The Geological History of the Pewsey Vale
Total Page:16
File Type:pdf, Size:1020Kb
189 THE GEOLOGICAL HISTORY OF THE PEWSEY VALE. By W. D. VARNEY, B.Sc. (Rl4<l May 6th, 1921.) PAGE. 1. INTRODUCTION 189 II. GENERAL DESCRIPTION 190 III. FOLDING OF THE PEWSEY ANTICLINE 191 (a) EVIDENCE OF PRE-EoCENE UPLIFT 191 (b) LATER UPLIFTS 192 IV. Tun PLATEAU GRAVELS •• 192 (a) CHARACTER OF THE GRAVELS 192 (b) AGE OF THE GRAVELS 194 (e) RELATION TO TilE PEWSEY VALE 195 V. THE REVERSAL OF TilE Avo:< DRAINAGE 196 VI. DEVELOP~ENT OF TilE BRISTOL Avox 197 VII. Tun KENKET. 199 VIII. THE LATER HISTORY OF TlIE PEWSEY VALE 201 IX. Sl.:~MARY OF CONCLl.:SlOKS 201 1. INTRODUCTION. N the centre of Wiltshire is the beautiful rural district I known as the Pewsey Vale, situated between the Marl borough Downs on the north, and the Salisbury Plains on the south. Devizes stands at the western end of the vale, and Pewsey near the centre of it. Its physical features were well described by A. J. Jukes-Browne,* who wrote thus :- " The Pewsey Vale is only a 'vale' when viewed from the range of hills which border its northern and southern sides. It is not a continuous valley occupied by a river running from one end to the other, but is rather a plain or plateau at a comparatively high level above the sea (from 350 to 540 fect); while the streams which rise within it do not flow out at either end, but through a gap in the southern escarpment." On page 43 of the same Memoir he states:- " The sources of the (Salisbury) Avon are to be found on the northern slopes of the Pewsey Vale and the river formed by the brooks which unite near Upavon flows southwards in a valley which has been cut through the escarpment on the south side of the Pewsey Vale. From this course and from the position of the gravels within the vale it is naturally inferred that, when the Avon began to cut its valley way the Pewsey Vale had not been developed, that the water shed lay to the north of this vale, and that from the water- • Geology of Devizes., Mem. GIOI. S"I'V.• 1905. p. I. 190 W. D. VARNEY, shed a nearly level or undulating plain sloped gradually southward across Wiltshire and Hampshire, In all pro bability the patches of . Clay-with-Flints' are remnants of this plateau. and it was on this surface that the River Avon began to cut its course. The present valley of the Avon has doubtless been developed pari passu with the development of the depression known as the Pewsey Vale." The same author also described the surface deposits which occur in the vale . Buckland,* in a paper" On the formation of the Valley of Kingsclere and other Valleys" described the Pewsey Vale as a " valley of elevation," the depression being caused by the re moval of rocks by denudation frc,n the weakened crest of an anticline. Though this may be partly the case, the present writer believes that ordinary fluviatile erosion has been largely respon siblc for the excavation and removal of the rocks of the vale. S. S. Buckman] described how the streams of the Pewsey Vale probably derive water from the Kenner by underground drainage, owing to th e lower elevation of the former valley, and he also showed that the west ern end ot the vale is being attacked by obsequent tributaries of the Severn drainage system. He predicted the further indentation of the vale by these streams until the Salisbury Avon must rise ncar the Upavon Gap (where it now leaves the vale), which will be further deepened by this obsequent stream. In 1865, T. Codringtonj described the occurrence of Eocene deposits just north of the Pewsey Vale, on successively lower zones of Chalk, and so deduced that the first uplift of the Pewsey anticline dated from pre-Eocene times. The object of the present paper is to pursue this subject further, and to investigate more fully the history of the Pewsey Vale, and to offer suggestions of a new kind as to its origin and relationship to the surrounding drainage systems. At the same time, the river systems of the district and their changes will also be discussed with regard to their bearing on the Pewsey Vale, and arising out of this, the development of the present Bristol Avon. II. GE~ERAL DESCRIPTION. The surface features, as described so well in the Survey Memoir already quoted, need no further description, unless it be to call attention to the sudden termination of the vale in the west by a steep slope down to the clay plain now occupied by the upper Bristol Avon. This steep slope is a continuation • Buckland, Trans . Geol. Soc., vol. 2, sere H" p. 1l9. t S. S. Buckman, Prot. Cotf<swold Nat. Field c.lub, vol. xiii. , pt. 3. p. ,86, et seq. : T. Codrington, Mag. lVilts . Arch. 6- Nat. Hist, Sor ., , 865. THE GEOLOGICAL HISTORY OF THE PEWSEY VALE. 191 of the lower part of the Chalk escarpment, and consists of Upper Greensand and Gault clays; but unlike the greater part of the escarpment, it has quite a number of small streams actively cutting into it, so that the end of the vale is irregular though abrupt. Viewed from the heights of the Cotteswolds about twelve miles to the west, the Pewsey Vale is seen as a great notch in the Chalk escarpment, similar to but much larger than others near it. Chalk hills and downs form the northern and southern margins, while westward the broad plain of the Bristol Avon, consisting of Kimmeridge and Oxford Clays, slopes gradually up to the Cotteswolds and the Oolitic escarpment. This plain is diversified by a low range of hills composed of Corallian rocks, and by low hills of ironsand of Lower Cretaceous age near the Chalk escarp ment. The watershed of the district roughly coincides with the Chalk escarpment, and passes through Devizes and the western end of the Vale of Pewsey. From it water drains east to the Kennet, south-cast to the Salisbury Avon, and west to the Bristol Avon. The latter finds its outlet from the broad clay vale to the Severn through a narrow gorge, at the head of which Bradford on-Avon stands, and which terminates some six miles nearer Bath. Mr. Harmer* characterised this gorge as an overflow channel from a great ice-dammed lake occupying the upper Avon valley. A careful study of the district, however, docs not at all support this view, and the available evidence suggests quite another origin for this interesting feature, to be dealt with hereafter. The Chalk and associated rocks of the whole area arc thrown into a series of gentle folds, with their axes running west to east. The Kennet Valley practically coincides with a shallow syncline continuous with the London Basin syncline, and the Pewsey Vale with an anticline in which the northern limb is steeper than the southern. From this anticline the dip, a gradual one southwards, is interrupted by anticlines, one faulted, at War minster and Wardour, and in the latter case the uplift is suffi -ciently great to have brought Jurassic rocks under denudation, as is also the case near the Pewsey Vale. At several places, both north and south of the vale, Eocene rocks are seen resting on the Chalk surfaces; these Reading Beds and also Clay-with-Flints rest on various horizons of the Chalk. III. FOLDING OF THE PEWSEY ANTICLINE. {a) EVIDENCE OF PRE-EOCDIE UPLIFT. It has already been shown] that Eocene rocks (Reading Beds) rest on an eroded surface of Chalk, in some places on divisions • Harmer,Quart. [ourn, Gtol. SOl'. , vol.lxiii.. IqC7, p. 484. t Mag. Wilts. Arch. & Nat Hist, Soc., vol. tx., p. 167, and ~lem. Grot. SUftl ., Country round ()evizes, 1905, p. 12. 19_ W. D. VARNEY, as low as the Terebratulina zone, and the nearer the approach to the Vale of Pewsey, the lower the zone immediately below the Tertiary rocks. Therefore the first uplift t ook place in the late Cretaceous and pre-Eocene times. Since at least 500 feet of Chalk are missing near the edge of the Vale, and if, as has been suggested, the depth at which the Upper Chalk was formed was probably 600 fathoms,* the total uplift must have been about 4,000 feet; whether the 500 feet of Chalk were re moved by submarine erosion or by sub-aerial erosion does not affect the final result. On the planed-down surface shallow water deposits of the early Eocene were laid down . Thus the first phase of the history of th e Pewsey Vale seems to have been an uplift from considerable depths to sea level or higher in late Cretaceous times, when the present anticline was initiated. (b) LATER UPLIFTS. Th e Pewsey anticline and the Kennet syncline are con temporaneous with the Weald anticline and the Lond on Basin syncline respectively. These were formed mainly during Miocene and early Pliocene times. West of th e Wealden area a land-mass was form ed, on which consequent strea ms probably flowed from the Welsh uplands; and th e drainage system so formed was the forerunner of the one now in existence. These rivers would doubtless form gravels, and though denudation during later Pliocene a nd Pleistocene times would remove most of them, some ma y be preserved, situat ed on what is now re latively high ground. IV. THE PLATEAU GRAVELS. (a) CHARACTER OF THE GRAVEL'S. Such gravels as those referred to in the preceding section, invariably found on the tops of hills, would naturally come under th e category of Plateau Gravels, similar to those described by the late Clement Reid in the Ringwood district of Hampshire'] associated with a now non-existent" Southampton River.