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Peat flows are not nearly so well documented in the international literature. However they are very prevalent inIreland and feature considerably in the database of past events. In Ireland they have also been called bogbursts or bog flows. As with other types of materials they may have an initial sliding mechanism beforebecoming a flow. Peat is a very complex material in engineering terms and this is discussed more fully in thechapter on the geotechnics of landslides.SlidesSlides involve the displacement of masses of material along well-defined surfaces of rupture called slip or shearsurfaces. The material moves en masse but is likely to break up with distance from the initial rupture point.Sliding is common in the British Isles due largely to the availability of earth materials which facilitate basalshearing (Department of the Environment,UK, 1994). Slides can be divided into rotational and translationalslides. However it may not be able to define the failure mechanism, particularly in older slides, so these mightbe classified as undifferentiated slides.Rotational Slides. These involve sliding on a shear surface which is concave upwards in the direction of movementwhere the displaced mass rotates about an axis which is parallel to the slope. The back or crown of the slideis marked by a crack or scarp slope which is concentric in plan. The displaced mass may flow further downslopebeyond the rupture surface to form a zone of accumulation at the toe of the total feature. However where the slipsurface dips into the hill, the downslope momentum may be arrested somewhat and the sliding stop. Rotationalslides can be single events or more commonly multiple events where there are sequential rotational slidesdown the slope. There is an extensive terminology on the anatomy of landslides (Anon, 1990). Fig. 2.2 illustratesthe anatomy of a slide and the terminology is included in Appendix 3.Translational Slides. These are also called planar slides. The mass of material moves downslope on a largelyplanar surface. There is little rotary movement and consequently little backward tilting of the earth materialswhich is characteristic of a rotational slide (Fig. 2.2). Translational slides can have very different impacts torotational slides. Where the slope is sufficiently steep and the shearing resistance along the slip surfaceremains low, the movement can continue on for a considerable distance. This is quite different to rotationalslides as described above. This has ramifications for risk assessment and planning controls. Translationalslides in rock usually occur along discontinuities such as bedding planes or joints. In the case of debris slidesfailure can occur on shallow shear surfaces at or near the base of the surface materials where there can bemarked changes in strength and permeability. Slopes where the discontinuities lie parallel or sub-parallel to theground surface would be more prone to translational sliding.Fig. 2.2 Landslide Features. (After Varnes, 1978)6