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Murrins Complex - Cashel OutwashTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: H562788, H581821
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, No Data, outwash

Summary of site:

The Murrins area records a complex series of events during the retreat of the final ice sheets of the last glaciation, deduced from a variety of landforms and extensive outcrops of deposits left behind. Two ice margins were involved, one retreating towards the Lough Neagh Basin to the east, the other south west towards Omagh and beyond. This account describes one of these landforms, an extensive glacial outwash plain 4 km long and up to 3.5 km wide extending from Murrinmaguiggan to Cashel Bridge. There are good exposures at both these extremes.

At Cashel Bridge two abandoned sand pits exposing between 12 and 16 m of deposits over faces up to 200 m long provide important evidence. The north pit section has thickly developed gravel and cobble conglomerates, their particles in direct contact, with some beds of grit and coarse sand and pods of thickly bedded sands. The upper portion of the face consists of massive (thick bedded) medium to coarse sands with some gritty, granular layer. Bedding planes become more frequent towards the top and the direction of flow of the water that deposited them was a few degrees either side of north. The south pit shows substantial lateral change in the nature of the sediment exposed over a distance of about 170 m. On the left of the face are massive gravel, cobble and boulder beds interspersed with thick sand pods, the whole topped off by sands. These deposits grade laterally into the central face where massive gravels and cobble conglomerate beds predominate with thin, discontinuous but sharply defined sand layers between them, again topped by somewhat less than a metre of sand. This sequence is abruptly terminated on the right of the face by a channel 10 m deep and 20 m wide filled, wall-to-wall, with clearly bedded grits and granules and medium grained sands. The axis of the channel trends slightly west of north. Both localities are the distant reaches of deposits washed out of a melting and retreating ice front by constantly changing volumes of meltwater and sediment load. They discharged into a shallow, temporary lake and the sediments moved in a variety of ways including underwater debris flows (where destabilised sediments dived down lake bed slopes and settled, coarse material first), settling of fine, suspended sands in still conditions and sheets flow where slopes of sediment crept with the currents. There is no evidence in the deposits of intense cold such as ice wedges, stones dropped from floating ice or the silting-in of large, static ice blocks. In both pits the final sediments are sands, suggesting that in the later stages the waning flow of meltwater could only carry finer material and perhaps there was less active sediment in the system. The Murrinmaguiggan pit is huge, covering an area of over 25 hectares with over 800 m of exposed sediment faces. Four exposures are particularly informative. The first reveals 7 m of deposits, mostly cobbles and boulders, some with matrix, some simply supporting each other, in loosely defined beds. In the lower part of the section there are intermittent sand beds, some massive. The most obvious feature in this section is a large channel cutting through all but a metre of the deposit, filled with a chaotic mix of bedded gravels, some cross-bedded, with cobbles forming vague lines. The second exposure has four divisions. The lowest consists of alternating gravels and sands with a contorted top but making a sharp contact with the second division which is made up of massive beds of even grained sand and silt, again with a sharp upper contact. The third contrasts strongly consisting of chaotic, loosely bedded cobbles, mostly in massive beds separated by medium grained sands to silts and contorted, gritty pods. This unit is channelled with their axes indicating a flow direction roughly to the west. Vaguely bedded grits with lenses of sand and silt make up the final division. The third exposure has a massive, 60 cm thick basal unit of even sized granules (2 to 4 mm in diameter) in which cobbles and boulders appear spasmodically. These larger rocks are bluntly angular and match the bedrock of the immediate vicinity, volcanics of the Tyrone Igneous Complex. This is followed by well-bedded sands, some cross-bedded, indicating a flow to the north or just west of north. The sequence has no break. In one area local currents have scoured the sediment creating a depression 5 m wide and a metre deep. The final exposure in the pit is over 40 m long and 6 m deep. It commences with more than 2 m of massive stratified medium-grained sands arranged in pods separated by granules and large pebbles filling cavities. Cross bedding shows currents meandering about 30 degrees either side of east. The top of this unit is crisp with a wavy surface and the sediments below are not deformed. The next unit is particularly striking and consists of two massive rafts of the local bedrock, one at least 22 m long and up to 1.7 m thick, the other 7 m long and 1.5 m thick. Both are heavily jointed (joints are undisplaced fractures) Tyrone Igneous Complex volcanics and elsewhere in the pit much smaller examples are enclosed within a massive, pasty diamict incorporating ice facetted and scratched pebbles and cobbles. The final unit is a 2.5 m thick mass of granules and pebbles cut by numerous channels most about a metre deep and up to 3 m across. The Murrinmaguiggan pit exposures reveal a landscape and a set of processes similar to those observed at Cashel Bridge except that here the dominant force was the ice front retreating to the south west. Again, meltwater was feeding sediment into a temporary glacial lake and deposits indicate that they were some distance from the ice sheet. The variety and rapid changes in the nature of the deposits are due to the amount and velocity of water released by the ice and its ability to transport its load of sediment. It obviously fluctuated widely, at time sweeping cobbles and boulders along, at others, still enough for suspended fine sediment to settle. The processes of sediment transport varied and included high density mass flow (when plumes of new sediment or destabilised existing sediment, plunge down underwater lake gradients in turbulent masses to settle on deeper, flatter lake beds), traction bed load (when water tumbles particles along in the current) and settling from suspension. Clearly, deposits were also being eroded and redistributed periodically as water volumes increased or streams changed course. Many of the sediments have very even grain sizes (described as well sorted) the result of sustained winnowing upstream, possibly in tunnels full of meltwater surging through the ice. The rock rafts could only have been transported on the bed of a glacier but they sit on a bed of obviously water-laid sediment. This clearly points to an ice re-advance at some stage. In this process the glacial ice freezes onto the bedrock, the ice continues to flow, exerting a powerful shearing stress on the rock which detaches in sheets or rafts along shallow fractures. The rafts are then transported in their frozen condition under the moving ice and in this case they rode over earlier glacial outwash. Finally, when the ice melted, the raft was stranded in later glacial deposits. The overall picture is of two ice masses retreating from the Murrins, one towards the Lough Neagh Basin, the other south westwards towards Omagh and beyond. They created a temporary closed basin in which their meltwaters accumulated to form a shallow glacial lake, its east and west flanks defined by high ground. The meltwaters and local heavy rain swept huge volumes of glacial debris into the lake partly filling it and it is the remnants of these deposits that can be seen in the sand pits and landforms of the present day These are important sites because they expose glacial sediments exhibiting a rich variety of processes and a history of events in a confined area of the upper Owenreagh valley during a period of rapid glacial retreat at the end of the last phase of glaciation in Ireland, the Midlandian.


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