Lower Bann Floodplain 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 Central Lowlands. This region owes its large-scale morphology to the early Tertiary subsidence of the Lough Neagh basin into the magma chamber from which the basalts that underlie much of the landscape originated. This has produced a largely centripetal drainage system from the rim of the basin into Lough Neagh that ultimately drains northwards via the Lower Bann. To the south of the Lough Neagh basin, the lowlands extend southwestwards along a Caledonian structural trend into the Monaghan-Clones depression. In the east of the region the lowlands extend northeastwards along the fault-guided Lagan Valley. There are no strong topographical barriers in the region and boundaries between LCAs tend to be subtle. The low gradients of the rivers, especially on the clay lowlands immediately around Lough Neagh, create inherent drainage problems and frequently it is only the slopes of the many drumlins that provide permanently dry sites. The Lough Neagh Basin was a major ice accumulation centre during the Late Midlandian and much of the lowland areas to the north and south of the Lough are dominated by extensive drumlin swarms.

The Lower Bann Floodplain comprises the flat floodplain landscape of the River Bann to the south of Coleraine, at a point where the Bann is joined by a series of tributaries, including the Macosquin River, the Aghadowey River, the Agivey River and the Ballymoney River. The floodplain is drained by numerous smaller streams and rivers and the low-lying fields are criss-crossed by a network of straight drainage ditches. A key element in the south of the LCA landscape is the Kilrea glaciofluvial complex that has aesthetic significance because of the variety of glacigenic morphology expressed within a relatively small area and the pristine nature of the majority of the topography, largely unspoilt by commercial aggregate workings, especially the area of Tully Hill.

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.

Tertiary (Antrim Lava Group) stratigraphic succession (between 50 and 60 million years old)

Stratigraphic Table (youngest rocks at the top of the table)

There are three rock/sediment successions covering roughly equivalent proportions of LCA53. In stratigraphic order.

Lower Basalt Formation and Upper Basalt Formation

The two Tertiary-aged basalt formations comprise a crudely-bedded succession of lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons (or boles). To the north of LCA 53 these two formations are separated by red, palaeoweathered beds and columnar basalts, not seen in this area. The basalts were erupted 55 million years ago as the North Atlantic opened. They are extensively quarried for construction materials, especially roadstone.

Lough Neagh Group

The Tertiary Lough Neagh Group comprises soft mudstones and lignites (soft coal). The mudstones have been mined for brick manufacture and the lignites may form an economic coal power resource.

The three formations of the area are brought into juxtaposition in this LCA by the major Tow Valley Fault, which runs NE - SW through the southeast of area.Lower Basalt Formation is restricted to the southeast of the fault.

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 it to contain limited areas of till deposited as Lough Neagh ice moved northwards along the Bann Valley during the drumlin readvance. These deposits are characterised by drumlins orientated approximately N-S, although precise orientation is dictated by local topography. However, as this ice retreated a significant deglacial complex was laid down that is important scientifically and for its sand and gravel resources.

The Kilrea Glaciofluvial Complex occurs in a 16 km long almost continuous linear zone from near Vow, 8 km southwest of Ballymoney, to Inishrush, part of which occupies 1.4 km2 in the south of this LCA. The general north/south orientation of the complex is subparallel to the axis of the present River Bann. The sands and gravels consist of subaqueous outwash, esker and deltaic ridges bordered on the margins by southeast/northwest trending drumlins and streamlined ridges. These landforms record deltaic deposition into a proglacial water body at the north of the LCA at Tully Hill during ice withdrawal towards the south. An esker, which adjoins the southern edge of the delta, marks the route of a subglacial feeder into the proglacial water body. The complex overlaps with LCA 54 to the north.

For further information, see:

Key Elements

Deglacial Complexes

Kilrea Glaciofluvial Complex

The complex is important in understanding the complexity of the recent glacial history of Northern Ireland and records a generally southward retreat of the Irish ice mass during the deglacial period. It demonstrates pristine ice-contact morphology and variable ice retreat rates during deglaciation and contains pristine examples of ice contact topography, unaltered by post-depositional meltwater erosion.