Summary of site:
The biological interest of the intertidal flats of Strangford Lough is well known. They provide extensive areas of feeding for migratory and resident birds and the sediment is host to a rich and varied invertebrate fauna. Despite this importance, remarkably little is known about the sediments themselves so, unfortunately, this account is limited to observation alone.
The lough is approximately 20 km long and about 9 km wide at its widest point. It is a sea lough fed through a single tidal channel 5 km long and about 1 km wide, the Strangford Narrows, entering its south east corner. The maximum tidal range is something over 3.5 m. The intertidal flats are most extensively developed in the northern half of the lough from the area just south of Greyabbey, extending up the east shore to just south of Newtownards in the extreme north and down the western shore where they form a 2 km belt extending as far south as Ardmillan.
The local landscape setting is classic drumlin “basket of eggs” topography, the drumlins standing predominantly on intensely folded Silurian greywackes, mudstones and sandstones relieved by a thin strip of Triassic Sherwood Sandstone Group rocks forming the eastern shore to just south of Greyabbey.
The two main factors determining the nature of the flats are waves and currents. The waves are generated by winds entirely within the lough since the Narrows are too confined to contribute significantly to wave energy. The orientation of the lough is roughly north/south with a prevailing south westerly to southerly wind direction. This configuration protects the western shore but with a fetch (the uninterrupted distance across open water) of 15 km, large waves can be raised in storm conditions, pounding the exposed, eastern shore.
The currents are at their most powerful in the Narrows and dissipate rapidly in the wider expanse of water between Killyleagh and Portaferry. A central current retains its identity to a line across the lough from Ringhaddy on the western shore but beyond that, in the area of extensive flats, currents are much weaker with only local effect.
The sediment shaped by these forces is presumed to have been derived from the deposits left behind as the ice sheets left the area, largely drumlins. The most degraded drumlins are on the eastern shore, some reduced to island remnants, others with steep, sapped western flanks testifying to a long history of erosion. The tidal flats created by these forces now cover an area of around 50 sq km.
At Greyabbey a surviving remnant of a once more extensive marsh passes seaward into the coarse sand and pebbly sheets forming the upper tidal flat which grades downwards into the main sand flat, dissected by small tidal channels. Current and wave ripples texture the surface of these deposits. At low spring tides peat and tree stumps are exposed revealing an earlier period of low sea level. There is evidence that the sand flats at Greyabbey are no more than a veneer overlying a red marine clay formed during the late glacial period.
The flats are at their most extensive between Newtownards and the mouth of the Comber River on the western shore. In this area they form a fringe 2 km wide backed in places by remnants of previously more extensive salt flats, now seriously eroded, forming an obvious scarp. Immediately below the scarp there is a high-tide beach of coarse sand grading outwards into sandy intertidal flats showing, at low tide, characteristic features of emergence, as water is shed over a large, almost flat area. These are largely complex and special ripple patterns, rills and miniature deltas. Remains of earlier, higher beaches can be seen in the area, one described from Rough Island east of Ringcreevy.
The flats in the Ardmillan area are muddy and the construction of causeways linking the drowned drumlins of Mahee Island, Nendrum, Watch Hill, Cross Island and Reagh Island with the western shore have drastically blunted the natural current flow and its transporting capacity in the area. However, there does appear to be seasonal erosion and deposition of soft mud.
In general throughout the lough, the more exposed the shore, the coarser the sediment as might be suspected.
The important sedimentary environment of the lough flats is crying out for research as are the drowned peats and woodland and the raised beach facets all of which offer great potential in the elucidation of past climates and sea level changes in the area.