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Written discussion on paper taken as read: 1 April 1966
269
WRITfEN DISCUSSION ON PAPER TAKEN AS READ: 1 APRIL 1966 by S. C. A. HOLMES
The Editor Dear Sir, In the paper by Dr. C. L. So on 'Some Coastal Changes Between Whitstable and Reculver, Kent' (Proc. Geo/. Ass., 77, 475-90), many interesting records of and speculations on erosional processes are elaborated, and a valuable review of the literature included. A fascinating history of coast erosion from Roman times indeed awaits further study. The area of the map on p. 476, the geology on which is taken from the New Series One-Inch Faversham (273) Sheet of the Geological Survey of Great Britain, was surveyed by S. C. A. Holmes (Six-Inch Library Public Reference Copies Kent 23 NW. and SW., 23 SE.) and S. Buchan (Six-Inch Copies Kent 24 SW., NW., NE.) in 1939 and 1946 and in 1938 respectively. A comparison between this New Series survey and the Old Series One-Inch Sheet 3 (Drift added 1872) is particularly illuminating in regard to Stud (or Studd) Hill, where it will be seen that a capping of gravel had, between the dates of the surveys, almost been eroded away; the Six-Inch Sheet Kent 23 SE. shows indeed the last relic of this patch of gravel and the landslip adjoining it as it appeared in 1946. It may also be noted that the earlier among the topographic maps show a slight promontory at Stud Hill. The broad pattern of coastal evolution between Seasalter and Reculver has been controlled by the nature of the Drift and Solid geology and its related topography, to produce a series of embayments or relatively rapid recessions where the rocks are soft or incoherent. As Dr. So points out, there is a tendency for these to be accentuated on their eastern sides. They are at Whitstable (a tract of Alluvium; not shown separately in fig. 1), Swalecliffe (Head Brickearth and Alluvium; the latter not separately distinguished), Hampton (Head Brickearth) and at or near the outcrop of the Lower London Tertiaries (mainly soft sands) emerging from just below shore level eastward of Beltinge. In contrast, the London Clay cliffs are relatively resistant, especially where they contain numbers of septarian claystone bands and carry (or carried) initially protective cappings of Head Gravel, as at Stud Hill. When attack by the sea develops, however, there can be no doubt that landsliding in the London Clay is, as Dr. So envisages, a major factor in stimulating erosion. The presence of ground water in the top 5 ft. or more of the London Clay is determined by the degree of fissuring, shrinkage cracking and localised pocketing of superficial material in the clay; that the latter, with spring water, was strongly in evidence around Stud Hill is an indication of-the effects of impeded drainage in disturbed, frost-heaved, ground subjacent to a deposit of periglacial Head Gravel. In contrasting rapid erosion of London Clay with a low rate for the Lower London Tertiaries in the area, Dr. So may be aware from his historical studies that since early in the nineteenth century the cliffs at Reculver have been protected under the auspices of Trinity House. Before that time, as described by William Boys in Duncombe, Bibliotheca Topographia Britannica (1784), p. 84 in the reference he quotes, erosion of the cliffs of Thanet Beds and Woolwich Beds was as intense as it had been since the sixteenth century. The relatively recent rapidity
270 of erosion of the London Clay cliffs is but another striking phase in the evolution of the coastline which happens to have coincided with modern human development of the area; it will be hoped that artificial protection will ensure a period of stability from erosional attack on the now existing resultant low headlands at Swalecliffeand Hampton.
s. C.
A. HOLMES
Institute of Geological Sciences Exhibition Road London S. W.7 REPLY BY DR. C. L.
so
The Editor Dear Sir, I am grateful to Mr. S. C. A. Holmes for the assurance he has made with regard to the influence of drift and solid geology on processes of coast erosion. In my study, coast recession has been inferred primarily from a comparison of topographical maps of various dates. Mr. Holmes's reference to the disappearance of a capping of gravel between two geological surveys, to which the study owes the bulk of its geological details, confirms my observation. It must be pointed out that in relating the broad pattern of coastal evolution to the distribution of superficial deposits, tracts of alluvium have been deliberately omitted from fig. 1. This is done partly to avoid overcrowding the map but largely to emphasise the role played by gravel and brickearth. I am fully aware of the need to refer to sea-defence works in the past, especially around Reculver. Unfortunately a substantial section on these, incorporated in the original manuscript, had to be deleted to keep the text to the desirable length. This section has since been rewritten and expanded to cover a wider field. It is hoped that it will shortly go to press and will render the study under consideration more intelligible. C. L.
so
Department of Geography and Geology University of Hong Kong Hong Kong
WRITfEN DISCUSSION ON PAPER TAKEN AS READ: 1 APRIL 1966 by S. C. A. HOLMES
The Editor Dear Sir, In the paper by Dr. C. L. So on 'Some Coastal Changes Between Whitstable and Reculver, Kent' (Proc. Geo/. Ass., 77, 475-90), many interesting records of and speculations on erosional processes are elaborated, and a valuable review of the literature included. A fascinating history of coast erosion from Roman times indeed awaits further study. The area of the map on p. 476, the geology on which is taken from the New Series One-Inch Faversham (273) Sheet of the Geological Survey of Great Britain, was surveyed by S. C. A. Holmes (Six-Inch Library Public Reference Copies Kent 23 NW. and SW., 23 SE.) and S. Buchan (Six-Inch Copies Kent 24 SW., NW., NE.) in 1939 and 1946 and in 1938 respectively. A comparison between this New Series survey and the Old Series One-Inch Sheet 3 (Drift added 1872) is particularly illuminating in regard to Stud (or Studd) Hill, where it will be seen that a capping of gravel had, between the dates of the surveys, almost been eroded away; the Six-Inch Sheet Kent 23 SE. shows indeed the last relic of this patch of gravel and the landslip adjoining it as it appeared in 1946. It may also be noted that the earlier among the topographic maps show a slight promontory at Stud Hill. The broad pattern of coastal evolution between Seasalter and Reculver has been controlled by the nature of the Drift and Solid geology and its related topography, to produce a series of embayments or relatively rapid recessions where the rocks are soft or incoherent. As Dr. So points out, there is a tendency for these to be accentuated on their eastern sides. They are at Whitstable (a tract of Alluvium; not shown separately in fig. 1), Swalecliffe (Head Brickearth and Alluvium; the latter not separately distinguished), Hampton (Head Brickearth) and at or near the outcrop of the Lower London Tertiaries (mainly soft sands) emerging from just below shore level eastward of Beltinge. In contrast, the London Clay cliffs are relatively resistant, especially where they contain numbers of septarian claystone bands and carry (or carried) initially protective cappings of Head Gravel, as at Stud Hill. When attack by the sea develops, however, there can be no doubt that landsliding in the London Clay is, as Dr. So envisages, a major factor in stimulating erosion. The presence of ground water in the top 5 ft. or more of the London Clay is determined by the degree of fissuring, shrinkage cracking and localised pocketing of superficial material in the clay; that the latter, with spring water, was strongly in evidence around Stud Hill is an indication of-the effects of impeded drainage in disturbed, frost-heaved, ground subjacent to a deposit of periglacial Head Gravel. In contrasting rapid erosion of London Clay with a low rate for the Lower London Tertiaries in the area, Dr. So may be aware from his historical studies that since early in the nineteenth century the cliffs at Reculver have been protected under the auspices of Trinity House. Before that time, as described by William Boys in Duncombe, Bibliotheca Topographia Britannica (1784), p. 84 in the reference he quotes, erosion of the cliffs of Thanet Beds and Woolwich Beds was as intense as it had been since the sixteenth century. The relatively recent rapidity
270 of erosion of the London Clay cliffs is but another striking phase in the evolution of the coastline which happens to have coincided with modern human development of the area; it will be hoped that artificial protection will ensure a period of stability from erosional attack on the now existing resultant low headlands at Swalecliffeand Hampton.
s. C.
A. HOLMES
Institute of Geological Sciences Exhibition Road London S. W.7 REPLY BY DR. C. L.
so
The Editor Dear Sir, I am grateful to Mr. S. C. A. Holmes for the assurance he has made with regard to the influence of drift and solid geology on processes of coast erosion. In my study, coast recession has been inferred primarily from a comparison of topographical maps of various dates. Mr. Holmes's reference to the disappearance of a capping of gravel between two geological surveys, to which the study owes the bulk of its geological details, confirms my observation. It must be pointed out that in relating the broad pattern of coastal evolution to the distribution of superficial deposits, tracts of alluvium have been deliberately omitted from fig. 1. This is done partly to avoid overcrowding the map but largely to emphasise the role played by gravel and brickearth. I am fully aware of the need to refer to sea-defence works in the past, especially around Reculver. Unfortunately a substantial section on these, incorporated in the original manuscript, had to be deleted to keep the text to the desirable length. This section has since been rewritten and expanded to cover a wider field. It is hoped that it will shortly go to press and will render the study under consideration more intelligible. C. L.
so
Department of Geography and Geology University of Hong Kong Hong Kong