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Ballygawley Delta Complex - Fallaghearn DeltasTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
Grid Reference: H608628
Rock Age: Quaternary
Other interest: No data, Deltaic sediments, Glacial deposit, deltas, delta

Summary of site:

At Fallaghearn, four deltas can be easily recognised by their flat top surfaces. They were built into a temporary ice-dammed lake and are dissected by deep, steep-sided meltwater channels that cut through the sands, gravels and pebbles of the deltas soon after the lake drained.

The delta deposits are heavily exploited for sand and gravel and long exposures, together totalling 1.5km, provide extensive evidence of their internal structures. Two exposures, one in the West Pit, the other in the East Pit, provide valuable information. The West Pit shows extensive pebble- and cobble-dominated deposits with their particles in contact and the spaces between partly filled with gravel and sand. The beds are not laterally continuous and they are interrupted at sharp boundaries by discontinuous beds and lenses of medium to fine grained sand. Within the pebble/cobble deposits, the angle of their front slopes during deposition (foresets) can be seen inclined at 20º. A separate exposure in the West Pit shows similar deposits near the base, but more laterally persistent, with foresets inclined between 10º and 30º. Towards the top they become discontinuous and much coarser. The East Pit shows more continuous beds with well defined, fine- and medium-grained layers incorporating pods of gravel and pebbles. To the south of the outcrop the continuity gives way to more complex cross-cutting bed contacts with strong contrasts in grain size. Stacked sand layers dip at anything from 10º to 50º and there are occasional rippled units dipping against the main trend. All these deposits are of the kind found in the vicinity of deltas, the more continuous usually at some distance from channel discharges and probably in deeper water. At water level on the delta surface the flow is tortuous, constantly seeking the slightest hint of gradient, and channels meander and scour new courses in the soft sediment. At times of high water supply the channels are capable of carrying gravel, pebbles and even cobbles, but as water passes out through each delta’s many mouths, the speed of the current falls off sharply and the load of particles immediately falls to the bed or is discharged down the steep front slope of the delta. The Fallaghearn deltas formed at the close of Ireland’s last glaciation and it is likely that the lake containing them drained over hours or days at the most, as glacial lakes tend to do, leaving them high and dry. Since then they have been progressively colonised by vegetation and eroded gently before the modern assault of the aggregates industry. How this site relates to others in the area is described in the Ballygawley site overview.

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