|Site Type: ||Coastal section|
|Site Status: ||ASSI, NNR|
|District: ||Down District Council|
|Grid Reference: ||J410350|
|Rock Age: ||Quaternary (Holocene, Midlandian Undifferentiated)|
|Rock Type: ||Gravel, Sand|
|Other interest: ||No data, Coastal processes, Estuarine deposits, bar, decalcified sands, gravel barrier, mudflat, sand dune, spit, tidal flat|
Summary of site:
The Murlough Complex consists of a lagoon behind two tracts of sand dunes. Four rivers drain into the lagoon, from south west to north east the Carrigs, the Moneycarragh, the Ardilea and the Blackstaff. It is this drainage that breaches the dune front through the Dundrum Channel, separating the Murlough dunes to the south west from the Ballykinler dunes to the east. As well as the inner bay (the lagoon), the outer face of the dunes forms the coastline of Dundrum Bay (the outer bay), an important sedimentary component of the system.
The complex is described in these four components, the inner bay, the Murlough dunes, the Ballykinler dunes and Dundrum Bay.
The lagoon is a tidal estuary with mud and sand flats. The Dundrum Channel separates two arms, the southern, about 2 km long and 500 m wide at its widest and the larger northern, about 3 km long and 1 km wide. Both arms are blanketed in sand and mud flats, only covered at high tide. The northern flats flood first and the south western flats later, the flood tide's flow restricted by its passage under the Downshire Bridge. Even at low tide the rivers retain water and most of the fresh water enters the southern arm of the system from the Carrigs and Moneycarragh rivers.
The Murlough dunes extend from the Shimna River, entering the sea through the south central part of Newcastle, for 5.5 km to the Dundrum Channel and are 1.5 km wide at their widest point near Murlough Farm at the north eastern end. The foundation of this area is a series of gravel ridges, running sub-parallel with the coastline, with recurved tips that can be seen in the back beach behind the dunes along the eastern shore of the southern limb of the lagoon. 12 ridges have been identified, highest at the back of the system where they achieve elevations between 8 and 9 m, descending to 5 m on the modern coastline. At the southern end the gravel particles are disk-shaped and to the east they are covered by a thin band of pale clay. Nearer the tip of the system, adjacent to the Dundrum Channel, they are more spheroidal and include local igneous rocks as well as altered Silurian sediments from the hinterland.
Immediately above the gravel ridges there are sand dunes inclined towards the sea. They range in height from 9 to almost 25 m above sea level. Enclosed within them are at least three buried soils which have been carbon dated from charcoal associated with arch'ological remains. The earliest are almost 4,800 years old, the others falling into the 2,000 years BP (before present) range.
Down the centre of the system is a series of 13 parabolic dunes, all more than 30 m high. Parabolic dunes are long, scoop-shaped hollows in the sand with their pointed ends to windward and they originate in sands with plant cover where the wind removes sand from hollows on the windward side and deposits it in the lea of the back slope. At Murlough they are aligned north/south with the prevailing wind. It is suggested that crescent-shaped dunes (called barchans), horns downwind, migrated inland from the shore, became stabilised by vegetation and the parabolic dunes formed in them as blow-outs. Some of the blowouts are deep enough to expose the gravel ridge foundations and form valleys from 300 to 500 m long. From medi'val arch'ological finds it can be shown that at least some of the parabolic dunes formed after 650 BP. In all cases the sand of the dunes is extremely fine with less than 3% shell content.
On the fringe, immediately behind the modern coastline, there are recent dunes that do not appear to be actively growing now and, although there was some dune development between 1969 and 1979, there has been nothing since. In fact there is severe erosion of dunes at the Dundrum Channel mouth and a spit near Murlough House has been breached.
The Ballykinler dunes show strong similarities with the Murlough system, particularly in their three morphological divisions. To the north on the landward side there is a flat plain between 7 and 12 m above sea level interpreted as a raised beach (caused by the rising of the land following the loss of its ice load at the end of the last glaciation) though there are no exposures of the underlying deposits so no certainty that the gravel foundation of the Murlough dunes is carried over to the east. The recurved spit of the dunes at the north end of the Dundrum Channel appears to have a cover of original dunes similar to those to the south west, rising to over 17 m in height. Along the shore of the inner bay there are gravel ridges with disk-like particles, evidently different from those on the other side of the channel, consisting largely of the local Silurian greywackes and sandstones. As at Murlough, a thin layer of clay drapes these deposits and is overlain by windblown sands containing two buried soils with shells and charcoal, clearly exposed near the tip of the spit. The south eastern section of the area, south of Tyrella House, is very low lying and near the water table. This creates wet areas, called slacks, between small dunes.
The south western triangle fronting the Dundrum Channel and the coast has a very different character. Here a complex dune field with individual dunes topping 30 m matches that in the Murlough complex with migrating dunes divided by long blowout valleys but differs in the nature of the valley floors which do not expose gravel ridge foundations but are smothered in shell debris near the coast, grading to sand further inland. Their alignment is south west to south south west. Another contrast is the sand in this system which is coarse with a high shell content. The good vegetational cover of the Ballykinler dunes renders them very stable.
Again, unlike the Murlough system, new dunes are developing at the south west tip near the mouth of the Dundrum Channel where three dune ridges have formed on the edge of the delta.
Dundrum Bay, the outer bay, is entirely sand dominated and boreholes drilled on the lower beach near the Slieve Donard Hotel show sand to almost 10 m depth. The sands are well sorted (of similar grainsize) slightly more variable on the beach than in the sea. The average tidal range in the bay is around 4 m rising to 7 m at exceptional spring tides. The sand is thought to be largely of glacial origin, washed off adjacent land over 10,000 years and reworked by the sea ever since.
The beach has a low angle and varies in width between 500 and 1,500 m. This shallowness, over hundreds of metres, dissipates wave energy under normal conditions long before it reaches the shore. There is a large delta formed by the ebb tide draining from the Dundrum Channel. Breaking waves create swash bars in the sand, usually parallel with the coast and over time they migrate inshore eventually stranding on the back of the beach on both dune systems although, at present, more commonly at Ballykinler. New dunes may originate from these deposits.
On the Murlough side of the channel the beach shows a ridge and runnel structure. The ridges normally parallel the coast although they can be diagonal and up to 5 can be seen at low spring tides.
At the back of the beach there is a gravel ridge that reflects storm waves although it is sometimes obscured by sand in the summer months. Human interventions attempting to protect sections of the Murlough system have not proved predictable and generally have resulted in erosion and longshore drift from the south west to the north east. In spit-like structures (long, narrow beach extensions attached to the shore at one end), with no new sand entering the system, the effect is to remove sediment from the base of attachment and to increase deposition at the tip and that appears to be the case here. There is a near-shore bar across the delta opposite the Dundrum Channel which could represent sands drifting to the north east from the Murlough system, migrating across the channel, to the Ballykinler shore.
Although it has been suggested that the sands of Dundrum Bay may form a closed recirculating system, no evidence has been collected to confirm or eliminate the possibility.
There is no overall synthesis of the evolution of the Murlough Complex but there are a number of observable features reflecting important events. The inaugural process, setting the baseline for the area, is taken as the departure of the ice after the late re-advance of the ice sheet that smeared the drumlins and related deposits across the hinterland. With the land still depressed from the loading effect of the ice, the sea would have reworked this considerable cache of glacial debris to generate a plentiful sediment supply which almost certainly incorporated material from the bed of the North Channel. Whether the recurved gravel ridges of Murlough date from this initial marine incursion is not known. A wave-cut notch in Dundrum village at 14 m above sea level is believed to mark the maximum height of the sea during these late glacial times and is dated at 15,000 years before the present. There is an undoubted spit structure to the dune system but how and when it was initiated is far from clear as is the influence of the falling sea level as the land eventually began to inch upwards in response to its unloading. Whenever they formed, the spits enclosed the inner bay providing the conditions for the accumulation of the sand and mud flats.
Much research remains to be undertaken in this Area of Special Scientific Interest, especially in the neglected dunes of Ballykinler. A systematic approach to the post-glacial history of events is an evident and major problem that may yet yield to investigation.
The complex is, in general, robust but proposals to protect the coast with defenceworks should be resisted. Their history in this area has proved neither predictable nor beneficial.