Garvagh Farmland 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 Garvagh Farmland is found to the east of the Glenshane Slopes. The area is dominated by rounded drumlins, with the characteristic smooth, elliptical form. Most are consistently aligned along a NW-SE axis, indicating the direction of ice flow in glacial times. River valleys, such as those of the Agivey River and the Grillagh River, tend to follow and widen slight gaps between the drumlins and minor tributaries wind around the small hills. There are often patches of marsh on the floodplains of some smaller rivers and in low-lying areas between the drumlins. There are often small woodlands on the ends of the drumlins and dense hedgerows around the base, giving visual emphasis to drumlin landforms.

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)

This LCA is a north-south tract from Garvagh to Magherafelt and comprises 95% Upper and Lower Basalt Formations of the Antrim Lava Group. In the southwest corner of LCA51 there occurs Neoproterozoic and Carboniferous strata. These three successions are usually in faulted or unconformable contact.

Neoproterozoic (Dalradian)

The sandstones and grits of the Glennelly Formation (roughly equivalent to the Fahan Grits of the Southern Highland Group in Co. Donegal) are found as a small area in the SW of the LCA. These sediments were deposited in a marine environment, buried to depths of 5km - 10km and subsequently metamorphosed in the Caledonian Orogeny (late Palaeozoic). The gritstones and sandstones (quartzites) of this succession are quarried elsewhere for construction materials.

Carboniferous (Upper Palaeozoic, from oldest to youngest)

This succession of conglomerates and sandstones, shales and thin limestones records the transgression of the Carboniferous sea onto older strata and the consequent establishment of marine conditions. These soft rocks provide poor-quality local building stones.

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 51 these two formations are separated by red, palaeoweathered beds and columnar basalts, not seen in this area. They are extensively quarried for construction materials, especially roadstone.

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.

Within the above context, the drift geology map for this LCA shows it to be dominated by the Midlandian till associated in this area with the northwards movement of ice out of the Lough Neagh Basin. This movement is indicated by large numbers of streamlined drumlins orientated approximately S-N, although precise orientation depends upon local topography. 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. Some have sharp crests, whereas others are more whaleback in profile. 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. Where drumlins are rock cored there may have been significant frost shattering prior to their shaping by ice flow. It is possible therefore to see tails of shattered debris within till leading away from the feature in the direction of flow (Davies and Stephens 1978). 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.

In the northeast of the LCA are limited deposits of glaciofluvial and glaciolacustrine deposits that are on the margin of the Kilrea deglacial complex. These were deposited as ice abalated and the ice margin retreated southwards at the end of the Midlandian. These deposits are described in detail in LCAs 52 and 53.

Key Elements

AONB

Very limited areas of the LCA lie within the Sperrin AONB (1968) in the far west and this is indicative of its scenic value.