Morphological mapping in the Axmouth-Lyme Regis Undercliffs, Devon

Abstract

Summary Morphological mapping has proved extremely useful in a renowned area of coastal landslipping. The undercliffs form a coastal strip nearly six miles long, and up to 500 m in width. The antiquity (possibly late Pleistocene in part), and the large scale of much of the landslipping have assisted in the development of a mature, luxuriant vegetation cover, which masks much of the ground detail. However, the main back and side scars have often remained clear from the air, enabling photogrammetry to be used to produce base maps at a scale of 1:2500. Ground details where added to the base maps from fieldwork clearly reveal the patterns of mass movement. Examples of the final geomorphological maps are presented. Some 19th century interpretations of the mechanisms of landslipping indicate the advantages of observing the features in a fresh, lightly vegetated state. The present programme of morphological mapping has, hopefully, revealed a sufficient quantity of the detail to enable a reassessment of the area in the light of modern theory. Detailed morphological mapping is also presented as a site investigation technique of considerable merit in areas of difficult access and terrain, or in protected areas.

The Cowms Rocks Landslide

“…The Cowms Rocks landslide is a complex feature which shows certain similarities to the well-known translational slide studied by Arber (1973) and Pitts (1979) at Dowlands on the south Devon coast. The morphology is remarkably similar, but whereas at Dowlands, slope instability became critical when the cliffs were undercut by the sea, the slopes at Cowms Rocks have experienced a much longer denudational history in which failure occurred some time ago.…”

“…During the Quaternary the hillslopes were covered with surficial deposits. There is evidence for Pleistocene valley floor bulging and hillside cambering throughout this part of North Derbyshire (Johnson 1965;1987;Stevenson and Gaunt 1971), but no glacial deposits have been found. In the Devensian Glacial, the northern part of the Derwent catchment remained largely ice-free even though the Irish Sea ice-sheet reached and covered the lower Pennine hills east of Manchester.…”

The temporal and spatial development of landslides in the axmouth‐lyme regis undercliffs national nature reserve, Devon

“…The main aims of this mapping were to produce a clear picture of the spatial distributions and complexity of the slope forms and processes, and to highlight geological controls on landsliding. The actual process of mapping in this area has been described elsewhere (Pitts, 1979a) and will not be repeated here. An overview of the various geomorphological units present in the Reserve was possible only after the completion of the mapping programme.…”

“…This map was based on the principles outlined by Waters (1958), with certain modifications. Negative slopes were included as a separate category, these being common in areas where some rotational component to the landslide mechanism exists (Pitts, 1979a). Slope categories maps provide a very succinct cartographic summary of a host of descriptive and interpretive data, such as those on geomorphological maps.…”

“…Vertical aerial photographs from nine sorties flown between 1946 and 1972 were also available for study. However, only relatively small amounts of detail could be gleaned from these, a problem which has already been discussed (Pitts, 1979a). Four sets of oblique aerial photographs covering the period 1956 to 1977 were also available, and proved useful when unclear or anomalous features appeared on the vertical cover.…”