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References CLARKE, M . R., A . J . DI XON & M. KUBALA . 1979. Th e sand and gravel resources of the Blackwater Valley (Aldershot) area . Description of l : 25000 sheets SU 85, 86 and parts of SU 84, 94, 95 and 96 . Miner. Assess . Rep. Inst. Ceol. Sci., No . 39. - - & - -. 1981. Th e Pleistocene braided river deposits in the Blackwater Valle y area of Berkshire and Hampshire , England . Proc. Geol. Ass . , 92, 13<}.-57. FISHER, P. F. 1982. A study of the Plateau Gravels of the western part of the London Basin. Unpublished PhD thesis Kingston Polytechnic. GIBBARD , P. L. 1977. Pleistocene history of the Vale of St . Albans. Phil. Trans. R. Soc., London., 8280,445-83. - - . 1979. Middle Pleistocene drainage in the Thames Valley. Geol. Mag. , 116, 35-44. - -. 1982. Terrace stratigraph y and dr ain age history of the Plateau Gravels of north Surrey, south Berkshire and

north Hampshire. Proc. Geol . Ass ., 93, 369--84. HARE, F . K., 1947. The geo mo rp hology of a part of the Middle Thames. Proc. Geol. A ss., 58, 294-339. LEOPOLD, L. B., M. G. WOLMAN and J . P. MILLER. 1964. Fluvial processes in Geom orphology , Fre eman, San Francisco. SQUIRRELL, H . C. 1976. The sand and gravel resources of the Thames and Kenner Valle ys, the country aro und Pangbourne, Berkshire: Description of 1 : 25 000 resource sheet SU 67. Miner. A ssess. Rep . Inst. Geol. Sci. , No. 21, 97 pp . THOMAS , M. F. 1961. River terraces and drainage development in the Reading area . Proc. Geol. Ass ., 72, 415-36. A . J. Dixon , Institute of Geological Sciences , Keyworth, Notts

Reply by the author The Editor, Dear Sir,

10 January 1983

I am grateful for the opportunity to reply to the two correspondents. I will reply to the points raised by Fisher in the same order as they were presented. Concerning the high Greensand chert content of the gravel in the north Surrey-south Berk shire area, Fisher is completely correct that they contain a greater frequency than is present in Blackwater or Mole Valley deposits. Although accepting none of my explanations, his comments do not take into account other lines of evidence: 1. He states that there is no evidence that the Whitewater ever carried chert -rich gravel. There are relativel y few deposits that can be reliabl y attributed to this stream. However, the Hazeley Heath gravels near Hartley Wintne y are undoubtedly of Whitewater origin and contain 10% Greensand chert. This supports Linton's (1930) suggestion that the Whitewater had a link with the Weald in the past. 2. The possibility that the high chert frequenc y is an artefact of the pebble count techn ique did not seem a tenable explanation to me. Nevertheless, Fisher 's evidence does not negate this possibility, it simply makes it less likely. Unfortunately , 'Fisher (1982)' was unavailable at the time of writing . 3. Regarding the reworking of Greensand chert from the Tertiary bedrock of the area , the correspondent correctly note s that this almost exclusively contains flint. However, the Barton Basement Bed, now termed Upper Bagshot by Curry et at (1978), does contain chert in significant quantities (Dewey & Bromehead, 1915). Bridgland (pers. comm.) comments that this bed at Stanners Hill near

Chobham contains 'both well rounded and poorly rounded Greensand chert . .. that if reworked into Pleistocene river gravel would be indistinguishable from material derived directl y from the Weald '. Further , the angular ity and roundness of clasts is unimp ortant since they predominantl y enter periglacial streams by solifluction (e.g . Church, 1972; French , 1976) and to a lesser extent by erosion of the river bed . Angularity of chert must the refore be expected to predominate following frost action dur ing downslop e movement. The writer found the above explanations convincing in the light of evidence available at the time. Fisher que stions the effect of the confluence of the Blackwater-Loddon stre am with the Mole-Wey stream near Weybridge (Gibbard, 1979). In doing so he make s three major errors in his table 1. Firstly, he presents average compo sitions for unit lithology based on the whole area of their outcrop . This cannot be appropriate since it ignores downstream changes . Secondl y, he disregards the input of of gravel from the River Mole, which on the basis of the authors' counts (Gibbard, 1979, table 1) carried about 10% Greensand chert (Tabl e 1). The high flint content would have certainly diluted the high Greensand chert frequencies of the Wey gravels (e.g . St. George's Hill). Th irdly, he ignores the compo sition of gravel in the Winey Hill outlier (St. George 's Hill Gravel ) , 4 km downstream of the confluence zone at Weybridge. The gravel here contains 19.9% Greensand chert, almost exactly that pred icted by mixing gravel derived from the Blackwater-Loddon , Weyand Mole valIeys at this stage (Table 1). On the basis of his calculations the correspondent calls into que stion the origin of Dollis Hill Gravel

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TABLE 1. Selected pebble counts of the St. George's Hill Gravel and equivalents (from Gibbard, 1979, table 1 and Gibbard, 1982, table 1).

% Greensand chert

% Vein

% Flint

St. George's Hill (Wey) Great Bookham Common (Mole) Staple Hill (Blackwater-Loddon)

77.35 89.38 65.53

23.22 10.81 34.46

0.57 0.30 0.00

predicted composition of gravel at confluence

77.52

22.83

0.19

Winey Hill (St. George's Hill Gravel)

79.29

19.92

0.80

composition. The nearest outcrop of this unit is 23 km downstream from the Weybridge confluence. The considerable distance is more than ample to allow incorporation of flint from Tertiary bedrock which would have certainly occupied the intervening ground. Equally, the increase in vein quartz can be explained by incorporation of local Pebble Gravel, which outcrops in the immediate vicinity of the Dollis Hill outliers. It is correct that my proposed reconstruction excludes any major supply of Greensand chert into Thames gravels upstream of Ware prior to the Late Anglian. High level (but not necessarily 'very early Quaternary') gravels indeed contain small amounts «3%) of Greensand chert. The origin of this chert is difficult to determine, but may have resulted from reworking of older gravel and Tertiary bedrock by minor streams in the proto-Kennet catchment, south of Reading. A far more serious problem is posed by the correspondents' suggestion that the gravels in the north Surrey-south Berkshire area represent an extension of the Wey. This would be an attractive hypothesis except that the gravels are unquestionably aligned in a generally west-east direction i.e. perpendicular to the Wey trend suggested by the correspondent. This alignment of the aggradations is supported by palaeocurrent data where available, a fact which must be taken into account in any interpretation of the palaeogeography. Fisher states that 'there is no evidence ... that any of these rivers (the Blackwater-Loddon, Mole and Wey) formed confluences with each other'. Implicit in this argument is that a northward flowing Wey must have joined the Thames somewhere between Henley and Ware; a recent reconstruction by Peake (1982) for example, placed the Wey along the Colne Valley. The writer has undertaken considerable investigations recently in the Middle Thames region, including pebble lithological counts of Thames gravels both west and east of the Colne Valley. Neither here nor anywhere else between Henley and Ware is there any

quartz

evidence at all for even a minor influx of either Greensand chert or 'Tertiary-type' rounded flint that would be expected if a tributary was confluent with the main river. In conclusion, the author feels that the criticisms put forward by Fisher underline the dangers in considering only one line of evidence i.e. pebble lithology. The inherent pitfalls in this approach are exemplified by his proposed alternatives, particularly that of the River Wey, which are extremely unlikely on the basis of all the evidence available. In reply to Dixon it seems there are a number of minor criticisms that arise out of a fundamentally different approach between his work and mine. This difference lies in the concept of aggradation in a fluvial environment and most importantly in the definition and usage of the term terrace. A river terrace may be defined as a 'topographic surface which marks the former valley floor level, i.e. the vestige of a floodplain' (Leopold, Wolman & Miller, 1964). This concept has been used in the past in southern England to develop morphostratigraphies based on the assumption that a series of terraces at progressively lower levels down a valley side represents a younging sequence. As a physiographic term, terrace should only be applied to surfaces and landforms. The former may be erosional or depositional and as such represent unconformities in a series of deposits. This was stressed by the Commission on Terraces and Erosion Surfaces of the International Geographical Union (Fairbridge, 1968). Frye & Willman (1962) state that the term terrace 'has no place in stratigraphic terminology because it literally means a topographic bench'. Fairbridge (1968) goes on to stress that a geological approach is essential for understanding the genesis and dating of fluvial deposits beneath terrace surfaces. These points were recently discussed by the author with reference to the Thames Valley (Gibbard, 1981), but they are equally applicable to much of southern Britain. The same concepts can also be applied to the use of so-called 'benches' or the bedrock surface beneath the

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deposits in question. Such surfaces again represent unconformities and are seldom horizontal or even. Depending on bedrock lithology the surface may be heavily channelled, potholed, and often have suffered the effects of solutional collapse, the latter particularly on Chalk. As a result the surface may have a topography that varies in age and in height with amplitudes of 2--4 m on average and even greater in exceptional cases. The only way to correlate fluvial deposits is to use the properties of the deposits themsel ves. Such features as topographic surfaces may be used to add subsidiary support to correlations based on geological analyses, but cannot be used alon e to determine them. My approach has been to apply lithostratigraphical principles based on lithological determinations, field inspection of sections and surfaces, palaeocurrent analyses, grain size analyses and accurate levelling of surfaces to construct and delimit sequences. The use of boreholes must be fixed with field control. Evidence of any type , especi ally palaeocurrent anal yses, cannot simply be ignored if it does not fit with already extant ideas. The correspondent question s the need to redefine the numbered 'terrace stages' as lithostratigraphical units. It is necessary because a reliable regional stratigraphy must be constructed using the deposits themselves and not depositional breaks. These deposits must be assigned to reference points from which they can be defined . One writer puts it thus 'consecutive numbering has frequently been used, but since the 100·feet Terrace is spoken alike as the first, the second or the third ... the interpretation of the literature is attended with much difficulty. For these reasons the adoption of local names for each area is preferable . . . pits, being well-known and identifiable , even if future , work should reveal higher or intermediate terraces' (Bromehead , 1912). For further comment on this topic see Fairbridge (1968) and Howard (1959). The reconstruction of fluvial deposit gradients should surely be determined by linking outcrops of demonstrably comparable lithology, and not based on the gradient of the modern stream (Johnson , 1944). Only in this way can a true representation of deposits' altitudinal distribution be obje ctively determined. It is of no consequence whether the gradient is determined by eye or by computer, since the latter will only arbitrarily link as many point s as possible without any

regard for individu al site considerations. The geologist can far better accept or reject correlations using his own experience of the deposits concerned. Following from this is Dixon 's rejection of the shallow gradients for the River Blackwater during the proposed stages compared to that of the modern stream. He has missed the point that the larger the river, in terms of water and sediment moved , the shallo wer the gradient. One only need compare the gradient of the Th ames (30-50 em krn" ) with that of the R iver Cain e (190 em krn"! and this with a Caine tributary (e.g . the Alderbourne: 250 cm km- 1) . If the Blackwater had headwaters in the Weald it would have been a much larger and therefore shallower graded river than the att enuated present stream. One last point concerning terminology. It is vital that terms, such as 'lower' Winter Hill terrace , be restri cted to the valley for which they were defined. Such assumptions of correlation have never been proposed by me and have not been demonstrated by Dixon. It is incorrect to refer aggradations in the Blackwater-Loddon system to this or any other Thames Valley unit. In addition , on the basis of detailed stratigraphical studies in the Middle Thame s Valle y, I do not accept the usage of the term s 'upper' and 'lower' Winter Hill terrace (sensu Hare , 1947) upstre am of Burnham, as extended by Sealy & Sealy (1956). Regarding the Caversham Channel, Clarke & Dixon (1981) were the first to suggest that the Black Park Terrace floored this feature, although Walder (1967) hinted that the Sealy's (1956) 'lower' Winter Hill terrace and their Black Park terrace were one and the same at Emmer Green golf course . The reference to Gibbard (1977, 1979) was to support the late Anglian age for the Black Park Gravel which has not been demonstrated by other authors. Lastl y, in considering the proposed confluence of the Kennet Silchester Gra vel with the Thames Black Park Gra vel at Hen ley, the correspondent states that borehole data (Squirrell, 1976) show that patches of plateau gravel, north of Tilehurst are of 'southern' (?western) origin with over 90% flint. The most likely origin of this material is not the Kennet , which was flowing south of Tilehurst plateau , but the River Pang. According to the same author gravel of the same comp osition underl ies the modern floodplain of this stream.

References BROMEHEAD, C. N. 1912. On the diversions of the Bourne near Chertsey. Summary of Progress for 1911. Geol. Surv . G.B. Appendix II. CHURCH, M. 1972. Baffin Island sandurs; a study of Arctic fluvial processes. Geol Surv. Canada, Bull . 216. CLARKE, M. R. & A. J . DIXON. 1981. The Pleistocene braided river deposits in the Blackwater Valley (Alder-

shot) area of Berkshire and Hampshire , England. Proc. Geol. Ass . 92,

1 3~57.

CURRY, D., C. E. ADAMS, M. C. BOULTER, F. C. DILLEY. F. E. EAMES, B. M. FUNNELL & M. K. WELLS . 1978. A correlation of the Tertiary rocks in the British Isles. Geol. Soc. Lond. Special Report No. 12. DEWEY, H. & C. E. N. BROMEHEAD. 1915. The

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geology of the country around Windsor & Chertsey. Mem. geol. Surv. U.K. FAIRBRIDGE, R. W. 1968. The encyclopedia of geomorphology. Reinhold Book Corp: New York. FRENCH, H. M. 1976. The periglacial environment. Longmans Group: London. FRYE, J. C. & H. B. WILLMAN. 1962. Morphostratigraphic units in Pleistocene stratigraphy. Bull. Am. Ass. Petrol. Geol., 46, 112-3. GIBBARD, P. L. 1977. Pleistocene history of the Vale of SI. Albans. Phil. Trans. R. Soc. Lond. B., 280, 445-82. - - . 1979. Middle Pleistocene drainage in the Thames Valley. Geol. Mag., 116, 35-44. - - . 1981. Views on the Pleistocene of the Thames Valley. Quat. Newsletter No. 34, 32-35. - - . 1982. Terrace stratigraphy and drainage history of the Plateau Gravels of north Surrey, south Berkshire and north Hampshire. Proc. Geol. Ass., 93, 369--84. HARE, F. K. 1947. The geomorphology of a part of the Middle Thames. Proc. Geol. Ass., 58, 294-339. HOWARD, A. D. 1959. Numerical systems of terrace nomenclature: a critique. J. Geol., 67, 239--43.

JOHNSON, D. 1944. Problems of terrace correlation. Bull. Geol. Soc. Am. 55, 793--818. LEOPOLD, L. B., M. G. WOLMAN & J. P. MILLER. 1964. Fluvial processes in geomorphology. W. H. Freeman & Co: San Francisco. LINTON, D. L. 1930. Notes on the development of the Western part of the Wey drainage system. Proc. Geol. Ass., 41, 160-74. PEAKE, D. S. 1982. The ground upon which Croydon was built. Proc. Croydon nat. Hist. & Scient. Soc., 17, 89--116. SEALY, K. L. & C. E. SEALY. 1956. The terraces of the Middle Thames. Proc. Geol. Ass., 76, 369--92. SQUIRRELL, H. C. 1976. The sand and gravel resources of the Thames and Kennet Valleys, the country around Pangbourne, Berkshire. Miner. Assess. Rep. lnst. Geol. Sci. No. 27. WALDER, P. S. 1967. The composition of the gravels near Reading, Berkshire. Proc. Geol. Ass., 78, 107-19. P. L. Gibbard, Subdepartment of Quaternary Research, Botany School, Downing Street, Cambridge CB23EA