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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 andReculver, Kent' (Proc. Geo/. Ass., 77, 475-90), many interesting records of andspeculations on erosional processes are elaborated, and a valuable review of theliterature included. A fascinating history of coast erosion from Roman timesindeed awaits further study.

The area of the map on p. 476, the geology on which is taken from the NewSeries One-Inch Faversham (273) Sheet of the Geological Survey of GreatBritain, was surveyed by S. C. A. Holmes (Six-Inch Library Public ReferenceCopies Kent 23 NW. and SW., 23 SE.) and S. Buchan (Six-Inch Copies Kent 24SW., NW., NE.) in 1939 and 1946 and in 1938 respectively. A comparison betweenthis New Series survey and the Old Series One-Inch Sheet 3 (Drift added 1872) isparticularly illuminating in regard to Stud (or Studd) Hill, where it will be seenthat a capping of gravel had, between the dates of the surveys, almost been erodedaway; the Six-Inch Sheet Kent 23 SE. shows indeed the last relic of this patch ofgravel and the landslip adjoining it as it appeared in 1946. It may also be notedthat the earlier among the topographic maps show a slight promontory at StudHill.

The broad pattern of coastal evolution between Seasalter and Reculver hasbeen controlled by the nature of the Drift and Solid geology and its relatedtopography, to produce a series of embayments or relatively rapid recessionswhere the rocks are soft or incoherent. As Dr. So points out, there is a tendencyfor these to be accentuated on their eastern sides. They are at Whitstable (a tractof Alluvium; not shown separately in fig. 1), Swalecliffe (Head Brickearth andAlluvium; 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 LondonClay cliffs are relatively resistant, especially where they contain numbers ofseptarian claystone bands and carry (or carried) initially protective cappings ofHead Gravel, as at Stud Hill. When attack by the sea develops, however, therecan be no doubt that landsliding in the London Clay is, as Dr. So envisages, amajor 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, shrinkagecracking 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-theeffects of impeded drainage in disturbed, frost-heaved, ground subjacent to adeposit of periglacial Head Gravel.

In contrasting rapid erosion of London Clay with a low rate for the LowerLondon Tertiaries in the area, Dr. So may be aware from his historical studiesthat since early in the nineteenth century the cliffs at Reculver have been protectedunder the auspices of Trinity House. Before that time, as described by WilliamBoys in Duncombe, Bibliotheca Topographia Britannica (1784), p. 84 in thereference he quotes, erosion of the cliffs of Thanet Beds and Woolwich Beds wasas intense as it had been since the sixteenth century. The relatively recent rapidity

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of erosion of the London Clay cliffs is but another striking phase in the evolutionof 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 ofstability from erosional attack on the now existing resultant low headlands atSwalecliffeand Hampton.

s. C. A. HOLMES

Institute of Geological SciencesExhibition RoadLondon S. W.7

REPLY BY DR. C. L. soThe Editor

Dear Sir,I am grateful to Mr. S. C. A. Holmes for the assurance he has made with regard tothe influence of drift and solid geology on processes of coast erosion. In my study,coast recession has been inferred primarily from a comparison of topographicalmaps of various dates. Mr. Holmes's reference to the disappearance of a cappingof gravel between two geological surveys, to which the study owes the bulk of itsgeological details, confirms my observation.

It must be pointed out that in relating the broad pattern of coastal evolution tothe distribution of superficial deposits, tracts of alluvium have been deliberatelyomitted from fig. 1. This is done partly to avoid overcrowding the map butlargely 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, especiallyaround Reculver. Unfortunately a substantial section on these, incorporated inthe 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 ishoped that it will shortly go to press and will render the study under considerationmore intelligible.

C. L. soDepartment ofGeography and GeologyUniversity of Hong KongHong Kong