Photo 4 (Return)

NE Isle of Portland, HM Prison 2001. A hard Portland Stone caprock rests upon Kimmeridge clay (dark material exposed in mod-slope). This stratigraphy results in deep-seated landsliding as large blocks become detached from cliff top and rotational slip planes develop deep within the underlying clay. Brunsden et al. (1996) have proposed that the weight of the caprock upon the Kimmeridge clay results in extrusion or outflowing of the clay laterally from the mid cliff face and underliff. Limestones and Portland cherts released from erosion of the toe deposits drift eastwards towards Portland Harbour (to the right) becoming intercepted in places by landslide lobes and boulder aprons.