Derg 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 Western River Basins. This region consists essentially of the connected river systems that drain the Carboniferous and Old Red Sandstone plateau of County Tyrone, as well as the foothills of the Sperrin Mountains to the east and Donegal to the west. The region extends from the Omagh Basin in the south, northwards along the lower Foyle valley. The Omagh Basin has particular significance as an ice centre during the Late Midlandian and is now largely covered by a complex mixture of glaciofluvial sands and gravels and drumlins overlying Rogen moraines. When the headwaters of these river systems rise together they have in the past been responsible for serious flooding at the bottleneck of Strabane. Although this has been mitigated by extensive drainage control works in and around the town.

The River Derg flows eastwards from the Killeter Uplands to join the Strule River near Ardstraw. The market town of Castlederg is at the principal crossing point in the centre of the Derg Valley. The broad valley is enclosed by an undulating landscape of rounded hills, many of which are capped with open moorland. The summits to the north of Castlederg have a particularly exposed character and are separated by extensive areas of blanket bog and marginal farmland. The River Derg has carved a relatively broad vale. There are many shallow tributary valleys and the landform is gently undulating, although the river floodplain itself is almost flat. The river is not embanked right along its course and there are extensive patches of peaty marsh and scrubby fen woodland at points where it is joined by narrow tributary streams. The landscape can therefore be summarised as one of gently rolling pastures on lower valley slopes and river floodplain, although a hummocky, morainic belt up to 12 km2 in area adds visually appealing topographic diversity to the valley of the Derg River east of Castlederg as far as the Foyle valley at Victoria Bridge.

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 covers the northwestern tip of Northern Ireland. The geology is dominated by Neoproterozoic metamorphic rocks with minor Carboniferous sedimentary rocks. The type location of Killeter (the Killeter Quartzite) is in the southwest of the LCA. Covers part of the Pettigoe Plateau - ASSI 065.

Faulting is ubiquitous in the Dalradian and common in the Carboniferous.

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 more than 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.

The Drift Geology map shows that the uplands in the west of the LCA are dominated by drift-free bedrock marked only by alluvial deposits along present-day water courses. In contrast, the east of the LCA is underlain by Late Midlandian till that was deposited by ice that moved north and northeastwards out of the Omagh Basin along the upper Foyle valley. The deglaciation of this eastern area was marked by the accumulation of 5.6km2 of the Foyle Valley sand and gravel complex as two linear deposits of Late Glacial hummocky moraine and outwash running northeast to southwest along the valley of the River Derg. The Foyle Valley complex, as a whole, is located along the axes of the Foyle, Mourne, Strule, Glenmornan and Derg river valleys and consists of a widespread assemblage of landforms genetically linked by formation during ice-margin retreat westward from the Sperrin valleys during the last deglacial cycle. Strong control on ice-margin configuration and meltwater drainage patterns was exercised by bedrock topography, serving to focus meltwater along the valley axes. This resulted in the formation of thick, flat-topped glaciofluvial terraces. Increases in sediment supply or temporary reductions in ice-margin retreat rates resulted in the accumulation of thick belts of hummocky moraine along the Derg river. Most of the complex can be found in LCA 27.

Key Elements

Deglacial Complexes

Castlederg hummocky moraine

A large, almost linear 11 km long by 1 km wide valley-parallel (WSW/ESE) belt of kettled topography, hummocks and valley-parallel ridge segments occurs mainly along the northern side of the Derg river valley from Mulvin Parks to Castlederg. The belt is continuous and occurs up to 60m O.D.. A smaller (2 km2), dissected valley-parallel belt of hummocks occurs on the southern side of the valley at Ballynaloan up to 70 m O.D.. Several old and now grassed exposures indicate that the hummocks consist of interbedded boulders, gravel and sand.