The micromorphology of quaternary river terrace deposits in the Kennet Valley, Berkshire, England

EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 9, 343-355 (1984)

THE MICROMORPHOLOGY OF QUATERNARY RIVER TERRACE DEPOSITS IN THE KENNET VALLEY, BERKSHIRE, ENGLAND

c. J. CHARTRES*

School of Geography, University of New South Wales, P . 0. Box I , Kensingion, N.S.W., Australia 2033

Received 18 November 1981 Revised 28 Ociober 1983

ABSTRACT

Three groups of micromorphological features are recognized in the river terrace deposits of the Kennet Valley. These are orthic pedological features, orthic pedological features disrupted by cryoturbation, and transport-inherited features. Fluvial and solifluction processes account for the presence of the transport-inherited features. Used in conjunction with sedimentological and geomorphological investigations the distribution, composition, size, shape, and relative proportions of micromorphological features are helpful in separating pedogenically-altered materials from undisturbed sediments, and in recognizing the Occurrence of more than one phase of soil development.

KEY WORDS Quaternary river terraces Soil micromorphology Soil development South central England

INTRODUCTION

Micromorphological observations have frequently been used to characterize palaeosols and to help to identify polygenetic soil profiles (see Mucher and Morozova, 1983 for a comprehensive review). To this end Brewer and Sleeman (1969) demonstrated that micropedological features and fabric arrangements can be used as diagnostic criteria of increasing soil development. Micromorphological studies in Britain have been successfully used to differentiate pre and post Devensian pedogenesis through the identification of palaeoargillic and argillic soil horizons (Bullock, 1974; Catt, 1979). Similarly, Chartres (1980) demonstrated that micromorphology could help determine the complex history of soil development in unglaciated areas, on a sequence of river terraces, in the Kennet Valley, central southern England. In the study of fluvial deposits, however, the question as to which features are pedological in origin and which are of sedimentary origin must always be considered. Similarly, difficulty with interpretation of micromorphological features may also arise when colluvial and soliflucted materials from the valley side encroach onto terrace surfaces. It is the aim of this paper to consider the nature and distribution of pedological features in the river terrace and other deposits of the Kennet Valley to try and establish whether undisturbed pedogenetic features can be distinguished from those formed elsewhere, which are subsequently eroded and redeposited.

THE MORPHOLOGY OF THE SOILS

The five sites investigated are located on the Hamstead Marshall Terrace, 47 m above the floodplain (one profile), the Thatcham Terrace, 9-10m above the floodplain (two profiles), the Beenham Grange Terrace, 2-3 m above the floodplain (one profile), and at Woolhampton on the floodplain itself (one profile) (Figure 1). Annotated profile diagrams are given in Figure 2.

Current address: CSIRO, Division of Soils, G.P.O. Box 639, Canberra, A.C.T. 2601, Australia.

0197-9337/84/040343-13$01.30 0 1984 by John Wiley & Sons, Ltd.

344 C. J. CHARTRES

Figure 1. Major terrace deposits (after Thomas, 1961). and sampling locations in the Kennet Valley

Previous studies (Chartres, 1975,1980) have shown that the soils of the Thatcham Terrace and above have palaeoargillic horizons overlain by periglacially disturbed Eb horizons, which have been mixed with Late Devensian loessial additions (Chartres, 1981). The soil of the lowest terrace has developed in possibly reworked loessial material and was not disturbed by periglaciation. On the floodplain many of the soils are still calcareous. reflecting a combination of high water-tables and limited leaching, calcareous parent materials, and a relatively short period of development during the Holocene. Kintbury and Woolhampton apart, details of the soils are given in Chartres (1980). At Kintbury, the second site on the Thatcham Terrace, the upper 0.70m of profile contains an Ap, and thin Eb and Bt horizons developed in a silty stone-free material (Figure 2). This soil overlies a bleached, disturbed gravel with a sandy silt loam matrix (2Eb, 070-1.50 m) which is separated from a gravel with a strong brown clay loam matrix (2Bt, 1.70-2.20m) by a clear wavy boundary. This latter unit overlies bedded, matrix-free gravels, which in turn have buried a chalky head deposit. On the basis of fabric and other studies, Cheetham (1976) suggested that the upper gravels containing the bleached and clayier horizons are a solifluction deposit that has moved onto the terrace from the neighbouring hill sides.

The soils of the floodplain are extremely variable and are developed in materials ranging from peats and marls to gravel. The profile examined near Woolhampton is developed in calcareous loams. It consists of an Ap horizon (M.20 m) of dark brown, stony, sandy clay loam overlying yellowish-brown to brownish-yellow sandy silt loams with rusty mottles. At 1.30 m, undisturbed, fluvially bedded gravels occur, and fluvial bedding was also detected in the lower part of the sandy silt loams. The profiles is calcareous throughout.

MICROMORPHOLOGY

Hamstead Marshall At Hamstead Marshall, micromorphological observations were made mainly on the 2Eb (050/140-1~20 m)

and 2Bt (1.OO/1.2(r1.70 m) horizons. The 2Eb horizon has predominantly poorly organized s-matrix materials (asepic to weakly insepic plasmic fabric) with some reddish-orange to yellowish-brown void (usually irregular vughs and interconnected vughs) and channel ferriargillans (Table I and Figure 3). At 0 4 0 4 5 0 m (Bt horizon)

RIVER TERRACE DEPOSITS 345

- SITE WOOLHAMPTON BEENHAM GRANGE

SOIL CLASSIFICATION

0

I

2

(GLEYICI BROWN CALCAREOUS ALLUVIAL SOIL-

TYPICAL ARGlLLlC BROWN EARTH

' . . .

"TEXTURE : S = sond Z =silt L =loom C =c lay

M Morrling

THATCH AM

STAGNOGLEYIC PALAEOARGILLIC BROWN EARTH-

**STONINESS

KINTBURY

TYPICAL ARGlLLlC BROWN EARTH kO-$7ml

. --------

12Bt 7.5YR5/6 SCd EA

HAMSTEAD MARSHALL

PALAEOARGILLIC BROWN EARTH,

Fluvially bedded gravels

Chalky (Quaternary) Head

I CHALKY HEAD I Reading Beds (Tertiary) o a 0

F = few ( 1-5 % s tones) C =common (6-15%stones) A = obundont (36-70% stones) EA=extremely a b u n d a n t ( 70%srones)

Figure 2. Morphological characteristics of the Kennet Valley Terrace profiles investigated. Flint is the predominant constituent of the gravels at all sites

ferriargillans are common, but from 0.5&1.20m (2Eb) they become increasingly rare. Also in the 2Eb horizon are yellow and red papules, pedorelicts (generally with well defined boundaries, which consist of oriented red clays with occasional embedded sand and silt grains), and sharp-bounded sesquioxide nodules, which are similar to features in the underlying clayier horizons. Though the ferriargillans in the upper part (2Eb horizon) of the profile are generally associated with vugh and channel margins, the overall organization of the matrix and other pedological features is disturbed, suggesting considerable disruption of the profile prior to formation of the reddish-orange to yellowish-brown ferriargillans. The disturbance is indicated particularly by the random distribution of many sharp-bounded red and yellow papules and blackish nodules in the matrix.

In contrast to the generally asepic matrix and overall porphyroskelic fabric of the 2Eb, virtually all the clays in the 2Bt horizons (l.OO/1*2O-1.70 m) are strongly oriented (omnisepic fabric). Detailed examination indicated that with few exceptions (possibly due to the weathering of glauconite grains), these clays are illuvial in origin

346 C. J. CHARTRES

Table I. The description and Occurrence of micromorphological features in Kennet Valley terrace deposits ~

Micromorphological feature Description Occurrence

Orthic pedological features Matrans (silty organoargillans)

Calcans

Yellowish-brown to reddish-orange ferriargillans and neoferriargillans

Yellow ferriargillans

Rusty brown mottles/concretions

Disrupted orthic features Cryoturbated yellow ferriargillans and papules

Cryoturbated rubified ferriargillans and papules

Rubified clayey pedorelicts

Embedded rubified grain ferriargillans

Transport inherited pedological features Embedded rubified grain ferriargillans

Rubified and black stained nodules

Lithorelicts a) Sarsens

b) Lithified shell fragments c) Tertiary clays

d) Marl and tufa concretions

Dirty brownish void and channel cutans consisting of clay, silt, and organic matter Layers of crystalline CaCO, deposited in voids and channels Clay and iron cutans predominantly in voids and channels

Strongly continuous internal fabric of yellow clay; cutans in voids and channels Irregular mottled areas, sometimes with concentric fabric

As for yellow ferriargillans, but are incorporated within the s-matrix and/or mixed with other disrupted material Strongly continuous internal fabric, masked by development of red col- ours (hues SYR-1OR). Often embedded in s-matrix and other disturbed material (e.g. yellow clay). Shapes variable, but distinct outer boundaries Irregular nodules and red (hues SYR-IOR) plasma separations with indifferentiated internal fabric Red (hues SYR-IOR) clay and iron cutans surrounding skeleton grains of flint and quartz. Frequently embedded in s-matrix and other disturbed materials

Red clay and iron cutans (as above) around skeleton grains embedded in the s-matrix Rounded nodules with undifferentiated internal fabric consisting of iron/manganese impregnated s- matrix and skeleton grains

Skeleton grains of quartzite

Skeleton grains of silicified shells Sharp bounded ‘pedorelict-like’ features with undifferentiated internal fabric Rounded fragments of crystalline calcareous material

Ap and Eb horizons

Soils derived in part from marls and tufa on the present floodplain Maximum development in Bt horizons (normal argillic) on all terraces 2Bt horizons on the Thatcham and Hamstead Marshall Terraces

All horizons with impeded drainage

2Bt horizons and as pedorelicts in 2Eb horizons at Thatcham and Hamstead Marshall Terraces

2Bt horizon and as pedorelicts in 2Eb horizon at Hamstead Marshall

2Eb and 2Bt horizons at Hamstead Marshall

2Bt horizons and rarely in 2Eb horizon at Hamstead Marshall

2Eb and 2Bt horizons at Kintbury

Common in 2Eb and 2Bt horizons at Kinbury. Also see in fluvial gravels on other terraces

All fluvially transported gravel deposits As for sarsens Near contact with the underlying Reading Beds (clayey facies) at Hamstead Marshall Floodplain only

HOWZON

2Eb

2Eb

=1

Orawls

a

2811

Eb

Bl

2Eb

281

Eb

Bl

Bl

RIVER TERRACE DEPOSITS

2.0 1 3.0

1.01

O Y

2.0 1

1.5 1

I t t t

i t l

ti

F I-

F

1 c

L Figure 3. The distribution of soil micromorphological features with depth in the Kennet Valley Terrace profiles. (Total clay contents based on particle size analyses; all other observations based on point counts of lo00 points per sampling depth. Percentage of voids and

skeleton grains are not shown)

(having banded and continuous fabric), but suffered serious disruption after emplacement (Figures 4 and 5). Furthermore, two fairly distinct types of illuvial clay were recognizable:

1. Intensely red (rubified), disrupted ferriargillans 2. Yellow, strongly oriented, disrupted ferriargillans

The red ferriargillans were sometimes observed as thick coatings around large flint skeletal grains (grain ferriargillans). These flints have been attacked by intensive chemical weathering and are stained black, particularly at their margins (Figure 6). However, most of the rubified illuvial clays are embedded in the yellow clay. Although the latter was itself frequently disrupted, large yellow ferriargillans were observed in places along the void and channel margins. These were apparently the source of the many papules seen both in the clay-enriched layer and in the overlying material. Many of the papules were banded indicating their origin as cutanic features (Figure 5).

348 C. J. CHARTRES

Figure 4. Photomicrograph of the 2Bt1 horizon at Hamstead Marshall. The dark manganiferous staining referred to in the text is clearly visible adjacent to the flint skeleton grain at bottom left. A disrupted yellow ferriargillan is clearly visible in the centre of the print. Non-

crossed polars.

Figure 5 . Photomicrograph of the 2Btl horizon at Hamstead Marshall. Some in-siru illuvial yellow clay occurs on the margin of the large void. The numerous subrounded to prolate and banded features are yellow papules. Non-crossed polars


,100Cc

Figure 6. Photomicrograph of the 2Bt, horizon at Hamstead Marshall illustrating a large weathered and dark stained flint grain with a red grain ferriargillan embedded in an s-matrix of oriented clay (omnisepic fabric). Crossed polars

With increasing depth, both the rubified and yellow illuvial clays become increasingly rare. By 2.40 m the only oriented clay occurs in occasional, distinct, well-rounded pedorelicts and Iithorelicts set in a totally asepic s-matrix. These occur in deposits, which show rudimentary, if slightly disturbed, sedimentary bedding.

Thatcham At Thatcham, thin, silty Ap (0-0.22 m), Eb (0.22-0.35 m), and Bt (0.354.45) horizons are separated from 2Bt

horizons by a clear and wavy boundary. The Ap, Eb, and Bt horizons all have silasepic plasmic fabric, contain occasional papules and pedorelicts, many of which have internal fabrics similar to those in the underlying material. Yellowish-brown to reddish-orange void (vugh) and channel ferriargillans are also common in the Bt horizon (Figure 3).

In the clay-enriched 2Bt horizons below the discontinuity numerous zones of well-oriented yellowish clay occur (Figure 7). The banded fabric of many of these clays again suggest an illuvial origin, and, in places, a few of these ferriargillans were partly within the margins of voids and channels. Stress cutans also occur along margins of some shear planes. In terms of colour and internal organization, the yellow oriented clays in the 2Bt, horizon contrast strongly with the yellowish-brown to reddish-orange ferriargillans seen both in the overlying material and also within the upper part of the 2Btl horizon.

Kintbury At Kintbury the upper 0.70m of silty stone-free material has an asepic to insepic plasmic fabric, with

yellowish-brown to reddish-orange void and channel ferriargillans increasing with depth. Matrans (silty organo-argillans; Mucher et al., 1972) also occur in the upper 0.20-0.50m of profile. Some clay papules and pedorelicts similar in nature to the matrix material also occur throughout the upper 0-70m of profile (Figure 3). Many of the aggregates in this material also have distinct boundaries, a feature possibly indicating the material was rolled or washed downslope. Similar features were reported from slopewash deposits in the Netherlands by Mucher (1974). The 'bleached' material (0.70-1.50 m; 2Eb) consists of large flints and sand grains set in an asepic to slightly insepic plasmic fabric. Some flints are surrounded by grain ferriargillans (Figure 8), redder in

350 C. J . CHARTRES

Figure 7. Photomicrograph of the 2Bt horizon at Thatcham. Note the complex fabric and the orientation of the majority of the yellow clay (omnisepic plasmic fabric). Crossed polars

I oop.

Figure 8. Photomicrograph of the 2Bt horizon at Kintbury. The large skeleton grain can be seen to be coated with a (red) grain ferriargillan. Noncrossed polars


Figure 9. Photomicrograph of the 2Bt horizon at Kintbury. Two subround sesquioxide indurated pedorelicts/nodules with sharp external boundaries embedded in a silasepic s-matrix. Note also the papule below the right hand nodule. Non-crossed polars

colour than the matrix, which itself is stained by iron oxides. Fine-textured cappings also occur on many of the ,

larger stones. A few yellowish-brown to reddish-orange void and channel ferriargillans were noted, but similar material also forms papules in the s-matrix. Occasional red papules also occur. The underlying clayier horizon (2Bt, 1.7C2.20 m) contains much more illuvial clay as reddish-orange to yellowish-brown ferriargillans (Figure 3). The s-matrix is iron stained and the plasmic fabric asepic to slightly insepic. Papules and sharp-bounded rubified sesquioxide nodules (Figure 9) are common in this horizon, which like the overlying horizon has a disturbed appearance. This disturbance appears to predate the illuviation features in these horizons.

Beenham Grange The profile at this site on the lowest terrace has no textural or mineralogical discontinuity within the

materials overlying the bedded fluvial gravels. The major micromorphological features are yellowish-brown to reddish-orange void (vugh) and channel ferriargillans (Figure 3). In the Bt horizon (0.7&1.00 m) these often occupy the major part of the voids and channels, but are not at all disturbed or disrupted. Also throughout this profile are fairly common rounded and sharp-bounded blackish sesquioxide nodules. The plasmic fabric is asepic to insepic throughout the upper metre at this site.

Woolhampton The upper 40 cm of this profile has a porphyroskelic fabric with asepic plasmic fabric. Numerous large sand

size, rounded grains of calcium carbonate occur within the s-matrix. Illuvial clays are not seen. Some brown mottles with diffuse boundaries also occur. Below 4&50 cm there is a distinct change. Some patches are extremely calcareous and secondary carbonate has been deposited in channels and voids. The matrix in the calcareous patches is asepic. However, other patches contain no visible carbonate. In these areas the clays are considerably better oriented and the fabric is insepic to occasionally mosepic. Such areas appear as small brown mottles when the slide is viewed as a whole. There are no signs of illuvial clay within the lower part of the profile.

352 C. J. CHARTRES

DISCUSSION

In the fluvial deposits of the Kennet Valley the problem of differentiating pedological features developed in situ (orthic pedological features of Brewer, 1964) from those inherited from older soils and deposits is further compounded by the periglacial disturbance of many of the older, pre-Devensian soil horizons. Furthermore, other pedological features may have been soliflucted, or colluvially reworked downslope on to the terrace surfaces.

Undisrupted, orthic pedological features A number of undisrupted, orthic pedological features occur in the terrace deposits. These include matrans (silty organo-argillans), calcans, yellowish-brown to reddish-orange ferriargillans, yellow ferriargillans, rusty mottles, and sesquioxide concretions (Table I). The matrans and yellowish-brown to reddish-orange ferriargillans occur on the margins of channels and voids, and are generally associated with horizons with asepic to insepic s-matrix plasmic fabrics. Furthermore, these illuvial features appear to be predominantly found in Devensian deposits, or in soil horizons containing Late Devensian loessial additions (Chartres 1980, 1981) and can be presumed to have developed during the Holocene period, as was suggested for similar features elsewhere by Bullock (1974). Calcans are restricted to the floodplain and form due to the translocation and redeposition of calcium carbonate, much of which was derived from local marl deposits. The rusty mottles and sesquioxide concretions included in the orthic group of features have diffuse outer boundaries, are often associated with channels and voids, and occur predominantly in soil horizons subject to imperfect drainage.

The yellow ferriargillans included in the group of undisrupted orthic pedological features only occur at Thatcham and Hamstead Marshall within 2Bt horizons, and are more commonly seen as disrupted features which are discussed in the following section.

Disrupted, orthic pedological features Included within this group of micromorphological features are yellow ferriargillans and papules (similar in

colour and properties to the undisturbed yellow ferriargillans), rubified ferriargillans and papules, rubified clayey pedorelicts, and embedded rubified grain ferriargillans (Table I). All these features occur predominantly within 2Eb and 2Bt horizons on the terraces above the Beenham Grange Terrace (2-3m). The concentration of such features within palaeoargillic horizons and their overlying eluvial horizons and the lack of similar features in underlying pedogenically little affected gravels suggests that they are most probably the disrupted remnants of old soil horizons rather than transported features. In some circumstances, biological activity can also cause disruption of in situ features and can prevent the formation of well-horizonated profiles (Hoeksma and Edelman, 1960). Such disturbance of 2Bt horizons in the Kennet Valley is thought unlikely because of the gravelly deposits would not make a very favourable habitat for many soil fauna, and secondly because micromorphological observations do not indicate phenomena such as pedotubles and fecal pellets, indicative of such activity. On the Hamstead Marshall Terrace, at least, fossil ice-wedge casts and highly contorted soil horizons boundaries were observed in the upper c. 2 m of gravels, and elsewhere on the terrace surface, clear polygonal crop-mark patterns were observed suggesting strong periglacial disturbance. Consequently, it is considered that periglacial activity was the most probable cause of disruption of the 2Eb and 2Bt horizons. Features such as silty cappings on stones (Fitzpatrick, 1956) and banded fabrics (Pawluk and Brewer, 1975) can be explained by analogy with soils found in present arctic environments. However, the effects of cryoturbation on profiles developed under previously temperate conditions are often inferred from the shape and distribution of the pedological features. Fedoroff (1 969) suggests the term ‘microcryoturbation’ to describe periglacial disturbance of micromorphological features. He argues that the comminution of skeleton grains and argillans, and the developments of grain ferriargillans can all result from microcryoturbation.

Evidence from the Kennet Valley suggests that the following effects can be attributed to cryoturbation processes:


1. The disruption of both rubified and yellow illuvial clay to produce irregularly shaped pedorelicts, papules, and embedded grain ferriargillans (Table I).

2. The incorporation of the above features within the overlying eluvial horizons.

Brewer (1964) uses the term ‘disrupted cutan’ to describe disturbed and redistributed cutanic material. The term ‘cryoturbated ferriargillans’ is used to describe such features in the older Kennet Valley soils affected by periglacial disturbance.

Evidence has been put forward by Dalrymple (1972) that ‘lehm’ fabrics, in which a high proportion of the plasma is oriented, develop by stress due to wetting and drying. Though there is evidence for such processes operating in some of the Kennet terrace soils (e.g. Thatcham, 2Bt horizon), the plasma separations produced are quite different from cryoturbated ferriargillans. Whilst the stress induced features are associated with shear planes, the disrupted types of features often occur embedded within other materials.

The extent to which periglacial disruption has occurred in the Kennet Valley depends upon the relative age of the profiles. With no dates except for the floodplain deposits, relative ages of the terraces can only be determined from the height of each terrace above the river. The fact that both the red and yellow illuvial clays in the 2Bt horizon at Hamstead Marshall are disturbed suggests two separate episodes of periglacial disruption (see Chartres, 1980). A few ferriargillans in the 2Bt horizons have apparently escaped the effects of microcryoturbation and appear relatively undisturbed. However, on macromorphological grounds, it is quite possible that the peds, or larger structural entities in which they occur, were moved by cryoturbation processes such as heaving and ice-wedge development.

The reddish-black iron and manganese (strong effervescence with H,O,) pedorelicts recognized in the 2Eb horizon at Hamstead Marshall, however, appear to have originated in the underlying 2Bt horizons. Similarly, pedorelicts of yellow oriented clays in the 2Eb horizon at Thatcham have been derived from the underlying 2Bt horizons. Both these types of pedorelicts are irregularly shaped and show no signs of rounding due to transport.

Also observed at Hamstead Marshall, particularly in the 2Bt horizon, are grain ferriargillans. Generally, these features smooth out irregularities in the shape of the grains they coat. Frequently the grains (flint and quartz) are embedded in the s-matrix, with no access to voids. The grain ferriargillans are generally reddish, yet in places the clay in the surrounding s-matrix is yellow or reddish-orange to yellowish-brown. Bullock and Murphy (1979), in a study of a soil developed in Plateau Drift at North Leigh, Oxfordshire showed that some such grain ferriargillans are features derived from elsewhere. A transport inherited origin for the embedded grain ferriargillans seems unlikely at Hamstead Marshall, where the most likely source is disruption in situ of previously illuviated clays. Furthermore, no such features were observed in the underlying unaltered fluvial gravels. Such features would not survive the high energy environment of the streams which deposited the gravels.

At Kintbury, though there are signs of periglacial disturbance in the soliflucted gravels (i.e. silty cappings on stones, and relatively common embedded grain ferriargillans, papules, and pedorelicts), these are set in an asepic to insepic soil matrix and the 2Bt horizon does not conform to the definition of a palaeoargillic horizon (Bullock, 1974) and possibilities other than the disturbance of older illuvial horizons have to be considered responsible for the properties of this profile. These are discussed in the following section.

Transport inherited pedological features On the Chalk downlands surrounding the Kennet Valley there are widespread deposits of Clay-with-Flints,

Plateau Drift (Loveday, 1962) and ancient, probably riverine deposits of Plateau Gravels. These deposits have all been reorganized pedologically. During the unstable phases of the Quaternary some of these materials could have been eroded and subsequently deposited within the river terrace sequence. Solifluction lobes mantling the Thatcham Terrace in several locations (e.g. Kintbury) offer the possibility of comparing the features of soliflucted soil with those of soils developed in situ on riverine deposits. However, because the terraces above the Beenham Grange level have been cryoturbated at least once, there is the further problem of distinguishing originally in sifu (orthic) pedological features, which have subsequently been disrupted by cryoturbation, from transported pedological features. Brewer (1964) lists several criteria which may be helpful

354 C. J . CHARTRES

in differentiating orthic and transport-inherited pedological features. These include the presence or absence of accretionary structures, shape, nature of their internal fabric compared with the surrounding materials, and clarity of their boundaries.

Lithorelicts observed in the soils include fragments of sarsens, abundant flint and silicified shell fragments from the Chalk (Brown et al., 1969), and clayey aggregates probably derived from the Reading Beds and London Clay (Table I). Such aggregates with sharp boundaries and strongly sepic fabrics were observed at Hamstead Marshall in the gravels immediately above the contact with the Reading Beds, and were presumably derived locally from the latter sediments. Though it is possible that finely divided chalk was also an initial constituent of the terrace gravels, it will have been removed after deposition by decalcification of the profiles. The presence of undisturbed sedimentary bedding below 1.70 m at Hamstead Marshall and also in the gravels at Beenham Grange, however, suggest that large chalk clasts were not a major constituent of the terrace gravels. Only on the floodplain are calcareous nodules found. These nodules are lithorelicts derived from the deposits of marl and tufa which occur above the fluvial gravels in some areas of the floodplain. At Kintbury, intensely rubified pedorelicts (Figure 9) with sharp, often rounded boundaries are common in both the 2Eb and 2Bt horizons. There is no evidence to suggest that those in the 2Eb were derived from the 2Bt horizon. In both horizons they are similar to ferruginous nodules seen in the Plateau Drift and other older deposits in the area. These nodules also occur in the underlying, pedologically little altered gravels, suggesting that they are transport-inherited features. They are extremely hard and indurated with sesquioxides, which probably enables them to survive riverine transport, though at Kintbury, faint haloes around some of the nodules suggest post-depositional solution of some of their constituents. Even in the pedologically altered horizons the internal s-matrix and fabric of the nodules is quite distinct from the surrounding materials, again indicating that they are transport-inherited rather than orthic features. Their source appears to be the Plateau Drift and Clay-with-Flints deposits upslope of the site, and the mode of transport on to the terrace was probably solifluction (Cheetham, 1976). Bos et al. (1971) reported a similar solifluctional and colluvial incorporation of rounded remnants of a Bt horizon (Eemian) into younger (Wiirm) deposits in West Germany. The rubified grain ferriargillans also observed at Kintbury in the 2Eb and 2Bt horizons do not appear to be cryoturbated orthic features as is the case Hamstead Marshall. More probably is that such features, which are somewhat similar to those reported by Bullock and Murphy (1969), were formed upslope in the soils of the Clay-with-Flints and Plateau Gravel and were then incorporated by solifluction into deposits found on the 9-10m terrace at Kintbury.

CONCLUSIONS

When used in conjunction with other pedological, sedimentological, and geomorphological data, micromorphological analyses can be a particularly effective means of distinguishing and separating soil horizons and sedimentary layers. In the Kennet Valley, not only do a wide range of micromorphological features help distinguish pedologically altered materials from pedologically unaffected deposits, but palaeoargillic horizons in pre-Devensian materials can be clearly differentiated from argillic horizons in Devensian deposits using the guidelines suggested by Bullock (1974). The present results, however, do suggest that the 2Bt horizon at Kintbury, which on field characteristics is palaeoargillic using Avery’s (1975) criteria, is on micromorphological grounds an argillic horizon and that the palaeoargillic appearance is due to the incorporation of older soil materials within the horizon by solifluction processes. Properties of pedological features such as shape, size, boundary characteristics, internal fabric, and their position relative to channels, vughs, and the s-matrix, as well as the relative proportion of different pedological features, can all be used as indicators of the origins of the soil material and to help elucidate profile history. Such analyses have much to offer sedimentologists, geomorphologists, and pedologists interested in Quaternary research.

ACKNOWLEDGEMENTS

This study was undertaken when the author was in receipt of a Reading University Postgraduate Scholarship, which is gratefully acknowledged. Thanks are expressed to Drs. P. Bullock and H. J. Miicher and Mr 1. M.


Fenwick for fruitful discussions regarding the work. The author is particularly indebted to Dr J. A. Catt for critically reading an earlier draft of the text and suggesting valuable improvements.

REFERENCES

Avery, B. W. 1975. ‘Soil classification in the Soil Survey of England and Wales’, J. Soil Sci., 24, 324-338. Bos, R. H. G., Jungerius, P. D., and Wiggers, A. J. 1971. ‘Solifluction and colluviation on the ice-pushed ridges of Uelsen, Kraft

Brewer, R. 1964. Fabric and Mineral Analysis of Soils, Wiley, New York. Brewer, R. and Sleeman, J. 1969. ‘The arrangement of constituents in Quaternary soils’, Soil Science, 107, 4 3 5 4 7 1 . Brown, G., Catt, J. A., Hollyer, S. E., and Ollier, C. D. 1969. ‘Partial silicification of chalk fossils from the Chilterns’, Geological

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Documents

The micromorphology of quaternary river terrace deposits in the Kennet Valley, Berkshire, England