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Butterlope GlenTyrone
View looking north from the south end of Butterlope Glen, Co. Tyrone.
Summary Full report
Site Type: Crag, Crags
Site Status: PASSI
Grid Reference: H493947
Rock Age: Precambrian (Dalradian)
Rock Name: Dart Formation, Dungiven Formation
Rock Type: Lava, Limestone, Pillow lava
Other interest: No data, No Data

Summary of site:

Butterlope Glen is a fine example of a deep valley that was cut across a watershed when an ice-dammed, high level glacial lake on one side breached the crest and drained into the adjacent valley on the other. The torrential flood of released water with its suspended load of abrasive sand, gravel and pebbles rapidly deepened the channel at the breach. Flow continued until the lake drained and the flow reduced to a trickle or no flow at all, leaving an elevated, deeply incised, short valley called a glacial overflow channel.

The principal interest of this account is the geology of the region exposed in the walls of the glacial overflow channel. The rocks of the area are ancient, of Precambrian age, belonging to the Argyll and Southern Highland Groups of the Dalradian stage. They are around 580 million years old and were formed at the South Pole, then ice-free, of the supercontinent called Gondwana. The Argyll Group has two subdivisions in this region, the Newtownstewart Formation below and the Dungiven above. The younger Southern Highland Group has four subdivisions, oldest to youngest; the Dart, Glenelly, Glengawna and Mullaghcarn Formations. In Butterlope Glen there are fine and extensive exposures of the Dungiven and Dart Formations. The entire succession of rocks at Butterlope is inverted, which means that the rocks that appear to be on top (and should therefore be youngest) are actually the oldest. This upside-down geology is at first confusing but needs to be understood to work out the sequence of events that is the key to understanding this span of mid-Dalradian time. The oldest rocks, the Dungiven Formation, are visible at the northern end of the glen where heavily deformed volcanics, mostly dusts of various compositions (converted to schists), chilled lavas (hyaloclastites) and stretched pillow lavas (pillow shaped masses of lava disgorged from submarine fissures) predominate. Some 200m from the northern end of the glen, dark blue and grey metamorphic limestones appear on top of the volcanics, parted periodically by grey sulphurous schists and altered mudstone (pelites). Again these are much deformed and folded with interesting cleavages and cleavage/bedding features. On top of this sequence are psammites (originally quartz sandstones), amphibole schists (formerly volcanic rocks) and a mixture of altered volcanic rocks. At this point the succession crosses the boundary into the Dart Formation and its lowest division, the Glenga Amphibolite Member (easily recognised by its silvery to greyish green amphibolite schists, the metamorphic equivalent of basaltic volcanics). These rocks are last seen at the hairpin bend immediately south of the glen. The original volcanics and sedimentary events that these rocks represent were deposited during an important period of change in the Dalradian. Towards the end of Argyll Group times the Earth’s crust was stretched and it thinned along a narrow zone, 2000km long. This fracture separated a huge section of the supercontinent of Gondwana, bringing the continent of Laurentia into independent existence. The ocean flooded into this rift that would eventually widen to become the Iapetus Ocean and soon afterwards a surge of volcanic activity breached the thin crust of the seabed. Sediments, largely sandstones, shales and mudstones were deposited between phases of submarine and continental eruptions and it is this mixture of rocks that we now see in Butterlope Glen. Laurentia (eventually to become modern North America, Greenland and the northern parts of Ireland and Britain) then began its slow northerly drift that would carry it over the equator 250 million years later. The deep burial of these rocks and their alteration by heat and pressure was caused by continental plate collisions in the Grampian orogeny of the early Ordovician period around 480 million years ago. The site has both national and international importance because it shows the relationship of different Dalradian successions within Ireland and between Ireland and Scotland. The long succession, with such a variety of rock types and structures in such a clearly exposed and compact setting, also makes this an ideal study and research environment. The abandoned quarries at the north end of the glen have been the victims of tipping but otherwise there appear to be no threats to the site.

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