliferous sediment sequence. It is unique in Ireland and has important palaeoenvironmental and palaeoclimatic implications for Quaternary research in northwestern Europe.

064 BANAGHER GLEN (ca 50% shared with LCA 29)

Largest serving intact block of semi-natural woodland in NI and the finest example of calcifuge oak woodland found. It has an important exposure of Upper Dalradian orthotectonic series and the major Sperrin fault passes through the site. The site is a complex system of river valleys. The underlying geology gives rise to a wide range of growing conditions ranging from strongly calcifuge types through to flushed calcicolous types.

Deglacial Complexes/Sand and Gravel Resources

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.

FRUITFIELD DELTA Complex

The Fruitfield Delta is nationally important for its evidence of high relative sea level late in the deglacial cycle. The relationship with the Ballykelly moraine provides a chronology of relative sea level following withdrawal of Scottish ice from the Lough Foyle coast. It is the only large-scale extraglacial delta on the north coast of Northern Ireland. Aggregate extraction has been limited and is not at present ongoing.

the 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.

MURNIES DELTA TEEAVAN (Faughan/Dungiven Basins Complex)

A glaciolacustrine deltaic complex is perched on the side of Teeavan Hill, on the northern flanks of the Sperrin range. The delta is two tiered and extensively quarried for its mineral aggregate reserves. The landform has excellent scientific merit and can be explained by the presence of an ice dam of Scottish provenance in the coastal lowlands of Lough Foyle during the late-glacial. This provides important regional chronostratigraphic evidence for late-glacial events. Intensive quarrying has destroyed much of its glacigenic morphology. This feature overlaps the boundary between LCAs 29 and 37.

Other sites/units identified in the Earth Science Conservation Review

327 BANAGHER GLEN QUARRY

Precambrian. Dungiven formation. Coarse brecciated limestone, interbedded with coarse-grained, black calcite and sedimentary structures exposed in a small quarry.

335 Dogleap/Limavady

Precambrian. Lithology and structure of Ballykelly Formation. Good exposures and access.

9 Ballykelly

Record of 3 distinct Quaternary events, from stratigraphy and morphology. Sistrakeel provides one of the oldest glacial sequences in Northern Ireland. Important for geological concepts and evidence of ice age events.

AONB

A small area of the north of the LCA lies in the North Derry AONB (1966), whilst a small area in the south lies in the Sperrin AONB (1968). These designations are indicative of the scenic quality of the landscape.