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Pleistocene Gravel Trains of the River Thames
Pleistocene Gravel Trains of the River Thames C. P. GREEN and D. F. M. MCGREGOR
GREEN, C. P. & D. F. M. McGREGOR, 1978 . Pleistocene Gravel Trains of the RiverThames. Proc. Geol. Ass .,89(2), 143- 56. The Gravel Trainsofthe Thames have been examined in an area between Bourne End (Bucks) and Bricket Wood (Herts), and have been compared with gravels at higher and lower levels. Pebble lithology and the roundness of flint pebbles have been determined in the half-phi size range 11·2-16'0 mm. The Mann-Whitney U test is used to measure the significance level of differences between sample data sets comprising different suites of gravels. The results indicate a variety of catchment changes before, during and after Gravel Train times, and suggest a re-interpretation of the drainage development of the area examined. 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 be recognised. Problems connected with the diversion of the Thames after Gravel Train times are discussed. Department of Geography , Bedford College (University of London) , Regent's Park, London, NWI 4NS.
CONTENTS page
1. 2. 3. 4.
INTRODUCTION ... SAMPLE PRE PARAnON AND ANALYSIS DISCUSSION ... CONCLUSION ACKNOWLEDGMENT REFERENCES
143 144
150 155 155 155
1. INTRODUCTION
The course of the Thames between Goring and the North Sea has changed substantially during the Pleistocene. Sherlock (1924) argued that an early course of the Thames had extended from Bourne 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, had blocked 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 the Thames through the Vale of St. Albans to the early Pleistocene Pebble Gravel stage, but he found no evidence for the persistence of this course later in the Pleistocene. He proposed therefore that glacial ice had advanced on two occasions into this area. On the first occasion , at the time of the Chiltern Drift glaciation, the Thames was diverted from the Vale of St. Albans into a more southerly course through the Finchley Depression. Stony clays between Chorleywood and Amersham, and eastwards into Hertfordshire were regarded by Wooldridge as boulder-clay and as evidence of this glaciation. The Higher and Leavesden Gravel Trains were related 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 point near Croxley Green. The Leavesden Gravel Train he described as falling towards the same point from the opposite direction, from the site of the supposed ice-front near St. Albans. Doubts about the role of glacial ice in this diversion have been expressed by Holmes (1965). The Lower Gravel 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 a second occasion, at the time of the glaciation which introduced chalky tills into this area (Eastern Drift glaciation of Wooldridge & Linton, 1955), the river was diverted from the Finchley Depression into its present course. Chalky tills occur as far west in the Vale of St. Albans as Bricket Wood, and as far south in the Finchley Depression as Finchley. The gravels of the Winter Hill terrace of the Thames were regarded by Wooldridge as confluent with the outwash deposits of the chalky till ice. This outwash reached the Thames along a route which is now occupied 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 near Hertford. His results show a relatively small proportion of durable stones of distant provenance in the chalky till. Significance is attached by both Sherlock and Wooldridge to the distribution of gravel between Bourne end and St. Albans. Features other than distribution, such as structure, and pebble size, shape and composition were not evaluated, apart from a general emphasis on the frequency of glacially-derived material. Subsequent accounts (Hare, 1947; Sealy & Sealy, 1955) have mainly concentrated on the morphology of terrace remnants. Important exceptions are the studies of Hey (1965) on the Pebble Gravels, and Walder (1967) on the Winter Hill stage near Reading. In these studies fairly precise stratigraphic distinctions are based on gravel composition, and a relatively large proportion of material of presumed Midland derivation is demonstrated at various levels above the Lower Winter Hill stage. In the present account gravels are described from an area between Bourne End in the west and the western limit of the chalky till in the Vale of St. Albans in the east (Fig. 1). Pebble composition and shape have been analysed in 37 samples from 31 sites. Duplicate samples were taken 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 the plough 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 in earlier studies: 1. Pebble Gravels (including the Westland Green Gravels) 2. The Higher Gravel Train 3. The Leavesden Gravel Train 4. The Lower Gravel Train 5. 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 on a 3·65 mm mesh and subsequently dried. The washed sample was sieved for 15 minutes to separate it into seven fractions at half-phi intervals. The sieve apertures used were 31,5, 22'4,
145
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
t N
o I
i'
o
Milt. I
I
6 I
I t iii
Kilomtlrrs
I
t i 10
+KINGSTON
Fig. 1. Location of sample points. Numbers refer to Table I. Contour at 122 m.
146
C. P. GREEN AND D. F. M. MCGREGOR
ft
600 ·68
550 Z
ILl
w
Q
·67
Z
ILl ILl
Z
IX
::J
·86
Pebble
0
Gravels
IX (:J
>-
ILl
...J )(
0
IX
III
0
Higher
G
m
180
160
Q
0 0 ~
500
ILl
450
140
400
120
,..
:0:
0
it
III
350
ravel
8/' ... 18
Train
";9 -21
100 300
250 200
80
60
150
40
20
o
Fig. 2. Sample points in relation to schematic profiles. Open symbols indicate the 'Leavesden' gravels. The dotted line represents 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 varies with particle size (Davis, 1958; Boggs, 1969) and the shape of particles is also partly determined by their size (Sneed & Folk, 1958) detailed comparisons between samples in this study are based on 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 earlier studies 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 mm fraction. (a) Pebble composition The pebbles in the gravels were separated into five classes (Table 1). Flint and Lower Greensand material are referred to as local. Material which is presumed to have originated outside the present catchment of the Thames is referred to as far-travelled. This includes all the materials classified in Table I as quartz, sandstone or other. Hey (1965) has reviewed the questions of both the 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 in the south Midlands. Thence it was carried through the Goring Gap by a river. ...' Hey evidently regards glacial ice as the most probable 'unknown means', and this view is accepted in the present account.
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
147
(i) Flint This is mostly derived within the present catchment of the Thames, either directly from the Chalk outcrop, or, particularly in the case of well rounded pebbles, from local Tertiary pebble beds. Small quantities may however have been introduced from further north at various time by glacial 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 be exceeded in amount by non-flint material. These gravels include the Westland Green series of Hey (1965), the Gravel Trains of Wooldridge (1938) and gravel on ground overlooking the Colne 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 structureless and compact, and usually have a 'dirty' matrix of clayey sand. They form the 'hoggin' of commerce, and are found as a surface horizon up to 3 m thick, either overlying 'clean' sandy gravels of 'ballast' quality, or mantling gentle slopes. T26 and T43 overlie T25 and T44 respectively. 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 are also 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 relatively less common than sandstone in the larger fractions (>16 mm), but becomes increasingly abundant in progressively smaller fractions. Potential sources of quartz pebbles are evident within the present catchment of the Thames, in the Lower Greensand and in Palaeogene formations. However the quartz pebbles in these sediments are mainly small « 11·2 mm). Larger pebbles are probably derived mainly from the Bunter Pebble Beds, in which quartz is common (Shrubsole, 1903), or from other sources outside the present catchment of the Thames. Such pebbles may therefore be regarded as far-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 have been regarded as 'Bunter'. However the sandstone pebbles in the gravels are enormously diverse, ranging from very hard, colourless orthoquartzites and metaquartzites, through the so-called Bunter types and a range of hard but less completely lithified sandstones, to pebbles of soft micaceous and feldspathic sandstones. How much of this material is of Bunter origin is difficult 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-coloured quartzites of otherwise similar lithology are present in T67 and T68. In the Westland Green Gravels 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 sandstone derived from the Lower Greensand of the Weald. Differentiation between chert and flint may be difficult in some cases. In this study classification as chert or cherty sandstone is based on the
148
C. P. GREEN AND D. F. M. MCGREGOR
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 twice among the 2890 far-travelled pebbles examined in the samples. Radiolarian cherts and silicified oolitic 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 sites shown in Fig. 1 (a)
Pebble Gravels
67
68
10
Westland Green 45
86
100
88·9 5·5 1·3 4·3
52·1 28·5 15·4 2·1 2·1
41·5 28·7 28·2 2·7 0·8
52·2 26·4 18·8 0·7 1.9
12·9 4·2 0·3
1·1 1·6 8·3
0·8 1·1 10·0
1·1 1·3 29·6
Higher Sample No.
Rock type (%) Flint Quartz Sandstone Lower Greensand Other Ratios f1int/(quartz + other far-travelled) quartz/other far-travelled other far-travelled/Lower Greensand Roundness (b)
00
0·44
0·44
0·54
0·41
0·43
Gravel Trains
HIGHER GRAVEL TRAIN LEAVESDEN GRAVEL TRAIN Sample No.
Rock type (%) Flint Quartz Sandstone Lower Greensand Other
12
40
55·0 19·5 25·4
82·5 7·0 5·6 2·8 2·1
Ratios f1int/(quartz + other far-travelled) quartz/other far-travelled other far-travelled/Lower Greensand
1·2 0·8
Roundness
0·38
00
83
84
85
55·2 65·2 58·7 71-0 66·8 60·2 19·6 14·9 13-2 13-0 12·1 15·6 18·9 16·4 23·9 14·0 18·6 20·4 0·8 0·8 1·1 2·4 2·8 1·8 1·2 1·9 3·4 1·2 1·4 3·5
90·4 4·7 4·9
97·7 0·7 1-1
41
5·6 0·9 2·8
1·3 0·9 8·0
0·40
0·36
69
2·0 0·8 10·2 0·36
70
1·5 0·5 34·1 0·30
72
2·5 0·9 19·0 0·35
79
2·1 0·6 18·2
1·6 0·7 9·0
0·6
9·4 1·0 oc
0·34 0·36 0·35
40·7 0·4 cc
0·32
149
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
(b) Pebble shape
The roundness of flint pebbles was examined in the 11-2-16'0 mm fraction using the visual chart of Krumbein (1941) in conjunction with a measured set of flint pebbles. Average sample size was 164 flint pebbles. In previous accounts of the Thames gravels (Hey, 1965; Walder, 1967) flint pebbles have been subdivided into rounded and angular classes. No direct comparison can be made however with earlier results as the basis of previous subdivisions was not defined. An indication of
(c)
Colne gravels 1
17
37
39
42
51
79·2 9·8 8·8 1·1 1·2
84·5 6·8 8·7
91·9 4·4 3·2
98·5
0·4
0·4
89·4 6·5 3·3 0·4 0·4
90·8 4·2 4·0 0·2 0·8
Ratios flint/( quartz + other far-travelled) quartz/other far-travelled other far-travelled/Lower Greensand
4·0 1·0 9·1
5·5 0·8
1l·5 1·2
65·7
8·8 1·8 9·3
1l·1 0·9 24·0
Roundness
0·38
0·33
Sample No.
Rock type (%) Flint Quartz Sandstone Lower Greensand Other
1·1
oc
oc
oc
0·33
0·30
0·36
0·34
LOWER GRAVEL TRAIN L EAVESDEN GRAVEL TRAIN
2
25
26
43
44
93·9 2·7 2·4 0·3 0·6
51·4 15·3 27·3 4·0 2·0
9·0 16·5 1·3 0·9 10'8 10·0
1·2 0·5 7·3
62·1 51·8 89·5 15·3 21·2 5·7 18·3 22·0 3·5 2·1 2·8 0·4 2·1 2·0 0·8
1·7 0·8 9·7
1·2 0·9 8·6
0·37
0·33
0·33
0·34
0·39
II
13
14
57·4 74·3 49·3 24·2 14·4 26·3 13·7 10·2 16·5 1·2 0·6 5·4 3·6 0·6 2·5
1·4 1·4 14·4 0·38
15 57·7 21·1 14·3 3·9 3·0
16
18
64·6 56·5 14·4 19·3 13-8 17·3 4·1 7·4 2·8
3·0 1·3 18·0
1·1 1·4 3·5
1·5 1·2 4·4
2·3 1·0 1·9
1·4 1·0 4·9
0·46
0·38
0·32
0·34
0·31
19
20
21
46
87
37·7 39·4 41·9 66·3 54·8 33·6 25·5 17·6 17·5 20·3 25·6 28·4 34·4 1l·4 21·0 1·5 4·2 3·4 2·1 1·8 1·5 2·4 2·7 1·5 3·0
0·6 1·2 18·1 0·43
0·7 0·8 7·3
0·8 0·5 10·9
2·1 1·2 8·0
0·36
ND
0·37
1·3 0·9 10·7 0·34
150
C. P. GREEN AND D. F. M. MCGREGOR
proportions in the present case can be given by defining 'rounded' flints as those having a roundness of O' 5 or greater. Average percentages of rounded pebbles defined in this way are as follows: Pebble Gravels, 46·72 per cent; Higher Gravel Train, 21·55 per cent; Lower Gravel Train, 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 are examined 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 glacial origin, these ratios may provide information on the sequence of glacial events. On their own these ratios are not particularly sensitive because they also reflect independent changes in the frequency 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 flint is provided by roundness values. (iv) The occurrence of Lower Greensand material in the gravels of the Thames has been discussed by previous workers (Wooldridge & Linton, 1955; Hey, 1965; Walder, 1967) and relates 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 U test 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 not be normally distributed. One-tailed probabilities are used as the direction of difference is often apparent. The results of the comparative test are set out in Tables II and III and are discussed in the following section. 3. DISCUSSION
A comparison in terms of pebble composition and shape between the main elements of the Gravel Trains and between the Gravel Trains and gravels at higher and lower levels indicates a number of significant catchment changes during the period in question. On the basis of this evidence a reappraisal of glacial events and drainage development in the middle Thames basin is possible. (a) Evidence of catchment changes Hey (1965) has shown that large amounts of far-travelled material first appear in the gravels of the Thames at the Westland Green stage. This far-travelled material, which contains sandstones from the Bunter Pebble Beds and abundant quartz, is considered by Hey to have entered the middle Thames basin through the Goring Gap. It seems probable that the far-travelled material was originally introduced into the Thames basin in glacial till or outwash, although the surface micro-morphology of quartz sand grains in the Westland Green Gravels of the middle Thames area (Hey, Krinsley & Hyde, 1971) suggests that the far-travelled material there is not of immediate glacial origin.
151
PLEI STOCENE GRAVEL TRAI NS OF THE RIVER THAMES
TAB LE II
Mann-Whitney U Test of sample data comparisons (l -tailed) Thames and Colne gravels
1. flint/( quartz + othe r far- travelled) 2. qu artz/ other far-trave lled 3. Lower Gr eensand (%) 4. oth er far-tra ve lled/ Low er Gr eensand 5. Roundness
a
b
77
Colne Lower Gravel v Gravels Train
Higher Low er G ravel v G ravel Tr ain Tra in
Pebble Higher Gravel v G ravel Tr ain c
a
b
c
NS
96
S
99 ·9 71
HS NS
99 ·9 98
HS HS
90 99
NS HS
99·9 66
HS NS
2 2
a
b
c
99 ·8
HS
2
68 99·9
NS HS
99 88
HS NS
2
a - significance le vel b - NS = not significant; S = significant (95%) ; HS = highl y significant (99"1.) c - grea ter set - 1 or 2 in co mparison und er re vie w, whe re S or HS
TABLE III
Mann-Whitney U Test of sample data comparisons (f -ta iled) - Lea vesden gravels Lower Lower Lea vesden Higher U pper Highe r G rave l v Gravel G ra vel v Le avcsdcn Gravel v Lea vesden Gravel Train Train Gravel Tr ain Tr ain Tra in Train
1. flint /(q uartz + ot her far-trave lled) 2. q uar tz/oth er far -travelled 3. Lower G reen sand (%) 4. other far-trav e lledl Lower Greensand 5. Round ness
I
b
c
a
b
c
2
89
NS
-
83
NS
-
93
NS
-
83
NS
-
NS
-
90
NS
-
87
NS
-
NS NS
-
59 88
NS NS
-
98 66
S NS
-
b
c
-
96
SS
NS
-
70
NS
66
NS
-
85
60 86
NS NS
-
83 91
b
c
86
NS
52
I
a
a
a
Lower U pper Le avesden Leavesden Gravel v Gravel T rain Train
-
1
a - significance level b - NS = not significa nt; S = significant (95%) ; HS = highly significant (99"1.) c - gea te r set - 1 o r 2 in co mparison under review, whe re S or HS
A comp arison betwe en the Pebbl e Gravels and the Higher Gravel Train suggests that catchment change s occurred between these stages. T he mean roundness of the flint pebbles and the quartz/other far-tra velled ratios are lower in the Higher Gravel Train. No change in the frequency of Lower Greensand material is seen. The decrease in the roundness of the flint in the Higher Gravel Train indicates an influx of relatively unworn flint. T his change is consistent with the 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 that fresh far-tr avelled mat erial was also reaching the Thames. This suggestion is supported by the 4
152
C. P. GREEN AND D. F. M. MCGREGOR
evidence that the provenance of the far-travelled material also changed. A less quartzose source in Higher Gravel Train times is indicated by the quartz/other far-travelled ratio. The similarity between the stages in terms of Lower Greensand content reflects the scarcity of this material at both levels. Between the Higher and Lower Gravel Train stages, changes are again apparent. There is an increase in the total amount of far-travelled material, which appears to reflect a relative increase in the amount of quartz. Also conspicuous is the change in the frequency of Lower Greensand material, which is augmented very significantly in the Lower Gravel Train. Roundness values in the Higher and Lower Gravel Trains are similar (0'36 and 0·35 respectively). The small vertical separation between the stages may explain this similarity since the influx of fresh flint as a result of dissection is likly to have been small and the derivation of Lower Gravel Train flint from the Higher 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 in the 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. The Leavesden Gravel Train he considered to be of eastern origin and to comprise the gravels of a short stream rising close to the site of a Chiltern Drift ice-front. The Chiltern Drift ice was thought 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 (Table III) indicate a complete absence of difference between them. The sites examined in the Leavesden area in the present study occur within the height ranges of both the Higher and Lower Gravel Trains. It is not possible to recognise, on the basis of elevation, a single group of gravels in the Leavesden area which might be confluent with the Higher Gravel Train (Fig. 2). It is possible however to separate the Leavesden sites into two groups (upper and lower) which appear to form easterly continuations of both the Higher Gravel Train and the Lower Gravel Train. 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 in the Leavesden area indicates no difference between them. Comparison between the upper and lower groups of gravel in the Leavesden area shows a similarity of roundness values and a significant contrast in the relative frequency of Lower Greensand material. Such results might be expected in a comparison between the Higher and Lower Gravel Trains. Additional contrasts which might be expected, in the absolute frequency of Lower Greensand, and in the other ratios evaluated, are almost certainly obscured in the analysis 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 the chalky 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 through the Vale of St. Albans persisted throughout Gravel Train times, in the Colne valley at levels below 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 of chalky 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 increased jlint/(quartz +other far-travelled) ratio in the latter. This influx of flints is associated with a slight
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
153
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 total amount of far-travelled material in the Colne gravels is relatively small and the average value of the quartz/other far-travelled ratio is not significantly different from the value in the Lower Gravel Train. It appears therefore that the chalky till ice has added little fresh far-travelled material to the system. The scarcity of far-travelled material in the Colne gravels is consistent with the observations of Perrin, Davies & Fysh (1973) and Rose (1974), who record that stony material in the chalky till itself is derived mainly from soft lithologies of Cretaceous and Jurassic age. (b) Stratigraphy
An attempt is made in the following paragraphs to place the evidence presented in this account in a stratigraphic context. Attention is given to the sequence of glacial events and to the diversion 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 to be of Anglian age (Turner, 1973; Gladfelter, 1975) as it is overlain at Hatfield by organic sediments of Hoxnian age. The chalky till glaciation can be related to the terrace succession of the Thames, and is usually regarded as approximately contemporary with the Winter Hill terrace. 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 be recognised in which glacially-derived material of western provenance is common. These are the Westland Green, Higher Gravel Train and Lower Gravel Train stages. All the gravels suggest torrential conditions of deposition. Such gravels in southern Britain are usually thought to indicate 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 were laid down. In terms of elevation the highest gravels of the Westland Green series are separated from the Gravel Trains by c.30 m, and the Westland Green Gravels appear also to be distinct from the Gravel Trains in terms of far-travelled content. It seems possible therefore that the Westland Green stage marks a separate phase of glaciation. The Westland Green stage has been correlated by Hey (1965) with gravels upstream from Goring which are at levels above the bulk of the Oxfordshire Northern Drift. The Northern Drift is inferred by Evans (1971) to be a product of the Baventian glaciation. Evans notes that parts of the 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 maximum in the Lower Gravel Train. This fact and the high absolute frequency of far-travelled material in the Lower Gravel Train are both consistent with a close approach of glacial ice of north-western provenance to the middle Thames basin in Lower Gravel Train times. In terms of far-travelled content the Higher Gravel Train differs from the Lower Gravel Train. There is also evidence of catchment changes affecting the supply of material from the Weald between Higher and Lower Gravel Train times. Thus the Gravel Trains may represent two further phases of glaciation. If a chronology such as that of Evans (1971) is correct, in which as many as 20 cool phases are
154
C. P. GREEN AND D. F. M. MCGREGOR
recognised in the Middle and Lower Pleistocene, then the precise relationships of the middle Thames evidence to specific climatic events or to the East Anglian stratigraphic succession are doubtful. If, on the other hand, there were fewer cool phases in the Lower and Middle Pleistocene, possible correlations between the high level gravels of the Thames and the East Anglian succession are limited in number. West (1968) suggests two cool phases (Thurnian and Baventian) in the Lower Pleistocene and one cool phase (Beestonian) in the pre-Anglian Middle 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 therefore unlikely that pre-Baventian ice reached the Thames basin. Thus the glacially-derived material in the pre-Anglian gravels of the Thames appears to relate to the Baventian and/or to the pre-Anglian Middle Pleistocene.
(ii) Diversion of the Thames The evidence presented in this account suggests strongly that the diversion of the Thames from the 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 of Wooldridge. If in fact the Thames persisted in the Vale of St. Albans until the Anglian glaciation then the river is unlikely ever to have occupied the Finchley Depression which appears to have been 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 the interpretation of related Pleistocene events in the middle and lower Thames basin. It is not the intention of the present authors to examine in this paper all the implications of their proposed alternative 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 and Lower Winter Hill and Black Park terraces) can be traced downstream as far as the valley of the Colne. The latest feature in the middle Thames area that could be related to a course through the Vale of St. Albans seems to be the Lower Winter Hill terrace. The earliest evidence of the diverted course of the Thames seems to be the so-called Kingston Leaf (Zeuner, 1959). This feature and consequently the diversion itself have not been satisfactorily related to the terrace succession of the middle Thames. The Kingston Leaf was regarded by Wooldridge (1958) as a downstream continuation of the Black Park terrace. Evans (1971) has pointed out that this proposal 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 earlier feature. The terrace succession could now be re-examined in terms of diversion from the Vale of St. Albans rather than from the Finchley Depression. 2. In examining the possible role of the chalky till (Anglian) ice in a diversion of the Thames from 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 Lower Winter Hill terrace, tentatively related by Clayton and Brown to the 'ponding' effect of 'Lake Hertford', could also be re-examined. 3. The possibility should not be overlooked that the Anglian ice played no part in the diversion of the Thames into its present course. Zeuner (1961) has suggested that the diversion indicates the capture of the middle Thames by a stream occupying the present course of the lower Thames. In general however little attention has been given in previous studies to the question of glacio-eustatic, or isostatic, effects associated with the Anglian glaciation, or to the influence on fluvial activity of changed hydrological conditions associated with that glaciation.
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
155
4. CONCLUSION
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. At Stage II far-travelled material became conspicuous. At Stage III little, or no, new far-travelled material was added, but existing material was re-worked and diluted with large quantities of flint. Wooldridge believed that more subtle stratigraphic distinctions could be made on the basis of 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 the characteristics of gravel composition and shape are shown to provide a detailed record of drainage development and changing environmental conditions, and are used, in conjunction with morphological evidence, in making fairly precise stratigraphic distinctions. Attention has been focused on the origin and significance of the Gravel Trains. It has been shown that whereas the Anglian (chalky till) ice entered the Thames basin from the north-east and supplied very little far-travelled material to the gravels of the Thames; glacially-derived material of pre-Anglian age is mainly, if not entirely, of north-western provenance. This far-travelled material is a significant component in the Thames gravels at all stages below the level of its first introduction in Westland Green times. It achieves a maximum frequency in Lower Gravel Train times. Thus, glacially-derived material was introduced into the Thames basin 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 the pebbly deposits of East Anglia and the work of Turner (1975) on the floral record of the Pleistocene suggest that a major advance of glacial ice occurred in the Baventian. It is probable therefore 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 of the early course through the Vale of St. Albans during Gravel Train times is indicated, and no separate Leavesden Gravel Train of eastern provenance can be substantiated. It is suggested that the Thames was diverted from a course through the Vale of St. Albans directly into its present course. ACKNOWLEDGMENT
We acknowledge with thanks a grant (to DFMM) forfieldwork from the Central Research Fund of the University of London. REFERENCES BOGGS, S. 1969. Relationship of size and composition in pebble counts. J. sedim. Petrol., 39, 1243-6. CLAYTON, K. M. & J. C. BROWN. 1958. The glacial deposits around Hertford. Proc. Geol. Ass., 69, 103-19. DA VIS, S. N. 1958. Size distribution of rock types in stream 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 the Australopithecines: stratigraphy, ecology and culture change in the Middle Pleistocene. Mouton. The Hague. HARE, F. K. 1947. The geomorphology of a part of the middle Thames. Proc. Geol. Ass., 58,294-339. HEY, R. W. 1965. Highly quartzose Pebble Gravels in the London Basin. Proc. Geol. Ass., 76, 40320. HEY, R. W. 1976. Provenance of far-travelled pebbles
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C. P. GREEN AND D. F. M. MCGREGOR
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 the Hertfordshire 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 the British Isles. Geol . Soc. Lond., Special Report No.4. KR UMBEIN, W. C. 1941. Mea surement and geological significance of shape and roundness of sedimentary particles. 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. A ss., 85, 239-58. SEALY, K. R. & C. E. SEALY. 1956. The terraces of the middle Thames. Proc. Geol. Ass ., 67,369-92. SHERLOCK, R. L. 1924. Th e superficial deposits of south Buckingham shire and south Hertfordsh ire, and the old course of the Thames. Proc. Geol. Ass., 35, 1-28. SHRUBSOLE, O. A. 1903. On the probable source of some of the pebbles of the Triassic pebble beds of south Devon and of the Midland counties. Q. Jl geol. Soc. Lond., 59, 311-33. SNEED, E . D. & R. L. FOLK. 1958. Pebble s in the Lower Colorado River , Texas; a stud y in particle morphogenesis. J. Geol., 66, 114--50. SPARKS, B. W., R. G. WEST, R. B. G. WILLIAMS & M. RANSOM. 1969. Hoxnian interglacial deposits near Hatfield, Herts. Proc. Geol. Ass., 80, 243-67.
TURNER, C. 1973. Eastern England, in Mitchell & others, A correlation of Quaternary deposits in the British Isles. Geol, Soc. Lond., Special Report No.4. TURNER, C. 1975. The correlation and duration ofMiddle Pleistocene interglacial periods in north-west Europe. In K. W. Butzer & G. L. Isaac : After the Australopithecines: stratigraphy, ecology and culture change in the Middle Pleistocene. Mouton. The Hague. WALDER, P. S. 1967. The composition of the Thames gravels near Reading , Berkshire. Proc. Geol. Ass ., 78, 107-19. WEST, R. G. 1968. Pleistocene geology and biology with especial reference to the British Isles. Longman. London . WOOLDRIDGE, S. W. 1938 . The glaciation of the London Basin and the evolution of the lower Thames dr ainage 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 Ice Age 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 in Britain: as represented by the Thames Valiey. John Baker. London . ZEUNER, F. E. 1959. The Pleistocene Period. 2nd . ed . Hutchinson. London. ZEUNER, F. E . 1961. The sequence of terraces of the lower Thames and the radiation chronology. Ann N. Y. Acad. Sci., 95, 377-80.
Received 30 September 1976 Re vised version received 1 March 1977
GREEN, C. P. & D. F. M. McGREGOR, 1978 . Pleistocene Gravel Trains of the RiverThames. Proc. Geol. Ass .,89(2), 143- 56. The Gravel Trainsofthe Thames have been examined in an area between Bourne End (Bucks) and Bricket Wood (Herts), and have been compared with gravels at higher and lower levels. Pebble lithology and the roundness of flint pebbles have been determined in the half-phi size range 11·2-16'0 mm. The Mann-Whitney U test is used to measure the significance level of differences between sample data sets comprising different suites of gravels. The results indicate a variety of catchment changes before, during and after Gravel Train times, and suggest a re-interpretation of the drainage development of the area examined. 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 be recognised. Problems connected with the diversion of the Thames after Gravel Train times are discussed. Department of Geography , Bedford College (University of London) , Regent's Park, London, NWI 4NS.
CONTENTS page
1. 2. 3. 4.
INTRODUCTION ... SAMPLE PRE PARAnON AND ANALYSIS DISCUSSION ... CONCLUSION ACKNOWLEDGMENT REFERENCES
143 144
150 155 155 155
1. INTRODUCTION
The course of the Thames between Goring and the North Sea has changed substantially during the Pleistocene. Sherlock (1924) argued that an early course of the Thames had extended from Bourne 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, had blocked 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 the Thames through the Vale of St. Albans to the early Pleistocene Pebble Gravel stage, but he found no evidence for the persistence of this course later in the Pleistocene. He proposed therefore that glacial ice had advanced on two occasions into this area. On the first occasion , at the time of the Chiltern Drift glaciation, the Thames was diverted from the Vale of St. Albans into a more southerly course through the Finchley Depression. Stony clays between Chorleywood and Amersham, and eastwards into Hertfordshire were regarded by Wooldridge as boulder-clay and as evidence of this glaciation. The Higher and Leavesden Gravel Trains were related 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 point near Croxley Green. The Leavesden Gravel Train he described as falling towards the same point from the opposite direction, from the site of the supposed ice-front near St. Albans. Doubts about the role of glacial ice in this diversion have been expressed by Holmes (1965). The Lower Gravel 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 a second occasion, at the time of the glaciation which introduced chalky tills into this area (Eastern Drift glaciation of Wooldridge & Linton, 1955), the river was diverted from the Finchley Depression into its present course. Chalky tills occur as far west in the Vale of St. Albans as Bricket Wood, and as far south in the Finchley Depression as Finchley. The gravels of the Winter Hill terrace of the Thames were regarded by Wooldridge as confluent with the outwash deposits of the chalky till ice. This outwash reached the Thames along a route which is now occupied 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 near Hertford. His results show a relatively small proportion of durable stones of distant provenance in the chalky till. Significance is attached by both Sherlock and Wooldridge to the distribution of gravel between Bourne end and St. Albans. Features other than distribution, such as structure, and pebble size, shape and composition were not evaluated, apart from a general emphasis on the frequency of glacially-derived material. Subsequent accounts (Hare, 1947; Sealy & Sealy, 1955) have mainly concentrated on the morphology of terrace remnants. Important exceptions are the studies of Hey (1965) on the Pebble Gravels, and Walder (1967) on the Winter Hill stage near Reading. In these studies fairly precise stratigraphic distinctions are based on gravel composition, and a relatively large proportion of material of presumed Midland derivation is demonstrated at various levels above the Lower Winter Hill stage. In the present account gravels are described from an area between Bourne End in the west and the western limit of the chalky till in the Vale of St. Albans in the east (Fig. 1). Pebble composition and shape have been analysed in 37 samples from 31 sites. Duplicate samples were taken 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 the plough 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 in earlier studies: 1. Pebble Gravels (including the Westland Green Gravels) 2. The Higher Gravel Train 3. The Leavesden Gravel Train 4. The Lower Gravel Train 5. 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 on a 3·65 mm mesh and subsequently dried. The washed sample was sieved for 15 minutes to separate it into seven fractions at half-phi intervals. The sieve apertures used were 31,5, 22'4,
145
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
t N
o I
i'
o
Milt. I
I
6 I
I t iii
Kilomtlrrs
I
t i 10
+KINGSTON
Fig. 1. Location of sample points. Numbers refer to Table I. Contour at 122 m.
146
C. P. GREEN AND D. F. M. MCGREGOR
ft
600 ·68
550 Z
ILl
w
Q
·67
Z
ILl ILl
Z
IX
::J
·86
Pebble
0
Gravels
IX (:J
>-
ILl
...J )(
0
IX
III
0
Higher
G
m
180
160
Q
0 0 ~
500
ILl
450
140
400
120
,..
:0:
0
it
III
350
ravel
8/' ... 18
Train
";9 -21
100 300
250 200
80
60
150
40
20
o
Fig. 2. Sample points in relation to schematic profiles. Open symbols indicate the 'Leavesden' gravels. The dotted line represents 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 varies with particle size (Davis, 1958; Boggs, 1969) and the shape of particles is also partly determined by their size (Sneed & Folk, 1958) detailed comparisons between samples in this study are based on 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 earlier studies 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 mm fraction. (a) Pebble composition The pebbles in the gravels were separated into five classes (Table 1). Flint and Lower Greensand material are referred to as local. Material which is presumed to have originated outside the present catchment of the Thames is referred to as far-travelled. This includes all the materials classified in Table I as quartz, sandstone or other. Hey (1965) has reviewed the questions of both the 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 in the south Midlands. Thence it was carried through the Goring Gap by a river. ...' Hey evidently regards glacial ice as the most probable 'unknown means', and this view is accepted in the present account.
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
147
(i) Flint This is mostly derived within the present catchment of the Thames, either directly from the Chalk outcrop, or, particularly in the case of well rounded pebbles, from local Tertiary pebble beds. Small quantities may however have been introduced from further north at various time by glacial 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 be exceeded in amount by non-flint material. These gravels include the Westland Green series of Hey (1965), the Gravel Trains of Wooldridge (1938) and gravel on ground overlooking the Colne 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 structureless and compact, and usually have a 'dirty' matrix of clayey sand. They form the 'hoggin' of commerce, and are found as a surface horizon up to 3 m thick, either overlying 'clean' sandy gravels of 'ballast' quality, or mantling gentle slopes. T26 and T43 overlie T25 and T44 respectively. 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 are also 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 relatively less common than sandstone in the larger fractions (>16 mm), but becomes increasingly abundant in progressively smaller fractions. Potential sources of quartz pebbles are evident within the present catchment of the Thames, in the Lower Greensand and in Palaeogene formations. However the quartz pebbles in these sediments are mainly small « 11·2 mm). Larger pebbles are probably derived mainly from the Bunter Pebble Beds, in which quartz is common (Shrubsole, 1903), or from other sources outside the present catchment of the Thames. Such pebbles may therefore be regarded as far-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 have been regarded as 'Bunter'. However the sandstone pebbles in the gravels are enormously diverse, ranging from very hard, colourless orthoquartzites and metaquartzites, through the so-called Bunter types and a range of hard but less completely lithified sandstones, to pebbles of soft micaceous and feldspathic sandstones. How much of this material is of Bunter origin is difficult 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-coloured quartzites of otherwise similar lithology are present in T67 and T68. In the Westland Green Gravels 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 sandstone derived from the Lower Greensand of the Weald. Differentiation between chert and flint may be difficult in some cases. In this study classification as chert or cherty sandstone is based on the
148
C. P. GREEN AND D. F. M. MCGREGOR
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 twice among the 2890 far-travelled pebbles examined in the samples. Radiolarian cherts and silicified oolitic 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 sites shown in Fig. 1 (a)
Pebble Gravels
67
68
10
Westland Green 45
86
100
88·9 5·5 1·3 4·3
52·1 28·5 15·4 2·1 2·1
41·5 28·7 28·2 2·7 0·8
52·2 26·4 18·8 0·7 1.9
12·9 4·2 0·3
1·1 1·6 8·3
0·8 1·1 10·0
1·1 1·3 29·6
Higher Sample No.
Rock type (%) Flint Quartz Sandstone Lower Greensand Other Ratios f1int/(quartz + other far-travelled) quartz/other far-travelled other far-travelled/Lower Greensand Roundness (b)
00
0·44
0·44
0·54
0·41
0·43
Gravel Trains
HIGHER GRAVEL TRAIN LEAVESDEN GRAVEL TRAIN Sample No.
Rock type (%) Flint Quartz Sandstone Lower Greensand Other
12
40
55·0 19·5 25·4
82·5 7·0 5·6 2·8 2·1
Ratios f1int/(quartz + other far-travelled) quartz/other far-travelled other far-travelled/Lower Greensand
1·2 0·8
Roundness
0·38
00
83
84
85
55·2 65·2 58·7 71-0 66·8 60·2 19·6 14·9 13-2 13-0 12·1 15·6 18·9 16·4 23·9 14·0 18·6 20·4 0·8 0·8 1·1 2·4 2·8 1·8 1·2 1·9 3·4 1·2 1·4 3·5
90·4 4·7 4·9
97·7 0·7 1-1
41
5·6 0·9 2·8
1·3 0·9 8·0
0·40
0·36
69
2·0 0·8 10·2 0·36
70
1·5 0·5 34·1 0·30
72
2·5 0·9 19·0 0·35
79
2·1 0·6 18·2
1·6 0·7 9·0
0·6
9·4 1·0 oc
0·34 0·36 0·35
40·7 0·4 cc
0·32
149
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
(b) Pebble shape
The roundness of flint pebbles was examined in the 11-2-16'0 mm fraction using the visual chart of Krumbein (1941) in conjunction with a measured set of flint pebbles. Average sample size was 164 flint pebbles. In previous accounts of the Thames gravels (Hey, 1965; Walder, 1967) flint pebbles have been subdivided into rounded and angular classes. No direct comparison can be made however with earlier results as the basis of previous subdivisions was not defined. An indication of
(c)
Colne gravels 1
17
37
39
42
51
79·2 9·8 8·8 1·1 1·2
84·5 6·8 8·7
91·9 4·4 3·2
98·5
0·4
0·4
89·4 6·5 3·3 0·4 0·4
90·8 4·2 4·0 0·2 0·8
Ratios flint/( quartz + other far-travelled) quartz/other far-travelled other far-travelled/Lower Greensand
4·0 1·0 9·1
5·5 0·8
1l·5 1·2
65·7
8·8 1·8 9·3
1l·1 0·9 24·0
Roundness
0·38
0·33
Sample No.
Rock type (%) Flint Quartz Sandstone Lower Greensand Other
1·1
oc
oc
oc
0·33
0·30
0·36
0·34
LOWER GRAVEL TRAIN L EAVESDEN GRAVEL TRAIN
2
25
26
43
44
93·9 2·7 2·4 0·3 0·6
51·4 15·3 27·3 4·0 2·0
9·0 16·5 1·3 0·9 10'8 10·0
1·2 0·5 7·3
62·1 51·8 89·5 15·3 21·2 5·7 18·3 22·0 3·5 2·1 2·8 0·4 2·1 2·0 0·8
1·7 0·8 9·7
1·2 0·9 8·6
0·37
0·33
0·33
0·34
0·39
II
13
14
57·4 74·3 49·3 24·2 14·4 26·3 13·7 10·2 16·5 1·2 0·6 5·4 3·6 0·6 2·5
1·4 1·4 14·4 0·38
15 57·7 21·1 14·3 3·9 3·0
16
18
64·6 56·5 14·4 19·3 13-8 17·3 4·1 7·4 2·8
3·0 1·3 18·0
1·1 1·4 3·5
1·5 1·2 4·4
2·3 1·0 1·9
1·4 1·0 4·9
0·46
0·38
0·32
0·34
0·31
19
20
21
46
87
37·7 39·4 41·9 66·3 54·8 33·6 25·5 17·6 17·5 20·3 25·6 28·4 34·4 1l·4 21·0 1·5 4·2 3·4 2·1 1·8 1·5 2·4 2·7 1·5 3·0
0·6 1·2 18·1 0·43
0·7 0·8 7·3
0·8 0·5 10·9
2·1 1·2 8·0
0·36
ND
0·37
1·3 0·9 10·7 0·34
150
C. P. GREEN AND D. F. M. MCGREGOR
proportions in the present case can be given by defining 'rounded' flints as those having a roundness of O' 5 or greater. Average percentages of rounded pebbles defined in this way are as follows: Pebble Gravels, 46·72 per cent; Higher Gravel Train, 21·55 per cent; Lower Gravel Train, 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 are examined 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 glacial origin, these ratios may provide information on the sequence of glacial events. On their own these ratios are not particularly sensitive because they also reflect independent changes in the frequency 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 flint is provided by roundness values. (iv) The occurrence of Lower Greensand material in the gravels of the Thames has been discussed by previous workers (Wooldridge & Linton, 1955; Hey, 1965; Walder, 1967) and relates 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 U test 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 not be normally distributed. One-tailed probabilities are used as the direction of difference is often apparent. The results of the comparative test are set out in Tables II and III and are discussed in the following section. 3. DISCUSSION
A comparison in terms of pebble composition and shape between the main elements of the Gravel Trains and between the Gravel Trains and gravels at higher and lower levels indicates a number of significant catchment changes during the period in question. On the basis of this evidence a reappraisal of glacial events and drainage development in the middle Thames basin is possible. (a) Evidence of catchment changes Hey (1965) has shown that large amounts of far-travelled material first appear in the gravels of the Thames at the Westland Green stage. This far-travelled material, which contains sandstones from the Bunter Pebble Beds and abundant quartz, is considered by Hey to have entered the middle Thames basin through the Goring Gap. It seems probable that the far-travelled material was originally introduced into the Thames basin in glacial till or outwash, although the surface micro-morphology of quartz sand grains in the Westland Green Gravels of the middle Thames area (Hey, Krinsley & Hyde, 1971) suggests that the far-travelled material there is not of immediate glacial origin.
151
PLEI STOCENE GRAVEL TRAI NS OF THE RIVER THAMES
TAB LE II
Mann-Whitney U Test of sample data comparisons (l -tailed) Thames and Colne gravels
1. flint/( quartz + othe r far- travelled) 2. qu artz/ other far-trave lled 3. Lower Gr eensand (%) 4. oth er far-tra ve lled/ Low er Gr eensand 5. Roundness
a
b
77
Colne Lower Gravel v Gravels Train
Higher Low er G ravel v G ravel Tr ain Tra in
Pebble Higher Gravel v G ravel Tr ain c
a
b
c
NS
96
S
99 ·9 71
HS NS
99 ·9 98
HS HS
90 99
NS HS
99·9 66
HS NS
2 2
a
b
c
99 ·8
HS
2
68 99·9
NS HS
99 88
HS NS
2
a - significance le vel b - NS = not significant; S = significant (95%) ; HS = highl y significant (99"1.) c - grea ter set - 1 or 2 in co mparison und er re vie w, whe re S or HS
TABLE III
Mann-Whitney U Test of sample data comparisons (f -ta iled) - Lea vesden gravels Lower Lower Lea vesden Higher U pper Highe r G rave l v Gravel G ra vel v Le avcsdcn Gravel v Lea vesden Gravel Train Train Gravel Tr ain Tr ain Tra in Train
1. flint /(q uartz + ot her far-trave lled) 2. q uar tz/oth er far -travelled 3. Lower G reen sand (%) 4. other far-trav e lledl Lower Greensand 5. Round ness
I
b
c
a
b
c
2
89
NS
-
83
NS
-
93
NS
-
83
NS
-
NS
-
90
NS
-
87
NS
-
NS NS
-
59 88
NS NS
-
98 66
S NS
-
b
c
-
96
SS
NS
-
70
NS
66
NS
-
85
60 86
NS NS
-
83 91
b
c
86
NS
52
I
a
a
a
Lower U pper Le avesden Leavesden Gravel v Gravel T rain Train
-
1
a - significance level b - NS = not significa nt; S = significant (95%) ; HS = highly significant (99"1.) c - gea te r set - 1 o r 2 in co mparison under review, whe re S or HS
A comp arison betwe en the Pebbl e Gravels and the Higher Gravel Train suggests that catchment change s occurred between these stages. T he mean roundness of the flint pebbles and the quartz/other far-tra velled ratios are lower in the Higher Gravel Train. No change in the frequency of Lower Greensand material is seen. The decrease in the roundness of the flint in the Higher Gravel Train indicates an influx of relatively unworn flint. T his change is consistent with the 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 that fresh far-tr avelled mat erial was also reaching the Thames. This suggestion is supported by the 4
152
C. P. GREEN AND D. F. M. MCGREGOR
evidence that the provenance of the far-travelled material also changed. A less quartzose source in Higher Gravel Train times is indicated by the quartz/other far-travelled ratio. The similarity between the stages in terms of Lower Greensand content reflects the scarcity of this material at both levels. Between the Higher and Lower Gravel Train stages, changes are again apparent. There is an increase in the total amount of far-travelled material, which appears to reflect a relative increase in the amount of quartz. Also conspicuous is the change in the frequency of Lower Greensand material, which is augmented very significantly in the Lower Gravel Train. Roundness values in the Higher and Lower Gravel Trains are similar (0'36 and 0·35 respectively). The small vertical separation between the stages may explain this similarity since the influx of fresh flint as a result of dissection is likly to have been small and the derivation of Lower Gravel Train flint from the Higher 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 in the 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. The Leavesden Gravel Train he considered to be of eastern origin and to comprise the gravels of a short stream rising close to the site of a Chiltern Drift ice-front. The Chiltern Drift ice was thought 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 (Table III) indicate a complete absence of difference between them. The sites examined in the Leavesden area in the present study occur within the height ranges of both the Higher and Lower Gravel Trains. It is not possible to recognise, on the basis of elevation, a single group of gravels in the Leavesden area which might be confluent with the Higher Gravel Train (Fig. 2). It is possible however to separate the Leavesden sites into two groups (upper and lower) which appear to form easterly continuations of both the Higher Gravel Train and the Lower Gravel Train. 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 in the Leavesden area indicates no difference between them. Comparison between the upper and lower groups of gravel in the Leavesden area shows a similarity of roundness values and a significant contrast in the relative frequency of Lower Greensand material. Such results might be expected in a comparison between the Higher and Lower Gravel Trains. Additional contrasts which might be expected, in the absolute frequency of Lower Greensand, and in the other ratios evaluated, are almost certainly obscured in the analysis 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 the chalky 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 through the Vale of St. Albans persisted throughout Gravel Train times, in the Colne valley at levels below 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 of chalky 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 increased jlint/(quartz +other far-travelled) ratio in the latter. This influx of flints is associated with a slight
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
153
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 total amount of far-travelled material in the Colne gravels is relatively small and the average value of the quartz/other far-travelled ratio is not significantly different from the value in the Lower Gravel Train. It appears therefore that the chalky till ice has added little fresh far-travelled material to the system. The scarcity of far-travelled material in the Colne gravels is consistent with the observations of Perrin, Davies & Fysh (1973) and Rose (1974), who record that stony material in the chalky till itself is derived mainly from soft lithologies of Cretaceous and Jurassic age. (b) Stratigraphy
An attempt is made in the following paragraphs to place the evidence presented in this account in a stratigraphic context. Attention is given to the sequence of glacial events and to the diversion 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 to be of Anglian age (Turner, 1973; Gladfelter, 1975) as it is overlain at Hatfield by organic sediments of Hoxnian age. The chalky till glaciation can be related to the terrace succession of the Thames, and is usually regarded as approximately contemporary with the Winter Hill terrace. 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 be recognised in which glacially-derived material of western provenance is common. These are the Westland Green, Higher Gravel Train and Lower Gravel Train stages. All the gravels suggest torrential conditions of deposition. Such gravels in southern Britain are usually thought to indicate 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 were laid down. In terms of elevation the highest gravels of the Westland Green series are separated from the Gravel Trains by c.30 m, and the Westland Green Gravels appear also to be distinct from the Gravel Trains in terms of far-travelled content. It seems possible therefore that the Westland Green stage marks a separate phase of glaciation. The Westland Green stage has been correlated by Hey (1965) with gravels upstream from Goring which are at levels above the bulk of the Oxfordshire Northern Drift. The Northern Drift is inferred by Evans (1971) to be a product of the Baventian glaciation. Evans notes that parts of the 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 maximum in the Lower Gravel Train. This fact and the high absolute frequency of far-travelled material in the Lower Gravel Train are both consistent with a close approach of glacial ice of north-western provenance to the middle Thames basin in Lower Gravel Train times. In terms of far-travelled content the Higher Gravel Train differs from the Lower Gravel Train. There is also evidence of catchment changes affecting the supply of material from the Weald between Higher and Lower Gravel Train times. Thus the Gravel Trains may represent two further phases of glaciation. If a chronology such as that of Evans (1971) is correct, in which as many as 20 cool phases are
154
C. P. GREEN AND D. F. M. MCGREGOR
recognised in the Middle and Lower Pleistocene, then the precise relationships of the middle Thames evidence to specific climatic events or to the East Anglian stratigraphic succession are doubtful. If, on the other hand, there were fewer cool phases in the Lower and Middle Pleistocene, possible correlations between the high level gravels of the Thames and the East Anglian succession are limited in number. West (1968) suggests two cool phases (Thurnian and Baventian) in the Lower Pleistocene and one cool phase (Beestonian) in the pre-Anglian Middle 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 therefore unlikely that pre-Baventian ice reached the Thames basin. Thus the glacially-derived material in the pre-Anglian gravels of the Thames appears to relate to the Baventian and/or to the pre-Anglian Middle Pleistocene.
(ii) Diversion of the Thames The evidence presented in this account suggests strongly that the diversion of the Thames from the 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 of Wooldridge. If in fact the Thames persisted in the Vale of St. Albans until the Anglian glaciation then the river is unlikely ever to have occupied the Finchley Depression which appears to have been 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 the interpretation of related Pleistocene events in the middle and lower Thames basin. It is not the intention of the present authors to examine in this paper all the implications of their proposed alternative 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 and Lower Winter Hill and Black Park terraces) can be traced downstream as far as the valley of the Colne. The latest feature in the middle Thames area that could be related to a course through the Vale of St. Albans seems to be the Lower Winter Hill terrace. The earliest evidence of the diverted course of the Thames seems to be the so-called Kingston Leaf (Zeuner, 1959). This feature and consequently the diversion itself have not been satisfactorily related to the terrace succession of the middle Thames. The Kingston Leaf was regarded by Wooldridge (1958) as a downstream continuation of the Black Park terrace. Evans (1971) has pointed out that this proposal 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 earlier feature. The terrace succession could now be re-examined in terms of diversion from the Vale of St. Albans rather than from the Finchley Depression. 2. In examining the possible role of the chalky till (Anglian) ice in a diversion of the Thames from 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 Lower Winter Hill terrace, tentatively related by Clayton and Brown to the 'ponding' effect of 'Lake Hertford', could also be re-examined. 3. The possibility should not be overlooked that the Anglian ice played no part in the diversion of the Thames into its present course. Zeuner (1961) has suggested that the diversion indicates the capture of the middle Thames by a stream occupying the present course of the lower Thames. In general however little attention has been given in previous studies to the question of glacio-eustatic, or isostatic, effects associated with the Anglian glaciation, or to the influence on fluvial activity of changed hydrological conditions associated with that glaciation.
PLEISTOCENE GRAVEL TRAINS OF THE RIVER THAMES
155
4. CONCLUSION
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. At Stage II far-travelled material became conspicuous. At Stage III little, or no, new far-travelled material was added, but existing material was re-worked and diluted with large quantities of flint. Wooldridge believed that more subtle stratigraphic distinctions could be made on the basis of 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 the characteristics of gravel composition and shape are shown to provide a detailed record of drainage development and changing environmental conditions, and are used, in conjunction with morphological evidence, in making fairly precise stratigraphic distinctions. Attention has been focused on the origin and significance of the Gravel Trains. It has been shown that whereas the Anglian (chalky till) ice entered the Thames basin from the north-east and supplied very little far-travelled material to the gravels of the Thames; glacially-derived material of pre-Anglian age is mainly, if not entirely, of north-western provenance. This far-travelled material is a significant component in the Thames gravels at all stages below the level of its first introduction in Westland Green times. It achieves a maximum frequency in Lower Gravel Train times. Thus, glacially-derived material was introduced into the Thames basin 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 the pebbly deposits of East Anglia and the work of Turner (1975) on the floral record of the Pleistocene suggest that a major advance of glacial ice occurred in the Baventian. It is probable therefore 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 of the early course through the Vale of St. Albans during Gravel Train times is indicated, and no separate Leavesden Gravel Train of eastern provenance can be substantiated. It is suggested that the Thames was diverted from a course through the Vale of St. Albans directly into its present course. ACKNOWLEDGMENT
We acknowledge with thanks a grant (to DFMM) forfieldwork from the Central Research Fund of the University of London. REFERENCES BOGGS, S. 1969. Relationship of size and composition in pebble counts. J. sedim. Petrol., 39, 1243-6. CLAYTON, K. M. & J. C. BROWN. 1958. The glacial deposits around Hertford. Proc. Geol. Ass., 69, 103-19. DA VIS, S. N. 1958. Size distribution of rock types in stream 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 the Australopithecines: stratigraphy, ecology and culture change in the Middle Pleistocene. Mouton. The Hague. HARE, F. K. 1947. The geomorphology of a part of the middle Thames. Proc. Geol. Ass., 58,294-339. HEY, R. W. 1965. Highly quartzose Pebble Gravels in the London Basin. Proc. Geol. Ass., 76, 40320. HEY, R. W. 1976. Provenance of far-travelled pebbles
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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 the Hertfordshire 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 the British Isles. Geol . Soc. Lond., Special Report No.4. KR UMBEIN, W. C. 1941. Mea surement and geological significance of shape and roundness of sedimentary particles. 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. A ss., 85, 239-58. SEALY, K. R. & C. E. SEALY. 1956. The terraces of the middle Thames. Proc. Geol. Ass ., 67,369-92. SHERLOCK, R. L. 1924. Th e superficial deposits of south Buckingham shire and south Hertfordsh ire, and the old course of the Thames. Proc. Geol. Ass., 35, 1-28. SHRUBSOLE, O. A. 1903. On the probable source of some of the pebbles of the Triassic pebble beds of south Devon and of the Midland counties. Q. Jl geol. Soc. Lond., 59, 311-33. SNEED, E . D. & R. L. FOLK. 1958. Pebble s in the Lower Colorado River , Texas; a stud y in particle morphogenesis. J. Geol., 66, 114--50. SPARKS, B. W., R. G. WEST, R. B. G. WILLIAMS & M. RANSOM. 1969. Hoxnian interglacial deposits near Hatfield, Herts. Proc. Geol. Ass., 80, 243-67.
TURNER, C. 1973. Eastern England, in Mitchell & others, A correlation of Quaternary deposits in the British Isles. Geol, Soc. Lond., Special Report No.4. TURNER, C. 1975. The correlation and duration ofMiddle Pleistocene interglacial periods in north-west Europe. In K. W. Butzer & G. L. Isaac : After the Australopithecines: stratigraphy, ecology and culture change in the Middle Pleistocene. Mouton. The Hague. WALDER, P. S. 1967. The composition of the Thames gravels near Reading , Berkshire. Proc. Geol. Ass ., 78, 107-19. WEST, R. G. 1968. Pleistocene geology and biology with especial reference to the British Isles. Longman. London . WOOLDRIDGE, S. W. 1938 . The glaciation of the London Basin and the evolution of the lower Thames dr ainage 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 Ice Age 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 in Britain: as represented by the Thames Valiey. John Baker. London . ZEUNER, F. E. 1959. The Pleistocene Period. 2nd . ed . Hutchinson. London. ZEUNER, F. E . 1961. The sequence of terraces of the lower Thames and the radiation chronology. Ann N. Y. Acad. Sci., 95, 377-80.
Received 30 September 1976 Re vised version received 1 March 1977