Tyrella Coastal Dunes Geodiversity Profile

Outline Geomorphology and Landscape Setting

The use of a cultural overlay in defining Landscape Character Areas (LCAs) means that they frequently subdivide natural physiographic units. It is common therefore for significant geomorphological features to run across more than one LCA. It is also possible in turn, to group physiographic units into a smaller number of natural regions. These regions invariably reflect underlying geological, topographic and, often, visual continuities between their component physiographic units, and have generally formed the basis for defining landscape areas such as AONBs. It is essential therefore, that in considering the 'Geodiversity' of an individual LCA, regard should be given to adjacent LCAs and to the larger regions within which they sit. In the original Land Utilisation Survey of Northern Ireland, Symons (1962) identified twelve such natural regions.

This LCA lies within the region described as the Uplands and Drift Covered Lowlands of Down and Armagh. The generally subdued relief associated with the underlying basement complex of highly folded Palaeozoic strata provides the unity of this region. Relative relief is provided in the north by the Silurian hills that overlook the lower Lagan Valley, The Newtownhamilton Plateau in south Armagh, the Caledonian igneous complex of Slieve Croob and the structural depression that underlies and defines Strangford Lough. Below ca 350m, there is an almost complete mantle of drumlins forming an internationally acknowledged type example of a 'drumlin swarm'.

The Tyrella Coastal Dunes is a strip of coastline that lies behind Dundrum (Outer) Bay on the South Down coast, between St John's Point in the east and Newcastle in the west. The area is dominated by extensive sand dunes. Their hummocky forms are covered in windswept Marram grass backed by flat farmland which supports rough grazing. A long sandy beach and the extensive mudflats at Dundrum Inner Bay together create a flat expanse which contrasts dramatically with the looming peaks of the Mourne Mountains. The beach becomes progressively stonier towards Newcastle. Landscape sensitivity is increased by the visibility of this low coastal strip from the Mournes and the high ground which backs Dundrum Bay. The dunes are extremely sensitive to development and require careful management to avoid erosion and the loss of rare and interesting plants. A conservation area around Tyrella Beach controls public access, vehicular access, grazing pressure and recreational use of the dunes, ensuring that minimum damage is suffered. The Murlough National Nature Reserve and Murlough ASSI lies within the 6,000 year old sand dune system to the west of Dundrum. The sand dunes also fall within the candidate Special Area for Conservation (cSAC). The area to the west of Dundrum Bay is designated as part of the Mourne Area of Outstanding Natural Beauty (AONB), while the area to the east falls within the Lecale Coast AONB.

Pre-Quaternary (Solid) Geology

The stratigraphy of this area is made up of the mapped formations in the table, the youngest of which usually overlie the oldest. The older formations can be upside down (tectonically inverted).

Stratigraphic Table (youngest rocks at the top of the table)

The area comprises 99% Lower Palaeozoic greywacke sandstones and shales, the remainder being Tertiary dykes and minor intrusives

The greywackes vary from a few centimetres to a few metres in thickness with a large proportion of rock fragments and a fine-grained matrix. They are interbedded with thinner beds of siltstone or mudstone, commonly arranged as fining-up cycles.

Quaternary (Drift) Geology

Northern Ireland has experienced repeated glaciations during the Pleistocene period that produced vast amounts of debris to form the glacigenic deposits that cover >90% of the landscape. Their present morphology was shaped principally during the last glacial cycle (the Midlandian), with subsequent modification throughout the post-glacial Holocene period. The Late Midlandian, the last main phases of ice sheet flow, occurred between 23 and 13ka B.P. from dispersion centres in the Lough Neagh Basin, the Omagh Basin and Lower Lough Erne/Donegal. The clearest imprint of these ice flows are flow transverse rogen moraines and flow parallel drumlin swarms which developed across thick covers of till, mostly below 150m O.D. during a period that referred to as the Drumlin Readvance. At the very end of the Midlandian, Scottish ice moved southwards and overrode parts of the north coast. Evidence for deglaciation of the landscape is found in features formed between the glacial maximum to the onset of the present warm stage from 17 and 13ka B.P. - a period of gradual climatic improvement. Most commonly these are of glaciofluvial and glaciolacustrine origin and include: eskers, outwash mounds and spreads, proglacial lacustrine deposits, kame terraces, kettle holes and meltwater channels (McCarron et al. 2002). During the Holocene, marine, fluvial, aeolian and mass movement processes, combined with human activities and climate and sea-level fluctuations, have modified the appearance of the landscape. The landforms and associated deposits derived from all of these processes are essentially fossil. Once damaged or destroyed they cannot be replaced since the processes or process combinations that created them no longer exist. They therefore represent a finite scientific and economic resource and are a notable determinant of landscape character.

The coast of Dundrum Bay has been described by Orford (in Whalley et al. 1985) as the largest east coast post-Midlandian sediment sink. Navas and Malvarez (in Knight 2002) describe the bay system at Dundrum as comprising three units. The first of these is Dundrum inner bay, a back barrier environment with extensive tidal mud and sand flats. Outside of this is an extensive Holocene dune system that includes the Murlough and Ballykinler complexes separated by a tidal channel linking the inner and outer bays. The outer bay includes the beaches, intertidal and offshore areas, as well as an extensive ebb tide delta at the mouth of the outlet from the inner bay. Beneath the dunes are a series of parallel beach ridges that Orford (1985) suggested during the rapid Holocene transgression (10 000 - 6 000 years B.P.), when rising sea level rolled onshore around the bay a barrier of re-worked fluvial sediment. Recent work by Orford and Murdy (in Knight 2002) favours a model in whereby the gravel ridges and dune dunes were built sequentially, as coupled units in which sand domination (a dissipative beach state) and gravel domination (a reflective beach state) alternated. Rather than one in which the complete ridge system was deposited first and then covered by the dunes. The beach at Dundrum is also noted for its classic ridge and runnel morphology.

The drift geology map of the LCA highlights the extent of the sand dunes in the west of the area, and also the presence of underlying raised beach deposits. In the east of the LCA there is a small extent of Late Midlandian till associated with ice that moved southeastwards across the region. The map also indicates a significant expanse of alluvial deposits on the coastal plain immediately inland of the Tyrella dune system.

Key Elements

ASSI

089 MURLOUGH (most of ASSI in this LCA)

A coastal complex exhibiting a range of gravel ridges, re-curved spit, dune systems, palaeosols and contemporary beach processes dating from between late glacial and modern times. This gives rise to an extensive range of coastal habitats, including scarce vegetation communities and rare plant species.

AONBs

The western part of this LCA lies within the Mourne AONB (1986), whilst the eastern part beyond the entrance to the inner Dundrum Bay is within the Lecale Coast AONB (1967). These designations are indicative of the scenic quality of the landscape.