The structural geomorphology of the Isle of Portland, southern England

Abstract

The Isle of Portland, southern England, is composed of gently folded Upper Jurassic Beds ranging from the Kimmeridge Clay through the Portland Group to the Lower Purbeck Group which are gently folded to form part of the northern end of the Shambles Syncline. The essential arrangement of the beds is for hard, jointed well-bedded and permeable Purbeck and Portland limestones to overlie the Kimmeridge Clay members. The jointing along NW-SE, NE-SW, N-S and E-W master and conjugate sets closely parallels the axis of the syncline and the NNE-SSW fault pattern of the Purbeck Anticline and is a major regional landform control at all scales from the occurrence and form of individual rock falls to the shape of the island itself. Portland has a large number of landslips, which have been mapped from colour air photography. Their spatial pattern is spectacularly related to the geological conditions of the island and varies in size and type in a systematic manner as the thickness of clay and the orientation of the dip changes with respect to the coastline orientation. The slips, which occur predominantly in the winter months after heavy rainfall, are of frequent occurrence and pose major hazards to engineering structures. The landslide pattern and the overall morphology of the island is used to suggest a tentative model of landscape evolution which emphasizes the process of lateral spreading, loading, clay extrusion and erosional unloading.