Lower Bann Valley 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 Valley landscape extends along the wider floodplain of the Lower Bann, from Ballymoney to Castledawson. It includes the lower reaches of the Moyola River and the Clady River, which converge with the Bann. The land is relatively low-lying, with a transition from shallow drumlins on the edges of the floodplains, to extensive flat pastures, bog and wet woodlands on the fringes of Lough Neagh and Lough Beg. These shallow drumlins form 'islands' surrounded by flat, open pastures, Stretches of the rivers are enclosed by embankments and are often hidden from view by extensive woodlands on wet, low-lying land. A key element in the landscape is the Kilrea glaciofluvial complex comprising the Tully Hill deltaic deposits in the north and esker ridges running through the centre and lying subparallel to the River Bann. The complex has aesthetic significance because of the variety of glacigenic morphology within a relatively small area, the pristine nature of the topography, largely unspoilt by commercial aggregate workings, the topographical contrast provided by the complex to the monotonous drumlin belts flanking the valley, the picturesque kettle hole/lake associations developed to the south of Kilrea and the extensive raised bog/topography associations developed near Inishrush. However, working pits are present in the Vow deposits, and the ready accessibility of the Tully Hill deposits indicates that they are vulnerable to exploitation
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)
various intrusives
| Upper Basalt Formation |
| Lower Basalt Formation |
This LCA is a north-south tract from Kilrea in the north to Toome in the south and comprises 99% Upper and Lower Basalt Formations of the Antrim Lava Group. In the southwest corner of LCA52 there occurs a dolerite intrusion
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 52 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.
Tertiary Dolerite Intrusion
Some 5km northwest of Casteldawson an elliptical outcrop of dolerite occurs. This is most likely to be a feeder tube for a post-Lower Basalt Formation aged volcano or lava flow.
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.
Although the drift geology map for the LCA shows the presence of Late Midlandian till laid down by northwards moving Lough Neagh ice as indicated by S-N orientated drumlins on the flanks of the valley - the area is dominated by glaciofluvial deposits formede as the ice ablated and the ice margin retreated southwards. This deglacial complex is important both 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 and occupies 10.6 km2 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. A small area of the Complex overlaps with LCA 53 to the north.
The south of the LCA, where it borders on Lough Neagh contains important diatomite deposits within the Toome ASSI.
Key Elements
ASSI/ASIs
131 WOLF ISLAND BOG
A very large area of intact lowland raised bog in two units, which together represent one of the largest remaining examples of lowland raised bog in Northern Ireland. Physiographical interest of the site relates to peat stratigraphy which is internationally significant. The site yields palaeoenvironmental data on possible Neolithic forest clearance, the decline of the native Scot's Pine, the effects of historical volcanic eruptions and botanical responses to climatic change.
053 CULNAFAY
Unique diatomite and associated materials deposited on historically (7500-5000yrs BP approx) seasonally inundated ground with the transition from diatomite to glacial till marking the former shoreline. This is the largest good quality deposit remaining in the area and is amongst the most important post-glacial freshwater diatomite deposits in the world. Diatomite is of international importance for biological, geological, hydrological and climatological reasons and it has considerable archaeological significance.
052 TOOME
Internationally important deposits of diatomite with changes in diatom species composition recording Holocene variations in water depth of the Lough Neagh basin. Associated organic material and geomorphological features also reflect hydrological variability within the Lough Neagh basin and climatic change within Northern Ireland.
Movanagher asi
Also known as Tully Hill, part of the Kilrea glaciofluvial complex, see below.
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 variable ice retreat rates during deglaciation and contains pristine examples of ice contact topography, unaltered by post-depositional meltwater erosion.
TULLY HILL Proglacial delta (Kilrea Glaciofluvial Complex)
A striking flat-topped delta unit immediately to the west of the River Bann, to the north of Kilrea. Landforms in the Tully Hill area are pristine examples of major deglacial landform types and clearly demonstrate links between the subglacial and proglacial environments.
Other sites/units identified in the Earth Science Conservation Review
324 BALLYMACOMBS MORE
Diatomite series underlying peat
