Pleistocene Gravel Trains of the River Thames

Pleistocene Gravel Trains of the River ThamesC. P. GREEN and D. F. M. MCGREGOR

GREEN, C. P. & D. F. M. McGREGOR, 1978 . Pleistocene Gravel Trains of theRiverThames. Proc. Geol. Ass . ,89(2), 143- 56. The Gravel Trainsofthe Thames havebeen examined in an area between Bourne End (Bucks) and Bricket Wood (Herts),and have been compared with gravels at higher and lower levels. Pebble lithologyand the roundness of flint pebbles have been determined in the half-phi size range11·2-16'0 mm. The Mann-Whitney U test is used to measure the significance level ofdifferences between sample data sets comprising different suites of gravels.

The results indicate a variety of catchment changes before, during and after GravelTrain times, and suggest a re-interpretation of the drainage development of the areaexamined. No diversion of the Thames from its early course through the Vale of St.Albans can be detected before or during Gravel Train times. On the contrary,continuations of both Higher and Lower Gravel Trains are apparent in the Vale of St.Albans, whereas a separate Leavesden Gravel Train of eastern derivation cannot berecognised. Problems connected with the diversion of the Thames after Gravel Traintimes are discussed.

Department ofGeography , Bedford College (University of London) , Regent's Park,London, NWI 4NS.

CONTENTSpage

1. INTRODUCTION ...2. SAMPLE PRE PARAnON AND ANALYSIS3. DISCUSSION...4. CONCLUSION

ACKNOWLEDGMENTREFERENCES

1. INTRODUCTION

143144150155155155

The course of the Thames between Goring and the North Sea has changed substantially duringthe Pleistocene. Sherlock (1924) argued that an early course of the Thames had extended fromBourne End in an east-north-east direction through the Vale of St. Albans into south Essex,Sherlock concluded that glacial ice, indicated by chalky tills in the Vale of St. Albans, hadblocked this course of the river and had diverted it into its present course.

The area was re-examined by Wooldridge (1938). He related the eastward course of theThames through the Vale of St. Albans to the early Pleistocene Pebble Gravel stage, but hefound no evidence for the persistence of this course later in the Pleistocene. He proposedtherefore that glacial ice had advanced on two occasions into this area. On the first occasion , atthe time of the Chiltern Drift glaciation, the Thames was diverted from the Vale of St. Albansinto a more southerly course through the Finchley Depression. Stony clays between Chorleywood and Amersham, and eastwards into Hertfordshire were regarded by Wooldridge asboulder-clay and as evidence of this glaciation. The Higher and Leavesden Gravel Trains wererelated by Wooldridge to a stage immediately following this event. The Higher Gravel Train

144 C. P. GREEN AND D. F. M. MCGREGOR

marks the course of the Thames eastward from Goring. It was traced by Wooldridge to a pointnear Croxley Green. The Leavesden Gravel Train he described as falling towards the same pointfrom the opposite direction, from the site of the supposed ice-front near St. Albans. Doubtsabout the role of glacial ice in this diversion have been expressed by Holmes (1965). The LowerGravel Train of Wooldridge (1938) lies to the south of the Higher Gravel Train at a lower level,and appears to follow a similar course from the west towards the Finchley Depression. On asecond occasion, at the time of the glaciation which introduced chalky tills into this area (EasternDrift glaciation of Wooldridge & Linton, 1955), the river was diverted from the FinchleyDepression into its present course. Chalky tills occur as far west in the Vale of St. Albans asBricket Wood, and as far south in the Finchley Depression as Finchley. The gravels of theWinter Hill terrace of the Thames were regarded by Wooldridge as confluent with the outwashdeposits of the chalky till ice. This outwash reached the Thames along a route which is nowoccupied by the valley of the Colne.

The occurrence near Hatfield of Hoxnian organic deposits overlying chalky till (Sparks, West,Williams & Ransom, 1969) has generally been taken to indicate an Anglian age for the till. Rose(1974) has examined the lithology and macro-fabric of the till at this site and at a site nearHertford. His results show a relatively small proportion of durable stones of distant provenancein the chalky till.

Significance is attached by both Sherlock and Wooldridge to the distribution of gravelbetween Bourne end and St. Albans. Features other than distribution, such as structure, andpebble size, shape and composition were not evaluated, apart from a general emphasis on thefrequency of glacially-derived material. Subsequent accounts (Hare, 1947; Sealy & Sealy, 1955)have mainly concentrated on the morphology of terrace remnants. Important exceptions are thestudies of Hey (1965) on the Pebble Gravels, and Walder (1967) on the Winter Hill stage nearReading. In these studies fairly precise stratigraphic distinctions are based on gravel composition, and a relatively large proportion of material of presumed Midland derivation isdemonstrated at various levels above the Lower Winter Hill stage.

In the present account gravels are described from an area between Bourne End in the westand the western limit of the chalky till in the Vale of St. Albans in the east (Fig. 1). Pebblecomposition and shape have been analysed in 37 samples from 31 sites. Duplicate samples weretaken in large pits (T15, T16 and T19, T20, T21) or where obvious lithological changes occurred(T25ff26, T37ff39, T43ff44). Representative samples were taken at each site from below theplough layer.

For comparative purposes the samples have been separated on the basis of elevation (Fig. 2)into five groups. These groups can be related to stratigraphic units recognised in the same area inearlier studies:1. Pebble Gravels (including the Westland Green Gravels)2. The Higher Gravel Train3. The Leavesden Gravel Train4. The Lower Gravel Train5. Gravels of the River Colne

2. SAMPLE PREPARATION AND ANALYSIS

Air-dry samples were sieved through a 3·65 mm mesh. The coarser fraction was then washed ona 3·65 mm mesh and subsequently dried. The washed sample was sieved for 15 minutes toseparate it into seven fractions at half-phi intervals. The sieve apertures used were 31,5, 22'4,

PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES 145

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i' I t iii t io Kilomtlrrs 10

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+KINGSTON tFig. 1. Location of sample points. Numbers refer to Table I. Contour at 122 m.


ft600

m

180

·68Z QILl

Q w 0·67 IX 0Z (:J ~ILl

ILl ·86>- ,..ILl ILlZ

Pebble ...J :0:IX Gravels )( 0::J 0 it0 IXIII 0 III

550

500

450

400

160

140

120

40150

200 60

80

o

100

20

250

350

300

Higher G ravel Train8/' ... 18 ";9 -21

Fig. 2. Sample points in relation to schematic profiles. Open symbols indicate the 'Leavesden' gravels. The dotted linerepresents the chalky till ice-front. South-west to north-east projection line.

16·0, 11'2,8'0,5-6 and 4-0 mm. Because the relative frequency of rock types commonly varieswith particle size (Davis, 1958; Boggs, 1969) and the shape of particles is also partly determinedby their size (Sneed & Folk, 1958) detailed comparisons between samples in this study are basedon the 11-2-16'0 mm fraction which provides a sample of adequate size (average 285 pebbles)from a manageable bulk sample of c.20 kg. Results may not be comparable in detail with earlierstudies in the same area. Hey (1965) examined pebbles in the 16-32 mm fraction, Walder(1967) examined all pebbles larger than 5 mm, while Rose (1974) examined the 4-16 mmfraction.

(a) Pebble composition

The pebbles in the gravels were separated into five classes (Table 1). Flint and Lower Greensandmaterial are referred to as local. Material which is presumed to have originated outside thepresent catchment of the Thames is referred to as far-travelled. This includes all the materialsclassified in Table I as quartz, sandstone or other. Hey (1965) has reviewed the questions of boththe ultimate and immediate origin of far-travelled material in the Thames gravels. He concludes(p, 417) that the far-travelled material was' ... gathered together by some unknown means inthe south Midlands. Thence it was carried through the Goring Gap by a river. ... ' Hey evidentlyregards glacial ice as the most probable 'unknown means', and this view is accepted in thepresent account.


(i) Flint

This is mostly derived within the present catchment of the Thames, either directly from theChalk outcrop, or, particularly in the case of well rounded pebbles, from local Tertiary pebblebeds. Small quantities may however have been introduced from further north at various time byglacial ice.

In all fractions, in all samples, flint is the most frequent single component. Flint is predominant in the higher Pebble Gravels (T67, T68) and in the Colne gravels (Tl, T17, T37, T42,T51). In the intervening altitudinal range gravels occur in which flint is less frequent and may beexceeded in amount by non-flint material. These gravels include the Westland Green series ofHey (1965), the Gravel Trains of Wooldridge (1938) and gravel on ground overlooking theColne valley (T15, T16) which Wooldridge (1938) described as the outwash of the Eastern Drift(chalky till) ice. In the same altitudinal range there are however gravels in which large proportions of flint occur (T26, T43, T84, T85). In the field such gravels are typically structurelessand compact, and usually have a 'dirty' matrix of clayey sand. They form the 'hoggin' ofcommerce, and are found as a surface horizon up to 3 m thick, either overlying 'clean' sandygravels of 'ballast' quality, or mantling gentle slopes. T26 and T43 overlie T25 and T44respectively. Similar compact, structureless, flinty gravels also occur at lower levels (e.g. T39),but they cannot be distinguished on the basis of composition because 'ballast' quality gravels arealso flinty at these lower levels (e.g. T37 which underlies T39).

(ii) Quartz

Quartz is an important part of the far-travelled component in all samples. Quartz is relativelyless common than sandstone in the larger fractions (>16 mm), but becomes increasinglyabundant in progressively smaller fractions.

Potential sources of quartz pebbles are evident within the present catchment of the Thames, inthe Lower Greensand and in Palaeogene formations. However the quartz pebbles in thesesediments are mainly small « 11·2 mm). Larger pebbles are probably derived mainly from theBunter Pebble Beds, in which quartz is common (Shrubsole, 1903), or from other sourcesoutside the present catchment of the Thames. Such pebbles may therefore be regarded asfar-travelled material.

(iii) Sandstone

Included here are the quartzites generally assigned to the Bunter Pebble Beds. In this study,smooth, hard, yellowish- and reddish-brown pebbles of thoroughly lithified sandstone havebeen regarded as 'Bunter'. However the sandstone pebbles in the gravels are enormouslydiverse, ranging from very hard, colourless orthoquartzites and metaquartzites, through theso-called Bunter types and a range of hard but less completely lithified sandstones, to pebbles ofsoft micaceous and feldspathic sandstones. How much of this material is of Bunter origin isdifficult to assess. In this study no subdivision of the sandstone class is attempted.

Bunter material is probably absent above the Westland Green level, although pale-colouredquartzites of otherwise similar lithology are present in T67 and T68. In the Westland GreenGravels and in gravels at lower levels Bunter material is invariably present.

(iv) Lower Greensand chert and cherty sandstone

Samples throughout the height range examined contain pebbles of chert and cherty sandstonederived from the Lower Greensand of the Weald. Differentiation between chert and flint maybe difficult in some cases. In this study classification as chert or cherty sandstone is based on the


presence of the following features: chalcedonic matrix, detrital mineral grains, sponge spicules,roughly pitted weathered surface, pale brown colour.

(v) Others

This class includes a wide range of rock types, most of which are represented only once or twiceamong the 2890 far-travelled pebbles examined in the samples. Radiolarian cherts and silicifiedoolitic material occur as well as small numbers of igneous, volcanic and metamorphic pebbles.Again it is difficult to know how much of this material comes from the Bunter Pebble Beds.

TABLE I Sample data obtained from 11.2-16.0mm fraction of Thames and Colne gravels. Location of sample sitesshown in Fig. 1

(a) Pebble Gravels

Higher Westland GreenSample No. 67 68 10 45 86

Rock type (%)Flint 100 88·9 52·1 41·5 52·2Quartz 5·5 28·5 28·7 26·4Sandstone 1·3 15·4 28·2 18·8Lower Greensand 4·3 2·1 2·7 0·7Other 2·1 0·8 1.9

Ratios 00

f1int/(quartz + other far-travelled) 12·9 1·1 0·8 1·1quartz/other far-travelled 4·2 1·6 1·1 1·3other far-travelled/Lower Greensand 0·3 8·3 10·0 29·6

Roundness 0·44 0·54 0·44 0·41 0·43

(b) Gravel Trains

HIGHER GRAVEL TRAIN

LEAVESDEN GRAVEL TRAIN

Sample No. 12 40 41 69 70 72 79 83 84 85

Rock type (%)Flint 55·0 82·5 55·2 65·2 58·7 71-0 66·8 60·2 90·4 97·7Quartz 19·5 7·0 19·6 14·9 13-2 13-0 12·1 15·6 4·7 0·7Sandstone 25·4 5·6 18·9 16·4 23·9 14·0 18·6 20·4 4·9 1-1Lower Greensand 2·8 2·8 1·8 0·8 0·8 1·1 2·4Other 2·1 3·5 1·9 3·4 1·2 1·4 1·2 0·6

Ratiosf1int/(quartz + other far-travelled) 1·2 5·6 1·3 2·0 1·5 2·5 2·1 1·6 9·4 40·7quartz/other far-travelled 0·8 0·9 0·9 0·8 0·5 0·9 0·6 0·7 1·0 0·4other far-travelled/Lower Greensand 00 2·8 8·0 10·2 34·1 19·0 18·2 9·0 oc cc

Roundness 0·38 0·40 0·36 0·36 0·30 0·35 0·34 0·36 0·35 0·32


(b) Pebble shape

The roundness of flint pebbles was examined in the 11-2-16'0 mm fraction using the visual chartof Krumbein (1941) in conjunction with a measured set of flint pebbles. Average sample sizewas 164 flint pebbles.

In previous accounts of the Thames gravels (Hey, 1965; Walder, 1967) flint pebbles havebeen subdivided into rounded and angular classes. No direct comparison can be made howeverwith earlier results as the basis of previous subdivisions was not defined. An indication of

(c) Colne gravels

Sample No. 1 17 37 39 42 51

Rock type (%)Flint 79·2 84·5 91·9 98·5 89·4 90·8Quartz 9·8 6·8 4·4 6·5 4·2Sandstone 8·8 8·7 3·2 1·1 3·3 4·0Lower Greensand 1·1 0·4 0·2Other 1·2 0·4 0·4 0·4 0·8

Ratiosflint/( quartz + other far-travelled) 4·0 5·5 1l·5 65·7 8·8 1l·1quartz/other far-travelled 1·0 0·8 1·2 1·8 0·9other far-travelled/Lower Greensand 9·1 oc oc oc 9·3 24·0

Roundness 0·38 0·33 0·33 0·30 0·36 0·34

LOWER GRAVEL TRAIN

EAVESDEN GRAVEL TRAIN

2 25 26 43 44 II 13 14 15 16 18 19 20 21 46 87

62·1 51·8 89·5 93·9 51·4 57·4 74·3 49·3 57·7 64·6 56·5 37·7 39·4 41·9 66·3 54·815·3 21·2 5·7 2·7 15·3 24·2 14·4 26·3 21·1 14·4 19·3 33·6 25·5 17·6 17·5 20·318·3 22·0 3·5 2·4 27·3 13·7 10·2 16·5 14·3 13-8 17·3 25·6 28·4 34·4 1l·4 21·0

2·1 2·8 0·4 0·3 4·0 1·2 0·6 5·4 3·9 7·4 4·1 1·5 4·2 3·4 1·8 2·12·1 2·0 0·8 0·6 2·0 3·6 0·6 2·5 3·0 - 2·8 1·5 2·4 2·7 3·0 1·5

1·7 1·2 9·0 16·5 1·2 1·4 3·0 1·1 1·5 2·3 1·4 0·6 0·7 0·8 2·1 1·30·8 0·9 1·3 0·9 0·5 1·4 1·3 1·4 1·2 1·0 1·0 1·2 0·8 0·5 1·2 0·99·7 8·6 10'8 10·0 7·3 14·4 18·0 3·5 4·4 1·9 4·9 18·1 7·3 10·9 8·0 10·7

0·37 0·33 0·33 0·34 0·39 0·38 0·46 0·38 0·32 0·34 0·31 0·43 0·36 ND 0·37 0·34

L


proportions in the present case can be given by defining 'rounded' flints as those having aroundness of O'5 or greater. Average percentages of rounded pebbles defined in this way are asfollows: Pebble Gravels, 46·72 per cent; Higher Gravel Train, 21·55 per cent; Lower GravelTrain, 22·14 per cent; Leavesden Gravel Train, 18·5 per cent; Colne gravels, 18·3 per cent.

(c) Comparative analysis

The analyses described here examine differences in terms of pebble composition and shape,either between gravels attributed to successive stratigraphic stages, or between gravels presumed to be of different provenance. The characteristics of composition and shape which areexamined can be related to the incidence of specific catchment changes:

(i) Changes in the frequency of far-travelled material are partly indicated by the flint/(quartz +other far-travelled) ratios. If, as seems probable, the far-travelled material is of glacialorigin, these ratios may provide information on the sequence of glacial events. On their ownthese ratios are not particularly sensitive because they also reflect independent changes in thefrequency of flint arising from the denudation of the Chalk outcrop.

(ii) The sequence of glacial events may be more clearly indicated by changes in the provenance of the far-travelled material. Such changes are partly indicated by quartz/other fartravelled ratios.

(iii) Additional insight into the denudation of the Chalk outcrop and the derivation of the flintis provided by roundness values.

(iv) The occurrence of Lower Greensand material in the gravels of the Thames has beendiscussed by previous workers (Wooldridge & Linton, 1955; Hey, 1965; Walder, 1967) andrelates to catchment changes in the area drained by tributaries originating in the Weald.

In order to test the significance of differences between sample data sets, the Mann-Whitney Utest is used. This versatile non-parametric test can be used with small, medium or large samples.It is simple to calculate and avoids the pitfalls of using a least-squares test on data which may notbe normally distributed. One-tailed probabilities are used as the direction of difference is oftenapparent.

The results of the comparative test are set out in Tables II and III and are discussed in thefollowing section.

3. DISCUSSION

A comparison in terms of pebble composition and shape between the main elements of theGravel Trains and between the Gravel Trains and gravels at higher and lower levels indicates anumber of significant catchment changes during the period in question. On the basis of thisevidence a reappraisal of glacial events and drainage development in the middle Thames basin ispossible.

(a) Evidence of catchment changes

Hey (1965) has shown that large amounts of far-travelled material first appear in the gravels ofthe Thames at the Westland Green stage. This far-travelled material, which contains sandstonesfrom the Bunter Pebble Beds and abundant quartz, is considered by Hey to have entered themiddle Thames basin through the Goring Gap. It seems probable that the far-travelled materialwas originally introduced into the Thames basin in glacial till or outwash, although the surfacemicro-morphology of quartz sand grains in the Westland Green Gravels of the middle Thamesarea (Hey, Krinsley & Hyde, 1971) suggests that the far-travelled material there is not ofimmediate glacial origin.

PLEI STOCENE GRAVEL TRAI NS OF THE RIVER THAMES 151

TABLE II Mann-Whitney U Test of sample data comparisons (l -tailed) Thames and Colne gravels

Pebble Higher Higher Lower Lower ColneGravel v Gravel Gravel v Gravel Gravel v Gravels

Tr ain Tr ain Tra in Train

a b c a b c a b c

1. flint/( quartz + othe r 77 NS 96 S 99 ·8 HS 2far- travelled)

2. qu artz/ otherfar-trave lled 99 ·9 HS 99·9 HS 2 68 NS

3. Lower Greensand (%) 71 NS 98 HS 2 99·9 HS4. oth er far-tra velled/

Low er Gr eensand 90 NS 99·9 HS 99 HS 25. Roundness 99 HS 66 NS 88 NS

a - significance levelb - NS = not significant; S = significant (95%) ; HS = highl y significant (99"1.)c - grea ter set - 1 or 2 in co mparison und er review, whe re S or HS

TABLE III Mann-Whitney U Test of sample data comparisons (f -ta iled) - Lea vesden gravels

Higher Lea vesden Higher Upper Lower Lower U pper LowerGrave l v Gravel G ravel v Leavcsdcn Gravel v Lea vesden Leavesden LeavesdenTr ain Train Tr ain Gravel Train Gravel Gravel v Gravel

Train Train Train Train

a b c I a b c I a b c a b c

1. flint /(quartz+ ot her far-trave lled) 86 NS - 96 SS 2 89 NS - 83 NS -

2. q uar tz/oth erfar-travelled 52 NS - 70 NS - 93 NS - 83 NS -

3. LowerG reen sand (%) 66 NS - 85 NS - 90 NS - 87 NS -

4. other far-trav elledlLower Greensand 60 NS - 83 NS - 59 NS - 98 S 1

5. Roundness 86 NS - 91 NS - 88 NS - 66 NS -

a - significance levelb - NS = not significant; S = significant (95%) ; HS = highly significant (99"1.)c - gea te r set - 1 or 2 in co mparison under review, where S or HS

A comparison betwe en the Pebble Gravels and the Higher Gravel Train suggests thatcatchment change s occurred between these stages. The mean roundness of the flint pebbles andthe quartz/other far-travelled ratios are lower in the Higher Gravel Train. No change in thefrequency of Lower Greensand material is seen. The decrease in the roundness of the flint in theHigher Gravel Train indicates an influx of relatively unworn flint. This change is consistent withthe large vertical separation and concomitant dissection between the stages. That the flint/(quartz +oth er far-travell ed) ratio remains unchanged, in spite of this influx of flint, suggests thatfresh far-tr avelled material was also reaching the Thames. This suggestion is supported by the

4


evidence that the provenance of the far-travelled material also changed. A less quartzose sourcein Higher Gravel Train times is indicated by the quartz/other far-travelled ratio. The similaritybetween the stages in terms of Lower Greensand content reflects the scarcity of this material atboth levels.

Between the Higher and Lower Gravel Train stages, changes are again apparent. There is anincrease in the total amount of far-travelled material, which appears to reflect a relative increasein the amount of quartz. Also conspicuous is the change in the frequency of Lower Greensandmaterial, which is augmented very significantly in the Lower Gravel Train. Roundness values inthe Higher and Lower Gravel Trains are similar (0'36 and 0·35 respectively). The small verticalseparation between the stages may explain this similarity since the influx of fresh flint as a resultof dissection is likly to have been small and the derivation of Lower Gravel Train flint from theHigher Gravel Train may have occurred.

The changes between the Higher and Lower Gravel Train stages appear to indicate simultaneous influxes of fresh material in Lower Gravel Train times from sources to the north and inthe Weald.

Far-travelled material in the Higher and Lower Gravel Trains is considered by Wooldridge(1938) to have reached the middle Thames from the west through the Goring Gap. TheLeavesden Gravel Train he considered to be of eastern origin and to comprise the gravels of ashort stream rising close to the site of a Chiltern Drift ice-front. The Chiltern Drift ice wasthought by Wooldridge to have blocked the former course of the Thames near St. Albans.

Comparisons between the Higher Gravel Train samples and the Leavesden samples (TableIII) indicate a complete absence of difference between them. The sites examined in theLeavesden area in the present study occur within the height ranges of both the Higher andLower Gravel Trains. It is not possible to recognise, on the basis of elevation, a single group ofgravels in the Leavesden area which might be confluent with the Higher Gravel Train (Fig. 2). Itis possible however to separate the Leavesden sites into two groups (upper and lower) whichappear to form easterly continuations of both the Higher Gravel Train and the Lower GravelTrain.

Comparison between the Higher Gravel Train and the upper group of gravels in the Leavesden area indicates a difference only in the somewhat insensitive jlint/(quartz+other fartravelled) ratio. Comparison between the Lower Gravel Train and the lower group of gravels inthe Leavesden area indicates no difference between them.

Comparison between the upper and lower groups of gravel in the Leavesden area shows asimilarity of roundness values and a significant contrast in the relative frequency of LowerGreensand material. Such results might be expected in a comparison between the Higher andLower Gravel Trains. Additional contrasts which might be expected, in the absolute frequencyof Lower Greensand, and in the other ratios evaluated, are almost certainly obscured in theanalysis by the large proportion of very flinty 'hoggin' samples. The widespread occurrence of'hoggin' in the Leavesden area probably reflects periglacial re-working of gravels during thechalky till glaciation when the ice-front was only two or three kilometres away.

Whereas the evidence in the Gravel Trains indicates that the course of the Thames throughthe Vale of St. Albans persisted throughout Gravel Train times, in the Colne valley at levelsbelow the Gravel Trains gravels of eastern provenance appear in which the content of fartravelled material is small (T1, T17, T37, T42, T51). These gravels are derived from the area ofchalky till deposition, and in some cases may be the outwash of the chalky till ice.

The most notable difference between Lower Gravel Train and Colne gravels is an increasedjlint/(quartz +other far-travelled) ratio in the latter. This influx of flints is associated with a slight


decrease of roundness. The decrease in the percentage of Lower Greensand material is obviously due to the derivation of the Colne gravels from sources outside the Weald. The totalamount of far-travelled material in the Colne gravels is relatively small and the average value ofthe quartz/other far-travelled ratio is not significantly different from the value in the LowerGravel Train. It appears therefore that the chalky till ice has added little fresh far-travelledmaterial to the system. The scarcity of far-travelled material in the Colne gravels is consistentwith the observations of Perrin, Davies & Fysh (1973) and Rose (1974), who record that stonymaterial in the chalky till itself is derived mainly from soft lithologies of Cretaceous and Jurassicage.

(b) Stratigraphy

An attempt is made in the following paragraphs to place the evidence presented in this accountin a stratigraphic context. Attention is given to the sequence of glacial events and to thediversion of the Thames from its course through the Vale of St. Albans.

(i) The glacial sequence

There seems to be general agreement that the chalky till in the Vale of St. Albans is most likely tobe of Anglian age (Turner, 1973; Gladfelter, 1975) as it is overlain at Hatfield by organicsediments of Hoxnian age. The chalky till glaciation can be related to the terrace succession ofthe Thames, and is usually regarded as approximately contemporary with the Winter Hillterrace. The Winter Hill terrace is in consequence assigned either to the Anglian (Kellaway,Worssam, Holmes, Kerney & Shephard-Thorn, 1973) or to the interglacial period (Cromerian)preceding the Anglian (Evans, 1971).

Above the Winter Hill level at least three distinct phases of gravel deposition can berecognised in which glacially-derived material of western provenance is common. These are theWestland Green, Higher Gravel Train and Lower Gravel Train stages. All the gravels suggesttorrential conditions of deposition. Such gravels in southern Britain are usually thought toindicate deposition in a cold environment (Wymer, 1968).

The large proportion of far-travelled material in the Westland Green Gravels (Hey, 1965)suggest that an ice-front may have approached the middle Thames area when the gravels werelaid down. In terms of elevation the highest gravels of the Westland Green series are separatedfrom the Gravel Trains by c.30 m, and the Westland Green Gravels appear also to be distinctfrom the Gravel Trains in terms of far-travelled content. It seems possible therefore that theWestland Green stage marks a separate phase of glaciation.

The Westland Green stage has been correlated by Hey (1965) with gravels upstream fromGoring which are at levels above the bulk of the Oxfordshire Northern Drift. The Northern Driftis inferred by Evans (1971) to be a product of the Baventian glaciation. Evans notes that parts ofthe Northern Drift complex may relate to the Lower Gravel Train of the middle Thames area.The present study shows that the relative frequency of far-travelled material reaches a maximumin the Lower Gravel Train. This fact and the high absolute frequency of far-travelled material inthe Lower Gravel Train are both consistent with a close approach of glacial ice of north-westernprovenance to the middle Thames basin in Lower Gravel Train times.

In terms of far-travelled content the Higher Gravel Train differs from the Lower GravelTrain. There is also evidence of catchment changes affecting the supply of material from theWeald between Higher and Lower Gravel Train times. Thus the Gravel Trains may representtwo further phases of glaciation.

If a chronology such as that of Evans (1971) is correct, in which as many as 20 cool phases are


recognised in the Middle and Lower Pleistocene, then the precise relationships of the middleThames evidence to specific climatic events or to the East Anglian stratigraphic succession aredoubtful. If, on the other hand, there were fewer cool phases in the Lower and MiddlePleistocene, possible correlations between the high level gravels of the Thames and the EastAnglian succession are limited in number. West (1968) suggests two cool phases (Thurnian andBaventian) in the Lower Pleistocene and one cool phase (Beestonian) in the pre-AnglianMiddle Pleistocene. Turner (1975) considers that at least one more cool phase in the preAnglian Middle Pleistocene is likely. Hey (1976) shows that the first major influx of fartravelled material in the pebbly deposits of East Anglia is in the Baventian. It is thereforeunlikely that pre-Baventian ice reached the Thames basin. Thus the glacially-derived material inthe pre-Anglian gravels of the Thames appears to relate to the Baventian and/or to thepre-Anglian Middle Pleistocene.

(ii) Diversion of the Thames

The evidence presented in this account suggests strongly that the diversion of the Thames fromthe Vale of St. Albans occurred after Gravel Train times and before the end of the chalky till(Anglian) glaciation. This finding clearly contradicts the two-stage diversion hypothesis ofWooldridge. If in fact the Thames persisted in the Vale of St. Albans until the Anglian glaciationthen the river is unlikely ever to have occupied the Finchley Depression which appears to havebeen blocked by Anglian ice at approximately the same time as the Vale of St. Albans.

The two-stage diversion hypothesis of Wooldridge has however clearly influenced theinterpretation of related Pleistocene events in the middle and lower Thames basin. It is not theintention of the present authors to examine in this paper all the implications of their proposedalternative hypothesis, but in the following paragraphs some points of interest are noted.

1. At levels below the Lower Gravel Train, the terraces of the middle Thames (Upper andLower Winter Hill and Black Park terraces) can be traced downstream as far as the valley of theColne. The latest feature in the middle Thames area that could be related to a course through theVale of St. Albans seems to be the Lower Winter Hill terrace. The earliest evidence of thediverted course of the Thames seems to be the so-called Kingston Leaf (Zeuner, 1959). Thisfeature and consequently the diversion itself have not been satisfactorily related to the terracesuccession of the middle Thames. The Kingston Leaf was regarded by Wooldridge (1958) as adownstream continuation of the Black Park terrace. Evans (1971) has pointed out that thisproposal is untenable as the Black Park terrace near Uxbridge is at about the same height a.D.as the gravel spreads near Kingston. He treats the Kingston Leaf as a separate and earlierfeature. The terrace succession could now be re-examined in terms of diversion from the Vale ofSt. Albans rather than from the Finchley Depression.

2. In examining the possible role of the chalky till (Anglian) ice in a diversion of the Thamesfrom the Vale of St. Albans, the significance of the supposed pro-glacial 'Lake Hertford'(Clayton & Brown, 1958) could be re-evaluated; and the unusually low gradient of the LowerWinter Hill terrace, tentatively related by Clayton and Brown to the 'ponding' effect of 'LakeHertford', could also be re-examined.

3. The possibility should not be overlooked that the Anglian ice played no part in the diversionof the Thames into its present course. Zeuner (1961) has suggested that the diversion indicatesthe capture of the middle Thames by a stream occupying the present course of the lowerThames. In general however little attention has been given in previous studies to the question ofglacio-eustatic, or isostatic, effects associated with the Anglian glaciation, or to the influence onfluvial activity of changed hydrological conditions associated with that glaciation.

PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES

4. CONCLUSION

155

In terms of composition the gravels of the Thames were subdivided by Wooldridge & Linton(1955) into three groups. At Stage I local material was almost exclusively predominant. AtStage II far-travelled material became conspicuous. At Stage III little, or no, new far-travelledmaterial was added, but existing material was re-worked and diluted with large quantities offlint. Wooldridge believed that more subtle stratigraphic distinctions could be made on the basisof terrace morphology, but not on the basis of gravel composition and lithology.

In the present account the broad threefold subdivision is still recognisable but in addition thecharacteristics of gravel composition and shape are shown to provide a detailed record ofdrainage development and changing environmental conditions, and are used, in conjunctionwith morphological evidence, in making fairly precise stratigraphic distinctions.

Attention has been focused on the origin and significance of the Gravel Trains. It has beenshown that whereas the Anglian (chalky till) ice entered the Thames basin from the north-eastand supplied very little far-travelled material to the gravels of the Thames; glacially-derivedmaterial of pre-Anglian age is mainly, if not entirely, of north-western provenance. Thisfar-travelled material is a significant component in the Thames gravels at all stages below thelevel of its first introduction in Westland Green times. It achieves a maximum frequency inLower Gravel Train times. Thus, glacially-derived material was introduced into the Thamesbasin at least once during the pre-Anglian Pleistocene, or possibly on three separate occasions(Westland Green, Higher Gravel Train, Lower Gravel Train). The work of Hey (1976) on thepebbly deposits of East Anglia and the work of Turner (1975) on the floral record of thePleistocene suggest that a major advance of glacial ice occurred in the Baventian. It is probabletherefore that glacial material relating to the Baventian is represented in the Thames gravels,and possible that material relating to other pre-Anglian cool phases is also represented.

The origin of the present course of the Thames has also been considered. The persistence ofthe early course through the Vale of St. Albans during Gravel Train times is indicated, and noseparate Leavesden Gravel Train of eastern provenance can be substantiated. It is suggestedthat the Thames was diverted from a course through the Vale of St. Albans directly into itspresent course.

ACKNOWLEDGMENT

We acknowledge with thanks a grant (to DFMM) forfieldwork from the Central Research Fundof the University of London.

REFERENCES

BOGGS, S. 1969. Relationship of size and compositionin pebble counts. J. sedim. Petrol., 39, 1243-6.

CLAYTON, K. M. & J. C. BROWN. 1958. The glacialdeposits around Hertford. Proc. Geol. Ass., 69,103-19.

DA VIS, S. N. 1958. Size distribution of rock types instream gravel and glacial till. J. sedim Petrol., 28,87-94.

EV ANS, P. 1971. Towards a Pleistocene time-scale.Part 2 of The Phanerozoic Time-scale-a supplement.Special Publication of the Geological Society no. 5,pp, 123-356.

GLADFELTER, B. G. 1975. Middle Pleistocene sedimentary sequences in East Anglia (United Kingdom).In K. W. Butzer & G. L. Isaac: After theAustralopithecines: stratigraphy, ecology and culturechange in the Middle Pleistocene. Mouton. TheHague.

HARE, F. K. 1947. The geomorphology of a part of themiddle Thames. Proc. Geol. Ass., 58,294-339.

HEY, R. W. 1965. Highly quartzose Pebble Gravelsin the London Basin. Proc. Geol. Ass., 76, 40320.

HEY, R. W. 1976. Provenance of far-travelled pebbles


in the pre-Anglian Pleistocene of East Anglia . Proc.Geol. Ass ., 87, 69-81.

HEY, R. W., D. H. KRINSLEY & P. J. W. HYDE.1971. Surface textures of sand-grains from theHertfordshire Pebble Gravels. Geol. Mag., 108,377-82.

HOLMES, A. 1965 . Principles of Physical Geology.2nd. ed . Nelson. London.

KELLAWAY, G. A. , B. C. WORSSAM, S. C. A.HOLMES, M. P. KERNEY & E. R. SHEPHARDTHORN. 1973. South-east England. In Mitchell &others, A Correlation of Quaternary deposits in theBritish Isles. Geol . Soc. Lond., Special Report No.4.

KR UMBEIN, W. C. 1941. Mea surement and geologicalsignificance of shape and roundness of sedimentaryparticles. J. sedim. Petrol., 11, 64-72 .

PERRIN, R. M.S., H. DA VIS & M. D. FYSH. 1973.Lithology of the Chalky Boulder Clay. Nature, phys.Sci., 245, 101-4.

ROSE, J . 1974 . Small-scale variability of some sedimentary propertiesofIodgementtill and slumped till.Proc. Geol. Ass., 85, 239-58.

SEALY, K. R. & C. E. SEALY. 1956. The terraces ofthe middle Thames. Proc. Geol. Ass ., 67,369-92.

SHERLOCK, R. L. 1924. The superficial deposits ofsouth Buckingham shire and south Hertfordshire, andthe old course of the Thames. Proc. Geol. Ass., 35,1-28.

SHRUBSOLE, O. A. 1903. On the probable source ofsome of the pebbles of the Triassic pebble beds ofsouth Devon and of the Midland counties. Q. Jl geol.Soc. Lond., 59, 311-33.

SNEED, E. D. & R. L. FOLK. 1958. Pebble s in theLower Colorado River , Texas; a study in particlemorphogenesis. J. Geol., 66, 114--50.

SPARKS, B. W., R. G. WEST, R. B. G. WILLIAMS &M. RANSOM. 1969. Hoxnian interglacial depositsnear Hatfield, Herts. Proc. Geol. Ass., 80, 243-67.

TURNER, C. 1973. Eastern England, in Mitchell &others, A correlation of Quaternary deposits in theBritish Isles. Geol, Soc. Lond., Special Report No.4.

TURNER, C. 1975. The correlation and duration ofMiddle Pleistocene interglacial periods in north-westEurope. In K. W. Butzer & G. L. Isaac : After theAustralopithecines: stratigraphy, ecology and culturechange in the Middle Pleistocene. Mouton. TheHague.

WALDER, P. S. 1967. The composition of the Thamesgravels near Reading , Berkshire. Proc. Geol. Ass ., 78,107-19.

WEST, R. G. 1968. Pleistocene geology and biology withespecial reference to the British Isles. Longman. London .

WOOLDRIDGE, S. W. 1938 . The glaciation of theLondon Basin and the evolution of the lower Thamesdrainage basin. Q. Jl geol. Soc. Lond., 94,627-67.

WOOLDRIDGE, S. W. 1958. Some aspects of the physiography of the Thames valley in relation to the IceAge and earl y man . Proc. prehist. Soc., 23, 1-19.

WOOLDRIDGE, S. W. & D. L. LINTON. 1955 .Srructure, surface and drainage in south-east England.Philip. London.

WYMER, J. J . 1968. Lower Palaeolithic Archaeology inBritain: as represented by the Thames Valiey. JohnBaker. London .

ZEUNER, F. E. 1959. The Pleistocene Period. 2nd . ed .Hutchinson. London.

ZEUNER, F. E . 1961. The sequence of terraces of thelower Thames and the radiation chronology. Ann N.Y. Acad. Sci., 95, 377-80.

Received 30 September 1976Re vised version received 1 March 1977

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Pleistocene Gravel Trains of the River Thames