September 29th - After the Ice, the deglaciation of the Ballinderry Valley.


Leaders: TONY BAZLEY and JAMES RUTHERFORD


The Ballinderry River rises in central Tyrone at the watershed dividing the drainage flowing to the west and east. The actual source is in Cam Lough at 200m. and flows eastwards into Lough Neagh at 20m. a distance of 24 miles. The valley is very shallow and resembles a peneplain, the product of glacial erosion. The surface features are the product of glacial deposition with outcrops of residual harder rocks usually of granite (eg. Craigballyharky mt. at 234m.) or metaphosed schist (eg. Corvanaghan mt. at 250m.)


The main purpose of this field study was was to examine the features of glaciation deposition. The Ballinderry valley was chosen as it is the most extensive area of this material in the north of Ireland and the numerous sand quarries give access to examine its content and there are numerous examples of eskers, deltas, kettlehole lakes and moraines. The location of these features shows the retreat of the ice is from west to east, from Cam Lough to the Lough Neagh basin. Thus it is logical to work from west to east.


We began with the eskers. These are subglacial ridges of sand and gravel washed out of the ice sheet by melt water streams and deposited in ridges in the stream bed under the ice, and the finest material (fine sand and silt) is washed out into the temporary lake, that forms at the edge of the ice, where it builds up into a delta. These deltas are the main source of good quality aggregate for the building trade. When the ice sheets melt the eskers stand out as sinuous ridges, sometimes used as good dry foundations for roads eg. the Davagh and Knockaleery ridges, the latter being a good example of a beaded esker (results from pauses in the retreat of the ice sheet). There are several fine examples of deltas in the valley. These are easily identified. They occupy prominent positions, have level surfaces and usually a steep slope on the downstream side, indicating a minor surge of the ice sheet (usually referred to as the ice contact slope).


Large quarries have been developed near Sultan, Evishanoran, and Killucan, at the western end of the valley, and nearer Cookstown at Drumshambo and Knockaleery. They display many of the typical features of deltas, eg. foresets and topsets with cross bedding.


Kettlehole lakes are another striking and numerous feature of the valley. These occur when large lobes of ice are buried in the till, and when they eventually melt the till subsides and the hollow becomes a lake, eg. Lough Bracken and sometimes occur in clusters as found around Cam Lough. They enhance the scenery and are sometimes adapted into fish farms. Prolonged large-scale extraction has had advantages and disadvantages. It has become a valuable secondary source of income in a region of marginal farmland. The Evishanoran deposits have a potentially high educational value, and there may be benefits for tourism. However if the extraction of sand and gravel proceeds on this scale it will seriously detract from the scientific, aesthetic and tourist value of the area. Consequently as sand and gravel are a finite resource it is important to draw up plans to protect this valuable landscape before active working encroaches any further.


Toney Bazley and James Rutherford (Geological Sectional Secretary)

 
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