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Murrins Complex - Lough Fingrean Frontal MorainesTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: H573777
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, No Data, moraine ridge

Summary of site:

Between Mulderg to the north west and Loughmacrory Hill to the south east there are six prominent north west/south east trending ridges up to 1.5 km long, 200 m wide and 25 m high. They have slightly winding tops, steep flanks and are between 100 and 200 m apart. They are also breached in places. They contrast strongly with the flat plain of glacial outwash deposits to the north and are closely associated with the complex of small lakes dotted throughout the area.

There are two exposures in these ridges, one at Murrinmaguiggan pit (see site 509 ű Cashel Outwash), the other near Lough Carn. The lowest bed seen in the Lough Carn outcrop is a pebble to cobble-sized conglomerate in a fine, sandy matrix, topped by an indistinctly bedded and intermittent granule unit about 30 cm thick. It has an indefinite upper contact with a continuous, capping diamict bed 40 cm to 1 m in thickness. Diamict is a rock of mixed grain sizes, often quite pasty and here it contains some randomly arranged, large pebbles. Small steep faults, both normal and reverse, interrupt the continuity of the conglomerate and sand units and are aligned slightly south of east. The deposits at both localities indicate an ice front environment in retreat towards the south and west with at least 6 periods of ôstandstillö when the rate of ice flow was matched by the speed of melt. In these circumstances all the debris released from the ice accumulates on the ice front as a mound often banked against the ice. The lower beds were deposited into standing water, probably a local ice-dammed lake, and the aligned larger rocks were a consequence of fast flowing water pouring from steeply inclined watercourses on or in tunnels (eskers) in the ice mass. Any fine matrix would have been carried away by this torrential flow. There is also the possibility that parts of the ridges were composed of debris squeezed up by the shearing of the ice bed along of the local bed rock. As the ice retreated from each ridge, the support it provided on the upstream side was lost and the sediments adjusted and settled by faulting along the unsupported line, roughly the crest line. The diamict is not affected by the faulting so it must be later, probably dumped on the ridge by slumping or debris flow (a process related to mudslides) of material raised from the ice bed by shearing. This belt of moraines is a prominent feature between its upland buttresses fronting the Murrins outwash plain and an ideal area for exploring the processes and sequence of events involved in the retreat of the margin of a major ice sheet at the close of the Midlandian, the last major glaciation in Ireland. The above account is a copy of the one you gave me on the floppy disk at the meeting on 8th October. I have adapted it to how I want to receive it from you so that we can easily incorporate your summaries into the database for outputting as web pages. We would like all the site summaries on the one word document rather than saved as separate files. There is no need for titles. All we need is the site number with a #in front of it. We do not want spaces between paragraphs (as I have done in this last section). When you finish a paragraph press Enter/Return to bring you to a new line and then immediately start the new paragraph without pressing Enter/Return again. No bullet points ű instead insert

    at the start of the list of bullets and then where you would expect the number to be inserted
  1. . It is vital to insert
at the end of your list.


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