Croagh and Garvary River 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 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 LCA itself is defined as a small area to the north east of Belleek has a rough and rugged appearance, despite being less than 150m above sea level. It lies on the edge of the Pettigoe Plateau, and is underlain by ancient metamorphic rocks. The plateau is scoured with many small loughs and rocky knolls, with the rounded summits of Croagh and Mallybreen Hill rising above. Blanket bog covers much of the landscape. The two principal rivers, the Garvary and the Woodford Rivers, have cut through small rounded glacial hills and terraces of sand and gravel. Lough Scolban and Keenaghan Lough lie on the southern boundary of the area and mark a change in the underlying geology between schist and limestone. Farming is concentrated on the till (boulder clay) soils of the lower slopes and valleys, where there is a dense pattern of hedged fields and small farms. On the plateau, patches of till are associated with old enclosures and clumps of trees around ruined farmsteads. Key elements in the landscape include: the isolated area of barren Pettigoe Plateau broken by small river valleys and the open moorland, small loughs and marginal agricultural land.

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 dominated by Moinian Lough Derg Formation (over 95%) with Carboniferous in the south.

The Moinian Lough Derg Formation comprises tightly folded and faulted quartz mica schists underlying 95% of LCA3. Originally sedimentary rocks, these schists formed during a period of deep burial and intense metamorphism some 750 million years ago. The schists are intruded by metabasites and serpentinites of unknown affinities. These dykes and other intrusions trend NW-SE and are probably late Caledonian in age. Exposed at Croagh Mountain (ESCR Site 309: amphibolites and granulites); Gadalough (ESCR Site 310: serpentinised basic intrusions); Larkhill (ESCR Site 311: pegmatites) and Keenaghan Lough (ESCR 209: Carboniferous unconformable upon Lough Derg Moinian).

The Carboniferous Keenaghan Shale is best known from the Tyrone - Fintona Block Carboniferous, so its presence here is unusual. The far eastern edge of the LCA is underlain by Ballyshannon Limestone (Chadian - Arundian, Tournaisian - Visean, Lower Carboniferous, 300-350m).

All the above strata were deformed during the end-Carboniferous phase of Variscan tectonic activity.

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. 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.

This limited area of terrain mainly comprises the peat blanketed upland of the Pettigoe plateau. Although largely drift-free, glacial striae and Roche Moutonnees (streamlined rock ridges with evidence of erosion/plucking on the downflow end) indicate that the ice flow that overrode the landscape in the Late Midlandian was approximately from the east. This is confirmed by pebble orientation within the lowland tills on the Lough margin. These orientations are, however, less consistent than the striae and possibly indicate a more local topographic control on deposition. The steep streams that drain the plateau into Lower Lough Erne are associated with alluvial sedimentation in their lower courses. However, the presence of sand and gravel terraces suggests that these rivers were active in the production of glaciofluvial deposits during the deglaciation of the area.

Key Elements

ASSIs

065 PETTIGOE PLATEAU

One of the largest expanses of blanket bog in NI, with a range of oligotrophic lake types, based on their aquatic macrophyte communities, and which support breeding populations of Golden Plover and Dunlin amongst others. The bog also exhibits the full range of structural features including a large number of well developed pool complexes, frequent acid flushes and a number of basin mires.

Other sites/units identified in the Earth Science Conservation Review

309 Croagh Mountain

Pre-Dalradian Precambrian. Outcrops of a wide variety of lithologies and structures of Lough Derg Group. Siliceous granulitic rocks with amphibolite and semi-pelitic components.

310 Gadalough

Precambrian. Lough Derg Group. Unusual altered and serpentinized relics of basic igneous intrusions.

311 Larkhill

Precambrian. Pegmatite veining in Lough Derg Group.

209 Keenaghan Lough

Carboniferous. Exposure of plane of unconformity between Keenaghan Shale Formation and Lough Derg Group.