A Discussion on valley slopes and cliffs in southern England: morphology, mechanics and Quaternary history - The development of an abandoned cliff in London Clay at Hadleigh, Essex

Abstract

Hadleigh Cliff forms part of a line of abandoned London Clay slopes, rising to a height of generally + 40 m o.d. or more, which extends westwards from Southend-on- Sea. The cliff, with its toe level originally at about —19 m o.d., was formed by strong fluvial erosion in the Middle and Late Devensian. By the latter part of the Late Devensian erosion had virtually ceased and since then the cliff has degraded in an episodic manner, largely in response to climatic changes. Four main stages of degradation, with intermediate periods of relative stability, have been recognized and dated, as follows: (1) Late-glacial, periglacial mudsliding, associated with a toe level of —19 m o.d. (2) Early Atlantic, temperate mudsliding, associated with a toe level which was rising with the continuing Flandrian aggradation, but lay on average at about —9m o.d. (3) Early Sub-Atlantic, temperate mudsliding, taking place to the present toe level of about + 3 m o.d. (4) A late 19th century, moderately deep-seated landslide in the crest of the slope, possibly caused in part by human interference. The times at which the first three of these stages of degradation occurred are believed to represent periods of generally increased mass movement activity in much of Britain and Europe. The present morphology of Hadleigh Cliff comprises a straight 20 scarp at the crest, an irregular and actively unstable 11° degradation zone, fronted by a smoother, quasi-stable accumulation zone inclined at about 8°. From a knowledge of the volumes and dates of the various colluvial units mantling the slope, reconstructions of earlier positions of the cliff profile are made. These indicate that during the last 10 000 years the inclination of the combined degradation zone and crest scarp has declined from about 19° to 13°, while that of the accumulation zone has remained relatively constant. The accompanying recession of the cliff crest has been approximately 50 m. From the pattern and dating of the various stages of colluviation, which increase both in age and in degree of fabric breakdown from crest to toe of the slope, it is clear that the cliff is degrading from the top. This is also reflected in the fact that the zone of weathered, in situ London Clay beneath the colluvium diminishes in thickness, in general, from bottom to top of the slope and is entirely absent beneath the late 19th century landslide. In an average year the potential evaporation at Hadleigh exceeds the rainfall. As a result soil moisture deficits are unusually high and appreciable pore-water tensions in the capillary zone probably exist even at times of maximum seasonal piezometric levels. Account is taken of these in the stability analyses that are carried out, which indicate that the accumulation zone has a factor of safety of around 1.05 in com­ parison with the value of unity obtaining in the currently unstable degradation zone. A comparison between the values of (pT indicated by the back analyses and those measured on the Hadleigh colluvium in ring shear shows the latter to be appreciably the lower: the discrepancy is reduced if the effects of pore-water tensions in the capillary zone are allowed for.