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DerryvreeFermanagh
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Summary Full report
Site Type: Lost, Roadside section
Site Status: PASSI
District: Fermanagh District Council
Grid Reference: H361390
Rocks
Rock Age: Quaternary (Pleistocene)
Rock Type: Till
Interest
Fossil Groups: Insects etc, Plant
Other interest: herb biozone, Glacial deposit, drumlin, subglacial, till

Summary of site:

Road realignment in 1969 required excavation of a drumlin and in the course of the work a unique section of deposits was exposed. It consists of lower and upper glacial tills separated by fresh water silts that contain a wide range of organic debris. Moss in one of the muddy silts has been carbon 14 dated at around 30,500 years old.

The lower boulder clay is the Derryvree Till, probably resting on the lime mudstones of the bedrock of the area which is the Carboniferous Carnteel Formation. Almost 90% of the till is fine sand and silt, the rest cobbles and pebbles, mostly of local Carboniferous rocks with about a third of Devonian Old Red Sandstone, probably from the north side of the Tempo-Sixmilecross Fault (about 8km to the north). The Derryvree Silts, up to 3m thick, were deposited in marshy conditions on top of the till after a break in time indicated by weathering of the till surface. Two small hollows can be seen on top of the till and the laminations in the silt parallel the contours of their beds. These silts contain thin layers of moss remains with poorly preserved but identifiable pollens. They indicate a landscape in the immediate area at that time dominated by mosses, grasses with some sedges and the odd willow and birch. Aquatic plants, such as pondweed and green algae, were also present. Tiny amounts of alder, hazel, birch, pine, spruce and elm pollen were also identified, suggesting distant forests. This is essentially a tundra flora, indicating a climate of short, cool summers with long winters and permanently frozen subsoil. The outer casings of 28 species of beetles were also identified; most are found today only in northern Britain on high hills or in northern tundra zones. The total absence of weevils suggests very hostile conditions. A sample of moss-rich mud from these silts yielded the carbon date. The glacial deposit above the silts is the Maguiresbridge Till, streamlined into drumlin form by the passage of ice. It is black-brown in colour and varies from 1m to 6m in thickness and, where the silts are absent, its contact with the lower till is sharply defined. The passage of ice has thrust these deposits, and planes of movement penetrate through the upper into the lower till in some places. The larger clasts in the upper till are mainly local Carboniferous rocks. The Maguiresbridge Till was formed below a thinning regional ice sheet that streamlined the deposits into drumlins, while at the same time causing deep shearing of the unconsolidated till. It was this ice sheet that formed the major drumlin fields of Ulster. This site is important because the carbon date separating the two Midlandian tills sets a chronological framework for the fluctuating climatic conditions of the time. The variety of plant material also provides an intriguing glimpse of the tundra environments of 30,000 years ago and the species that had by then colonised the area. For all these reasons Derryvree is a key site for the understanding of Midlandian events and should be preserved. The cutting is now overgrown but could be re-exposed, if necessary, for future research.


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