Earth Science Conservation Review
| Mourne Plain Complex - Overview | Down |
| Site Type: | Coastal section, Inland exposure, Pit |
| Site Status: | |
| Grid Reference: | J31923166,J33443167,J32703098 |
| Google maps: | 54.2168,-5.97798 |
| Rocks | |
|---|---|
| Rock Age: | Quaternary (Late Midlandian) |
| Interest | |
| Other interest: | Glacial deposit, Glaciomarine deposit, Glaciotectonic, ice lobe, moraine ridge, outwash, raised beach |
Description:
The low-lying Mourne Plain is underlain by Silurian greywacke sandstones and mudstones, with the exception of a small area of Carboniferous limestone at the entrance to Carlingford Lough (Wilson, 1972). The bedrock geology is rarely exposed, except at a few coastal locations, as it is almost completely covered by thick glacigenic deposits e.g. the morainic ridge at Ballymartin (334500 317000) attains a local topographic height of 60m within 300m of the coast, while the Cranfield moraine at Tullyframe (326300 316800) rises to 70m above the general level of the lowland to the southwest. These may be regarded as minimal depths of material within the more prominent moraines. This contrasts with the steeply rising, largely drift-free bedrock slopes of the Mourne Mountains granite massif to the north, which dominates the local topography. Together the two areas supply the majority of clasts found within the glacigenic deposits.
The morphology of the area contains three principal glacigenic topographic elements:
1) Curvilinear recessional moraines tending generally from northwest to southeast are related to late Midlandian ice retreat from the Plain westward into Carlingford Lough. The ridges in the east of the Plain are widely spaced and discontinuous whereas further west they occur as tightly spaced occasionally intersecting or crosscutting ridges and segments.
2) Spreads of proglacial outwash, which occur at lower altitudes in the more extensive inter-morainal lows contain enclosed depressions.
3) A series of shoreline notches and benches are cut into the outwash and moraine ridges by both lateglacial and postglacial marine highstands.
A number of exposures have been chosen to illustrate the range of sedimentological processes, which together have resulted in the morphological diversity of the Mourne Plain. Sections have been selected which display the sedimentological complexity and variability typical of the Plain's three major morphological elements. The landforms are subdivided, classified and described firstly by morphological type, from west to east.
MOURNE PLAIN MORAINES
For site specific information on the Mourne Plain Moraines see the following sites:
Key Site 441 - Cranfield Moraine Key Site 442 - Glenloughan Pit Key Site 443 - Tullyframe Pit Key Site 444 - Moore Lodge moraine
MOURNE PLAIN OUTWASH
The most extensive area of outwash on the Mourne Plain lies between 12 and 18m O.D. and measures approximately 1.6km2 in area. The outwash is triangular in plan view, is bounded to the west and north by the ridge of the Cranfield moraine and to the east by a moraine ridge at Ballynahattan (328400 312200). To the south the outwash is notched by a lateglacial strandline at 19m a.s.l. Large areas of the original surface of the outwash plain have been removed as a result of aggregate extraction.
For site specific information on the Mourne Plain Outwash see the following site:
Key Site 445 - Sandpiper Pit
KETTLEHOLES
A large number of up to 1km2 interconnected depressions (kettleholes) occur 1km to the west of Ballykeel (323300 316800), within a wide undulating spread which forms the eastern side of the Kilkeel River valley. The largest of these kettleholes contained a lake, which eventually drained to the south along a well-marked channel. A depression at Nace (333000 315800) in a low relief spread contains a poorly exposed section consisting of crossbedded sand with occasional fine gravel stringers. The palaeoflow, measured from cross-bedding, was towards 260deg. The top of the exposure is at ca. 30m O.D., the altitude of a late-glacial marine notch, and is in a depression at the landward end of a channel which dissects the marine notch. Isolated enclosed depressions are also found throughout the Plain (e.g. 329600 313200).
RAISED BEACHES
In the area between the Cranfield Point and Ballymartin moraine ridges, the seaward margins of the moraine ridges on the Mourne Plain are cut by up to four late-glacial and postglacial beach notches. Spreads and ridges of sand and gravel are eroded to produce a series of steps subparallel to the current coastline. The most continuous of these notches occurs at 20m O.D. at Dunnaval (2.1km long) and 30m O.D. east of Kilkeel (1.5km long).
It is noted that notches at these elevations are absent from inside the area bounded by the Cranfield moraine.
Between Cranfield Point and Slatemill Road (329300 312500) the undulating terrain is notched at 10m a.s.l., up to 300m inland. This is at a similar height to a raised beach found within the Cranfield moraine, and thus clearly post-dates the moraine. This shoreline is more irregular than the late-glacial notches, and is not as well-marked, probably the result of rapid postglacial isostatic adjustment. A lower notch at 6m a.s.l. is less continuous, and formed an embayed strandline. An almost continuous low ridge (<10m O.D.) trends northeast from Cranfield point for approximately 1km, parallel to the current coastline. Similar landforms occur within the Cranfield moraine in the Soldiers Point and Greencastle areas.
For site specific information on the Mourne Plain Raised Beaches see the following site:
Key Site 446 - Kilkeel Cliff
OVERVIEW
The moraines and associated outwash couplets on the Mourne Plain record stages in the decay of the late Midlandian ice sheet, which had its ultimate source in the Lough Neagh-Irish Midlands axis. As the level of the ice sheet fell below the level of the eastern Mourne, topographic control increased in importance, and it is likely that the Mournes were completely encircled by ice during initial downwastage. Moraines record periods of stillstand during this retreat, and exposures such as that at Moore Lodge indicate that a number of oscillations of the ice front were superimposed on this general pattern.
Charlesworth (1955) identified the Cranfield moraine as a limit of the major readvance which fronts the drumlin swarms of County Down. Alternative limits for the readvance have been presented by Stephens and McCabe (1977) based on the existence of lateglacial shorelines between certain moraine ridges. These lateglacial shoreline notches are the result of the interplay of glacioeustatic and glacioisostatic processes (Stephens & McCabe, 1977). The moraines at Cranfield and Ballykeel were felt to be the most likely candidates for the terminus of readvance ice, which entered Carlingford Lough from the northwest. However, the pinching out of the high lateglacial shoreline at Ballykeel may simply be the result of coastal erosion removing evidence of its continuation to the east or due to the presence of bedrock at the surface. The truncation of moraine ridges at ca. 30m O.D. at Kilkeel, and at ca. 20m O.D. at Dunnaval confirms this conclusion. It is suggested that the Cranfield moraine marks the terminus of the Carlingford ice lobe.
The topography of the Cranfield moraine records a distinctly lobate form in the ice margin, and sediments at Tullyframe and Glenloughan record outwash processes at the ice front. A similar lobate form has been described for a similar ice lobe centred in Dundalk Bay (McCabe et al., 1987). The processes recorded in the sediment pile at the Sandpiper pit are related to the ice front position in a glacioisostatically depressed basin, where it is likely that the ice mass would terminate in a marine environment (Eyles & McCabe, 1989). This interpretation is supported by the presence of Late-glacial raised beaches immediately outside the Cranfield moraine limit. This confirms that readvance ice entered the late glacial sea. Rapid ice flow into Carlingford Lough is inferred from valley and coastal drumlins in the Warrenpoint area. Massive diamict overlying marine mud on the coast at Cranfield Point records readvance over ice distal muds and provides critical stratigraphic evidence of a major readvance.
The raised beach deposits assist in the definition of the sequence of events during the later stages of deglaciation. Higher beaches are evidence of the lateglacial marine transgression, which accompanied the retreat of the ice sheet from the Plain, as an isostatically depressed lowland area, became available for erosion and inundation. The sediments exposed along the coastline record an emergent sequence of shallow marine and beach environments. The cobble and boulder line at the base of the succession is a boulder pavement containing reworked clasts (Eyles, 1987). Cross-cutting striations on a glacially bevelled upper surface suggests later overriding by pack ice.
Relatively thick low-angle cross stratified sand is typical of the transition between offshore and shoreface zones (Bourgeois & Leithold, 1984). Cross stratification records rapid deposition of sediment eroded from shoreface bars, and hummocky cross- stratification is consistent with rapid deposition from oscillatory flows below fair weather base (Bourgeois, 1980). Small channels are scours which are frequent in this zone of wave build up and rapid changes in flow regime. Interstratification of sand and gravel is typical of lower shoreface (Bourgeois & Leithold, 1984). Stacked sheets of gravel and coarse sand are characteristic of the upper shoreface zone, and are the product of reworking by waves and marine currents (Nemec & Steel, 1984). Overall coarsening upward of gravel culminates in beachface deposits of shape and size sorted gravels. Textural uniformity and lateral persistence of individual beds are typical of this zone (Clifton, 1973). The coarseness of many of the beds is the result of storm events, which winnow finer sediments to produce overbeach deposits. Emergent beach facies exposed in the Kilkeel cliff are evidence for a gradual lowering of relative sea level as isostatic rebound outstripped the input from retreating ice sheets. Truncation of the Cranfield moraine by lateglacial beaches indicates that glacier ice was still present at this site while rebound was under way. Postglacial raised beaches found both within and without the Cranfield moraine indicate that relative lowering of sea level continued after the final deglaciation of the area.
IMPORTANCE
The complex of morainic ridges and outwash sequences of the Mourne Plain demonstrates important geomorphological and glaciological concepts linked to the decay of tidewater ice margins in the Irish Sea. It forms the only well-described terrestrial topographic record of a marine based ice lobe (McCabe et al., 1984). The spatial sequence, sedimentological content and spacing of the morainic, ice marginal sediment piles demonstrate aspects of the ice mass condition and phased nature of generally rapid retreat. The Cranfield moraine contains a stratigraphy indicating a readvance of ice from Carlingford Lough onto the plain. The dating of glaciomarine muds in the sequence using biostratigraphic data will hopefully verify that the readvance was related to a significant readvance of the Irish ice sheet around 15kyr B.P. recorded from other sites around the Irish coastline (McCabe, 1996). The raised beach notches cut into the deposits indicate the influence of a marine highstand in the area synchronous with the readvance event. The Cranfield area represents a critical location for;
1) examining the relation of marine influences upon a terrestrial ice mass,
2) demonstrating the importance of stratigraphy in reconstructing ice mass dynamics, and
3) the relation of the Carlingford readvance to millennial time-scale events in the circum-North Atlantic (cf. McCabe, 1996) which will attach an international importance to the coastal glaciomarine site network in Northern Ireland.
The morphology of the area contains three principal glacigenic topographic elements:
1) Curvilinear recessional moraines tending generally from northwest to southeast are related to late Midlandian ice retreat from the Plain westward into Carlingford Lough. The ridges in the east of the Plain are widely spaced and discontinuous whereas further west they occur as tightly spaced occasionally intersecting or crosscutting ridges and segments.
2) Spreads of proglacial outwash, which occur at lower altitudes in the more extensive inter-morainal lows contain enclosed depressions.
3) A series of shoreline notches and benches are cut into the outwash and moraine ridges by both lateglacial and postglacial marine highstands.
A number of exposures have been chosen to illustrate the range of sedimentological processes, which together have resulted in the morphological diversity of the Mourne Plain. Sections have been selected which display the sedimentological complexity and variability typical of the Plain's three major morphological elements. The landforms are subdivided, classified and described firstly by morphological type, from west to east.
MOURNE PLAIN MORAINES
For site specific information on the Mourne Plain Moraines see the following sites:
Key Site 441 - Cranfield Moraine Key Site 442 - Glenloughan Pit Key Site 443 - Tullyframe Pit Key Site 444 - Moore Lodge moraine
MOURNE PLAIN OUTWASH
The most extensive area of outwash on the Mourne Plain lies between 12 and 18m O.D. and measures approximately 1.6km2 in area. The outwash is triangular in plan view, is bounded to the west and north by the ridge of the Cranfield moraine and to the east by a moraine ridge at Ballynahattan (328400 312200). To the south the outwash is notched by a lateglacial strandline at 19m a.s.l. Large areas of the original surface of the outwash plain have been removed as a result of aggregate extraction.
For site specific information on the Mourne Plain Outwash see the following site:
Key Site 445 - Sandpiper Pit
KETTLEHOLES
A large number of up to 1km2 interconnected depressions (kettleholes) occur 1km to the west of Ballykeel (323300 316800), within a wide undulating spread which forms the eastern side of the Kilkeel River valley. The largest of these kettleholes contained a lake, which eventually drained to the south along a well-marked channel. A depression at Nace (333000 315800) in a low relief spread contains a poorly exposed section consisting of crossbedded sand with occasional fine gravel stringers. The palaeoflow, measured from cross-bedding, was towards 260deg. The top of the exposure is at ca. 30m O.D., the altitude of a late-glacial marine notch, and is in a depression at the landward end of a channel which dissects the marine notch. Isolated enclosed depressions are also found throughout the Plain (e.g. 329600 313200).
RAISED BEACHES
In the area between the Cranfield Point and Ballymartin moraine ridges, the seaward margins of the moraine ridges on the Mourne Plain are cut by up to four late-glacial and postglacial beach notches. Spreads and ridges of sand and gravel are eroded to produce a series of steps subparallel to the current coastline. The most continuous of these notches occurs at 20m O.D. at Dunnaval (2.1km long) and 30m O.D. east of Kilkeel (1.5km long).
It is noted that notches at these elevations are absent from inside the area bounded by the Cranfield moraine.
Between Cranfield Point and Slatemill Road (329300 312500) the undulating terrain is notched at 10m a.s.l., up to 300m inland. This is at a similar height to a raised beach found within the Cranfield moraine, and thus clearly post-dates the moraine. This shoreline is more irregular than the late-glacial notches, and is not as well-marked, probably the result of rapid postglacial isostatic adjustment. A lower notch at 6m a.s.l. is less continuous, and formed an embayed strandline. An almost continuous low ridge (<10m O.D.) trends northeast from Cranfield point for approximately 1km, parallel to the current coastline. Similar landforms occur within the Cranfield moraine in the Soldiers Point and Greencastle areas.
For site specific information on the Mourne Plain Raised Beaches see the following site:
Key Site 446 - Kilkeel Cliff
OVERVIEW
The moraines and associated outwash couplets on the Mourne Plain record stages in the decay of the late Midlandian ice sheet, which had its ultimate source in the Lough Neagh-Irish Midlands axis. As the level of the ice sheet fell below the level of the eastern Mourne, topographic control increased in importance, and it is likely that the Mournes were completely encircled by ice during initial downwastage. Moraines record periods of stillstand during this retreat, and exposures such as that at Moore Lodge indicate that a number of oscillations of the ice front were superimposed on this general pattern.
Charlesworth (1955) identified the Cranfield moraine as a limit of the major readvance which fronts the drumlin swarms of County Down. Alternative limits for the readvance have been presented by Stephens and McCabe (1977) based on the existence of lateglacial shorelines between certain moraine ridges. These lateglacial shoreline notches are the result of the interplay of glacioeustatic and glacioisostatic processes (Stephens & McCabe, 1977). The moraines at Cranfield and Ballykeel were felt to be the most likely candidates for the terminus of readvance ice, which entered Carlingford Lough from the northwest. However, the pinching out of the high lateglacial shoreline at Ballykeel may simply be the result of coastal erosion removing evidence of its continuation to the east or due to the presence of bedrock at the surface. The truncation of moraine ridges at ca. 30m O.D. at Kilkeel, and at ca. 20m O.D. at Dunnaval confirms this conclusion. It is suggested that the Cranfield moraine marks the terminus of the Carlingford ice lobe.
The topography of the Cranfield moraine records a distinctly lobate form in the ice margin, and sediments at Tullyframe and Glenloughan record outwash processes at the ice front. A similar lobate form has been described for a similar ice lobe centred in Dundalk Bay (McCabe et al., 1987). The processes recorded in the sediment pile at the Sandpiper pit are related to the ice front position in a glacioisostatically depressed basin, where it is likely that the ice mass would terminate in a marine environment (Eyles & McCabe, 1989). This interpretation is supported by the presence of Late-glacial raised beaches immediately outside the Cranfield moraine limit. This confirms that readvance ice entered the late glacial sea. Rapid ice flow into Carlingford Lough is inferred from valley and coastal drumlins in the Warrenpoint area. Massive diamict overlying marine mud on the coast at Cranfield Point records readvance over ice distal muds and provides critical stratigraphic evidence of a major readvance.
The raised beach deposits assist in the definition of the sequence of events during the later stages of deglaciation. Higher beaches are evidence of the lateglacial marine transgression, which accompanied the retreat of the ice sheet from the Plain, as an isostatically depressed lowland area, became available for erosion and inundation. The sediments exposed along the coastline record an emergent sequence of shallow marine and beach environments. The cobble and boulder line at the base of the succession is a boulder pavement containing reworked clasts (Eyles, 1987). Cross-cutting striations on a glacially bevelled upper surface suggests later overriding by pack ice.
Relatively thick low-angle cross stratified sand is typical of the transition between offshore and shoreface zones (Bourgeois & Leithold, 1984). Cross stratification records rapid deposition of sediment eroded from shoreface bars, and hummocky cross- stratification is consistent with rapid deposition from oscillatory flows below fair weather base (Bourgeois, 1980). Small channels are scours which are frequent in this zone of wave build up and rapid changes in flow regime. Interstratification of sand and gravel is typical of lower shoreface (Bourgeois & Leithold, 1984). Stacked sheets of gravel and coarse sand are characteristic of the upper shoreface zone, and are the product of reworking by waves and marine currents (Nemec & Steel, 1984). Overall coarsening upward of gravel culminates in beachface deposits of shape and size sorted gravels. Textural uniformity and lateral persistence of individual beds are typical of this zone (Clifton, 1973). The coarseness of many of the beds is the result of storm events, which winnow finer sediments to produce overbeach deposits. Emergent beach facies exposed in the Kilkeel cliff are evidence for a gradual lowering of relative sea level as isostatic rebound outstripped the input from retreating ice sheets. Truncation of the Cranfield moraine by lateglacial beaches indicates that glacier ice was still present at this site while rebound was under way. Postglacial raised beaches found both within and without the Cranfield moraine indicate that relative lowering of sea level continued after the final deglaciation of the area.
IMPORTANCE
The complex of morainic ridges and outwash sequences of the Mourne Plain demonstrates important geomorphological and glaciological concepts linked to the decay of tidewater ice margins in the Irish Sea. It forms the only well-described terrestrial topographic record of a marine based ice lobe (McCabe et al., 1984). The spatial sequence, sedimentological content and spacing of the morainic, ice marginal sediment piles demonstrate aspects of the ice mass condition and phased nature of generally rapid retreat. The Cranfield moraine contains a stratigraphy indicating a readvance of ice from Carlingford Lough onto the plain. The dating of glaciomarine muds in the sequence using biostratigraphic data will hopefully verify that the readvance was related to a significant readvance of the Irish ice sheet around 15kyr B.P. recorded from other sites around the Irish coastline (McCabe, 1996). The raised beach notches cut into the deposits indicate the influence of a marine highstand in the area synchronous with the readvance event. The Cranfield area represents a critical location for;
1) examining the relation of marine influences upon a terrestrial ice mass,
2) demonstrating the importance of stratigraphy in reconstructing ice mass dynamics, and
3) the relation of the Carlingford readvance to millennial time-scale events in the circum-North Atlantic (cf. McCabe, 1996) which will attach an international importance to the coastal glaciomarine site network in Northern Ireland.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Mourne Plain Complex - Overview, County Down, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=440. Accessed on 2024-12-26 |
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