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Three principal categories of slope instability can be recognised:1. Rotational Landslips2. Mudflows and Debris Flows3. Rock Falls1. Rotational LandslipsAt a number of locations (Fig. 7.1) the edge of the Antrim Basalt Plateau consists of large scale, deep-seated,multiple rotational landslip features. The mechanisms behind this instability are directly related to the geologicalsuccession and the geomorphological processes that subsequently sculpted the landscape. The plateau edgeis capped by hard “competent” rocks: basalt lavas (Antrim Lava Group, Tertiary) and chalk (Ulster WhiteLimestone Formation, Cretaceous). These rocks overlie softer, less competent, impermeable mudstone:(Waterloo Mudstone Formation, Jurassic and the Penarth Group & Mercia Mudstone Groups, Triassic) (Fig.7.2 and Plate 7.1).Fig. 7.2 Generalised Landslip Model Co Antrim (Crown Copyright)During the last ice age, ice sheets flowed along the edge of the plateau eroding the soft mudstones rocks at itsbase. This undercut and oversteepened slope became inherently unstable. Along the plateau edge, failurestook place on the vertical and steeply inclined surfaces within the chalk and basalt and along more shallowlyinclined surfaces within the underlying more plastic mudstones (Fig. 7.2).The large, older rotational landslide blocks have, through time, reached a state of equilibrium. These areas dohowever remain susceptible to ground movement and remain at risk of rock falls, shallow slumps and translationalslides. In north and west Belfast an extensive area of palaeo-landslips continues to be a significant constrainton development. Where previous development has taken place on landslipped ground, dwellings and infrastructurefrequently suffer from cracking and disruption of foundations as a result of continuing ground movements.Stephens (1964) linked the large scale slip features at Benevenagh, County Londonderry and at Glenarm, CoAntrim to glacial action. Carney (1974) carried out a study of the landslide complexes along the Antrim coastusing aerial photographs and applied his interpretation of the geomorphological history of the area as follows:“In late Tertiary times the lava plateau was affected by a series of erosion cycles seen as bench levels at 600m,310m and 250m. Formation of the landslip escarpment may have occurred towards the end of this period afterthe junction between the Ulster White Limestone Formation and Waterloo Mudstone Formation had beenexposed to marine erosion”.During the early stages of glaciation in Northern Ireland, ice spread across Antrim from centres in Scotland andIreland. The “Irish” ice spread eastwards across the Antrim Plateau and “Scottish” ice moved southwardsacross parts of Co. Antrim and Co. Down. A combination of the eastwards moving Irish ice and topographyrestricted the spread of the Scottish ice to the northern and eastern coastal fringes of Antrim (Bazley, 2004).Glacial erosion removed much of the pre-glacial landslip debris along the edge of the Antrim escarpment. Bythe time the ice had melted and retreated, the edge of the escarpment had been oversteepened and unconstrainedthus precipitating a new phase of landslipping.73