Chines on the Isle of Wight: Channel Adjustment and Basin Morphology in Relation to Cliff Retreat

Abstract

Deeply incised valleys are common landforms along the cliffed coastline of Dorset and Hampshire, a coastline which bears the brunt of the erosive effects of waves, wind and running water as is evident from this study of the Isle of Wight chines. A model was developed in order to examine stream profile and knickpoint recession based on the theories of Culling (1957) and Leopold et al. (1964). Measurements of stream power, rate of cliff retreat and area of drainage basin were interpreted to determine the sequential development of chines as distinct from their inland valleys. It was concluded that chines are in a continual state of flux in contrast to their upper valleys and the relative role of cliff and catchment characteristics have important consequences for their durability. CLIFF RETREAT has featured very little in geographical literature, and the writing has usually been in a descriptive form, as shown by the studies of May (1964) and Steers (1953). The great length of eroding cliff coastline delimiting the British Isles offers the opportunity to study the effects of cliff retreat as a boundary condition to geomorphology's fundamental unit, the drainage basin. The concept of an equilibrium, or balance, existing between the effect of a downcutting river, and processes of erosion on the slopes, with respect to the basin characteristics and the inputs to the basin, is long established. As most rivers terminate at the coast, another variable, that ofthe retreating coast, should be incorporated into the concept. This study attempts to examine the balance between basin morphology and channel adjustment on the one hand, and cliff retreat on the other, with a view to understanding the presence of the 'chines' on the south coast of the Isle of Wight. 'Chine is a local word for a deep fissure or gully cut in soft rock by a stream descending steeply to the sea' (Stamp, 1966). In fact, this definition is too narrow and was extended to include some features not occupied by streams. Suggestions as to the cause of chines have varied from landsliding (Lyell, 1867), to enlargement after rainfall (Gardner, 1879), wind erosion (Englefield, 1816) and spring sapping (Bristow, 1889). Bury (1920) commented that where the chines were lengthening, headward retreat was greater than cliff recession. He found the 'double structure' of a chine within a more open valley to be common (Fig. la) and claimed that the chines were formed recently, being the result of present climatic conditions, and their size was a function of the power of their streams. The open valleys above were formed by much larger volumes of water. He suggested that the different levels of chines were caused by sea-level changes, an argument later supported by Steers (1953). The most recent reference to cliff retreat and river erosion was made by Cotton (1947) when he referred to valleuses in Normandy. He stated, 'cliff recession is the sole cause of the rejuvenation that is commonly developed in the lower valleys of small streams entering the sea'. Description of problem and area The south-west coast of the Isle of Wight between St Catherine's Point and Freshwater Bay is a cliff line formed in Cretaceous strata. Structurally these exhibit an eroded anticline, the main axis extending from the Purbeck and Weymouth anticlines in the west through to Brook Point on the Island. Here the oldest deposits, Wealden Marls, are revealed in the cliff and beach. Either side, the rocks become increasingly younger, sloping in the western limb at more that 20? and on the east at 2?-5?. At intervals along the cliff top, superficial deposits include blown sand, alluvium, valley brickearth, gravel terraces and plateau gravel. Inland the chalk downs delimit an area -? Miss Flint is Land Survey Assistant for the Dorset Heritage Coast Team, Dorset County Council, Dorchester. This paper won the 1980 RGS Essay Prize, and was re-drafted for publication in 1981.