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BallinderryTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: X26833812, X26293782
Rocks
Rock Age: Quaternary
Interest
Other interest: No data, Glacial deposit, Glaciotectonic, cross-valley moraine, delta, esker, flat-topped delta, delta

Summary of site:

This complex landscape of glacial retreat features is centred on the Ballinderry River valley. It extends about 25km east-west, from an area just south of Cookstown in the east along the entire length of the river and just beyond its source in the west, to include Loughmacrory Hill. Individual sites extend 4-5km either side of the river.

As the regional ice sheets of the last stages of the last ice age (the Midlandian) began to melt and retreat to their local ice centres, an elaborate series of glacial deposits was left in their wake. In this area, the deposits have been sequenced in time and seven phases have been recognised and described. A map is desirable to aid interpretation. Phase 1. A ridge extending north from near Teebane for about 5km consists of boulders and gravel with an internal arrangement indicating a high pressure water flow from south to north. It is interpreted as an esker (a choked tunnel within a glacier) at the head of the Ballinderry valley. Phase 2. As the ice continued to melt, thinning and retreating as it decayed, it separated into lobes, one retreating into the head of the Owenreagh valley to the west, a second south along the line of Granagh Burn, and a third into the upper Ballinderry valley. A large glacially-ponded lake formed in the ice-free lowland with a surface at 180m above sea level. Ice retreating west fed melt water into the lake, creating a small delta at Creggan, while debris from the southerly retreating ice created the much larger delta at Killucan. Eskers are found associated with both. Phase 3. The flat tops of deltas west of Evishanoran Mountain and at Murnells (3km to the south east) suggest that the ice-ponded lake survived and reached a new shore level at 235m above present sea level. The highland mass of Evishanoran on Cregganconroe was ice-covered for a time and surrounded by water to the west, north and east. Late in this phase the ice retreated east from Scalp towards Pomeroy. The winding terraced ridges called kames near Lough Mallon and Black Lough are evidence of this recession and the ice masses trapped in the sediments melted and are now expressed as water-filled kettle holes in the same area. Phase 4. As the ice continued to recede eastwards down the Ballinderry Valley, it dammed a lake which drained and stabilised at progressively lower levels. The evidence survives in the form of flat-topped terraces of sediment that defined each new and lower shoreline. The highest, at Gortaclady, is 220-200m above base level, followed by Cavanoneill at 190-180m and Kildress at 140-130m. The terraces were probably much more extensive than at present and each would have been eroded to provide sediment for the next in the sequence. There is some evidence at Dunnamore of a further terrace in this stage, now reduced to a degraded valley fill. Phase 5. A cross-valley ridge, just over 3km long, to the north of Drumshambo to the south south west, is now heavily dissected. It is composed of water-deposited cobbles, pebbles and sands that appear to have been thrust by a minor ice advance after their original deposition. The eastern slope is steep, suggesting contact with the advancing ice. Phase 6. Between Carchoney Cross Roads and Crockadoo is a further ridge with Knockaleery forming its most prominent feature. It appears to have been built by coalescing fans of debris carried into a lake from several stream sources along the ice front. Again, as at the Muntober/Drumshambo ridge, there is evidence of ice contact on the steep eastern slope and that a glacial push during a minor re-advance deformed the deposits. This ridge defines the eastern limit of the large structures in the area. Phase 7. Immediately west of Cookstown on the west bank of the Ballinderry River is the Black Hill Ridge. Unfortunately, the paucity of exposure complicates its interpretation. The ridge could be either another feature caused by a glacial re-advance pushing into sands and gravels or, alternatively, a terminal moraine representing a period when the retreating glacier stabilised and melted at the rate of ice flow. The flat lowland west and south-west of the hill is a fragment of a plain of sand and gravel deposits spread by melt water pouring from the front of the ice sheet. South of Cookstown a north-west trending ridge from Derryloran in the west to Rockhead Hill appears to link to another trending north east from Rockhead Hill to Drummond Wood. This is an esker with an east to west water flow from an ice centre in the Lough Neagh area. Conservation of landscapes on this scale presents many problems, particularly in the face of pressures for sand and gravel extraction, but careful planning control could restrict activity to areas concealed from the main roads and viewpoints. It may well be worth attempting to preserve the concentration of interest (esker, ponded water and flat topped delta) of the delightful landscape around Lough Doo before active working encroaches any further.


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