Previous SiteNext Site

BenburbTyrone
No picture
Summary Full report
Site Type: Stream section
Site Status: ASSI
District: Dungannon District Council
Grid Reference: H810521
Rocks
Rock Age: Quaternary (Gortian, Pleistocene)
Interest
Fossil Groups: Plant
Other interest: No data, Interglacial deposit, interglacial peat, pollen analysis, till, lacustrine

Summary of site:

The Pleistocene ice age lasted from 1.6 million years ago to about 10,000 years ago, although many experts believe that it is not yet over.

The Pleistocene was not simply one sustained glacial period but a series of glaciations separated by warmer interglacials. During the glaciations, massive ice sheets spread over the land and oceans (in the northern hemisphere the last one covered all of Canada, extending down through the Great Lakes region, all northern Europe and Asia and most of the British Isles) and in the interglacials the ice retreated probably to positions similar to today’s. The present warm climatic period could be nothing more than the latest interglacial. Ice sheets are powerfully destructive, tending to obliterate from the surface of the land almost all indications of previous surface processes. Occasionally a rare set of circumstances will preserve pockets of evidence of earlier events and this site near Benburb is just such an example. In the bank of a small tributary of the River Blackwater, 500m south west of the village and in two further places in the immediate vicinity, a peat bed can be seen beneath a coarse glacial deposit. Because it lies below the glacial deposit, it must be pre-glacial in age. The glacial layer probably represents the final stages of the last glaciation, the Midlandian. It was also thought that the peat occurred below the local drumlin field. Drumlins are small egg-shaped hills of glacial debris that form when the ice sheets thin to a point where they are no longer able to plough such material along their bases but start to glide over and mould it instead. This raised the possibility that the peat could have been smeared, or at least distorted so that it was no longer in its original sequence. The peat was drilled and a complete core, 4.15m long, was recovered and examined for plant remains. The lowest 30cm yielded fresh water aquatic plants typical of northern latitude pond or small lake environments, with alder and birch on the margins. Five zones with characteristic plant remains followed. The first was dominated by pine and birch pollen (making up over 90% of the sample) but with some alder, willow, juniper and hazel, with grasses and sedges. The second was characterised by pine, yew and alder, but also included hazel, birch, ash and elm, with grasses. The third zone was typified by a reduction in wetland vegetation, with pine forming only 15% of the sample and deciduous forest species such as oak, ash and elm increasing strongly. The following sub-zone was marked by the appearance of silver fir with rhododendron. The final sub-zone was predominantly yew forest, with spruce becoming significant for the first time. This sequence tells a cohesive story and indicates that the peats were relatively undisturbed by glacial activity. These deposits are important not simply because they were the first peats to be found sealed beneath glacial deposits (only one further example has been found since), showing them to be interglacial, but also because they share broad similarities with peats found in the Republic of Ireland and dated to the Gortian stage. The Gortian is not the last interglacial but precedes the glaciation before that, called the Munsterian. This would make the peats more than 200,000 years old. Although there are differences in detail between the Benburb deposits and other Gortian sites, such as high levels of elm pollen throughout and the restriction of box to the third zone (in the south it survives into the silver fir sub-zone following), in general it conforms to the Gortian cycle of vegetation. It commences with a cool, wet climate progressing through two phases of temperate forests by the end of the fifth zone. Slight differences in the balance of vegetation could be due to local climatic conditions in the north and east during Gortian times. The circumstances reflected at Benburb - very old peats sealed beneath glacial deposits - make the site unique in Northern Ireland and of national importance. The minor differences in the composition of the forest progression when compared with those in the south give added research importance, aiding the understanding of the processes affecting plant distribution during the Gortian. All peat sites are fragile but at Benburb the peat is offered immediate protection by its glacial cover. Any developments requiring excavation, drainage or the passage of heavy equipment pose a risk and should be notified to the Environment and Heritage Service (DoENI). The site could also be obscured by tree planting or the dumping of rubbish.


Previous SiteNext Site

|Home|