Derrykillultagh 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 Derrykillultagh LCA is a rolling, relatively elevated farmland landscape on the margins of the Antrim basalt plateau. The area is characterised by rounded hills and shallow slopes, although there are some gullies. The southern boundary of the LCA is marked by a steeper escarpment slope. Many slopes are divided by straight, narrow glens that form a ladder pattern in views from below. The plateau descends gently to the west, where there is a gradual transition, through the Upper Ballinderry Plateau (LCA 109) to the claylands on the edge of Lough Neagh. Towards the east, where the plateau becomes increasingly more elevated, the soils are of poorer quality. The landscape is sensitive to change in the more open, elevated parts of the plateau, and on the steeper slopes to the south The landscape can therefore be summarised as one of rolling, slightly acidic farmland on the margins of the Basalt Summits. It largely comprises rounded hills, with fairly shallow slopes, but with a steeper escarpment to the south. A key element in the landscape is the overall structural control exerted by the underlying lava flows.

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.

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

Triassic sandstones comprise red, purple and brown cross-stratified sandstones, siltstones with minor clay beds and partings. The sandstones are mostly soft and poorly-consolidated. Outcrop restricted to fault-bounded areas (about 5% of the LCA) to the southeast of LCA110 (western parts of the Lagan Valley). Below the Cretaceous - Tertiary escarpment and above the Sherwood Sandstones in the southeast of LCA110, low hills are underlain by soft sedimentary rocks of the Triassic Mercia Mudstone Group - about 5% of the LCA. This group comprises occasionally calcareous red silstones and mudstones with subordinate grey-green siltstones, mudstones and anhydrite. Centimetre-thick grey or red sandstones may occur. The Mercia Mudstone Group is predominantly red-brown and unfossiliferous in the lower parts of the exposed succession, becoming grey-green, sometimes fossiliferous and sometimes carbonate-cemented toward the top. The beds form an aquiclude, soft and contain anhydrite.

The Cretaceous succession is found in linear, fault-bounded exposures below the Tertiary basalt escarpment of LCA110. Basal greensands are recorded from the base of the succession. The presence of the Ulster White Limestone Group is known from isolated exposures, old limekilns and marlpits in the area.

The Tertiary basalt formations comprise a crudely-bedded succession of lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons (or boles) that dominate LCA110. The basalts rest unconformably on the older formations. They are extensively quarried for construction materials in this area, especially roadstone. In the far north of LCA110, these two formations are separated by red, palaeoweathered beds and ashfalls of the Interbasaltic Formation. Within the Lower Basalt Formation a mapped volcanic unit occurs in the south of LCA110. The Lower Basalt Formation is the dominant rock type of the area.

Isolated exposures of the Upper Basalt Formation occur in the north of the LCA in faulted contact.

NE-SW and minor NW-SE oriented faults dominate the outcrops of Tertiary rocks and juxtapose all the above formations.

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 be underlain for the most part by Late Midlandian till laid down by ice that moved rapidly across the area from a centre in the Lough Neagh Basin. As long ago as 1939, Charlesworth in his seminal paper on the glaciation of northeast Ireland identified a small number of drumlins in this area and their orientation can be used to confirm the eastwards/southeastwards flow of this ice. Within Northern Ireland drumlins take a variety of forms; some are rounded in plan, although the majority are elongated in the direction of ice flow. Although most drumlins are composed of glacial till or tills, a small number are 'drumlinoid features' are rock-cored and some are composed of sand and gravel. It is generally accepted that the drumlins of Northern Ireland were formed by deposition beneath fast flowing ice. In the majority of cases this has resulted in a thick layer of Upper (younger) Till overlying a core of Lower (older) Till. This pattern has been observed across Northern Ireland, apart from a limited area in the north of County Down. The precise temporal relationship between the two tills has not been definitively resolved, but Davies and Stephens (1978) refer to an organic layer between the tills in County Fermanagh that has been dated at 30 500 ± 1170/1030 years B.P. and shelly material between the tills on the Ards Peninsula dated at 24 050 ± 650 years B.P.. However, these deposits only indicate that the Lower Till is older than the dates obtained.

Of considerable interest, though only of very limited areal extent, are a series of glaciofluvial deposits that occur in the extreme southeast of the LCA. These are part of the Lagan Valley Deglacial Complex. This is a discontinuous belt of glaciofluvial and glaciolacustrine deposits that occurs for 40km along the axis of the Lagan valley from Belfast WSW to Aghalee, Co. Antrim.

There are also a number of other, isolated areas of glaciofluvial deposition in the north of the LCA that may be related to ice marginal conditions at the end of the Midlandian. Davies and Stephens (1978) consider that the final stages of ice-wasting in the east of Northern Ireland probably involved wide scale stagnation, downwasting and withdrawal inland towards the Lough Neagh lowlands (p.176). This would have left upland areas such as the Antrim Plateau ice free and surrounded by encircling ice.

The drift geology map also identifies linear deposits of alluvium associated with the dip slope drainage of the plateau.

Key Elements

Deglacial Complexes

LAGAN VALLEY DEGLACIAL COMPLEX,

The Lagan Valley Deglacial Complex is highly important in understanding the complexity of deglacial processes. Streamlined landforms along the margins of the valley and glacially moulded bedforms indicate ice advance and episodes of fast ice flow from the west. Glaciolacustrine deposits indicate that during initial deglaciation the lower valley contained an ice-dammed lake, probably impounded by Scottish ice in outer Belfast Lough. A lobe of Irish ice located in the valley, related to ice pressure from the Lough Neagh Lowlands contained subglacial conduits now recorded by eskers that probably supplied sediment to the Malone deltaic sands that now underlie most of south Belfast. The phased retreat of the ice lobe further westward is recorded by cross-valley, ice-contact ridges. During the final deglaciation, drainage was to the west, indicating a reversal in the drainage gradient probably due to isostatic depression of the Lough Neagh Lowlands during the last glacial cycle.