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The Caesar's Camp Gravel—an early Pleistocene fluvial periglacial deposit in southern England
The Caesar's Camp Gravel-an early Pleistocene fluvial periglacial deposit in southern England M. R. Clarke and P. F. Fisher CLARKE. M. R . & P. F. FISHER . 19R3. The Cae sar's Camp Gravel-an early Pleistocence fluvial periglacial deposit in southern England . Proc. Geol. Ass .. 94 (4), 345-355 . New information from shallow boreholes and from recent exposures has allowed reinterpret ation of the age and origin of the Caes ar' s Camp Gravel , near Aldersho!. The deposit compri ses two main sedimentary facies: a cobble-gr avel facies compo sed principally of nodular flints. and a sand-clay facies occurring as thinner interbedded units. The composition of the gravel and its heavy mineral content are distinct from those of the gra vels seen on Netle y Heath with which it was once correlated . The depo sit is thought to represent fluvial periglacial aggradation related to an early Pleistocene cold phase. pre-dating the main terrace sequence in the area and post-dating the ne arby high-level marine deposits of late Pliocence/early Pleistocene age. A well defined palaeosol occurs in the upper part of the gravelly deposits and is overlain by silty fine sands presumed to be of loessic origin .
M. R. Clarke, Institute of Geological Sciences, Keyworth . Nottingham NGI25GG P. F. Fisher, School of Geography, Kingston Polytechnic. Penrhyn Road , Kingston-uponThames KTI 2EE
1. INTRODUCTION Nearly one hundred years have passed since the high-level deposits of Caesar's Camp, to the southwest of Aldershot (Fig. 1), were noted as forming 'among the most conspicuous of the gravel-capped hills' of the Thames Valley (Prestwich, 1890, p. 161). The nature and origin of these deposits provoked considerable debate during the late 19th century (Irving . 1890; Monkton, 1892), but the tirst full description of them was by Bury (1922). He concluded (p . 97) that the size and condition of the large nodular flints (which make up a large proportion of the gravels) 'afford a strong prima facie case , if nothing more , in favour of marine action' . He thus supported the view of Gilbert (1920) that deposits at a similar height. on the north side of the Thames at Little Heath , ncar Berkhamsted, in the Chilterns, were marine. A marine origin was not endorsed by Dines and Edmunds (1929, p. 108) in the Geologic al Survey memoir for the Aldershot and Guildford area; they felt that ' definite and conclusive solution s of the problems (of the origins of the Caesar's Camp Gravel) are still wantin g'. Nonetheless, Bury 's conclusions tended to gain acceptance , and the depo sits of the Hale plateau were used by Wooldridge (1927) and Wooldridge and Linton (1955) to indicate the southwestern limit of a high-level Pliocene shoreline. The marine origin of the Caesar's Camp Gravel was further emphasised by Wooldridge's assertion that the deposit had yielded to him the characteristic Diestian suite of heavy minerals, typical of the Lenham Beds in Kent. This was later questioned by Grove s (1931) whose analytical results suggested that the heavy mineral suites of the two deposits were different in several respects.
Little further work has been published on the nature of the Caesar's Camp Gravel, but Montford (1966) recorded his impression that the deposits are of glacial origin . More recently the altitudinal limits of Wooldridge's high level shoreline , (the 'Calabrian'), have been called into question (John , 1980). This paper presents new inform ation obtained from shallow boreholes drilled in the Caesar 's Camp Gravel and from field inspections. of recent workings in the deposit , by the authors in 1976 to 1980. 2. METHODS The working of the Caesar's Camp Gravel as a local source of aggregate has exposed a number of large pit sections which have been available for inspection over the last ten years. The most interesting sections are visible in a pit (822494) at the western end of the deposit (locations Y and Z. Fig. 2), where the nature and form of the Caesar's Camp Gravel are best demonstrated. Additional information has been obtained from five, 8-inch diameter boreholes specially drilled (in 1976), by Shell and Auger rig, in the Caesar's Camp Gravel at the locations shown in Fig. 2. This work was undertaken by the Institute of Geological Sciences as part of a national assessment of sand and gravel resources commissioned by the Department of the Environment (Clarke , Dixon & Kubala , 1979). Bulk samples of the depositis have been collected from the pit sections and the boreholes. to provide material for study . The particle size distribution curves (Fig. 3) are based upon sieve analyses carried out using a wet sieving method conforming to BS 812 and pipette analyses for the silt and clay size fractions . The residues of these analyses together with unused
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M. R. CLARKE AND P. F. FISHER
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material, have been used to provide the 8-32 mm size fraction for the pebble counts (Table 1), The heavy mineral analyses (Table 2) were carried out by separation (using bromoform) of the very fine sand fraction (0.063 mm to 0.125 mm of Wentworth, 1922) of material obtained from the boreholes. They are compared (Table 2) with similarly obtained material from boreholes drilled in the terrace deposits of the Blackwater valley and with the frequencies (of a slightly different size fraction) reported for deposits of Headley Sand (John, 1980). Soil thin sections were prepared of samples of the clay-enriched material which forms the palaesol at location W (830492), 3. THE TEXTURE OF THE DEPOSITS
The pit sections available for inspection at locations X and Y (Fig. 2) show that the deposits at Caesar's Camp comprise three main lithological units: (a) The Cobble-gravel facies
The most prominent and characteristic feature of the Caesar's Camp Gravel is their massive nature and the abundance of large nodular flints which form the
distinctive cobble-gravel facies (Fig. 4). The flint cobbles are frequently larger than 0.2 m (Fig. 3a) and generally have a thin white patina concealing the grey, black or brown interiors. The cobbles are nodular, showing little damage from their primary condition, although horns are generally broken or worn away. They appear to have travelled only a short distance from their source, and are similar to the proximal massive gravels (Gm) described by Miall (1978). The matrix consists mainly of yellow and orange, fine and medium grained sub angular quartz, with some coarse quartz and flint sand. Generally the cobble-gravel facies forms one unit ranging from 0.8 m to 1.5 m in thickness; in some parts of the pit it is separated by beds of fine and medium grained quartz sand (see below). There is no evidence of wide variations in grading within each gravel unit, and the junction between the gravels and adjacent sandy beds is generally sharp both above and below the unit. Locally, the gravel contains thin silty clay laminae (samples CS 9 and CS 10) which pass upwards into more massive clayey fine sands up to 0.5 m thick, for example, sample CS 11 (Fig. 3b). It is noteworthy that all of the material recovered from the boreholes
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has suffered a loss of coarse gravel and cobbles due to the crushing of material by the drilling process. This can be illustrated by the grading curves shown in Fig. 3a; the bulk samples of in-situ gravel (CS 5 and CC 5) from pit sections at locations Y and X (823492) respectively. contain about 30% of cobble size (+64 mm) flints. whereas this material is poorly represented in the borehole samples. (b) The sand-clay facies The Caesar's Camp Gravel contains numerous thin sandy layers interbedded within the cobble-gravel facies; these sand bodies appear to represent three modes of deposition described below. (i) Horizontal, planar-bedded sands Planar-bedded sands (up to 1.5 m thick) are seen both overlying and underlying the gravel facies in the northern face of the main exposure (Yon Fig. 2), The sands are composed mainly of beds of pale yellow or orange fine-grained quartz; their grading characteris-
tics are typified by samples CS 2, CS 3, CS 7 and CS 8 (Fig. 3c) which appear to represent the (Sh) facies described by Miall (op. cit). In some places. the sands display alternating bands (up to 3 ern thick) of colour which apparently reflect variations in grain size; samples of CS 2 and CS 7 (orange stained) contain slightly more clay than the pale yellow sands of samples CS 3 and CS 8 (Fig. 3c). Locally the planar-bedded sands are overlain by the cobble-gravel facies. In several places at location Y. the horizontally bedded sands are deeply eroded by steep-sided channels ('gullies') up to 1.5 m in depth and 1.0 m wide. which are filled with material similar to the overlying cobble-gravel (Fig, 5). (ii) Cross-bedded sand lenses Small (up to 1.0 m in length and 0.3 m in thickness) cross-bedded and planar-bedded lenses of fine and medium sand (the Sp and St facies of Mia!! commonly occur within the gravel facies; they are apparently
348
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CAESAR'S CAMP GRAVEL
TABLE 1. The composition of the Caesar's Camp Gravel expressed as a percentage (by number) of the 8-32 mm material No. of pebbles (+8mm)
Sample location
Rounded flint
179 167 846 310 338 3368 20017
Borehole 84 NW 1 Borehole 84 NW 2 Borehole 84 NW 3 Borehole 84 NW 4 Borehole 84 NW 5 Sample CC5 at Location 'X' River Terrace Deposits Blackwater Valley (mean of nine samples)
Angular flint
7.2 91.1 16.2 81.4 9.3 90.6 9.1 90.3 24.9 69.1 (not separated) (not separated)
more numerous in the western face of the pit (location Z). Samples CS 12 and CS 13 (Fig. 3d) taken respectively from the middle and base of one of these sand lenses, suggest a fining upwards of grain size within the sand lenses. Similarly samples CS 14 and CS 15 taken from the top and base of a larger
Flint (total)
Cherty sandstone
98.3 97.6 99.9 99.4 94.0 98.3 89.8
Vein-quartz
Ironstone 1.7 2.4
1.2 0.5 9.6
0.1 0.3 1.5 0.5 0.5
0.3 3.3 0.7 0.1
cross-bedded sand unit (approximately 1 m thick and up to 10 m long) also demonstrate a fining upwards. (iii) Channel infill sands and silts Fine sands and silts, similar to the Ss tacies of Miall, are seen in two pit sections, infilling channel-like
TABLE 2. Heavy minerals of the 0.063 to 0.125 mm (v. fine sand) fraction of the Caesar's Camp Gravel Caesar's Camp Gravel
Heavy minerals
Zircon Tourmaline Rutile Muscovite Kyanite Staurotite Sillimanite Monazite Garnet Epidote Others Opaques (as a percentage of the total heavy mineral content) Total number of grains
Borehole (84 NW2)
Borehole (86 NW5)+
Percentages by number 48.09 (71.59) 55.10 (71.05) 6.12 (7.89) 2.29 (3.41) 5.34 (7.95) 4.08 (5.26) 32.8 22.45 2.04 (0.63) 1.5 (2.27) 3.8 (5.68) 8.16 (10.52) 1.5 (2.27) 2.04 (2.63) 4.6 (6.82)
Terrace * deposits of the Blackwater Valley
Borehole (84 NW 3)+
Borehole (84 NW 1)+
31.83 (53.53) 7.51 (12.63) 7.81 (13.13) 40.54 2.40 (4.04) 5.40 (9.09) 0.60 (1.01) 3.60 (6.06) 0.30 (0.50)
44.79 (61.43) 8.33 (11.43) 6.25 (8.57) 27.08 5.2 (7.14) 3.12 (4.28) 2.08 (2.86) 3.12 (4.28)
41.62 18.29 8.62 11.91 7.5 3.77 1.43 2.32 2.42 1.52
Headley** Sand
52.16 12.74 22.52 0.3 5.02 6.1 0.6
74.71
90.29
37.52
81.10
84.76
0.56 71.84
518
505
533
508
5,243
7,847
+ Second sample from the bottom of each borehole was separated-values in brackets are calculated on a muscovite-free basis. ** From John, 1980 Table IlIa-mean of 5 samples in range 0.0075-0.09 m. * From unpublished data mean of 5 samples.
350
M. R. CLARKE AND P. F. FISHER
Fig. 4. The cobble-gravel facies of the Caesar's Camp Gravel.
depressions. At location V (827495) in the northern margins of the deposit, a broad channel of about 5 metres width and 0.8 m depth, could be seen within the cobble-gravel facies (Fig. 6). This channel contains a sequence of planar-bedded and ripple cross laminated fine to medium-grained sands (sample CC 6 in Fig. 3f) which pass upwards into laminated clays and silts. In the western face of the main pit (location Z) sands and silty clays of a similar type, also fill a complex channel-like depression within the cobble gravel facies. The deposits (Fig. 7) reach 3 m in thickness and about 10 m in lateral extent; they comprise upper and lower silt/clay members, separated by a cobble-gravel unit up to 1 m thick. The lowermost member is over 1 m thick and exhibits parallel concave bedding in pale grey and orange silts and clays (represented by samples CS 17 to CS 22 in Fig. 3e) showing no obvious trend in grain-size variation. These deposits are overlain in turn by the cobble-gravel, and the upper sequence of silts and clays, which also demonstrate perfect parallel bedding in relation to the concave upper surface of the gravel unit. The samples CS 23 to CS 26 from the upper silt/clay unit (Fig. 3f) contain more fine sand and less clay than those from the lower silt/clay unit. The dark staining at the base of this upper unit
appears to be due to a modern pedological concentration of iron and manganese compounds above the junction with the bed of cobble gravel. (c) Palaeosol and Loam
Locally the deposits of Caesar's Camp Gravel are overlain by a silty clay loam up to 0.8 m thick, as at location W (830492). This fine-grained material (mean grading curves CC 1 and CC 2 in Fig. 3e) is thought to be of aeolian origin; it overlies (and hence postdates) a rubified palaeosol which occurs in the upper part of the Caesar's Camp Gravel. The upper part of this palaeosol is characterised by massive columnar structures up to 20 em across with yellow (10 YR 1/4-0n the Munsell colour chart: Geological Society of America, 1975) faces and yellowish red (7.5 YR 6/8) interiors with red (10 R 4/6) mottles and secondary platy structures. Lower horizons are not columnar, but show platy structures with red (10 R 4/6) mottles. Thin sections of the palaeosol show well oriented clay coats surrounding skeletal grains (similar to the free grain cutans of Brewer, 1964), many disrupted clay coats (papules) and a banded fabric reminiscent of the 'silt droplets' of Romans and Robertson (1974). In contrast, the overlying deposits of loam only rarely
C A E SA R'S C A MP GR A VEL
351
Fig. 5. Gully erosion of the inter-b edded fine and medium sands by the cobble-gravels.
contained oriented clay part icles which generall y occur as void argillans (Brewer, 1964). 'Modern' soils similar to podzol s of the Southampton series (Ja rvis et al. , 1979) have also developed to a depth of 1 rn, into the top of the Caesar 's Camp Gr avel although they are more commonly formed in the upper part s of the localised deposits of loam. 4. DISCUSSION (a) The texture and composition of the deposits The mean particle size distr ibutions of the material proved in three IMAU boreholes (84 NW 1, NW 3 and NW 5) are very similar (Fig. 3a). In contrast, borehole SU 84 NW 2, drilled in the solifiucted material at the southern margin of the depo sit, proved its base at + 152.8 m (+501 ft) 0 .0 ., and the borehole samples contained a high proportion (56% ) of clay to fine sand (-0.25 mm size material) which presumably had been incorporated from the underlying Tertiary bedrock. The position and basal height of + 168.6 m (+ 553 ft) QD. of the dep osits in the fifth borehole (SU 84 NW 4) suggest that they too are soliflucted from the main deposit. This is supported by the mean gradin g results (Fig. 3a) which show that the material , at this site is similar to a bulk sample of soliftucted sand and gravel
(CS 4) taken from pit section Y (822494) in the upper part of the main gravel sprea d. Thus it would appea r that the mater ial formi ng the Hungry Hill spur represents a large solifiucted spread of gravel derived from the central are a. The composition of the (8-32 mm) material fro m the five boreholes drilled in the Caesar's Camp Gravel is compared to the composition of the terrace dep osits in the Blackwat er Valley in Tabl e 1. The most striking feature of the relative proportions of the components in the Caesar's Camp Gravel is the almost entire lack of chert y sandstone debris , deri ved from the Lower G reensand . In only one sample is there more than 1% of sandstone , compared with an average of 10% in samples from the terr ace depo sits of the Blackwater Valley (Clarke & Dixon , 1981). Th e sparse occurrence of Lower Greensand debri s in the Cae sar's Ca mp Gr avel also contrasts strikingly with results from the Ranm ore Gra vel of the North Downs where a variation, from 20% in the west near East Horsely, to 3% near Chipstead in the East , has been record ed (John, 1980). Contrary to the suggestion (Bury , 1922) that cherty materi al is absent from the central plain of the Hale Plateau, small amounts have been found in sample s from Borehole 84 NW 5 and from the exposures at location X. Although the frequenc y of
352
M . R. CLA R K E A ND P . F. F IS HER
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Fig. 6. Channel-infill fine sands and clays, interbedded with the cobble-grave l facies at location V.
quartz pebbl es is relati vely constant in both the Caesar's Camp Gra vel and the terrace deposits of the River Blackwater, the frequency of ironstone varies considerably. The proven ances of four of the five components recogni sed in the Caesar's Camp Gr avel are given by Clarke and Dixon top, cit); the angular flint is derived from the Chalk, the rounded flints are derived from Tertiary pebble beds, and chert and cherty sandstones are derived from the Hythe Beds of the Weald (Middlemiss, 1978). The probable source of the vein-quartz and ironstone is thought to be pebble beds within the Cretaceous strata of the Weald, although some ironstone material may be derived from ironpans which occur within the Caesar's Camp Gravel itself. The possibility that these dep osits once contained considerable amounts of chalk debri s cannot be ignored; subsequent decalcification could have accounted for some of the present compaction of the deposit. (b) The elevation of the deposits The borehole results show that the basal surface of the depo sit (recognised by its sharp junction with the underl ying Tert iary Beds) is highly irregular and that
the sprea d of sand and gravel is affected by solifluxion towards its margins. Three of the boreh oles (IGS registered numb ers SU 84 NW 1, NW 3 and NW 5), lying within the 'central plain' (Dines & Edmunds, 1929), show that the base of the gravel lies at heights of betwe en +176.8m (+580 ft .) and +1 84.1m (+604 ft.) 0 .0 . The highest deposit s occur towards the west. The significance of the altitudinal position of these deposits is best measured in relation to the surrounding topography and the nature of the nearby drift depo sits. The summit surface level of the Caesar's Camp Plate au , at a height of about + 187 m 0 .0. is approximately 40 m above the projected surface level of the highest terrace (11th Terrace) of the Blackwat er Valley (Clarke & Dixon , 1981) and some 30 m higher than any part of the Hog 's Back (Chalk) ridge to the south east. The Caesar' s Camp Gravel is clearl y a remn ant spread of material which relates to a period when the surrounding landscape lay at least 40 m above that seen today. The nearest drift deposits occurrin g at a similar height to the Caesar's Camp Gravel are those at Netley Heath , some 22 km to the east , where deposits of marine origin (the Headle y Formation of John , 1980) containing a Red Crag faun a (Chatwin, 1927) are found .
C AES A R ' S CAMP G R AV E L
353
Fig. 7. Channcl-infill sands and clays at location Z .
(c) The heavy mineral analyses In general , the heavy miner al content of the Caesar's Camp Gravel bears a stro ng similarity to that of the Headley Sand on Netley He ath : zircon is generally the most frequent hea vy mineral in all samples, followed by roughl y equal amounts of rutile and tourmaline. However, in the Caesar's Camp Gravel , garnet is present in small amounts and monazite is more frequent th an reported by John (1980) for the Headley Sand. The frequencies of muscovite in samples from the Caesar's Camp Gravel and from the terrace deposits of Blackwater Valley are substantially higher than those of the Headley Sand . Thus, the samples from the deposits of Caesar's Camp Gravel do appear to have a mineralogy distinct from the presumed marine deposits on the North Downs , as the analyses of Groves (1931) suggested. The major difference between the sample s of river terrace deposits from the Blackwater Valley and the Caesar's Camp Gravel is the frequency of occurrence of musco vite noted above , and which Davie s (1915) reported as an important constituent of the Tertiary deposits, but only a minor component of the Cretaceous. Either this implies the incorporation of Eocene dep osits near to the base of the Caesar 's Camp Gravel, or that the extent of Eocene material in the
catchment area was greater at the time of deposition of the Cae sar's Camp Gr avel than when the Blackwater river was laying down its terrace dep osits. The mineralogy is otherwise con sistent with an origin in the Weald , since no minerals were observed for which a source cannot be found in the Lower Greensand (Davie s, 1915; Wood , 1956).
(d) The origin of the deposits A number of sedimentary features seen within the deposits of Caesar's Camp Gravel suggest depo sition in cold climatic conditions. The finely laminated silts and clays described above, reflect periodic deposition which may be related to annual, seasonal or even diurnal variation in run-off and sediment supply. The pre senc e of what appear to be icc-rafted flint pebbles (rdrop-sto nes') in the silts and sands suggests that the lamin ation may be due to climatically controlled fluctuation s in a per iglacial en vironment. Similarly the colour-banded fine sands (samples CS 2, 3, 7 and 8) also suggest a periodicity of sed iment supply which could be attributed to the variation in the volume of meltwaters. The erosional gravel-filled channels (gullies) are remarkable for the steepness of their margin s and the lack of incorporation of the adj acent sands (Fig. 5).
354
M . R . CLA R K E A N D P . F . FI SH ER
Thi s suggests that the gullies were formed when the sandy beds were still frozen . The bases of these features appear to be rounded , as if the erosional process was carried out by mass-movement of the in-situ gravel facies, rather than under more open fluvial conditions. The size and abundance of th e flint cobbles. and the un bedded and poorly sorted condition in which the y are found all suggest that the Caesar 's Camp Gravel was formed in periglacial conditions. The severity of the climatic conditions which led to the formation of these dep osits, may be illustrated by comp arison of the size of material in the cobble-gravel facies (largest particle size about 350 mm) to the size of gravel (largest particle size about 32 mm) in the nearby terrace deposits of the Blackwater Valley which are thought to have been laid down in an aggrading periglacial braided stream (Clarke & Dixon. 1981). The comparison of the Caesar's Camp Gravel to the torrent gravels ('Schotter') of the lower Alpine Valleys appears to be as valid today as when it was first made by Irving (p . 559) in 1890. It is thought that the red colours of the palaeosol relate to interglacial pedogenesis (Catt , 1979) , the band ed fabri c and plat y structures are interpreted as being crea ted by ground ice cond itions. (Romans & Robertson . 1974). The loam may represent reworked loess of late De vensian age (Catt , 1978) .
large volume s of meltwater which are most easily explained by periglacial conditions. Th e height of the depos its (+ 180 m 0 .0.) is significantly greater than the surro unding early Pleistocene river terrace deposits, the highest of which is projected to lie at about + 140 m 0 .0. in the vicinity of Aldershot. The apparent lowering of the topograph y of the area by some 40 metres during the period between the deposition of the Caesar's Camp Gravel and that of the 11th Terrace of the Blackwater river must have taken a considerable time . The age difference bctwen these deposits is emphasised by the difference in the heavy mineral assembl ages and stone litholo gies of the se deposits. Further, the Caesar's Camp Gr avel shows few mineralogical affinities with the marine deposits of the Headley Form ation (John, 1980) which lie between heights of 150 to 180 m 0.0. on Netley Heath, 22 km to the west of the Hale Plateau. It is suggested that the deposits of Cae sar 's Camp Gravel post-date the 'Calabrian' transgression and earl y Pleistocene marine deposition (Red Cr ag) but pre -date the seque nce of Pleistocene terraces of the river Blackwater. If this is accepted, the Caes ar 's Camp Gr avel represents some of the earlie st Pleistocene periglacial deposits to be found in so uthern England .
s, CONCLUSIONS
ACKNOWLEDGEMENTS
Although the absolute dating of the Caesar's Camp Gravel rem ains uncertain. the nature of the deposit and its topographic relationships allow some general conclusions to be made . The sedimentological evidence suggests that the deposits are related to a cold phas e flood-event (or events) , flowing northward off the Chalk dipslope of the North Downs. It is suggested that the size of the clast s and the massive bedding of the gravels indicate
We would like to thank Dr. G. M. Brown, Director. Institute of Geological Science s (NERC) for permi ssion to publish this paper, Dr. R. G. Thurrell, Mr. B. C. Wor ssam , Dr. D . John and Dr. J . Lowe have read the manu script and made helpful suggestio ns. The borehole information made available for this stud y was obt ained from an asses sment of sand and gravel resources of the area , commissioned by the Department of the Environment.
References BREWER. R. 1964. Fabric and Mineral Analysis of soils. Wiley. London. BURY. H. 1910. The denudation of the western end of the Weald. Q. JI. Geol. Soc. London, 66, 640- 92. - -. 1922. Some high-level gravels of north-east Hampshire. Proc. Geol. Assoc., 33, 81-103.
CAIT. J . A. 1978. The contribution of loess to soils in Lowland Britain. In (S. Limbrey & J. G. Evans: eds). The effects of man on the landscape: the Lo wland zone. CBA Research Report No. 21. 12- 20. - - . 1979. Soils and Quaternary Geology in Britain. Journal of Soil Science, 30, 607-42.
CHATWIN. C. P. 1927. Fossils from the iron sands on Netley Heath (Surrey), Mem. Geol. Sur. Summ. Prog. 1926, 154- 7.
CLARKE, M. R. & A. J. DIXON. 1981. The Pleistocene
braided river deposits in the Blackwater Valley area of Berkshire and Hampshire. England. Proc. Geol. A ssoc., 92, 139-5 7. -
- , - - . & M. KUBALA. 1979. The sand and gravel resources of the Blackwater Valley (Aldershot) area. Description of 1:25 000 sheets SU 85, 86. and parts of SU 84,94.95, and 96. Miner, Assess. Rep. lnst. Geol. Sci.,
39.
DAVIES. G. M. 1915. The rocks and minerals of the Croydon regional survey area. Proc. Croydon Nat. Hist. Sci. Soc., 8, 5-96. DINES. H. G. & F. H. EDMUNDS. 1929. The Geology of the Country around Aldershot and Guildford. Mem . Geol. Surv. U.K.
GEOLOGICAL SOCIETY OF AMERICA, 1975. Rock Color Chart-(Netherlands: Huyskes-Enschede).
CAESAR'S CAMP ORA VEL
GILBERT, C. J. 1920. On the Occurrence of Extensive Deposits of High-Level Sands and Gravels resting upon the Chalk at Little Heath near Berkhampstead. Q. Jl. Geol. Soc. Lond., 75,32-50. GROVES, A. W. 1931. On the unroofing of the Dartmoor Granite and the distribution of detritus in the sediments of southern England. Q. Jl. Geol. Soc. Lond., 87, 62-94. IRVING, A. 1890. Note on the Plateau Gravels of East Berks and West Surrey; their Age, Composition, and Structure. Q. Jl. Geol. Soc. Lond., 46,557-64. JARVIS, M. G., J. HAZELDEN, & D. MACKNEY. 1979. Soils of Berkshire. Soil survey Bulletin No.8, Harpenden. JOHN, D. T. 1980. The soils and superficial deposits on the North Downs of Surrey. In The Shaping of Southern England. London, Academic Press. MIALL, A. D. 1978. Lithofacies types and vertical profile models in braided river deposits: A Summary. In (Miall, A. D: ed) Fluvial Sedimentology, Canadian Society of Petroleum Geologists Memoir, No.5, 597-604. MIDDLEMISS, F. A. 1978. The Cherts in the Hythe Beds (Lower Cretaceous) of South-east England. Proc. Geol. Assoc.. 89,283-98.
355
MONKTON, H. W. 1892. On the Gravels South of the Thames from Guildford to Newbury. Q. Jl. Geol. Soc. Land.. 48, 29-59. MONTFORD, H. M. 1966. Field meeting to Farnham, Surrey, Proc. Geol. Assoc.. 77,381-3. PRESTWICH, J. 1890. On a southern drift in the Thames Valley and its relation to the Westleton beds etc. Q. Jl. Geol. Soc. Land.. 46, 155-81. ROMANS, J. C. C. & L. ROBERTSON. 1974. Some aspects of the genesis of alpine and upland soils in the British Isles. In (G. K. Rutherford; ed.). Soil Microscopy., Kingston, Canada, 498-510. WENTWORTH, C. K. 1922. A scale of grade and class terms for clastic sediments J. Geol., 30, 377-92. WOOD, G. S. 1956. The heavy mineral suites of the Lower Greensand of the western Weald. Proc. Geol. Assoc., 67, 124-37. WOOLDRIDGE, S. W. 1927. The Pliocene history of the London Basin. Proc. Geol. Assoc., 38,49-132. - - & D. L. LINTON. 1955. Structure, surface and drainage in south-east England. George Philip, London.
M. R. Clarke, Institute of Geological Sciences, Keyworth . Nottingham NGI25GG P. F. Fisher, School of Geography, Kingston Polytechnic. Penrhyn Road , Kingston-uponThames KTI 2EE
1. INTRODUCTION Nearly one hundred years have passed since the high-level deposits of Caesar's Camp, to the southwest of Aldershot (Fig. 1), were noted as forming 'among the most conspicuous of the gravel-capped hills' of the Thames Valley (Prestwich, 1890, p. 161). The nature and origin of these deposits provoked considerable debate during the late 19th century (Irving . 1890; Monkton, 1892), but the tirst full description of them was by Bury (1922). He concluded (p . 97) that the size and condition of the large nodular flints (which make up a large proportion of the gravels) 'afford a strong prima facie case , if nothing more , in favour of marine action' . He thus supported the view of Gilbert (1920) that deposits at a similar height. on the north side of the Thames at Little Heath , ncar Berkhamsted, in the Chilterns, were marine. A marine origin was not endorsed by Dines and Edmunds (1929, p. 108) in the Geologic al Survey memoir for the Aldershot and Guildford area; they felt that ' definite and conclusive solution s of the problems (of the origins of the Caesar's Camp Gravel) are still wantin g'. Nonetheless, Bury 's conclusions tended to gain acceptance , and the depo sits of the Hale plateau were used by Wooldridge (1927) and Wooldridge and Linton (1955) to indicate the southwestern limit of a high-level Pliocene shoreline. The marine origin of the Caesar's Camp Gravel was further emphasised by Wooldridge's assertion that the deposit had yielded to him the characteristic Diestian suite of heavy minerals, typical of the Lenham Beds in Kent. This was later questioned by Grove s (1931) whose analytical results suggested that the heavy mineral suites of the two deposits were different in several respects.
Little further work has been published on the nature of the Caesar's Camp Gravel, but Montford (1966) recorded his impression that the deposits are of glacial origin . More recently the altitudinal limits of Wooldridge's high level shoreline , (the 'Calabrian'), have been called into question (John , 1980). This paper presents new inform ation obtained from shallow boreholes drilled in the Caesar 's Camp Gravel and from field inspections. of recent workings in the deposit , by the authors in 1976 to 1980. 2. METHODS The working of the Caesar's Camp Gravel as a local source of aggregate has exposed a number of large pit sections which have been available for inspection over the last ten years. The most interesting sections are visible in a pit (822494) at the western end of the deposit (locations Y and Z. Fig. 2), where the nature and form of the Caesar's Camp Gravel are best demonstrated. Additional information has been obtained from five, 8-inch diameter boreholes specially drilled (in 1976), by Shell and Auger rig, in the Caesar's Camp Gravel at the locations shown in Fig. 2. This work was undertaken by the Institute of Geological Sciences as part of a national assessment of sand and gravel resources commissioned by the Department of the Environment (Clarke , Dixon & Kubala , 1979). Bulk samples of the depositis have been collected from the pit sections and the boreholes. to provide material for study . The particle size distribution curves (Fig. 3) are based upon sieve analyses carried out using a wet sieving method conforming to BS 812 and pipette analyses for the silt and clay size fractions . The residues of these analyses together with unused
345
346
M. R. CLARKE AND P. F. FISHER
85
80
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Caesar's Cam p Gravel
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Fig.!. The location and local geology of the Caesar's Camp Gravel.
material, have been used to provide the 8-32 mm size fraction for the pebble counts (Table 1), The heavy mineral analyses (Table 2) were carried out by separation (using bromoform) of the very fine sand fraction (0.063 mm to 0.125 mm of Wentworth, 1922) of material obtained from the boreholes. They are compared (Table 2) with similarly obtained material from boreholes drilled in the terrace deposits of the Blackwater valley and with the frequencies (of a slightly different size fraction) reported for deposits of Headley Sand (John, 1980). Soil thin sections were prepared of samples of the clay-enriched material which forms the palaesol at location W (830492), 3. THE TEXTURE OF THE DEPOSITS
The pit sections available for inspection at locations X and Y (Fig. 2) show that the deposits at Caesar's Camp comprise three main lithological units: (a) The Cobble-gravel facies
The most prominent and characteristic feature of the Caesar's Camp Gravel is their massive nature and the abundance of large nodular flints which form the
distinctive cobble-gravel facies (Fig. 4). The flint cobbles are frequently larger than 0.2 m (Fig. 3a) and generally have a thin white patina concealing the grey, black or brown interiors. The cobbles are nodular, showing little damage from their primary condition, although horns are generally broken or worn away. They appear to have travelled only a short distance from their source, and are similar to the proximal massive gravels (Gm) described by Miall (1978). The matrix consists mainly of yellow and orange, fine and medium grained sub angular quartz, with some coarse quartz and flint sand. Generally the cobble-gravel facies forms one unit ranging from 0.8 m to 1.5 m in thickness; in some parts of the pit it is separated by beds of fine and medium grained quartz sand (see below). There is no evidence of wide variations in grading within each gravel unit, and the junction between the gravels and adjacent sandy beds is generally sharp both above and below the unit. Locally, the gravel contains thin silty clay laminae (samples CS 9 and CS 10) which pass upwards into more massive clayey fine sands up to 0.5 m thick, for example, sample CS 11 (Fig. 3b). It is noteworthy that all of the material recovered from the boreholes
347
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Fig, 2, The topography of the Caesar's Camp Gravel and sample locations,
has suffered a loss of coarse gravel and cobbles due to the crushing of material by the drilling process. This can be illustrated by the grading curves shown in Fig. 3a; the bulk samples of in-situ gravel (CS 5 and CC 5) from pit sections at locations Y and X (823492) respectively. contain about 30% of cobble size (+64 mm) flints. whereas this material is poorly represented in the borehole samples. (b) The sand-clay facies The Caesar's Camp Gravel contains numerous thin sandy layers interbedded within the cobble-gravel facies; these sand bodies appear to represent three modes of deposition described below. (i) Horizontal, planar-bedded sands Planar-bedded sands (up to 1.5 m thick) are seen both overlying and underlying the gravel facies in the northern face of the main exposure (Yon Fig. 2), The sands are composed mainly of beds of pale yellow or orange fine-grained quartz; their grading characteris-
tics are typified by samples CS 2, CS 3, CS 7 and CS 8 (Fig. 3c) which appear to represent the (Sh) facies described by Miall (op. cit). In some places. the sands display alternating bands (up to 3 ern thick) of colour which apparently reflect variations in grain size; samples of CS 2 and CS 7 (orange stained) contain slightly more clay than the pale yellow sands of samples CS 3 and CS 8 (Fig. 3c). Locally the planar-bedded sands are overlain by the cobble-gravel facies. In several places at location Y. the horizontally bedded sands are deeply eroded by steep-sided channels ('gullies') up to 1.5 m in depth and 1.0 m wide. which are filled with material similar to the overlying cobble-gravel (Fig, 5). (ii) Cross-bedded sand lenses Small (up to 1.0 m in length and 0.3 m in thickness) cross-bedded and planar-bedded lenses of fine and medium sand (the Sp and St facies of Mia!! commonly occur within the gravel facies; they are apparently
348
M . R. CLA R K E A ND P. F. FI SH ER
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Fig. 3. Mean grading analyses of samples from the Caesar's Camp Gravel.
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349
CAESAR'S CAMP GRAVEL
TABLE 1. The composition of the Caesar's Camp Gravel expressed as a percentage (by number) of the 8-32 mm material No. of pebbles (+8mm)
Sample location
Rounded flint
179 167 846 310 338 3368 20017
Borehole 84 NW 1 Borehole 84 NW 2 Borehole 84 NW 3 Borehole 84 NW 4 Borehole 84 NW 5 Sample CC5 at Location 'X' River Terrace Deposits Blackwater Valley (mean of nine samples)
Angular flint
7.2 91.1 16.2 81.4 9.3 90.6 9.1 90.3 24.9 69.1 (not separated) (not separated)
more numerous in the western face of the pit (location Z). Samples CS 12 and CS 13 (Fig. 3d) taken respectively from the middle and base of one of these sand lenses, suggest a fining upwards of grain size within the sand lenses. Similarly samples CS 14 and CS 15 taken from the top and base of a larger
Flint (total)
Cherty sandstone
98.3 97.6 99.9 99.4 94.0 98.3 89.8
Vein-quartz
Ironstone 1.7 2.4
1.2 0.5 9.6
0.1 0.3 1.5 0.5 0.5
0.3 3.3 0.7 0.1
cross-bedded sand unit (approximately 1 m thick and up to 10 m long) also demonstrate a fining upwards. (iii) Channel infill sands and silts Fine sands and silts, similar to the Ss tacies of Miall, are seen in two pit sections, infilling channel-like
TABLE 2. Heavy minerals of the 0.063 to 0.125 mm (v. fine sand) fraction of the Caesar's Camp Gravel Caesar's Camp Gravel
Heavy minerals
Zircon Tourmaline Rutile Muscovite Kyanite Staurotite Sillimanite Monazite Garnet Epidote Others Opaques (as a percentage of the total heavy mineral content) Total number of grains
Borehole (84 NW2)
Borehole (86 NW5)+
Percentages by number 48.09 (71.59) 55.10 (71.05) 6.12 (7.89) 2.29 (3.41) 5.34 (7.95) 4.08 (5.26) 32.8 22.45 2.04 (0.63) 1.5 (2.27) 3.8 (5.68) 8.16 (10.52) 1.5 (2.27) 2.04 (2.63) 4.6 (6.82)
Terrace * deposits of the Blackwater Valley
Borehole (84 NW 3)+
Borehole (84 NW 1)+
31.83 (53.53) 7.51 (12.63) 7.81 (13.13) 40.54 2.40 (4.04) 5.40 (9.09) 0.60 (1.01) 3.60 (6.06) 0.30 (0.50)
44.79 (61.43) 8.33 (11.43) 6.25 (8.57) 27.08 5.2 (7.14) 3.12 (4.28) 2.08 (2.86) 3.12 (4.28)
41.62 18.29 8.62 11.91 7.5 3.77 1.43 2.32 2.42 1.52
Headley** Sand
52.16 12.74 22.52 0.3 5.02 6.1 0.6
74.71
90.29
37.52
81.10
84.76
0.56 71.84
518
505
533
508
5,243
7,847
+ Second sample from the bottom of each borehole was separated-values in brackets are calculated on a muscovite-free basis. ** From John, 1980 Table IlIa-mean of 5 samples in range 0.0075-0.09 m. * From unpublished data mean of 5 samples.
350
M. R. CLARKE AND P. F. FISHER
Fig. 4. The cobble-gravel facies of the Caesar's Camp Gravel.
depressions. At location V (827495) in the northern margins of the deposit, a broad channel of about 5 metres width and 0.8 m depth, could be seen within the cobble-gravel facies (Fig. 6). This channel contains a sequence of planar-bedded and ripple cross laminated fine to medium-grained sands (sample CC 6 in Fig. 3f) which pass upwards into laminated clays and silts. In the western face of the main pit (location Z) sands and silty clays of a similar type, also fill a complex channel-like depression within the cobble gravel facies. The deposits (Fig. 7) reach 3 m in thickness and about 10 m in lateral extent; they comprise upper and lower silt/clay members, separated by a cobble-gravel unit up to 1 m thick. The lowermost member is over 1 m thick and exhibits parallel concave bedding in pale grey and orange silts and clays (represented by samples CS 17 to CS 22 in Fig. 3e) showing no obvious trend in grain-size variation. These deposits are overlain in turn by the cobble-gravel, and the upper sequence of silts and clays, which also demonstrate perfect parallel bedding in relation to the concave upper surface of the gravel unit. The samples CS 23 to CS 26 from the upper silt/clay unit (Fig. 3f) contain more fine sand and less clay than those from the lower silt/clay unit. The dark staining at the base of this upper unit
appears to be due to a modern pedological concentration of iron and manganese compounds above the junction with the bed of cobble gravel. (c) Palaeosol and Loam
Locally the deposits of Caesar's Camp Gravel are overlain by a silty clay loam up to 0.8 m thick, as at location W (830492). This fine-grained material (mean grading curves CC 1 and CC 2 in Fig. 3e) is thought to be of aeolian origin; it overlies (and hence postdates) a rubified palaeosol which occurs in the upper part of the Caesar's Camp Gravel. The upper part of this palaeosol is characterised by massive columnar structures up to 20 em across with yellow (10 YR 1/4-0n the Munsell colour chart: Geological Society of America, 1975) faces and yellowish red (7.5 YR 6/8) interiors with red (10 R 4/6) mottles and secondary platy structures. Lower horizons are not columnar, but show platy structures with red (10 R 4/6) mottles. Thin sections of the palaeosol show well oriented clay coats surrounding skeletal grains (similar to the free grain cutans of Brewer, 1964), many disrupted clay coats (papules) and a banded fabric reminiscent of the 'silt droplets' of Romans and Robertson (1974). In contrast, the overlying deposits of loam only rarely
C A E SA R'S C A MP GR A VEL
351
Fig. 5. Gully erosion of the inter-b edded fine and medium sands by the cobble-gravels.
contained oriented clay part icles which generall y occur as void argillans (Brewer, 1964). 'Modern' soils similar to podzol s of the Southampton series (Ja rvis et al. , 1979) have also developed to a depth of 1 rn, into the top of the Caesar 's Camp Gr avel although they are more commonly formed in the upper part s of the localised deposits of loam. 4. DISCUSSION (a) The texture and composition of the deposits The mean particle size distr ibutions of the material proved in three IMAU boreholes (84 NW 1, NW 3 and NW 5) are very similar (Fig. 3a). In contrast, borehole SU 84 NW 2, drilled in the solifiucted material at the southern margin of the depo sit, proved its base at + 152.8 m (+501 ft) 0 .0 ., and the borehole samples contained a high proportion (56% ) of clay to fine sand (-0.25 mm size material) which presumably had been incorporated from the underlying Tertiary bedrock. The position and basal height of + 168.6 m (+ 553 ft) QD. of the dep osits in the fifth borehole (SU 84 NW 4) suggest that they too are soliflucted from the main deposit. This is supported by the mean gradin g results (Fig. 3a) which show that the material , at this site is similar to a bulk sample of soliftucted sand and gravel
(CS 4) taken from pit section Y (822494) in the upper part of the main gravel sprea d. Thus it would appea r that the mater ial formi ng the Hungry Hill spur represents a large solifiucted spread of gravel derived from the central are a. The composition of the (8-32 mm) material fro m the five boreholes drilled in the Caesar's Camp Gravel is compared to the composition of the terrace dep osits in the Blackwat er Valley in Tabl e 1. The most striking feature of the relative proportions of the components in the Caesar's Camp Gravel is the almost entire lack of chert y sandstone debris , deri ved from the Lower G reensand . In only one sample is there more than 1% of sandstone , compared with an average of 10% in samples from the terr ace depo sits of the Blackwater Valley (Clarke & Dixon , 1981). Th e sparse occurrence of Lower Greensand debri s in the Cae sar's Ca mp Gr avel also contrasts strikingly with results from the Ranm ore Gra vel of the North Downs where a variation, from 20% in the west near East Horsely, to 3% near Chipstead in the East , has been record ed (John, 1980). Contrary to the suggestion (Bury , 1922) that cherty materi al is absent from the central plain of the Hale Plateau, small amounts have been found in sample s from Borehole 84 NW 5 and from the exposures at location X. Although the frequenc y of
352
M . R. CLA R K E A ND P . F. F IS HER
.-
Fig. 6. Channel-infill fine sands and clays, interbedded with the cobble-grave l facies at location V.
quartz pebbl es is relati vely constant in both the Caesar's Camp Gra vel and the terrace deposits of the River Blackwater, the frequency of ironstone varies considerably. The proven ances of four of the five components recogni sed in the Caesar's Camp Gr avel are given by Clarke and Dixon top, cit); the angular flint is derived from the Chalk, the rounded flints are derived from Tertiary pebble beds, and chert and cherty sandstones are derived from the Hythe Beds of the Weald (Middlemiss, 1978). The probable source of the vein-quartz and ironstone is thought to be pebble beds within the Cretaceous strata of the Weald, although some ironstone material may be derived from ironpans which occur within the Caesar's Camp Gravel itself. The possibility that these dep osits once contained considerable amounts of chalk debri s cannot be ignored; subsequent decalcification could have accounted for some of the present compaction of the deposit. (b) The elevation of the deposits The borehole results show that the basal surface of the depo sit (recognised by its sharp junction with the underl ying Tert iary Beds) is highly irregular and that
the sprea d of sand and gravel is affected by solifluxion towards its margins. Three of the boreh oles (IGS registered numb ers SU 84 NW 1, NW 3 and NW 5), lying within the 'central plain' (Dines & Edmunds, 1929), show that the base of the gravel lies at heights of betwe en +176.8m (+580 ft .) and +1 84.1m (+604 ft.) 0 .0 . The highest deposit s occur towards the west. The significance of the altitudinal position of these deposits is best measured in relation to the surrounding topography and the nature of the nearby drift depo sits. The summit surface level of the Caesar's Camp Plate au , at a height of about + 187 m 0 .0. is approximately 40 m above the projected surface level of the highest terrace (11th Terrace) of the Blackwat er Valley (Clarke & Dixon , 1981) and some 30 m higher than any part of the Hog 's Back (Chalk) ridge to the south east. The Caesar' s Camp Gravel is clearl y a remn ant spread of material which relates to a period when the surrounding landscape lay at least 40 m above that seen today. The nearest drift deposits occurrin g at a similar height to the Caesar's Camp Gravel are those at Netley Heath , some 22 km to the east , where deposits of marine origin (the Headle y Formation of John , 1980) containing a Red Crag faun a (Chatwin, 1927) are found .
C AES A R ' S CAMP G R AV E L
353
Fig. 7. Channcl-infill sands and clays at location Z .
(c) The heavy mineral analyses In general , the heavy miner al content of the Caesar's Camp Gravel bears a stro ng similarity to that of the Headley Sand on Netley He ath : zircon is generally the most frequent hea vy mineral in all samples, followed by roughl y equal amounts of rutile and tourmaline. However, in the Caesar's Camp Gravel , garnet is present in small amounts and monazite is more frequent th an reported by John (1980) for the Headley Sand. The frequencies of muscovite in samples from the Caesar's Camp Gravel and from the terrace deposits of Blackwater Valley are substantially higher than those of the Headley Sand . Thus, the samples from the deposits of Caesar's Camp Gravel do appear to have a mineralogy distinct from the presumed marine deposits on the North Downs , as the analyses of Groves (1931) suggested. The major difference between the sample s of river terrace deposits from the Blackwater Valley and the Caesar's Camp Gravel is the frequency of occurrence of musco vite noted above , and which Davie s (1915) reported as an important constituent of the Tertiary deposits, but only a minor component of the Cretaceous. Either this implies the incorporation of Eocene dep osits near to the base of the Caesar 's Camp Gravel, or that the extent of Eocene material in the
catchment area was greater at the time of deposition of the Cae sar's Camp Gr avel than when the Blackwater river was laying down its terrace dep osits. The mineralogy is otherwise con sistent with an origin in the Weald , since no minerals were observed for which a source cannot be found in the Lower Greensand (Davie s, 1915; Wood , 1956).
(d) The origin of the deposits A number of sedimentary features seen within the deposits of Caesar's Camp Gravel suggest depo sition in cold climatic conditions. The finely laminated silts and clays described above, reflect periodic deposition which may be related to annual, seasonal or even diurnal variation in run-off and sediment supply. The pre senc e of what appear to be icc-rafted flint pebbles (rdrop-sto nes') in the silts and sands suggests that the lamin ation may be due to climatically controlled fluctuation s in a per iglacial en vironment. Similarly the colour-banded fine sands (samples CS 2, 3, 7 and 8) also suggest a periodicity of sed iment supply which could be attributed to the variation in the volume of meltwaters. The erosional gravel-filled channels (gullies) are remarkable for the steepness of their margin s and the lack of incorporation of the adj acent sands (Fig. 5).
354
M . R . CLA R K E A N D P . F . FI SH ER
Thi s suggests that the gullies were formed when the sandy beds were still frozen . The bases of these features appear to be rounded , as if the erosional process was carried out by mass-movement of the in-situ gravel facies, rather than under more open fluvial conditions. The size and abundance of th e flint cobbles. and the un bedded and poorly sorted condition in which the y are found all suggest that the Caesar 's Camp Gravel was formed in periglacial conditions. The severity of the climatic conditions which led to the formation of these dep osits, may be illustrated by comp arison of the size of material in the cobble-gravel facies (largest particle size about 350 mm) to the size of gravel (largest particle size about 32 mm) in the nearby terrace deposits of the Blackwater Valley which are thought to have been laid down in an aggrading periglacial braided stream (Clarke & Dixon. 1981). The comparison of the Caesar's Camp Gravel to the torrent gravels ('Schotter') of the lower Alpine Valleys appears to be as valid today as when it was first made by Irving (p . 559) in 1890. It is thought that the red colours of the palaeosol relate to interglacial pedogenesis (Catt , 1979) , the band ed fabri c and plat y structures are interpreted as being crea ted by ground ice cond itions. (Romans & Robertson . 1974). The loam may represent reworked loess of late De vensian age (Catt , 1978) .
large volume s of meltwater which are most easily explained by periglacial conditions. Th e height of the depos its (+ 180 m 0 .0.) is significantly greater than the surro unding early Pleistocene river terrace deposits, the highest of which is projected to lie at about + 140 m 0 .0. in the vicinity of Aldershot. The apparent lowering of the topograph y of the area by some 40 metres during the period between the deposition of the Caesar's Camp Gravel and that of the 11th Terrace of the Blackwater river must have taken a considerable time . The age difference bctwen these deposits is emphasised by the difference in the heavy mineral assembl ages and stone litholo gies of the se deposits. Further, the Caesar's Camp Gr avel shows few mineralogical affinities with the marine deposits of the Headley Form ation (John, 1980) which lie between heights of 150 to 180 m 0.0. on Netley Heath, 22 km to the west of the Hale Plateau. It is suggested that the deposits of Cae sar 's Camp Gravel post-date the 'Calabrian' transgression and earl y Pleistocene marine deposition (Red Cr ag) but pre -date the seque nce of Pleistocene terraces of the river Blackwater. If this is accepted, the Caes ar 's Camp Gr avel represents some of the earlie st Pleistocene periglacial deposits to be found in so uthern England .
s, CONCLUSIONS
ACKNOWLEDGEMENTS
Although the absolute dating of the Caesar's Camp Gravel rem ains uncertain. the nature of the deposit and its topographic relationships allow some general conclusions to be made . The sedimentological evidence suggests that the deposits are related to a cold phas e flood-event (or events) , flowing northward off the Chalk dipslope of the North Downs. It is suggested that the size of the clast s and the massive bedding of the gravels indicate
We would like to thank Dr. G. M. Brown, Director. Institute of Geological Science s (NERC) for permi ssion to publish this paper, Dr. R. G. Thurrell, Mr. B. C. Wor ssam , Dr. D . John and Dr. J . Lowe have read the manu script and made helpful suggestio ns. The borehole information made available for this stud y was obt ained from an asses sment of sand and gravel resources of the area , commissioned by the Department of the Environment.
References BREWER. R. 1964. Fabric and Mineral Analysis of soils. Wiley. London. BURY. H. 1910. The denudation of the western end of the Weald. Q. JI. Geol. Soc. London, 66, 640- 92. - -. 1922. Some high-level gravels of north-east Hampshire. Proc. Geol. Assoc., 33, 81-103.
CAIT. J . A. 1978. The contribution of loess to soils in Lowland Britain. In (S. Limbrey & J. G. Evans: eds). The effects of man on the landscape: the Lo wland zone. CBA Research Report No. 21. 12- 20. - - . 1979. Soils and Quaternary Geology in Britain. Journal of Soil Science, 30, 607-42.
CHATWIN. C. P. 1927. Fossils from the iron sands on Netley Heath (Surrey), Mem. Geol. Sur. Summ. Prog. 1926, 154- 7.
CLARKE, M. R. & A. J. DIXON. 1981. The Pleistocene
braided river deposits in the Blackwater Valley area of Berkshire and Hampshire. England. Proc. Geol. A ssoc., 92, 139-5 7. -
- , - - . & M. KUBALA. 1979. The sand and gravel resources of the Blackwater Valley (Aldershot) area. Description of 1:25 000 sheets SU 85, 86. and parts of SU 84,94.95, and 96. Miner, Assess. Rep. lnst. Geol. Sci.,
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DAVIES. G. M. 1915. The rocks and minerals of the Croydon regional survey area. Proc. Croydon Nat. Hist. Sci. Soc., 8, 5-96. DINES. H. G. & F. H. EDMUNDS. 1929. The Geology of the Country around Aldershot and Guildford. Mem . Geol. Surv. U.K.
GEOLOGICAL SOCIETY OF AMERICA, 1975. Rock Color Chart-(Netherlands: Huyskes-Enschede).
CAESAR'S CAMP ORA VEL
GILBERT, C. J. 1920. On the Occurrence of Extensive Deposits of High-Level Sands and Gravels resting upon the Chalk at Little Heath near Berkhampstead. Q. Jl. Geol. Soc. Lond., 75,32-50. GROVES, A. W. 1931. On the unroofing of the Dartmoor Granite and the distribution of detritus in the sediments of southern England. Q. Jl. Geol. Soc. Lond., 87, 62-94. IRVING, A. 1890. Note on the Plateau Gravels of East Berks and West Surrey; their Age, Composition, and Structure. Q. Jl. Geol. Soc. Lond., 46,557-64. JARVIS, M. G., J. HAZELDEN, & D. MACKNEY. 1979. Soils of Berkshire. Soil survey Bulletin No.8, Harpenden. JOHN, D. T. 1980. The soils and superficial deposits on the North Downs of Surrey. In The Shaping of Southern England. London, Academic Press. MIALL, A. D. 1978. Lithofacies types and vertical profile models in braided river deposits: A Summary. In (Miall, A. D: ed) Fluvial Sedimentology, Canadian Society of Petroleum Geologists Memoir, No.5, 597-604. MIDDLEMISS, F. A. 1978. The Cherts in the Hythe Beds (Lower Cretaceous) of South-east England. Proc. Geol. Assoc.. 89,283-98.
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MONKTON, H. W. 1892. On the Gravels South of the Thames from Guildford to Newbury. Q. Jl. Geol. Soc. Land.. 48, 29-59. MONTFORD, H. M. 1966. Field meeting to Farnham, Surrey, Proc. Geol. Assoc.. 77,381-3. PRESTWICH, J. 1890. On a southern drift in the Thames Valley and its relation to the Westleton beds etc. Q. Jl. Geol. Soc. Land.. 46, 155-81. ROMANS, J. C. C. & L. ROBERTSON. 1974. Some aspects of the genesis of alpine and upland soils in the British Isles. In (G. K. Rutherford; ed.). Soil Microscopy., Kingston, Canada, 498-510. WENTWORTH, C. K. 1922. A scale of grade and class terms for clastic sediments J. Geol., 30, 377-92. WOOD, G. S. 1956. The heavy mineral suites of the Lower Greensand of the western Weald. Proc. Geol. Assoc., 67, 124-37. WOOLDRIDGE, S. W. 1927. The Pliocene history of the London Basin. Proc. Geol. Assoc., 38,49-132. - - & D. L. LINTON. 1955. Structure, surface and drainage in south-east England. George Philip, London.