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Devonian - Tedd Formation; Tedd Cross Roads SegmentFermanagh
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Summary Full report
Site Type: Inland exposure
Site Status:
District: Fermanagh District Council
Grid Reference: H263616
Rocks
Rock Age: Devonian (Middle Devonian, Upper Devonian)
Rock Name: Fintona Group, Tedd Formation
Rock Type: Conglomerate, Mudstone, Sand, Sandstone, Siltstone
Interest
Fossil Groups: Miospore, Miospores
Other interest: dessication cracks, fault, ripple marks, No Data, alluvial plain, lacustrine

Summary of site:

The Fintona Block is a large, fault-defined area of Devonian and Carboniferous rocks whose corners are the north-eastern edge of Lower Lough Erne in the north west, Lisbellaw in the south west, Omagh to the north east and Pomeroy in the east. Towards the north-west corner, with Tedd Cross Roads marking its southern edge, is a lenticular area on the geological map about 3.5km long by 600m wide, pinching out east and west. It is defined north and south by components of the Cloughfin Fault.

Within this small area, and unique to it, is a suite of Devonian rocks - the Tedd Formation. Neither base nor top of the sequence is seen. The rocks are inclined steeply to the south at about 60 from the horizontal. The edges of the beds appear as a series of low, parallel ridges across the landscape. A thickness of about 500m of rock can be seen here; it is described as a fining upwards sequence (meaning that the coarse grain sized rocks are at the base and grain size becomes progressively finer towards the top - reasonably interpreted as part of a continuously progressing process). At the base, pebbly, coarse grained sandstones dominate, gradually giving way to medium grained sandstones and then, in the top 200m, beds become thinner and consist of siltstones and finally mudstones. These rocks formed in a large desert basin, probably originally near the margin but progressively further out as the basin silted up and enlarged. The sediments were derived from the surrounding uplands over a considerable period, washed in by flash floods that accompanied rare torrential desert downpours. As the floodwater lost its velocity in the wider basin it was less able to carry coarse grains, so the sediment became progressively finer towards the middle. There it dried rapidly, cracking and curling in the intense heat awaiting the next seasonal rains. The green mudstone marks the transition from sheet floods to lake conditions as residual water formed temporary desert lakes like modern playas. It was in these lakes that the microscopic spores used to date these rocks accumulated. Land plants were in their early stages of evolution in the Devonian but clearly there were enough in this hostile environment to leave a trace. When the full report was prepared they were thought to indicate a middle to late Devonian age but subsequent research now proves them to be early in the period, between 390 and 400 million years ago. At that time the entire Irish and British areas were part of a gigantic supercontinent moving imperceptibly northwards across the equator. A newly formed, towering mountain range on the scale of the modern Himalayas ran north-south on its eastern side. Scotland and Northern Ireland were part of the south-eastern flank of the system, partly mountain and partly rain-shadow desert. The Tedd Formation is coarser than the other desert sediments of the early Devonian on the northern sectors of the Fintona Block and its fining upwards sequence is restricted to the Tedd Cross Roads lens. No stratotype has been described. The outcrops stretch across pasture and there are no obvious threats.


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