The Garrison Lowlands 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 on the northwestern border of the region described as the Plateau and Valley Lands of Fermanagh. This is a series of sharply defined plateau blocks separated by steep-sided, glacially deepened, lake strewn valleys. The morphology of the Carboniferous uplands is largely controlled by the presence of a series of gritstone caprocks. Beneath these, well-jointed limestones have allowed the development of extensive subterranean drainage systems as well as a variety of surface karst phenomena for which the region is internationally renown.

The Garrison Lowlands, in the far west of Fermanagh, are bounded by Lough Melvin and the County River. The land rises to the east, and a mixed geology has resulted in poorly drained lowland soils, with exposed sandstone on ridges. Long rounded ridges of glacial deposits, becoming flatter further west, and are separated by small rivers which head west in narrow wooded gorges, through circular lakes, bogs and wet meadows. The village of Garrison, is a focus of recreational activity and lies in a picturesque setting at the head of Lough Melvin, where views southwards are dominated by distinctive 'ladder farms' on pronounced slopes. Numerous raths are scattered throughout the area. The north shores of Lough Melvin are low-lying and exposed with boulder and shingle beaches fringed by thickets and reed beds; low ridges project into the Lough forming points and islands. This edge landscape is highly sensitive to change. The Lough itself has unusually clean water. Karstic activity within the LCA is indicated by an active cave system in the Dartry Limestone at Ground Bridge. This is a member of the Garrison karst unit that extends into LCA4.

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)

This LCA is comprised of fossiliferous Carboniferous sedimentary rocks with the exception of the Tertiary Garrison Sill. The Mullaghmore Sandstone dominates the northwestern third with the other limestone, shale and sandstone formations forming varying proportions of the underlying geology. Dartry Limestone is exposed at Cashel Quarry (ESCR Site 190).

All the Carboniferous strata were deformed during the end-Carboniferous phase of Variscan tectonic activity. Major faults and folds have their strike NE-SW, along pre-existing, Caledonian trends.

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 over 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 BP. 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 BP. - 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. 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 a drumlin covered lowland area underlain by Late Midlandian till. The orientation of the drumlins suggests that the final direction of ice flow that was responsible for their formation was from the southeast. Although drumlins to the north of Doagh, suggest that ice may also have moved into the area from the east. 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',some 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 BP. and shelly material between the tills on the Ards Peninsula dated at 24 050 ± 650 years BP. However, these deposits only indicate that the Lower Till is older than the dates obtained.

Key Elements

ASSI

139 Ground Bridge

The Ground Bridge on the Roogagh River at Tullybelcoo is a short active river cave. The passage is approximately 45m long and the Roogagh River flows through an outcrop of the Carboniferous Dartry Limestone. The sink is very close to the contact between the Dartry Limestone and the Quarry Sandstone Member. The Dartry Limestone dips southeast at 15° and is thinly bedded (approximately 30-40cm) with the roof of the cave developed on the bedding. This is a unique karstic feature in Northern Ireland.

Other Karst Sites

Natural bridge over the County River.

This natural bridge is situated on the County River on the border between County Fermanagh and County Leitrim leaving only half the feature in Northern Ireland. It is adjacent to a scheduled archaeological site and the bridge must have been significant in the siting of the adjacent monastery. The bridge is developed in a small outcrop of the Knockmore Limestone, has a span of approximately 8m and is 4m in breadth. There are foot caves in both north and south banks and a short joint controlled rift at right angles to the river bank cuts the cliff lined bank next to the bridge.

Other sites/units identified in the Earth Science Conservation Review

190 Cashel

Carboniferous. Exposure of upper beds of Dartry limestone formation, with a thin fossiliferous palaeokarst horizon near base.