WRITTEN CONTRIBUTION TO THE DISCUSSION

OF A PAPER PREVIOUSLY READ: 6 APRIL 1962

The Editor

Dear Sir,

Mr. F. J. W. Holwill's paper (Proc. Geol. Ass., Lond., 73, 281) on the limestonesof the Ilfracombe Beds is an important contribution to the knowledge of Devonianstratigraphy in south-west England. Unfortunately I was unable to be present whenthe paper was read and, therefore, I should now like to comment on two importantaspects of his contribution.

The problem of calculating thicknesses in the Ilfracombe Beds is not discussedby Mr. Holwill, despite the fact that these beds have been subjected to cleavagefolding. As Cloos (1947) has shown, in areas of cleavage folding it is necessary toanalyse the deformation before original stratigraphical thicknesses can beobtained. Examples of cleavage folds have been described from the North Devoncoast by de Sitter (1958) and, in discussing cleavage folds generally, he hasemphasised that a total compression of between somewhat less than 30 %and amaximum of 70% is required for their development. Evans (1922, 203) fullyrealised the difficulties in calculating thicknesses of the North Devon rocks, andstressed that his estimates of thickness were very approximate. His estimate of1290 feet for the sequence between the Newberry Beds and David's Hole Beds(inclusive) is reduced to about 350 feet by Mr. Holwill (1963, 291). This reducedthickness has been obtained by detailed measurements of the sequence afterallowing for the effects of the folds. However, in rocks subjected to cleavagefolding it is not sufficient to unravel the folding, for internal adjustments havealso occurred in the rocks, as emphasised, for example, by the 'slurred ooliths'described and figured by Mr. Holwill (1963, 284, and plate 13A). Therefore, Mr.Holwill's thicknesses must be regarded as tentative values until the deformation inthe rocks has been completely analysed.

In the discussion of the stratigraphical position of the limestones, Mr. Holwillhas indicated (1963, 291) that the Combe Martin Beach Limestone has a coralfauna 'not previously recorded from Devonian strata in the British Isles (with theexception of a doubtful species of Metriophyllum from South Devon)'. Mr.Holwill seems to have overlooked a somewhat similar fauna from South Devon(including Syringaxon and ?Metriophyllum, along with species of Thamnopora,Alveolites and Pleurodictyum), recorded by Smith (1953) in Lower Devonianslates between Plymouth Sound and Start Peninsula. Also, Mr. Holwill incor­rectly listed Dorothy Hill's (1939) Western Australian Devonian coral fauna asincluding Syringaxon when, in fact, only Barrandeophyllum, Metriophyllum,Thamnopora and Alveolites are common to both the Combe Martin BeachLimestone and the Western Australian occurrences. Hill considered this WesternAustralian fauna indicated an Upper Givetian or Lower Frasnian horizon,possibly the latter. Such a long-range correlation with the type-successions ofEurope, applied to such narrow limits as the Upper Givetian or Lower Frasnian,is a rather speculative practice. Even more doubtful is a correlation of the CombeMartin Beach Limestone (in the nominal type-area of the Devonian) with anhorizon near the Middle-Upper Devonian boundary because it contains fourcoral genera in common with Western Australian 'Upper Givetian or LowerFrasnian' beds, as Mr. Holwill has suggested. With the possible exceptions of thesimilarity of the laccophyllids with those in the upper Middle Devonian ofBohemia and the closeness of Metriophyllum to M. bouchardi Edwards & Vaime

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from the Frasnian at Boulonnais, the coral evidence cited by Mr. Holwill does notindicate dating closer than Middle or lower Upper Devonian. Until specificassignments from each limestone formation are given, neither restricted agedeterminations nor the position of the Givetian-Frasnian boundary can bederived from the coral faunas. There is a strong possibility that the faunaldifferences between the Jenny Start Limestone (with its compound rugose corals)and the Combe Martin Beach and David's Stone Limestones (with their small,solitary rugose forms) result from a change in environmental conditions duringsedimentation of the respective limestones, rather than representing a time­significant faunal change coinciding with the Givetian-Frasnian boundary.

REFERENCESCLODS, E. 1947.Oolite Deformation in the South Mountain Fold, Maryland. Bull. geol.

Soc. Amer., 58,843-918.DE SITTER, L. U. 1958. Boudins and Parasitic Folds in Relation to Cleavage and

Folding. Geol. en Mijnb., N.S., 20, 277-86.EVANS, J. W. 1922. The Geological Structure of the Country round Combe Martin,

North Devon. Proc. Geol. Ass., Lond., 22, 201-28.HILL, D. 1939. Western Australian Devonian Corals in the Wade Collection. J. roy.

Soc. W. Aust., 25 (8), 141-52.HOLWILL, F. J. W. 1963. The Succession of Limestones within the Ilfracombe Beds

(Devonian) of North Devon. Proc. Geol. Ass., Lond., 73, 281-93.SMITH, S. 1953. Notes on Corals from Lower Devonian Rocks of S.W. Devonshire.

Trans. R. geol. Soc., Cornwall, 18 (3), 299-308.

B. D. WEBBY

Department ofGeologyThe University, Bristol 8

REPLY BY TIlE AUTHOR

I am grateful to Dr. B. D. Webby for his comments on my paper. Although theproblem of measuring thicknesses in the highly folded and cleaved IlfracombeBeds was not discussed, I am only too conscious of the limitations which areinvolved. The measurements given in my paper were made at right angles to thebedding surfaces and on a single normal limb of a fold. These measurementswere found to agree tolerably well with those made by Evans over limited parts ofthe succession (e.g. Evans, 1922, p. 212, para. 5). The fundamental reason for thedifference in total thickness of the sequence between the Newberry and theDavid's Hole Beds as measured by Evans (1922) and myself is that he consideredthere were five units in this part of the succession, whereas I recognise only two ofthese. Later, Evans himself dropped the 'Slates and Limestones of Oakestor Bay'(Evans, 1929) from his succession and my own work has shown that the David'sHole Beds and the Red Limestone Series are to be correlated; likewise the JennyStart Beds and Newberry Beds. This then is the basic reason for our differentthicknesses, not any failure to make allowance for compression. Incidentally, Dr.Webby errs in suggesting that Evans thought his measurements of this part of thesuccession were only approximations; he specifically excepted these minordivisions when he wrote: 'the estimates of the thickness of the strata are intendedmerely to give some idea of the probable thickness' (Evans, 1922, 203).

I accept Dr. Webby's correction regarding the record of Syringaxon by Hill(1939) and I agree with him that the faunal evidence for dating the CombeMartin Beach Limestone is not as strong as could have been wished. Nevertheless,he himself accepts that the fauna is indicative of a Middle or Lower Upper