Benburb

Also within

Tyrone County

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Habitats

Fast flowing river and stream

Geology & landscape

Geological Feature: Drumlin · Geological Period: Quaternary · Rock Type: Glacial Till

Landscape processes: Glacial, Peatland / Mire

Description

Benburb Pleistocene The interglacial peat deposit at Benburb is sealed by glacigenic diamict. It is the only interglacial peat deposit presently known in Northern Ireland. The peaty sequence at Benburb is sealed by a drumlin forming till and therefore predates the last cold stage. Floristically its pollen content is similar to about ten other interglacial sites in Ireland which are thought to belong to the Gortian temperate stage. The Gortian interglacial dates from some 300-420,000 years before present B.P. and is generally assumed to predate the penultimate, Munsterian, glaciation. The original exposure of the interglacial peat was discovered in the bank of a small tributary of the River Blackwater. Most of the peat is sealed beneath a few metres of glacial till and is only partly exposed along the banks of the tributary stream. The peat is compressed and the lack of firm stratigraphic control and poor exposures does not rule out the possibility that the peat is not in situ. The lack of clear bedding planes and the fairly uniform nature of the peat suggests that nonconformities are absent. The apparent setting is of organic materials accumulating in a lake. The four major pollen assemblage zones contain the colonising, early and late temperate stages of this interglacial cycle, though the final phase has been truncated or is a non-sequence. Floristic studies of the sequence highlight many of the problems involved in the correlations between interglacial sequences in Ireland and those in England. Differences between Benburb and other Gortian sites may well reflect geographic variability in floristic composition within the same interglacial. Macrofossils from the basal 'pollen assemblage zone' the pollen-free PAZ 0 series show that Slender Naiad Najas flexilis was the most abundant species. The majority of seeds recovered from this level are from submerged or floating aquatics. PAZ 1 has pine Pinus spp. >80% and birch Betula spp. 10% as the greatest contributors. Non-arboreal pollen includes grasses Gramineae 50%, sedges Cyperaceae 10%, aquatics and members of a rich herbaceous flora. The presence of land plants around the pond is indicated by the fruits of alder Alnus glutinosa, downy birch Betula pubescens and silver birch Betula pendula. PAZ 2 comprises only three spectra, pine Pinus spp. and yew Taxus spp. pollen are strongly represented. Alder Alnus spp. levels rise to 36% and grasses Graminae fall to 10%. Levels of Coryloid pollen rise to a peak of 10% and birch Betula spp. falls to below 5%. Above this PAZ 3, pine Pinus spp. pollen falls from 25% to around 15% and the wetland vegetation is supplemented by mixed deciduous forest comprising oak Quercus spp., ash Fraxinus spp. and elm Ulmus spp.. The presence of holly Ilex spp. and ivy Hedera helix, indicates that this zone represents the warmest part of the interglacial cycle. Finally, in PAZ 4A, Fir Abies spp. pollen is present for the first time and suggests that the vegetation surrounding the area was predominantly forest comprising this species. Rhododendron Rhododendron spp. and species X also are distinctive fossils at this level. Leaves of European silver-fir Abies alba are typically present. Within the uppermost portion of this level PAZ 4B Fir Abies spp. declines, as does alder Alnus spp. and Coryloid pollen, leaving yew Taxus spp. as the dominant arboreal species. spruce Picea spp., which is present throughout the diagram in small quantities, rises to a maximum in this zone. Independent pollen zonation confirms that the Benburb sequence is the first fossiliferous interglacial series to be recorded in the north of Ireland. The basic pollen assemblages are similar to those of Gortian sites elsewhere in Ireland and include the association of fir Abies spp., yew Taxus spp., species X and Rhododendron Rhododendron spp. in the late temperate stage. However, certain differences exist between the assemblages at Benburb and other Gortian sites in Ireland. These may be interpreted in several ways. Differences in their floristic composition may reflect a geographic variation within the same interglacial. Alternatively it could be argued, that the presence of only one till above the interglacial peats suggests a last interglacial age a pre-Midlandian age. However, the critical stratigraphic marker of hornbeam Carpinus spp. pollen is not present. https://www.daera-ni.gov.uk/publications/benburb-assi

Species mentioned

Plant communities