| Previous Site | Next Site |
Earth Science Conservation Review
| Murrins Complex - Glensawisk Burn Delta | Tyrone |
| Site number: | 508 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | Status: | TBC |
| Grid Reference: | H532786,H557833 End(s) of linear site, Not Entered | ||
| Google maps: | 54.65318,-7.17576 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Description: | |
This area extends from Lough-a-Tirrive (H532786) to Fallagh Upper (H557833) (5km long) and forms a valley between the upland areas of Mullaghcarn and Mulderg (H540789) drained by Glensawisk Burn. The valley reaches approximately 3km wide at the junction with the Owenreagh River. Glacial sediments infill a pre-existing bedrock valley. The sediments are characterised by flat spreads (between 180-220m a.s.l.) of deltaic and glaciofluvial sand and gravel which have been heavily dissected by glacial meltwater to a depth of 40-60 m. Upvalley these sediments onlap onto bedrock; downvalley they onlap onto localised soliflucted diamict. Peat has developed on all substrate types to a depth of 1-2m. Near Stradowan Bridge (H535799), 2km north of Mountfield, are a series of three linked former sand and gravel pits, totalling up to 300m long and 150m wide. The pits are dug at 210-220m into the eroded upper delta/outwash surface, which reaches 235m a.s.l. (Dardis, 1986). All pits offer exposure of similar sediment sequences. The most northern pit, termed the Stradowan pit, is examined in detail here. There is one main exposure face. Exposure A. Facies description This exposure (42m long and <7m high) shows clast and matrix-supported gravel beds interbedded with massive and vaguely stratified well-sorted sand beds. These facies occur in stacked planar or cross-bedded sequences separated by laterally-continuous planar erosional contacts. Within each major sequence, facies are separated by sharp or diffuse non-erosional contacts. Beds are often dipping (towards 320§). Gravel beds are composed of sorted granules and small pebbles (<6cm diameter) supported by a minor component (30%) of granules and coarse sand. Beds (20-40cm thick) are often inversely graded and show marked lateral changes in texture and sorting. Local clast-supported or openwork pods also occur. Tabular sand beds (20cm thick) are composed of massive, well-sorted medium-grained sediment. Few internal bed structures are seen and there are no lateral or vertical changes in texture. A channel (1m high and 3m across) infilled with clast and matrix supported pebble to granule-sized clasts occurs between major facies sequences. The channel is composed of three stacked, superimposed (probably reactivated) channel infills separated by a conformable and diffuse sandy layer which overlies an erosional surface. Beds comprising the infill are normally graded and change upwards from imbricate openwork pebbles (<12cm diameter) dominating the basal channel to vaguely-stratified matrix-supported granules dominating the upper channel. Overall channel axis is aligned approximately 025-205§. Facies interpretation This exposure records the deposition of a variety of well-sorted sediments in a subaqueous depocentre by massflow and grainflow processes (cf. Rust and Romanelli, 1975). Variations in flow pulse energy can account for the observed facies variability and continuity (Church and Gilbert, 1975). High-energy pulses are associated with massive, matrix-deficient gravel pods and lenses, often with erosional basal contacts. Planar basal contacts demonstrate a dominant longitudinal component of the flow regime in which basal scouring occurs. Sediment sorting is due to lower energy underflow sediment pulses, often relating to cross-bed formation with gravity flow sorting down the cross-bed slope (Church and Gilbert, 1975). Massive, well-sorted and unstructured sand facies, associated with waning flows are characterised by diffuse lower and sharp, sometimes erosional, upper contacts. Stacking and overlapping of these different sediment sequences is characteristic of a shallow-water aggrading delta or outwash environment (Rust and Romanelli, 1975) with variable rates and locations of sediment input. The incised channel infilled with imbricate (a(p)) and clast-supported pebbles may result from a series of high-energy flow events sourcing from an ice margin further upvalley. The Glensawisk Burn delta was identified by Charlesworth (1924) and Colhoun (1972) as resulting from southward ice retreat through Lough-a-Tirrive towards Mountfield. Dardis (1986), following Charlesworth's (1924) hypothesis of deltaic sedimentation in 'Lake Gortin', proposed that sediment was deposited into an ice lake at 235m a.s.l. which was impounded by an unstated mechanism in the Owenreagh valley and which extended south as far as the Fintona Hills. Dardis (1982, 1986) correlated the Glensawisk Burn delta with others elsewhere, such as at Todd's Leap, Evishanoran and Grouse Lodge. The Glensawisk delta surface was fed by only a small, local source at the head of the valley (i.e. near Lough-a-Tirrive), and was laterally constrained by adjacent deglaciated areas (Dardis, 1986). However, there is little evidence for subaerial runoff from these areas. An interpretation of southward ice retreat is followed here based on evidence of moraine ridges composed of local angular material near Lough-a-Tirrive. Evidence of ice-marginal activity, recorded in other sections in the Stradowan area, includes bed contortion and shortening, shearing of clay layers, water escape structures and some large-scale open folding. | |
| Notes: | |
For site specific information on other sites within the Murrins Complex see the following sites: Key Site 509 - Murrins Complex - Cashel Outwash Key Site 510 - Murrins Complex - Lough Fingrean Frontal Moraines Key Site 511 - Murrins Complex - Laght Hill Retreat Moraines Key Site 512 - Murrins Complex - Coolesker/Beragh Eskers For overview information and references on the Murrins Complex see the following site: Key Site 479 - Murrins Complex - Overview For general information and references on the sand and gravel landscapes of N. Ireland, and for directions to other regional sand and gravel sites, see the following site: Key Site 439 - Introduction to the Sand and Gravel Landscapes of N. Ireland This site is also a Site of Local Nature Conservation Importance. See Key Site 730 - Glensawisk Burn. | |
| Keywords | |
|---|---|
| Rocks: | Gravel, Sand |
| Geomorph: | deltas |
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Murrins Complex - Glensawisk Burn Delta, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=508. Accessed on 2026-11-06. |
| Previous Site | Next Site |