Summary of site:
The north of Ireland is famous for its glacial land forms, with landscapes dominated by glacial processes which have left their mark in a variety of ways. The Annalong Valley, carved into granite of the Mourne Mountains in its upper reaches and opening on to a lowland underlain by slightly altered Silurian sediments to the south, provides important evidence of the behaviour of the mountain glaciers during the last great continental glaciation in Ireland, the Midlandian.
There were two quite different kinds of glaciers at work in the area during the Midlandian: mountain glaciers accumulating initially as snowfields on high mountain uplands and flowing down valleys; and continental or regional glaciers (as in the Antarctic today) which blanketed the surrounding land in slow-flowing ice sheets.
Critical evidence for the limits of ice activity are moraines (dumps of glacial rubbish scoured out by the ice and released as the ice melts). If the ice is in steady retreat during climatic warming, the debris is released fairly evenly over the ground and recognisable moraines are not created; but if the rate of flow is matched by the rate of ice melt, while the end of the glacier appears to be stationary, glacial debris is continually released at the same point. Over a few years of climatic equilibrium it accumulates into pronounced linear mounds, usually curved to the shape of the glacial snout. Such ridges are called terminal moraines and mark the temporary arrests of ice sheets. As ice melts back over a landscape or into a glacial valley, a series of such moraines can be left in its wake that can be dated relative to each other (the furthest from the ice source being the oldest).
Should the climate deteriorate again during this process, glaciers may be re-energised. When this happens, old moraines are cut through or eliminated by advancing ice and new moraines may form. In these circumstances, it is the cross-cutting relationships that assist in dating, the most intact structure cutting another being the later of the two.
A mountain glacier, mainly fed by the ice fields in the corrie at the Castles of Commedagh and augmented by smaller glaciers from corries at Cove Lough, Slievelamagan, Blue Lough and Binnian Lough, occupied the Annalong Valley during the Midlandian. This glacier spread on to the continental ice sheet that flowed around the Mournes on the lower ground. Here a moraine of the ice sheet is clearly cut by the lobe of the furthest terminal moraine of the Annalong Valley glacier. This establishes that the mountain glacier remained active after the general glacial retreat.
Two main glacial events are recognised in the Annalong Valley. The first was the major event when the glacier completely filled the valley and spread out as a tongue on to the low ground, interrupting a moraine of the retreating continental ice sheet. This is presumed to have occurred at the height of the last glaciation, around 22,000 years ago. Later, as the climate again deteriorated towards the end of the Midlandian, a small glacial re-advance cut some of the earlier moraine structures within the valley.