| Site Description |
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| Highlights: |
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| The high plateau formed by the Antrim Basalts forms a dramatic backdrop to the scenic incision of the steep-sided Glenshesk valley. The rural valley contains assemblages of morainic ridges, fans and glaciofluvial terraces formed in association with meltwater draining westward and northward from successive ice margin positions located across the valley. In the mid to upper valley at Greenan (314300 434300), a prominent outwash fan slopes across the valley from an apex to the east and dominates the valley fill topography. The highest part of the fan is located at the margins of the Antrim plateau 1.5km to the east of the valley axis near the eastern end of a rock-cut, possibly Nye-type channel. Erosional remnants of glaciofluvial terraces and flat bottomed, steep-sided meltwater channels containing fills of outwash and alluvium occur at lower levels within the Glenshesk and Inver Burn valleys. Variation in retreat rates resulted from continual but variable ice pressure from an ice mass located to the east or southeast. It is inferred that the deposits, preserved in a topographic low, indicate a generally eastwards retreat of the ice margin. |
| Description: |
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| The deposits are described generally from west to east, across the Glenshesk river valley, in chronological sequence. |
| 1. Glenbank ridge A 0.5km long, 10m high, fluvially modified ridge occurs at 110m O.D. on the western bank of the Glenshesk river southeast of Glenbank House (313600 435200). The ridge is oriented SW/NE across the Glenshesk valley at its junction with the Inver Burn valley, which drains westward. A small area (<0.2km2) of hummocky kettled topography occurs immediately to the southeast of the ridge. |
| Glenbank exposure |
| A disused pit in the ridge at (313700 434500) exposes three interbedded facies. The lower facies consists of coarse, fairly well-sorted cobble gravel within 6m wide by 1m deep cross-cutting channels with palaeoflows generally towards 10§N. Above a sharp contact, facies two consists of horizontally bedded silty sand and occasional 3-4cm thick beds of small-scale, planar cross-bedded sand. Palaeoflow, from ripple orientation and cross-bedding was highly variable, generally to between 220 and 285§N. One 5-10cm thick bed of finely-bedded sand is convoluted. Facies 3 is 60-80cm thick, occurs above sharp contacts with both facies 1 and 2 along the section and consists of horizontally and small-scale planar cross-bedded, pebble to cobble gravel with a palaeoflow, from a-axis imbrication to 20§N. The exposure contains three ice-wedge pseudomorphs which are up to 1.7m deep by 0.8m wide. |
| 2. Doonfin Ridges A complex of onlapping and cross-cutting cross-valley ridges which decrease in elevation eastwards from 160 to 150m O.D. are located at Doonfin, 0.25km south of the Glenbank ridge. The largest ridge which is oriented WNW/ESE, is approximately sub-parallel to the Inver Burn valley and occurs at up to 169m O.D. The 1km long ridge is smooth-crested and asymmetrically shaped, with over-steepened slopes on its northern flank. No exposure occurs in the ridge. Three smaller ridges occur to the southwest of the ridge and are oriented generally SW/NE. The flat to gently sloping Greenan fan (described below) onlaps onto the eastern margins of the complex at (313800 433600). |
| 3. Tenaghs ridge A 15-20m high, steep-sided, flat-topped cross-valley ridge occurs as a topographically dominant feature across the Glenshesk valley at Tenaghs (313900 433200). The ridge, which has been extensively modified by fluvial erosion, is deeply incised by a steep-sided meltwater channel now occupied by the Glenshesk river. No large exposures occur in the ridge. Small river bank cuttings and minor slumps on the ridge sides show that the ridge consists of two principal facies. Facies 1 consists of massive, poorly sorted gravel and facies 2, which comprises up to 80% of the ridge, consists of horizontally bedded sand and silty sand interbedded with cross-bedded sandy pebble-gravel. Palaeoflow, based on cross-bedding, was to 260§N. Clasts were composed mainly (85%) of schist and basalt, probably of local origin. |
| An arcuate ridge of low relief occurs along the rising drift-free slope of Oughtbristacree Mt (316800 432400) forming the western margin of the mid-Glenshesk valley at 180m O.D. The <2m high bank is overlain at its northern end by the western edge of the Tenaghs ridge. |
| 4. Greenan Fan A large (1.2km2) gently sloping (<15§) fan-shaped spread dominates the topography in the mid-Glenshesk valley. The fan apex occurs at 180m O.D. at the western end of a 10m deep rock cut channel in the northern spur of Oghtbristacree Mt. (316800 432400). The fan decreases in elevation westwards from its apex on the northwest slopes of Oghtbristacree Mt. to a flat-topped portion located along the valley axis from Glenshesk Bridge (314100 435800) to Tenaghs (313800 433300) located at 150m O.D. The flatter-topped toe of the deposit is dissected by the Glenshesk river, forming steep river banks for approximately 1.5km. The northern margin of the deposit is also dissected by the westward draining Owencam and Greenan rivers which flow into the Glenshesk from the highlands to the east. On the western side of the Glenshesk river, the toe of the fan onlaps onto the eastern end of the largest ridge at Doonfin. |
| 5. Lower terraces A localised belt of dissected, flat-topped fragments occur at up to 60m O.D., 10-15m above the level of the flat valley floor, on the western side of the Glenshesk river around Drumeeny (313200 438900). The valley floor plain is 2.7km long by 0.5km wide between (313500 439300) and Broughmore (314100 437300). The valley narrows southwards and is confined on both sides by largely drift-free steep bedrock walls up to 80m deep for 1.5km. |
| Importance: |
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| The deposits in the Glenshesk valley form an isolated, concentrated belt of glaciofluvial deposits located in a steep-sided, sparsely populated valley, which extends from the uplands of the Antrim plateau to the north coast at Ballycastle Bay. The area is designated as an Area of Outstanding Natural Beauty in recognition of the diverse landscapes, habitats, flora and fauna found in the region [DOE(NI), 1988]. The glaciofluvial deposits of the Glenshesk valley add to the scientific importance of the region by recording a local, generally west to east withdrawal of ice across the Antrim plateau uplands and east coast of Ireland during the last deglacial cycle. The deposits indicate that the backwastage rate of the ice lobe, which occupied the valley, was variable and resulted in the accretion of cross-valley ridge complexes probably during periods of reduced retreat rates. Variability in retreat rates indicates continual but variable ice pressure from a larger ice mass located to the east during this deglacial phase. The moraine ridge/meltwater channel/outwash terrace association in the Inver Burn valley form an example of a classic deglacial landform assemblage, a moraine/outwash couplet. The Greenan fan forms one of the largest examples of a proglacial outwash fan in Northern Ireland. Its obvious association with a rock-cut Nye type subglacial channel also adds to its scientific value. It is inferred from the dissection of deposits along the valley axis and the formation of glaciofluvial terraces in the lower valley that the coastal area was free of Scottish ice at this time. |
| Interpretation: |
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| The interpretation of the deposits in the Glenshesk valley is based principally on landform morphology and the pattern of landform assemblages as sediment exposures are poor. The deposits are inferred to have been formed during the phased retreat of an ice lobe generally eastwards from the Inver Burn depression across the Glenshesk valley towards highlands to the east. This resulted in the deposition of morainic belts followed by resedimentation as valley-fill outwash sequences from meltwater escaping northwards along the Glenshesk valley. |
| 1. Glenbank and Doonfin ridges The cross-valley ridges at Glenbank and Doonfin are located at opposite sides of a flat-bottomed, steep-sided topographic low which appears to have been over deepened by ice advancing from the highlands to the east. |
| The exposure at Glenbank shows coarse, poorly-sorted outwash sediments associated with deposition in channels related to ice-marginal effluxes and finely bedded, well-sorted sands associated with deposition from effluxes in more distal ice-margins or possibly changes in the transported sediments. It is inferred from the sediment sequence and kettled topography to the southeast of the ridge that the core of the ridge formed from the amalgamation of a series of ice-marginal 'kame' mounds. The upper fluvial facies were deposited as the ice-margin retreated eastwards across the valley, with extensive fluvial modification and melting of buried ice masses resulting in the steep-sided morphology. The composition and sediment geometry of the largest ridge at Doonfin is unknown. However, from its proximity to the Glenbank ridge, orientation sub-parallel to the Inver Burn channel and over-steepened, ice-contact northern slopes, it is inferred to have formed at the margins of an ice mass located in the Inver Burn channel. |
| The ridges are probably the lateral remnants of a continuous cross-valley, arcuate morainic belt. The central valley-axis part has been removed by later meltwater erosion along the Inver Burn channel. The smaller ridges along the eastern bank of the Glenshesk river appear to onlap onto the larger Doonfin ridge and are oriented subparallel to the Glenshesk valley, indicating their probable formation at the lateral margins of an ice tongue oriented north/south along the valley axis, coincident with the formation of the ridge at Tenaghs. |
| 2. Tenaghs ridge The undulating to flat-topped cross-valley ridge Tenaghs, based on the apparent sequence of high-energy, ice-marginal sediments (massive, possibly debris-flow diamicts) and lower-energy sediments (well-sorted sand and gravel), is inferred to have formed as an ice-contact glaciolacustrine ridge associated with a reduction in the retreat rate of an ice-tongue from the upper reaches of the Glenshesk valley. This is inferred from the Tenaghs ridge cross-cutting a valley-side ridge which occurs at up to 180m O.D., indicating a more southerly extent of ice in the valley at an earlier stage. The flat to undulating topped Tenaghs ridge possibly represents the elevation of a proglacial lake impounded against the rising bedrock to the south as the ice tongue retreated northwards in this part of the valley. A rock-cut channel to the west of the ridge may represent the location of an overflow channel which regulated the elevation of the lake. The steep-sided morphology of the ridge is probably a result of meltwater erosion. |
| The dissection of the Tenaghs ridge and valley fill to the north indicates that meltwater erosion was a powerful agent in the late glacial modification of probably all deposits in the Glenshesk valley. The high meltwater discharges from snow and ice melt may also have removed stratified sediments from south of the Tenaghs ridge. The Lower terraces probably represent the remnants of the resedimented outwash deposited by the northward draining meltwater. |
| 3. Greenan Fan The large fan shaped deposit banked against the rising slopes of the Antrim plateau to the east of the valley is inferred to be a proglacial outwash fan composed of coarse, poorly sorted debris flows from ice-marginal sources. Much of the sediment observed to compose the spread is poorly rounded and is of local origin. The deeply incised rock-cut channel to the east of the fan is probably of subglacial origin and formed the principal conduit of meltwater and feeder of subglacial debris westwards from an ice mass located to the east. The fan onlaps onto and is banked against the ice marginal ridges in the valley and therefore post-dates them, supporting a west to east retreat of ice margins across the valley axis. |
| The observations of Hill and Prior, 1968, indicate that there was a southeast/northwest advance of ice across the Antrim plateau uplands and therefore ice pressure from this direction onto the north coast during the last glacial cycle. These conclusions fit with a model of generally west to east retreat of ice margins across the Glenshesk valley during the deglacial period. |
| Notes: |
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| 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 |