189 THE GEOLOGICAL HISTORY OF THE PEWSEY VALE. By W. D. VARNEY, B.Sc.
(Rl4 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 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. " The axi s of the Pewsey Vale (but not the anticlinal axis), points westwards almost directly to the Bradford Gorge of the Bristol Avon , and on the tops of the highest hills bounding it gravels of the Plateau Gravels t ype are to be found (Fig. 22). They were first noticed on Farleigh Down by C. H. Weston.j and again by Prestwich§ in his account of th e " Westleton Beds," being ascribed by him to those beds, of Pliocene age . The des criptions given by these authors, however , are very incompl ete since the constituents throwing most light on the source of the •A. J. Jukes-Browne. Bu ild' tlt 0' tloe British Isles (1911). p. 325. t Mem. Geol, Sur v., Ringwood Sheet. ' 902 . t C. H . Weston, QU4rl. J oum . Gtal. Soc., vol. 1850, p, H 9· § Prestwich, Quart. Journ. Geo!. Soc., vol. 1890, p. '43. THE GEOLOGICAL HISTORY OF THE PEWSEY VALE. 193 deposits are not even mentioned. Consequently their theories as to the origin of the gravels are necessarily misleading. The present writer has now found identical deposits south of the Avon Gorge, on Bathampton Down. They form a surface deposit covering these hills at a height of 500 feet to 650 feet FIG. 22.--CoNTOt;R MAP OF THE BRADFORD GoRGE. Areas with white dots show positions of Gravels. above the Avon in the gorge, resting on Great Oolite Limestone. The matrix is a reddish-brown sandy loam, and contains :- 1. Numerous angular and subangular flints. 2. Flint pebbles, often broken by frost. 3. Many quartz pebbles, white, pink and brown in colour. w. D. VARNEY, 4. Fine and coarse sandstones, in flat pebbles, 5. Occasional pieces of conglomerate, containing pebbles similar to NO.3. 6. Fragments of Palseozoic Shales, resembling shales from the Coal Measures. 7. Small pieces of coal. (Rare). 8. Rolled and subangular pieces of local oolitic limestone. The flints ar e thickly patinated, and frost-shattered , sug gesting prolonged exposure to weathering. much more so than the few flints which occur in the valley gravels below. Th e qu artz pebbles were derived, apparently, from a conglo merate, pieces of which were transported entire from their seat of origin. along with numbers of sandstone fragments. The conglomerate and sandstones suggest strongly that they were derived from Old Red Sandstone deposits. such as occur in South Wales, North Somerset and Gloucestershire. Such an origin would account for the sandy nature and red colour of the matrix, the presence of fragments of coal and shale. all of which mu st have come from the west or north-west ; and th e compar ative rarity of the soft er rocks points to the fact that the gravels ar e some considerable distance from their original source. Again. a current required t o transport such pieces of conglo merate and pebbles up to two inches in diamet er must have been fairly strong , and so must have come from high ground. The evidence of the pebbles shows this could only have been South Wales, and so one arrives at the conclusion that in these gravels we have remains of a river which origi nally flowed east wards from the Welsh uplands (Fig. 25a). Further. the abundance of Chalk flints in these gravels shows that the Chalk exte nded much farther west than it does now; most likely the river which deposit ed the gravels flowed in a broad valley with Chalk on either side, from which the flints were derived, simil ar to the Pewsey Vale at present, (b) THE AGE OF THE GRAVELS. As point ed out by West on, these gravel deposits differ from those forming the undoubted Pleistocene terraces in the valley below, in several respects. I. The valley gravels contain relatively few flints, while such st ones are numerous on the hill-tops. 2. Remains of Elephas, Rhinoceros, Sus, Equus, Bos and Ursus have been obtained from the valley, but the Plateau Gravels have yielded no organic remains, and no human artifacts. Some specimens show what ma y be traces of flaking, but no undoubted imple ment s or worked flint s occur. 3. The Pl at eau Gravels occur at heights of 500 to 600 feet THE GEbLOGICAL HISTORY OF THE PEWSEY VALE. 195 above the river, while the terraces are only a few feet above it. 4. As has been pointed out already, the flints of the higher gravels are thickly patinated, and in many cases are weathered even to the centre, while the flints of the river terraces are comparatively fresh. Everything points to the much greater age of the Plateau Gravels, and they seem to be of pre-Pleistocene age, so that they presumably date from at least Pliocene times. This is the date assigned by Clement Reid to the Plateau Gravels in the Ring wood district, and in all likelihood the river flowing out of the Pewsey Vale was a tributary of Reid's "Southampton River," if not the main stream. Another conclusion appears justified. If this explanation of the Farleigh and Bathampton Down Gravels is correct they must have been formed before the Bradford Gorge was cut, and, in fact, before the present west-flowing Avon existed. R....Wfl. .. Ia - ~ 3 of f FIG. 23.-SECTION FROM PEWSEY VALE TO FARLEIGlI DOWN. (c) RELATION TO THE PEWSEY VALE. The question next to be considered is :-What became of this east-flowing river when it proceeded beyond the Cotteswolds ? It probably flowed eastwards, as a consequent stream, in the direction of the Pcwsey Vale, where it would take advantage of the line of weakness of the Pewsey anticline. It is a signi ficant fact that if the floor of the Pewsey Vale were produced westwards, it would approximately reach the same hill-tops on which the gravels described above are found. The floor of the vale rises westwards from Wilsford towards Lydeway with an average slope of 17 feet per mile (Fig. 23) ; continuing this rise westwards for about 14 miles (the distance from Lydeway to Farleigh Down), it would just reach that plateau, a little over 600 feet high, where the gravels occur. Such a coincidence hardly seems accidental. Therefore it appears likely that the suggested river flowed through the Pewsey Vale, and that it played an important part in excavating that wide valley. The outlet from the vale was probably the Avon Gap at Upavon, which bears every sign of great age in its gentle curves and 196 W. D. VARNEY, considerable width, though this may be due partly to the softness of the Chalk in which it has been made. Therefore, as stated in the Survey Memoir, this valley is as old as the vale itself.* The only other possible outlet to the Vale for such a river is at the eastern end, through the lowest part near Savernake. The Great Western Railway takes advantage of this in leaving the Vale of Pewsey for the Kennet valley. The lowest point. however, is over 5(j() feet high, and the Pewsey Vale, as shown by its gravels (see Section VIII.), had. already been cut much below this level by early Pleistocene times. Therefore the possibility of an outlet at the eastern end into the Thames Valley is rather remote. If at any time this was the way the drainage left the vale, it was at a very early stage in its history, and other facts seem to show that the present Upavon Gap was the original outlet. Where the Salisbury Avon leaves the Pewsey Vale it turns almost due south. This may be due to the existence of a shallow syncline in the Chalk, with an almost north to south trend, formed possibly, as a result of earth movements which Sir Aubrey Strahan describes as Charnian.t These movements caused folding in a system of N.N.W. to S.S.E. lines, practically coincid ing in direction with a gentle depression which causes patches of Lower Chalk to be preserved in a line crossing the vale in this direction, pointing to the Upavon Gap. The syncline is also indicated by the fact that the junction of the Lower Chalk with the Upper Greensand is lowest in these Chalk outliers and in the Avon valley south of Rushall. This depression would be sufficient to deflect the supposed river southwards, till it flowed into the old" Southampton River" demonstrated so clearly by Clement Reid. That such gentle folds do influence rivers is also shown by H. Bury'[ in the Wealden area, and by A. J. Jukes-Browne§ writing in a paper on the Teign Valley in Devon. The Pewsey Vale lies midway between these two areas, and would be affected most likely by similar movements, which were probably of earlier date than the east-to-west folds. II V. THE REVERSAL OF THE AVO~ DRAINAGE. The next problem to solve is :-What caused the beheading of the river from the Welsh uplands and so resulted in the re versal of the drainage from cast to west, leading to the growth 'Of the Bristol Avon, which is an obsequent stream? The most likely river to capture the hear! waters of the 'old Welsh river was the Severn, as it worked backwards as a sub- • Geology of Deviees, l\ftm~ Geol.Su'fl ., IC)0,5, p, 45. t A. Strahan, II Origin of the River System of South Wales," Qua~t. [ourn, Geol, 50r., vol, 1I902, p, 213. ; H. Bury, Qua,t. [our« Gcol. Soc., vol, 1010. P 658. l A. J. Jukes-Browne, Qua,t. [ourn, Gtol. s«; vol. '9"4, P 33" I, Ibid, p. 332. THE GEOLOGICAL HISTORY OF THE PEWSEY VALE. J97 sequent stream, in the manner suggested by W. M. Davis.· It seems fairly plain that it has beheaded the Thames in this way. The beheading of the former stream would take place earlier, thus allowing time for the greater subsequent changes which have taken place. The shortening of the first river certainly took place after the Farleigh Down gravels were formed, and since these are most likely of Pliocene age, the development of the Severn to its present proportions, and the formation and growth of the Bristol Avon took place probably in late Pliocene and Pleistocene times. VI. DEVELOPMENT OF THE BRISTOL AVON. After the capture of the headwaters of the Welsh river (Fig. 2Sb) an obsequent would begin to occupy its valley near the Severn. This was the embryonic Avon, which, cutting back wards into a fairly steep valley-edge composed of Jurassic rocks, made as the Severn valley was deepened, was able to work rapidly, thus still further shortening the already beheaded con sequent stream. As it cut downward too, it imposed its valley on underlying folded Palreozoic rocks. making the Clifton Gorge. It cut a valley eastwards deep into the Chalk and associated rocks which extended much further west th an th ey do at present (as shown by the flints in the Farleigh Down Gravels). The next step was the sending out of branches along the strike of the softer rocks-resembling subsequent streams, which worked north and south, especially north. On the north side two of these, now known as the By Brook and the Upper Avon res pectively, were able to intercept and behead consequent streams which flowed eastwards on the Chalk surface, thus gaining in volume and power, and the latter stream became the chief of the upper part of the Avon system. One of the rivers so beheaded was the Kennet, as will be shown later, and it has also captured the headwaters of other Thames streams (Fig. 2SC) . As shown above, the Upper Avon is really a strike stream developed from the obsequent river which forms the Lower Avon, flowing from east to west. According to the t erminology and definitions of W. M. Davis, and generally adopted by geolo gists and geographers, it is not really a subsequent stream (which flows into a consequent ), though it behaves like one, flowing along the strike of soft rocks and beheading true con sequents which flow from west to east. It, is very rarely that a tributary of an obsequent stream be comes an important part of a river system, and in this respect the Upper Avon and the By Brook are almost, if not quite, unique in Britain. In order to show that such streams were -JW. M. Davis, U Development of certain English Rivers,' Geograpkicat Lourn., vol. v., J895, P.1H· 198 W. D. VARNEY, formed after the obsequent rivers into which they flow, they might well be termed post-obsequent. A post-obsequent stream can then be defined as a strike stream flowing into an obsequent, and developed after it. This distinguishes it from a subsequent, which is a strike-stream flowing into a consequent river. A study of the escarpment at the western end of the Pewsey Vale, south of Devizes, illustrates the way in which the early Avon excavated its valley, formed the Bradford Gorge, and gradually cut it s way through the Oolitic rocks of the Cotteswold area. What took place there is being repeated now at the Cretaceous escarpment , and the truly obsequent part of the Bristol Avon, known as the Semington Brook, is still pursuing its work of cutting its valley in the eastward dire ction, into the valley of the earlier, consequent river-namely, the Pewsey Vale. In doing so it is gradually absorbing more and more of the drainage which goes eastwards, diverting it into the Severn Basin. A fine example of this occurs at Stert, about two miles east of Devizes. Rising between these places is a small stream which flows south and then east wards, and which , after flowing through a relatively wide, shallow valley, descends into one which is deep, steepsided and narrow, known by the people of the locality as " St ert Valley," and noted as a place of consider able beauty. Just below St crt, this vall ey and the stream it contains take a sudden turn, first to the south and th en to the west, so that the direction of flow is practically reversed. The rivulet then joins the Scmington Brook, which receives a number of others draining the escarpment, and similarly cutting deep valleys in it. The sharp bend marks the place where the Stert Stream was beheaded, and just above it is a distinct notch in the valley side indicating the course taken by the Stert stream before the capture took place. The old course through this deserted valley leads directly t o one of the branches of the Salisbury Avon, which now rises in almost level ground, in marshy land ncar Lydeway. It seems obvious that the stream draining St ert Valley was originally one of the headwaters of the Salisbury Avon, now diverted into the Bristol Avon. 'the notch mentioned above stands high up ab ove the stream, showing that the deep Stert Valley was formed after the change of drainage took place. This valley is the much-deepened bottom of a wider one, which is really an extension of the Pewsey Vale, and the extra deepening took place as a result of the river capture described above. The slope down which the water flowed changed from a dip slop e to a scarp slope, resulting in greatly increased excavating powers , with the consequent forma tion of Stert Valley with its deep sides. Thus the waters of the Bristol Avon are gradually extending THE GEOLOGICAL HISTORY OF THE PEWSEY VALE. 199 eastward, and, as S. S. Buckman" remarks, they will probably reach the Upavon Gap sooner or later, and further deepen it. The resulting valley or gorge will then be analagous with the Bradford Gorge through the Cotteswolds. VII. THE KENNET. H is only intended here to describe the upper Rennet, and to suggest some phases of its early history. Near Beckhampton, between Marlborough and Devizes, the Kennet on being traced backward is found to take a sudden turn to the north toward its source, which lies north of Winterbourne Bassett. The main road to Devizes and Bath at Beckhampton follows a valley for a short distance, in a direct line with the Kennet's west to east course. This valley, now dry, passes between Morgan's Hill and Calstone Down towards Heddington, forming a distinct notch in the Chalk escarpment (Fig. 24). It points westwards towards a stream flowing from the Cotteswolds to the Upper Avon, and further west to another which flows into the By Brook, the other post-obsequent part of the Bristol Avon system. These two small streams once formed one continuous consequent, which was twice beheaded by the two post-obsequent rivers named above. Before the capture took place it must have flowed towards the Kennet through the high-level valley which now is the only remnant of its course on the Chalk remaining west of Beckhampton. W. M. Davis] states briefly that the Avon probably beheaded the Kennet, but here is more definite proof of his statement. This has been dealt with at length because it seems an almost parallel case with what most likely took place in the Pewsey Vale, the chief difference being that of magnitude. The Kennet could hardly have risen further west than the Cotteswolds, or if it did, no traces of its extension in that direction remain; since it was smaller, and since it flowed in a synclinal basin, it could not cut down through the Chalk to the Greensands below. But here is a case of a high-level valley formed in a very similar way to that suggested for the Pewsey Vale. It also ends abruptly in the west, at the edge of the Chalk, showing that it formerly extended further west, before the Chalk had been cut back to within its present limits. Like the Kennet, the Thames, Wylie, Stour and Frome. all flow in a very similar direction to the long axis of the Pewsey Vale, and this coincidence of direction seems to suggest that the last-named valley was formed by a river with a similar direction, and there seems to be sufficient reason, already stated, for its turning southwards to leave the Pewsey Vale. • s. S. Buckman, Proc, Cots. NtJt. Hisl . • 11<1 Field Club, vol. xxiii., pt. 3, p. 19o t W. M. Davis, Geog, lou"'., 1895. vol. v., p. 145. PROC. GEOL. Assoc., VOL. XXXI!., PART 4, I92I~ i .. 200 W. D. VARNEY, \Ol ~ 10I ~ i .. ,.j . ~ ~ ,.j 8''t . > 10I zI '"';II 'f ;II ~ J>l:c '"'1>0 0 .' 0:'"' p..< 0: f>l ::>"" 10I :c '"' '"0 ~"" 0: g:::> ;II .. 0 I ... ci Ii: THE GEOLOGICAL HISTORY OF THE PEWSEY VALE. 201 VIII. THE LATER HISTORY OF THE PEWSEY VALE. Within the Pewsey Vale north of the village of Pewsey, are several patches of coarse gravel, occupying small plateaux at a height of 80 or 90 feet above the alluvium at Pewsey," but about 500 feet below the northern rim of the vale. This latter fact shows that the Pewsey Vale had been excavated to a depth of about 500 feet before these gravels were deposited. They descend to lower heights above the river in ~laces, so the estimate of 500 feet is a moderate one. Very primitive flint implements have been found in these gravels, indicating that they are of early Pleistocene age. Thus bv early Pleistocene times the vale had a depth of at least 500 'feet, and since then it has only been deepened 50 to 90 feet. Much of the denudation of the Pewsey Vale must have taken place, therefore, in Pliocene times, and this estimate coincides with that for the age of the Farleigh Down and Bathampton Down Gravels, and so strengthens the argument for the common origin of those gravels and the Pewsey Vale, After the formation of the early Pleistocene gravels erosion again took place, and the vale was deepened almost to its present level; river gravels were deposited near the present stream courses, and in them later Pleistocene fossils have been found. A further uplift then took place and these gravelly deposits were eroded, terraces being formed, after which newer alluvium was laid down. In the west of the vale there are widespread deposits of loam and drift consisting of chalky clay, greensand and chalk pebbles, evidently washed from the surrounding hills. Most of the steep slopes of the Chalk facing the Vale (and also facing the Cottes wolds) are grooved by deep, steep-sided, flat-bottomed valleys, many showing a composite structure with terraces, indicating successive periods of excavation. They are quite dry now, but their steep sides suggest very rapid erosion, and they are most likely the product of erosion due to melting snow flowing over frozen ground. The material so removed has been spread over the western end of the vale, forming the above-described surface deposits. The absence of flints from these (except for very small fragments) is probably due to there being no flint-bearing Chalk within several miles of this end of the vale. Their abrupt termination westwards again points to the Vale having formerly extended much further in that direction. IX. SUMMARY OF CONCLUSIONS. I. The Vale of Pewsey, as stated by Buckland, is closely connected with a west-to-east trending anticline, movement most likely taking place along an old line of folding. The first proved • Geology of Devzes, Me",. Geol, 51" •. , '905. p. 45. oN N ~-' ~J \ \ [-'oj ;.E ~ ~ Z (tj ~ FIG. 25A. FIG. 25B. FIG. 2:J.-MAPS ILj:.USTRA1TNG THE SUGGESTED CHANGES I~TIlE DRAINAGE SYSTI>~SOF l'H)i: ]3RISl'OL AVON Al'lD THj!: SALISJlURY AVON. THE GEOLOGICAL mSTORY OF THE PEWSEY VALE. 2°3 ,I ,, , , I \ I I 2°4 W. D. VARNEY, uplift was in late Cretaceous and pre-Eocene times, followed by a depression after much Chalk had been removed by erosion. Further folding took place during Miocene and perhaps early Pliocene times, during which the anticline assumed its present proportions. 2. The line of weakness of the anticlinal area was probably utilised not later than in Pliocene times by a river flowing west to east from the Welsh uplands, across what is now the Severn Valley, the Cotteswold area, and the Avon Vale, and which excavated the Pewsey Vale by ordinary river erosion. The river left this valley by an opening to the south, its course here being determined by a shallow syncline possibly formed as a result of gentle movements along an old" Charnian " axis, nearly north to south. It then flowed into the ancient " Southampton River," and may have been the means of con tributing quartz pebbles and fragments of Palseozoic rocks to the gravels of that river.* The corresponding remains in the Vale of Pewsey have been removed by subsequent erosion. though occasional far-travelled pebbles of rocks quite foreign to the district can be found on the north side of the vale. 3. This old river was first shortened by being beheaded by the Severn, and later by the development of an obsequent stream, which grew into the present Bristol Avon, in its old valley; the Avon threw out strong post-obsequent branches which helped to remove the Chalk from what is now the Avon Vale. They also beheaded the streams flowing to the east over the Chalk from the Cotteswold region, including the Kennet; and in the case of the Kennet part of its old high-level valley still exists, forming a notch in the Chalk escarpment, while por tions of its headwaters can still be traced with tolerable certainty (Fig. 2sd). Moreover, the tributary rivulets which are really the head waters of this reversed system of drainage are actively cutting still further east in the neighbourhood of Devizes, causing still more water to flow toward the Bristol Channel instead of in an easterly direction. Therefore it is doing at the Cretaceous escarpment precisely what it did at the Oolite ridge, that is, cutting a gorge which will be analagous with the Bradford Gorge. 4. The large river from the west brought gravels, some of which are preserved on Farleigh Down and Bathampton Down, on either side of the gorge of the Bristol Avon, and they form a valuable clue to the history of the whole river system of the neighbourhood. S. The Pewsey Vale was excavated almost to its present depth by early Pleistocene times, since which time it has only been deepened S0 to go feet, while its western end has been greatly modified by the streams of the Bristol Avon system. • C. Reid, "p. m., p. 30. THE GEOLOGICAL HISTORY OF THE PEWSEY VALE. 205 6. At various cold periods in more recent times, quantities of chalky mud have been swept into the vale from the hills round it by swiftly-flowing water which cut the now dry, terraced valleys in the steep chalk slopes overlooking it. The material so removed was spread over the floor of the vale forming the so-called .. landwash."* 7. The excavation of the vale by the cast-flowing river ceased during the late Pliocene or early Pleistocene times, and it therefore follows that the Severn, which caused the reversal of the drainage, was in existence in Pliocene times. 8. Some deductions can be made concerning the recession of the Chalk escarpment. Though the Farleigh Down Gravels were deposited on the Oolites, the flints in it show that Chalk, and Chalk of high horizons, must have existed in the neighbour hood. At that time, therefore, the Chalk escarpment was somewhere near the present Oolite ridge, and may even have extended west of it. If the Pliocene age of the Farleigh Down Gravels is correct, the Chalk escarpment has receded about 14 miles during later Pliocene, Pleistocene and Recent times. The principal factors causing it to recede were post-obsequent strike streams, the chief of which now forms the Upper Avon, and its obsequent and consequent tributaries. In conclusion, the author wishes to acknowledge indebtedness to Prof. H.H. Swinnerton, for much valuable advice while doing the work for this paper, and while writing it ; to Dr. A. E. True man, for many suggestions and encouragement; and Prof. S. H. Reynolds, for use of books in his library. • Geolo/lY 01 Devises, Mem. GeM. Su",., 1905, p. 43.