469

AUTHOR'S REPLY

The author would like to thank Mr. Gray for communicating the informationobtained from the exploratory boreholes sunk in the vicinity of the A.229Chatham road at the east end of the West Section of the Maidstone By-Pass andthe result of his work on the cutting. The reversed nature of the fault which he hasdemonstrated at this locality becomes less marked eastwards, but is still apparentin the Grange Lane Cutting Section 5 (1960, 372). The disturbance of the Gault,although marked at the faulted southern edge of the outcrop, does not extend toany great distance northwards. Although the exposure of the Lower Gault wasnot extensive in Section 6 at the Boarley Bridge (1960, 372), it showed little dis­turbance and possessed a dip of about r northwards. Farther to the south-eastin the north-west quadrant of the A.249 clover-leaf Section 4 (1960, 369), theangle of dip was found to be but a few degrees towards the NNE. near the junc­tion of the quadrant with the A.249 Sittingbourne road. It increased, however,to 25°-30° near the edge of the Gault outcrop which is cut by three minor strikefaults with throws in the direction of dip. The southern fault affects the Folke­stone Beds and the Gault-Lower Greensand Junction only while the other twoaffect the Lower Gault. On the north side of the By-Pass 200 feet west of thejunction of the north-east quadrant and about 700 feet along the strike fromSection 4, a single strike fault with a dip throw of a few inches could be seen andthis affected the Gault-Lower Greensand Junction only. On the south side of theBy-Pass this fault was no longer apparent (1960, 366). The officers of the Geo­logical Survey, who at that time did not have the benefit of the By-Pass sections,accurately terminated the strike fault at the point represented on Sheet 288. Thecutting of the By-Pass at this locality has shown, however, that the southern edgeof the Gault outcrop does not swing eastwards in the manner they indicated butcontinues farther on a strike bearing of 126° (true) before turning eastwards.

The principal fault which affects the southern edge of the Gault outcrop,therefore, becomes more important as one proceeds north-westwards from theA.249 Clover Leaf towards Aylesford.

H. G. OWEN

54 Auckland Rise,London, S.£.19.

The Editor

Dear Sir,

In a recent paper, 'Pleistocene Sea-levels in North Devon' (Proc. Geol. Ass.,Lond., 71, 169), the author, Miss M. A. Arber, states that two low-level surfacesof erosion are present, the Hele Surface (150feet plus to about 100 feet plus) andthe Croyde Surface (at about 50 feet). Both these surfaces are attributed to marineplanation of a periglacial deposit, the head, and the high sea-levels are stated as,'not likely to be of a later date than the last Interglacial, and the head may there­fore belong to the Penultimate Glaciation. The head itself is younger than theglacial erratics of North Devon, which may therefore belong to a still earlierglaciation' (p. 175).

There can be no doubt whatever that the head deposits (the plural is useddeliberately) are younger than the large glacial erratics of North Devon, becauseit can be shown that the head deposits overlie both the large erratics and theraised beach deposits in which many of the erratics lie. G. F. Mitchell has recentlypublished a paper (Advanc. Sci., 1960) in which it is suggested that the Mainor Lower Head dates from the Riss or Saale glacial period (the PenultimateGlaciation of Zeuner, 1959).

PROC. GEOL. ASSOC., VOL. 72, PART 4, 1961 31

470

Recent investigations show that the Main Head of the coastal seotions aroundBarnstaple Bay is a frost rubble deposit of great thickness, representing the pro­duct of a period of severe periglacial climate. The material consists of blocks ofall sizes of slate, sandstone and quartz, set in a sandy matrix, and where it has notbeen subsequently disturbed by later frost action the rock fragments have apreferred orientation downslope, often lying at gentle angles to the horizontaland projecting out of the face of the cliff when seen in section. From the higherground inland the marerial has moved down the old convex coastal slope as a kindof sludge, and spread out as a great 'apron' or solifluxion terrace with a concaveinner margin at the foot of the old coastal slope. The surface of this 'apron' tendsto lie at very low angles of less than three degrees of slope, forming an almostlevel 'shelf' or terrace. This is well seen at Saunton Down and Baggy Point(south side). The surface of this terrace is always composed of head, sometimesoverlying considerable thicknesses of raised beach shingle and sandrock (well­bedded sands, with shingle lenses and layers). No marine deposits have been foundto cap the Main Head, and in no place can the coastal 'aprons' or solifluxionterraces be shown to owe their final 'terrace-like' form to marine planation.

This is just as true in the river valleys draining to the coast, where the rivershave trenched the head deposits which flowed off the valley's sides to fill thevalleys to a considerable depth; very careful analysis of the sections is necessary todistinguish between the true river (alluvial) terraces and the dissected solifluxiondeposits, especially when the aim is to make interpretations as to the levelattained by sea-levels during the Pleistocene.

Furthermore, on the coast around Barnstaple Bay, at Saunton Downend andMiddleborough, for example, the surface of the Main or Lower Head has beendisturbed by later frost action; in places it is overlain by a layer of sand, abovewhich another layer of Younger or Upper Head can be seen; and the UpperHead may consist of multiple layers of frost rubble; frost wedges can be seen todissect the upper head at a number of points near Middleborough. Capping allthese head deposits is a variable thickness of sand and slate fragments, perhapswith some small pebbles, this representing a wash of material from higher up theslope, and this uppermost layer is known to contain Mesolithic flint flakes.

Close examination of these surfaces thus shows them to be without marinedeposits, and their morphology to be very uneven, with frequent changes ofaltitude; and at many places the heads can be traced in unbroken sequence fromthe present cliff section up slope across the surfaces to the old coastal slope at therear of the solifluxion terrace. If further proof is needed then it may be pointed outthat similar terraces can be observed in many parts of southern England, southWales and southern Ireland, that is, in areas which lay, for the most part, outsidethe limit of the glacial advances of the Riss (Saale) and Wiirm (Weichsel) periods;the phenomena has been recorded by many authors.

In point of detail, the 'Hele Surface' 'around Hele and Bickington' as reportedby Miss Arber (p. 171) is not cut across head deposits; the Hele-Ellerslie­Bickington rock ridge is capped by sands and gravels, in places probably some3Q-40 feet thick at least; the deposits show strong cryoturbation, convolutionsand frost wedges, but are in primary position. Mitchell (1960) has suggested thatthey represent an aggradation phase of the Taw-Torridge system in the earlyPleistocene, and the frost action probably took place during the Riss (Saale)glacial period.

The Anchor Wood river cliff (p. 172) cut in rock on the left bank of the Tawdownstream of Barnstaple, cannot have any significance in separating the Hele andCroyde surfaces; it is simply the undercut edge of the Hele-Bickington rock ridge.

InterglacialMindel/Elster

471

The Croyde Surface (p. 172) is essentially a solifluxion terrace in the CroydeValley; and around Fremington (lying to the south of the Hele-Bickingtonridge) the land between 50 and 200 feet consists for the most part of solifluxiondeposits-sandy, stony sludges, not unlike some boulder clays-which at theFremington clay pits can be seen to cap the chocolate boulder clay. Only at thecoast near Fremington Quay can the raised beach gravels be seen to reach thesurface, but here there is no typical head of slate rubble-the ground surfacesimply consists of the exposed raised beach (its surface deeply disturbed bycryoturbation), which can be seen in section to the west of Fremington Quay inthe cliffs, in the railway cutting to the east, and in the cliff near the old lime kilnof Penhill Point. This raised beach, equivalent to the beach sealed below MainHead on the outer coast, can be traced southwards below the Fremingtonboulder clay near Combrew Farm. Thus the 50 foot contour lies partly uponraised beach material (which is older than the boulder clay, and the Main Headof the outer coast) and partly upon solifluxion material which also seals off thetrue Fremington boulder clay. There is no trace whatever of an old cliff-line cuteither in the solifluxion materials or in the boulder clay near Fremington, yet aglance at the Ordnance Survey map will show that this area would have beencompletely open to a marine incursion which exceeded 50 feet 0.0.

To sum up, there is, in my opinion, no evidence for the planation of the headby marine transgressions, and the sequence of events around Barnstaple Bay mayrather be stated as follows:

Post-Glacial Alluvium, hillwash; Mesolithic implements.Wiinn/Weichsel Younger Upper Head and frost wedges.Interglacial No trace of marine deposits.Riss/Saale Disturbance of surface of Main Head (frost cryoturbation).

Main Head.Fremington Boulder Clay.Raised beach (deposit from 20--60 feet 0.0.).Large erratics reach the coast (Saunton Red Granite; Croyde(Freshwater Gut) granite-gneiss).

Early Pleistocene Cutting of the low rock platforms below 50 feet on whichthe large erratics rest.Hele-Ellerslie-Bickington sands and gravels.

REFERENCES

MITCHELL, G. F. 1960. The Pleistocene History of the Irish Sea. Presidential Addressto Section C, British Association, Cardiff, 1960. Advanc. Sci., 68.

STEPHENS, N. 1960. The Quaternary Deposits around Barnstaple Bay, North Devon:their stratigraphy and correlation. Paper read to Section C, British Association,Cardiff, 1960. Unpublished.

NICHOLAS STEPHENS,Department ofGeography,The Queen's University of Belfast.

AUlHOR'S REPLY

I made clear in my paper on 'Pleistocene Sea-levels in North Devon' that Ioffered my own conclusions simply as working hypotheses until more evidencewas forthcoming, and I welcome the fact that Mr. Stephens and Mr. Mitchellhave already carried so much further the study of what I termed the Hele andCroyde Surfaces, as well as having investigated the stratigraphical details of the