| The Dundrum Bay complex includes the two dune systems at Murlough and Ballykinler, which separate Dundrum Inner Bay from the Outer Bay in the northwestern Irish Sea.|
| The Murlough and Ballykinler dune systems are based on a complex area of raised marine gravel barrier ridges formed across Dundrum Bay, from re-worked glacial material, in the late Pleistocene and early Holocene. Interpretations of the development of the marine gravel features of Murlough have been used in the elucidation of sea-level changes in the north of Ireland.|
| The gravel ridges are overlain by primary prograded aeolian dunes of early-Holocene age, the surface of which show long periods of stability indicated by the presence of well developed palaeosols. The seaward extent of these ancient dunes became buried under an extensive transgressive dune sheet and parabolic dunes, probably in the early Mediaeval period. Limited modern development of dunes continues by localised reworking of sediments associated with the development of migrating swash bars on the ebb-tide delta of the channel.|
| Dundrum Outer Bay is a wide gently shelving bay with a meso/ macro-tidal and low to moderate wave energy environment. The bottom sediments indicate a closed system with little or no modern supply and are considered relict and of glacial origin.|
| Both Ballykinler and Murlough have a low angle dissipative wide sand beach. The Murlough foreshore is noted for the well developed ridge and runnel beach system which is less well developed on the Ballykinler shore. An extensive ebb-tide delta occurs at the mouth of the channel from the Inner Bay at Dundrum, on which swash bars periodically form and migrate inland until stranded on the back shore of either system. A nearshore bar occurs at the mouth of the Dundrum channel.|
| The sand beach along much of the Murlough system is backed by a reflective gravel ridge or storm beach, of variable longshore extent, not normally exposed at either proximal or distal ends of the spit. An exposure of well sorted and closely laid gravel pavement occurs at the mouth of the Shimna river in Newcastle.|
| Dundrum Inner Bay is an estuarine lagoon, connected to the Outer Bay by a tidal channel. It is the modern remnant of a previously more extensive estuary.|
| The Dundrum Bay complex includes four units; the Murlough and Ballykinler dune systems, and Dundrum Outer and Inner Bays.|
| Dundrum Bay, situated on the SE coast of Co. Down, extends from St. John's Point in the east (J527336) to the Mourne Mountains in the SW. That area between Craigalea rocks (J448350) and the mouth of Glen River, Newcastle (J376305) is included in this report.|
| The Murlough System lies between Dundrum Outer and Inner Bays, extending from Newcastle to the Dundrum channel connecting the two bays (J418348). The full extent of the system, includes an area presently managed as the Royal Co. Down Golf Course, at the SW, Newcastle end, (J385325 approx centre) and the Murlough NNR at J405345 (approx centre). The Murlough system extends from the Shimna river, it is 5.5km long and 1.5km wide at the widest point at the northern end. It is backed to the NW by the intertidal waters of Dundrum Inner Bay and by the low-lying land of the Maghera Plain to the SW.|
| The Ballykinler dunes are opposite to Murlough across the Dundrum channel at the northeastern end of Dundrum Bay (J425355 approx centre). They are backed on the northern shore by Dundrum Inner Bay, and by land of the Lecale peninsula, at the village of Ballykinler (J435365). They are 2.5km long and 1.6km wide.|
| Dundrum Inner Bay is a medium sized estuary with extensive tidal mud and sand flats, fed by four small rivers. It lies to the landward side of the Murlough and Ballykinler dune fields, which in the past formed across Dundrum Bay confining an area of inter- tidal estuary. The former extent of the estuary under higher sea levels is unconfirmed.|
| Background: The first recognition of the complexity of development of the Murlough/Ballykinler area was given on the Geological Survey map of 1871, where the presence of raised beaches, or ancient sea bay was indicated, landward of both Ballykinler and Murlough dune systems, together with the representation of the gravel ridges in the Murlough System.|
| The area first became of interest in the 1950-60's, when Stephens carried out survey and investigative work on the geomorphological development of the distal end of the Murlough System particularly the raised gravel ridges in the area which later became Murlough NNR. He also mapped the limits of marine transgression on the Maghera Plain interpreting them, as well as the gravel ridges, as of mid-Holocene date related to the maximum marine transgression considered to be between 6000-5000 B.P. Stephens published little detailed information from these investigations though general comments can be found in Davies and Stephens (1978). Nevertheless the work, together with investigations of the buried soils of Murlough (Cruickshank, 1980), and data accumulating from archae- ological evidence and 14C datings influenced Stephens in his interpretation of Holocene sea level changes on the east coast of Ireland. (Mitchell & Stephens, 1974; Stephens & McCabe, 1977).|
| Stephen's interpretation of the development of the Murlough System as a spit structure, briefly described by Whatmough (1976) and Shepherd and Whatmough (1980), remained extant until questioned by Orford (1981). Here the possibility of a more complex development beginning in the late-glacial period was suggested, as were possible alternative interpretations of the structure of the Murlough System. Little further systematic study has been carried out since.|
| No recorded information is available for the structure and development of the Ballykinler Dunes. Orford (1981) highlights the lack of knowledge of the origin of the Ballykinler Complex, but assumes it to have a structure similar to that of the Murlough System.|
| The sediments of Dundrum Outer Bay are described by Erwin et al. (1978), who also speculated about the origin of the sediments and their age. Several levelling surveys of the foreshore adjacent to Murlough NNR have been carried out but few published (Adcock, 1973; Graham 1971; Hannon, 1970).|
| Physical characteristics of the Inner Bay are briefly summarised by Roberts, Montgomery and Ellis (1989) as is hydrological information in Roberts and Montgomery (1989). The former extent of the northern Inner Bay was investigated by Hood (1986).
| I - MURLOUGH SYSTEM|
| The northern shoreline of the Murlough System, from Slidderyford Bridge to north of Keel Point Bridge, exhibits a series of small arcuate bays related to horn-shaped promontories. These appear to be recurved ends of a fan-shaped series of ridges with low relief, lying at the back of Murlough Farm (3-4m O.D), which were visible on aerial photographs taken in 1978 following a period of prolonged drought. Cliff exposures on the Inner Bay shore near to the Downshire Bridge show asymmetrical ridge structures in sub- parallel dip section, which have a gravel dominated core of disk- shaped clasts. Farther to the east similar low-lying gravel deposits are overlain by a band of pale clay of unknown origin, which is itself overlain by aeolian sands of shallow depth.|
| Seaward of this fan lies a ridge of low sand dunes, 6-9m O.D., through the centre of Murlough Farm. This trends to the NE, and joins with higher dunes which form a strongly recurved point at the N point and which lie on a gravel basement. Sections through these deposits show a more spheroidal gravel, with no overlying clay, but well developed podsolised surfaces with iron pans.|
| The main gravel ridges of Murlough NNR, together with their extensive dune cover of complex topography, extend from the Mourne coast at Newcastle, trending SW-NE. From Slidderyford northward they lie to the SW of the Murlough farmland which has long been under cultivation. This area of low relief appears free from both beach gravel development and significant sand dune cover, and was considered likely to have been originally an area of salt-marsh. An exploratory bore hole (Orford, 1981) revealed a light sand cover of sheet form overlying sediments more typical of a fluvial environment, possibly of a flashy discharge, or a migrating river system, and no evidence of marine clays.|
| Gravel barrier: Mitchell and Stephens (1974) identify a descending series of 12 sub-parallel gravel ridges underlying the main dune system, trending SW-NE. The height of the ridges exposed in dune blowout floors lie at +8-+9m O.D. at the rear of the system and at a maximum of +5m O.D. close to the sea. These ridges are well exposed in the proximal and middle section of the system. The recurved ends of some of these ridges can be seen at Slidderyford and behind Murlough Farm. Few exposures occur in the distal part of the system where they are limited to the bases of deep blowout hollows. Gravel is also exposed in cliffed dune sections along the northern shore of the system at the North Point, extending to the boat house.|
| The edge of the system in the Outer Bay is a storm gravel beach up to 4m in height forming the upper part of the foreshore. The longshore extent of this beach varies but is not normally exposed at either distal or proximal ends of the system; its exposure has however been progressively extending northwards in recent times.|
| The gravel ridges are clast-supported, composed of spheroidal clasts derived from a wide range of geological origins, dominated by Silurian greywackes and hornfels, Tertiary granites and porphyries from the Mournes and Newry granodiorites. Numerous erratics also occur; of particular note are Ailsa Craig micro- granite, Cretaceous flint nodules and Tyrone granites. Shape and size sorting is evident across the gravel ridges; flat disks are uncommon. Shepherd and Whatmough reported a decrease in pebble size in the old ridges along the system from SW-NE, indicating longshore drift in the development of the gravel ridges.|
| Sand dunes: The landward dunes are typical of a primary low-angled, seaward dipping, sand sheet, of low relief ranging in altitude between 9- 18m O.D, with one dune at 24.5m O.D. The surface of these dunes show well developed soils, particularly brown calcareous sands, brown podzols and ferric podsols, present as both ground soils and buried palaeosols (Cruickshank, 1980). The altitude of the buried palaeosols declines to 4.5m at the seaward edge of the sheet, where it is overlain by dunes more typical of a transgressive sand sheet. 14C dates from charcoal associated with archaeological material and incorporated within these palaeosols range from 4775+/-140 B.P. to 600 B.P, though the majority of sites have given dates prior to 2000 B.P. (Table 1 Carter, 1982) and are found at ~14m O.D. Three separate layers of palaeosols can be identified representing up to three periods of prolonged stability in the dune system.|
| Edwards (1978) investigated the palynology of the buried soil horizons, but found them inconclusive due to the poor state of preservation of the pollens. He also found that major differences exist between soil horizons, with some indicating graminaceous vegetation. An Ericacea/Rosacea/Artimisia assemblage was found at one site with a radiocarbon date of 2825+/-14 B.P. (UB412).|
| A series of 13 high parabolic dunes occur down the centre of the system, which are morphologically typical of dunes formed on a secondary transgressive dune sheet. All but one occur in Murlough NNR. They all exceed 30.5m in height, with the highest at 37.5m. They are oriented on a N-S axis, with the original windward erosion face indicating southerly winds to be the major erosional force. This corresponds to the longest fetch for the wind of over 1300km, to the Bristol Channel. The present form of these high dunes indicates the inshore migration of barcanoid dunes and transverse blowouts, probably reworking the original sediments and overwhelming the landward prograded dune sheet to the north and exposing the underlying gravel beaches to the south. The valleys so created rarely exceed 300m in length, with one some 500m. As the buried palaeosols can be located in the bases of many of the erosion faces of these parabolic dunes and residual knolls, the extent of the primary dune system appears to have been at least as extensive as the modern dune field. Pottery and coins incorporated within the surface of the buried palaeosols, indicate the formation of the overlying parabolic dunes to post date the early Mediaeval period 650 B.P.|
| The sediments of both sets of dunes are very similar, averaging 2.2-2.4 phi (Adcock, 1973) with a very low shell content of 2-3%.
The coastal edge of the system is characterised by an irregular series of low modern dunes and foredune ridges, none of which are presently actively accreting sediment. Nairn (1977) compared maps dating from 1837 to 1974, and showed that the development of the Murlough House Spit is of recent origin, and shows periodic accumulation and transport of sediment at the channel end of the beach. Embryo-dune and foredune development on the Murlough shore last took place between 1969-79, following the migration inshore and stranding of a swash bar (Hannon, 1970). At present the dunes at the channel mouth are suffering from severe erosion, with the possibility of the distal end of the Murlough House Spit being breached. The development of the dunes on the Murlough side of the Dundrum Channel are closely influenced by the position of the river channel and development of swash bars on the Ballykinler shore.
There is some evidence for the development of two earlier recurved spits, at the North Point and Boat-House Point.|
| II - BALLYKINLER DUNES|
|The dune field at Ballykinler can be divided into three morphologically distinct units, similar to those of Murlough. At the landward side lies a flat plain, identified as a flat terrace of raised beach (GSI, 1871), lying between 7.5-12.5m O.D. Nothing is known of the sediments or structure of this area. It has been under agricultural use until recent times.|
|The northwestern end of the area, including what appears to be a large recurved spit end at the northern end of Dundrum channel, has a low relief similar to the primary dune field at the northern end of Murlough, ranging from 10-17.5m O.D. Cliff sections exposed along the Inner Bay shoreline show a series of southerly dipping marine gravel beds with clasts aligned and sand supported, over- lying marine sands with scattered gravel inclusions. The clasts appear to originate from a different population to those of the main gravel ridges of Murlough, being composed of flat disks with greywacke and sandstones predominating, with some granodiorites. The gravel bars are overlain by a layer of clay, similar to that found at Murlough, which is further overlain by aeolian sands with two well developed palaeosols and a podsolised ground surface similar to those found at Murlough. Localised exposures of marine shells and charcoal, probably indicating past human activity, are found incorporated in the podsols at the tip of the spit.|
|The southeastern end of the system is also of low relief, with wet slacks exposed between the low dunes. The southwestern triangle of the dunes is of a very different character being a topographically complex dune field, in which the highest dunes attain heights of 27.5-32m O.D. As on Murlough this is characteristic of a trans- gressive dune field, with long valleys created by the retreating dunes. The deflation surface flooring these valleys is of a different character to those of Murlough, grading from shelly small gravels at the seaward end of valleys, to sand further inland. There is no evidence of underlying gravel bars similar to Murlough. The dune valleys at the landward and western ends of the system are wet slacks where the water table is exposed. The align- ment of the dune valleys is SSW-SW. The sand incorporated within this dune field is coarse and shelly, unlike the fine shell-free sands of Murlough.|
|The dunes of Ballykinler are near fully vegetated and very stable. There is little opportunity to study dune sediments, and no exposure of underlying palaeosols can be found. The coastal dunes are formed from shell-rich sands, overlying and grading to fine sands in the central dune area. Dune development continues at the south western tip of the system and along the shoreline of the Outer Bay. At least three dune ridges have formed around the dune edge on the ebb-tide delta in recent times.|
|III - DUNDRUM OUTER BAY|
|Erwin et al. considered the primary source of sediment in the Bay to be of glacial origin, either as meltwater channels or morainic deposits. The sediments adjacent to Ballykinler and Murlough are sand dominated. The beach sands are mesokurtic fine sands. The sublittoral sands are well sorted leptokurtic and very fine, or poorly sorted very fine sands. Ripples extend to 10m depth.|
|The tidal range in the bay is 3.9m at mean springs, but extends to 6.9m at extreme low spring tides. The littoral zone of the Bay is a wide low angle dissipative beach ranging from 400-1500m in width on the Murlough shore. An extensive ebb-tide delta is associated with the mouth of the Dundrum channel, draining Dundrum Inner Bay. Swash bars periodically form at the mouth of the channel and migrate inland until stranded on the backshore of either system. Protection thus given to the upper shore initiates deposition of sediments and the development of embryo dunes. The stranding of such a bar is described by Hannon. They are more normally found on the Ballykinler side of the delta. A nearshore bar lies across this delta.|
|The Murlough beach is noted for a well developed ridge and runnel beach formation. Up to five ridges may be exposed at extreme low spring tides. These normally lie parallel to the shore but may become oblique. Sediment size of the sands of the beach has been found to vary between 131-136.6 u (Hannon, 1970) and 1.60-300 phi (Adcock, 1973) with 5-6% shell content. Borehole data GSNI D 61/ 233-235, taken from holes to -16.40m in depth, located down the beach at the Slieve Donard Hotel pipeline, show the underlying sediments to be mostly unsorted sands and gravels with numerous cobbles. A very soft blue/grey clayey silt occurs between -9.75 to -11.3m at the top of the beach and at -7.3m to -9m at mid beach level. No datum points are given.|
|The sand beach of much of the Murlough System is backed by a reflective gravel ridge of variable longshore extent, but not normally exposed at either the proximal or distal ends of the system. An exposure of well sorted and closely laid gravel pave- ment occurs in association with the mouth of the Shimna River in Newcastle. Map evidence suggests that this is a permanent feature. An exposure of poorly sorted gravels to cobbles overlying an iron- stained lithified base occurs periodically at the southwestern end of the beach, extending from the Glen River northeastwards along the beach adjacent to the Royal Co. Down Golf Course. Though this would seem to be a recent phenomenon, exposed by the disappearance of beach sands at Newcastle, map evidence suggests it to be the more normal state of the beach. Transient partial sand ridges overlie these exposures in the summer months. Man made structures constructed over the last 150 years at Newcastle, above MHW will undoubtedly have affected the natural coastal processes and sediment transport at the proximal end of the Murlough System. Beach protection works e.g. the construction of a set of groynes, and marram planting were carried out by the golf club in the 1930s as a response to erosion caused by the downdrift scour from the concrete sea wall constructed in front of the Slieve Donard Hotel.
The present day sediment transport patterns within Dundrum Bay are unknown; it is probable that the bay is a closed sediment cell, with little or no new sediment being supplied. Given a stationary or rising sea level, together with longshore drift, but a lesser supply of sediment, it seems likely that the existing sediments will be reworked to satisfy sediment transport requirements, with the consequence of shoreline erosion taking place. Spit-type structures are particularly vulnerable, in which the proximal zone becomes cannibalised and narrow, while the distal end acts as a sediment sink and becomes wider. Recent patterns of erosion and deposition on both the Murlough and Ballykinler beaches indicate that this may well have been taking place. Monitoring of the Newcastle/Murlough beach by repeated beach profiling is presently under way as part of an EC project Ecopro. It is possible that the nearshore bar associated with the ebb-tide delta is transporting sediment between the two sides, but no information on this exists. A circulatory sediment transport system in the bay could also have developed, but no information is currently available on this.
IV - DUNDRUM INNER BAY
Dundrum Inner Bay is a fairly typical medium sized estuary with extensive tidal mud and sand flats. It has two arms which extend on a SSW-NNE axis behind the Murlough/Ballykinler dune systems. The larger northern part is ~3km long and up to 1 km wide. The smaller southern arm extends for ~2km to the SW and is up to 0.5km wide, draining to the Outer Bay by a channel. The southern arm of the bay is constricted to a narrow channel by the construction in 1874 of a causeway linking the Murlough System with a rocky promontory on the landward shore at Keel Point. Much of the bay is high level sand and mud flats, covered only by the highest parts of the tide. The main river channels however retain water at the lowest tides. The average depth of the system ranges from 0.5-6m at HWST. The incoming tide tends to fill the northern bay at the early stages with the southern bay being covered only at the later stages of the tide, where the tidal movement is impeded through the narrow channel at the Downshire Bridge.
Most of the fresh water enters the southern Inner Bay. Four rivers drain into the bay, the major freshwater input is from the Carrigs and Moneycarragh Rivers. The Blackstaff and Ardilea Rivers input combined is estimated to be 17.7% of the total freshwater input. The Blackstaff River has well developed meanders with the develop- ment of ox-bow lakes, one of which has recently been in-filled.
No fully investigated explanation of the origin and structure of the Dundrum Bay Complex is available. Past attempts have looked only at parts, particularly of the Murlough System and the Maghera Plain, with little scientific note being taken of the Ballykinler System. The time datum for morphological development of the Dundrum Bay Complex is usually taken as being after the last major re-advance in the Midlandian ice, with the subsequent development of Murlough and Ballykinler Systems placed in the context of Ulster's isostatic and eustatic variation.
The "Drumlin Re-advance" of Stephens and McCabe (1977) left extensive drift deposits across south Co. Down which were concentrated in the present day Dundrum Bay. These heterogeneous deposits, when reworked provided the abundant sediments for the beach/dune deposits of the complex. The presence in the Murlough gravel ridges of glacial erratics such as Ailsa Craig micro- granite and abundant flint nodules also suggests that glacial debris from the Irish Sea Basin ice has been incorporated in the system. The wave-cut notch/platform at 14m O.D., found through Dundrum village, is taken to be the possible maximum transgression limit of the Late-Glacial ~15,000 B.P. (Stephens & McCabe, 1977) and is regarded by them as probably the lowest of the Late-Glacial shorelines in SE Ulster.
Mitchell and Stephens (1974) considered the formation of Murlough to be a spit structure developing as a drift aligned prograding beach gravel plain overlain by storm ridges, which they relate to falling sea level following the maximum mid-Holocene transgression dated by them to 6000-5000 B.P. The progressive elongation of the spit was responsible for the development of estuarine conditions on the Maghera Plain with later infilling with estuarine and alluvial deposits. 14C dates of 4770 and 3625 B.P. in the over- lying dunes gave a minimum time for the ridge formation. No interpretation of the landward structure of Murlough, or the development of the Ballykinler System was given.
The Dundrum Bay complex of structures covers a prolonged period of coastal development reflecting the sea level changes from the late Pleistocene, following the withdrawal of the late Midlandian ice sheet from NE Ireland at 18,000 B.P., to modern times. The modern coastal processes continue to be of considerable interest, but mans interference could jeopardise the natural dynamic equilibrium of the area which should be allowed to continue to naturally readjust to change.|