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Roddan's PortDown
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Summary Full report
Site Type: Coastal section
Site Status: PASSI
District: Ards Borough Council
Grid Reference: J641658
Rocks
Rock Age: Quaternary (Pleistocene)
Rock Type: Clay, Gravel, Sand
Interest
Fossil Groups: Ostracods
Other interest: late-glacial sequence, post-glacial pollen zone, Beach deposit, Late-glacial deposit, Marine deposit

Summary of site:

Buried beneath the foreshore gravels at Roddanís Port between two truncated drumlins is a sequence of rocks that attracted early attention because stumps of Scots Pine could be seen in a bed of peat. It attracted renewed attention in the 1960s when it became the subject of a thorough research investigation and, with some of the earliest carbon dates, established a sequence of events at the very end of the last glaciation (the Midlandian).

The sequence has three main divisions. The first was traced from below modern sea level to about 2 m above high water mark and consists of around 4.5 m of red clay on top of the glacial material of the drumlins. There are faint laminations but no strong bedding planes and wisps of gravel are present. The only fossils found were ostracods, all of species still alive. A grey clay follows containing the pollen of birch, pine and willow and the remains of algae. The second major division is an organic mud compressed into slabs. Sedges and grasses permeate this deposit with plentiful calcareous algae. Shells of freshwater snails occur in discontinuous layers. Pollen of birch, juniper and crowberry increase through this section and grassland species of Romex are strongly represented. No warm climate species are present and mugwort, rock-rose and fennel pondweed confirm a cool climate. Pollens from the top layer of the mud show an increase in grasses and a decline in juniper and crowberry. Above the organic mud is the third division, 90 cm of grey sand with a strew of shale fragments ranging from angular to rounded. Thin seams of vegetation contain leaves of the least willow and hair moss. Pollen is sparse but joint pine, now a southern European species, was found. A wide range of water plants indicates a moist, cool, green landscape with buttercups in places. Interesting finds at this level were remains of the arctic tadpole shrimp, at present only found in ponds and lake margins in arctic situations. The sequence is completed by a band of clay containing seeds of pondweed and water-milfoil. Then follows a reed peat bed about 20 cm thick containing birch bark. The sequence at Roddanís Port has yielded considerable amounts of evidence of plant colonisation and change from the retreat of the glaciers around 16,000 years ago to the landscapes about 9-10,000 years ago. The red clays at the base of the sequence are marine in origin and probably span the bleak period following the ice retreat to about 12,000 years ago when the land began to rise, adjusting to the loss of its load of ice sheets. Plant remains from near the top of the grey clay above gave a carbon date of a little over 12,000 years, a time of arctic climate and tundra landscapes. The organic mud with its enriched flora signals an improved climate. The species present suggest mild sub-arctic conditions and an open landscape of tree studded tundra and crowberry heath. The organic material from the top of the mud yielded carbon dates between 11,500 and 11,700 years. The grey sand probably represents an accumulating mantle of waterlogged material that moved down the slopes of the drumlins in alternate periods of freeze and thaw. Its flora indicates a return of cold conditions and a tundra landscape with dwarf willow. The last 13,000 years was divided into 8 zones by a Danish geologist, Knud Jessen, based on vegetational changes and the zones referred to in the full report of this site are based on his. The red clay equates with Zone I, called the Older Dryas; the organic mud with Zone II, the Allerod period and the grey sands with Zone III, the Younger Dryas. This zonal system has since been superseded by a scheme based on a complete vegetational history of Northern Ireland recorded from Sluggan Bog (site 102). There are 10 zones in this classification but the first three equate to Jessenís and are considered late glacial. The first is named the Older Salix herbacea, the second the Woodgrange Interstadial and the third the Nahanagan (Woodgrange) Stadial. There is only one other known site where a late glacial land sequence is sandwiched between marine deposits reflecting changes in sea level due to ice loss and global events. Such sites provide fundamental research information and with current concerns about climate change they are likely to remain valuable and relevant records. Although at present obscured, this site should be protected.


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