Newney Green Quarry

[TL 648 065]

D.R. Bridgland

Highlights

This site provides evidence for the route of the pre-Anglian Thames through East Anglia and for the major glaciation that led to the diversion of the river into its modern course. Superimposed warm-climate and cold-climate soils separate Thames gravels and Anglian till at Newney Green and represent an important element of the regional stratigraphy.

Introduction

Newney Green Quarry is a key site that has been used to establish the lower Middle Pleistocene succession of the Chelmsford area (Rose et al., 1976, 1978). This sequence comprises fluvial deposits, part of the Kesgrave Group, overlain by various products of glaciation during the Anglian Stage, including outwash gravel, coversand and till. The site has also provided exceptional opportunities to study a buried soil horizon that occurs at the top of the Kesgrave Sands and Gravels. This horizon, of considerable stratigraphical significance, comprises superimposed warm- and cold-climate fossil soils, the Valley Farm and Barham Soils respectively (Rose et al., 1976, 1985a, 1985b; Rose and Allen, 1977; Kemp, 1985a).

(Table 5.2) Clast-lithological composition of the gravels described in Chapter 5, Part 1.

The site at Newney Green is also of importance for studies of the glacial (Lowestoft Formation) deposits. The site is the type locality of the Newney Green (Till) Member (see below), a subdivision of the Lowestoft Formation proposed by Whiteman (1987, 1990; in Allen et al., 1991). Although there has been considerable work on the palaeosols and till at Newney Green, the regional stratigraphical relations of the underlying Kesgrave Group gravels have been largely overlooked. This same situation exists throughout central Essex, where the various formations of the Low-level Kesgrave Subgroup are largely buried by Anglian Stage glacial deposits, precluding a regional synthesis using normal mapping techniques. A recent evaluation of borehole data (Whiteman, 1990) promises, however, to resolve this problem.

Description

Sections at Newney Green have varied considerably over the 15 years of the quarry's life. In particular, the till sequence is missing in lower areas, where it has been removed by post-Anglian erosion. The stratigraphical sequence is set out in (Table 5.4) (see also (Figure 5.6)).

The Kesgrave Sands and Gravels (1) are represented by the lowest and most extensive Pleistocene unit present at Newney Green. This comprises predominantly cross-stratified sands and gravels with foreset orientations indicating an eastward palaeocurrent direction (Rose et al., 1978). The sedimentary characteristics of this deposit point to deposition by a braided river (Rose et al., 1976). It has a gravel composition typical of the Kesgrave Group, with conspicuous cobble-sized clasts of volcanic rocks of the type attributed by Hey and Brenchley (1977) to sources in North Wales.

The Valley Farm Soil is well-developed at the top of the fluvial deposits, except in areas where the till has a strongly erosive base and directly overlies the lower parts of the Kesgrave gravels. This soil is apparent as a clay-rich, reddened horizon, with patches of grey mottling. This represents the illuvial horizon of an argillic soil, the formation of which required a temperate climate. The higher layers of this soil were presumably removed by erosion prior to the deposition of the overlying sediments. Other indications of clay illuviation in this temperate soil are the presence of clay skins around gravel clasts and clay infillings of voids. The red coloration comes from haematite, the formation of which is evidence of a warm temperate environment with high seasonality (J. Rose, pers. comm.). Whiteman (1990) recognized five separate horizons within this soil in part of the Newney Green Pit, but these appeared to be primarily of sedimentary origin, reflecting original differences in parent lithologies.

The reddened Valley Farm Soil is usually deformed by the superimposed periglacial Barham Soil, which manifests itself as a complex of regularly spaced involutions, with occasional ice wedge pseudomorphs (Rose et al., 1976, 1985a; Rose and Allen, 1977). Other characteristic features are frost-cracks, sand-wedges, fractured and vertically orientated stones, silty-clay cappings of sand grains and features associated with ground-ice development, such as platy aggregates and banded fabrics (for summary, see Rose et al., 1985a). The cores of the involutions and wedges are often filled with coversand, in which wind-faceted stones (ventifacts) may be found (Rose et al., 1978). During the working of the quarry, removal of the Lowestoft Till revealed, from time to time, the polygonal nature of involutions and wedges within the periglacial soil (Rose et al., 1978, 1985a; (Figure 5.7)). Possible remnants of higher, humic horizons have been observed on rare occasions, preserved within the cryoturbation structures of the periglacial soil. These were found to have a very low organic content and probably represent the diffuse humic horizons of an Arctic Brown Soil (J. Rose, pers. comm.). No pollen or microfauna has been found in them.

The glaciofluvial gravel (bed 4) occurs as localized lenses between the buried soil horizon and the till. It can be distinguished from the underlying Kesgrave gravels on the basis of its clast and heavy-mineral content and, within individual beds, its poorer sorting (Rose et al., 1978). The clast component of this gravel includes Chalk and other (exotic) non-durable material, of types characteristic of the overlying till complex (Whiteman, 1990). The heavy minerals are particularly diagnostic; a rich assemblage has been recognized (Catt, in Rose et al., 1978), including poorly durable varieties such as apatite and collophane. This assemblage is comparable with that from the coversands, but contrasts markedly with the mineral suite from the Kesgrave Sands and Gravels, which is restricted to residual grains such as tourmaline and zircon. This change from durable, residual minerals to a more varied assemblage, including easily weathered types, suggests an input of fresh material into the region. It therefore provides important evidence for the association of the upper gravel with the glaciation that deposited the overlying till (Rose et al., 1978).

The Lowestoft Till uniformly covers the highest ground throughout the area of the Newney Green workings. In early descriptions it was reported to be generally homogeneous and structureless, with local banding; other recorded variations include lenses of flow till, identified at the base of the main (lodgement) till unit and differences in the frequency of calcareous material, particularly Chalk clasts (Rose et al., 1978). Rose et al. also noticed that the lower part of the till was less calcareous, its gravel-sized component being dominated by flint and quartzites reworked from the underlying Kesgrave Group deposits. This distinction was later used by Whiteman (1987; in Allen et al., 1991) to define separate lower (Chalk-poor) and upper (chalky) divisions, his Newney Green and Great Waltham Members (respectively). These members are themselves subdivided (Whiteman, 1987, 1990; in Allen et al., 1991). The Newney Green Member is made up of a lower, laminated deformation till, comprising sheared and remobilized material from the palaeosols and coversand (beds 2 and 3), and an upper homogenized till of similar composition (also including much reworked material from the underlying sands and gravels). Subdivision of the Great Waltham Member reflects the decalcification of the upper part of this very chalky member.

Coarse, chalky gravel together with laminated sands, silts and clays have recently been observed occupying an irregular and undercut depression in the surface of the till, possibly pointing to a final phase of Anglian deposition in the area (Whiteman, 1990). The clast composition of the gravel resembles that of the underlying glacial beds. Whiteman considered that these deposits had been let down into the upper part of the till after deposition, perhaps in response to the melting of a block of ice incorporated in the underlying sequence. They may thus be considered to represent a kettle hole infill.

A conservation section was identified at Newney Green in 1988, as commercial operations were drawing to a close. A preliminary excavation revealed that all the essential elements of the stratigraphy for which the site is important are present (Figure 5.8). The complex palaeosol is well-developed in the northern part of the section, but is cut out by the erosive base of the glacial deposits to the south. Incorporated in the cryoturbation features of the Barham Soil are unweathered gravels and sands of presumed Kesgrave affinities, as well as reddened material from the Valley Farm Soil. A bed of grey clay is also incorporated in these structures, mottled with the red coloration of the warm-climate palaeosol. This material appears to be derived from the London Clay bedrock; it may have been redeposited in standing water, but a more likely explanation is that it was brought to its present level, prior to the cryoturbation episode, by diapirism, a phenomenon that has been observed at this site previously (Rose et al., 1978; Whiteman, 1990). Whiteman (1990) noted that both London Clay and till have been injected into the lower sands and gravels by this process. An upper orange sand, probably coversand, is also present in the cores of involutions and, unusually, as a small undisturbed remnant above the cryoturbated palaeosol (Figure 5.8).

Strong lateral deformation of the cryoturbation structures of the Barham Soil can be recognized in the GCR section (Figure 5.8). This may be related to similar lateral deformation of such structures previously observed at this and other sites (Rose et al., 1978; Allen, 1983, 1984), attributed to a glaciotectonic effect caused by the pressure of the overriding ice sheet. The tilting of the upper parts of the structures to the south (Figure 5.8), in keeping with the notion that the ice was moving generally southward, lends support to this interpretation.

Interpretation

There is a considerable history of debate on the stratigraphy and origin of the gravels underlying the Lowestoft Till of southern East Anglia, dating back to the original definition of the 'Glacial Beds' by Wood (1867; Wood and Harmer, 1868). Much of the published material dealing with these deposits incorrectly assumes that they are of glaciofluvial origin (Clayton, 1957; Baker, 1971; Wilson and Lake, 1983). The idea that most of the gravels underlying the till sheet of Essex and Suffolk are the product of the early Thames, rather than the Anglian glaciation, dates back to the beginning of the century, to the views of Salter (1896, 1905). It is only recently, however, that this interpretation has found widespread acceptance, through the work of Rose et al. (1976; Rose and Allen, 1977), who gave them the name Kesgrave Sands and Gravels. A recent regional synthesis of these early Thames deposits (Whiteman, 1990, pers. comm.) has suggested that the gravel at Newney Green represents the Waldringfield Gravel, previously recognized in south-east Suffolk and north-east Essex (Allen, 1983, 1984; Bridgland, 1988a; (Figure 5.2)).

The early Thames terrace gravels of the Kesgrave Group in fact extend well beyond the limits of the Anglian glaciation and can be demonstrated at many sites to be pre-Anglian. The principal evidence for this is the preservation of the warm-climate Valley Farm Soil in the upper levels of the Kesgrave deposits, beneath the Anglian till (see above). The degree of reddening and clay enrichment recorded from various sites suggests that fully interglacial conditions would have been necessary for its formation (Rose and Allen, 1977; Kemp, 1985a). Kemp (1983, 1985a) discovered, from a study of the micromorphological properties of the Valley Farm Soil at several sites, that it has a complex history of formation, with alternating periods of rubification and disruption of its fabric by periglacial processes. Different degrees of complexity are found at various sites, with the greatest number of climatic cycles indicated where the soil is developed on formations of the High-level Kesgrave Subgroup. Unfortunately, it is difficult to determine the precise number of climatic fluctuations responsible for a particular soil type, but Kemp's work provided important new evidence that the stratigraphical sequence first outlined by Rose et al. (1976) was oversimplified. It is now accepted that the Valley Farm Soil, as recognized on different formations within the Kesgrave Group, represents different numbers of climatic cycles, reflecting the deposition of the Kesgrave Sands and Gravels over a considerable period of Pleistocene time (Rose, 1983a; Kemp, 1985a; Rose et al., 1985b).

Whiteman (1990) found evidence from micromorphological analysis of the palaeosol (unit 2) at Newney Green for only a single period of illuviation, uninterrupted by any significant disturbance prior to the formation of the superimposed Barham Soil. Subsequent to the formation of the various disruption features associated with the Barham Soil, illuviation of calcium carbonate together with further clay had apparently occurred. However, Whiteman concluded that this later illuviation episode postdated the emplacement of the till, since he could envisage no potential source for the calcium carbonate other than Chalk clasts in the glacial deposits. Since illuviation appears, according to this interpretation, to have affected the palaeosol horizon after its burial beneath the till, Whiteman (1990) was forced to conclude that the thickness of till at Newney Green (maximum 3.75 m) was insufficient to isolate the fossil soil from post-Anglian pedogenic activity. His failure to recognize pre-Anglian disturbance of the soil fabric at Newney Green is difficult to reconcile with his attribution of the gravel there to the Waldringfield Formation. Stratigraphical evidence from north-eastern Essex suggests that several climatic cycles intervened between the deposition of the Waldringfield Gravel and the Anglian Stage glaciation (Bridgland, 1988a; see above, Introduction to Part 1). A possible explanation is that the upper layers of the gravel may have been stripped during pre-Anglian periglacial episodes, a process that is likely to have affected the upper surfaces of Kesgrave Group terrace formations throughout the area. Only on the most stable land surfaces would the full potential complexity of the Valley Farm Soil have been realized.

The superimposed Barham Soil represents the final modification, by periglacial processes during the early Anglian Stage, of the complex Valley Farm Soil. It was initially termed an Arctic structure soil (Rose et al., 1976; Rose and Allen, 1977), but this term was subsequently found to be too restrictive, as a wider range of pedogenic properties was observed at different sites, and the Barham Soil was recognized as a complex and variable periglacial soil (Rose et al., 1985a, 1985b). When it is developed on the complex Valley Farm Soil, it may be difficult or impossible to distinguish certain of the features of the Barham Soil, such as the fracturing of clasts and the break up of clay skins (Rose et al, 1985a), from pre-existing features of the Valley Farm Soil, formed by earlier periglacial disruption (Kemp, 1985a). The characteristic features of the Barham Soil are particularly well-developed at Newney Green (Figure 5.8). For example, the polygonal pattern of the larger-scale structures, reminiscent of landscapes in modern tundra regions, is superbly illustrated on the exhumed upper surface of the Barham Soil (Figure 5.7).

The Barham Soil is closely associated throughout the area of its occurrence with various glacially-derived wind-blown sediments, namely coversands and loess (Rose et al., 1976, 1985a; Rose and Allen, 1977). These are usually preserved in wedges and in the cores of involutions within the Barham Soil (Figure 5.6), but occasional remnants are found, apparently in situ, between the Barham Soil and the overlying glacial sediments (see (Figure 5.8)). Further evidence of the importance of aeolian activity during the early Anglian is the occurrence of wind-polished or faceted pebbles in the coversand.

The overriding of the Barham Soil, later in the Anglian, by the ice sheet that deposited the Lowestoft Till, resulted in glaciotectonic deformation of the pre-existing features, sometimes producing overfolds, shears and nappe-like structures (Rose et al., 1985a). Whiteman (1987; in Allen et al., 1991) recognized a basal 'deformation till', immediately overlying the palaeosol horizon, at Newney Green (Table 5.4) and at other nearby sites. He noted that it is often difficult to determine the junction between the deformed palaeosol horizon and the overlying deformation till. Ideally, however, there is a plane of décollement separating this lowest division of the till from the underlying, glacio-tectonically deformed sediments. This plane of décollement serves as the most effective lower boundary of the till sequence. In the GCR section, glaciotectonic effects can be observed in the form of lateral deformation of the structures within the Barham Soil (Figure 5.7), broadly reflecting ice movement towards the south or south-east.

Whiteman (1987, 1990; in Allen et al., 1991) has subdivided the Anglian till of central Essex into a number of facies-related beds, the gross lithological properties of which suggest that the sequence as a whole represents two distinct members of the Lowestoft Till Formation. The sections at Newney Green have been of considerable importance in the definition of these members, the lower of which is named after the site. The higher, stratigraphically younger 'Great Waltham Member' is named after another site in the Chelmsford area, 8 km to the north of Newney Green.

The Newney Green Member, largely derived from the underlying gravels, palaeosol and coversand, is commonly banded throughout, not just in its lower 'deformation till' division, in which the banding is attributed to shearing and attenuation of layers within the original sediment (see above). Many different mechanisms to explain banding in tills have been proposed and the origin of this phenomenon in the upper part of the Newney Green Member is uncertain at present (see Allen et al., 1991).

The overlying Great Waltham Member is a compact grey, chalky till of classic 'Chalky Boulder Clay' type, although Whiteman subdivided it into lower and upper units, the latter representing its weathered and decalcified form. Decalcified Great Waltham Till may account for the full sequence in areas where the till cover is thin. In some instances differences between the lower and upper divisions of the Great Waltham Member cannot be explained entirely as the result of weathering. This led Whiteman (in Allen et al., 1991) to suggest that differences in the direction of ice-movement and, therefore, in provenance are implicated.

The junction between the Newney Green and Great Waltham members is generally sharp, but Whiteman (1987, 1990; in Allen et al., 1991) found no indication that they represent more than a single ice advance. There is no evidence as yet to relate this advance to the more complex glacial sequence in south-west Essex and Hertfordshire, where four separate ice advances occurred during the Anglian Stage (Cheshire, 1986a; Chapter 3, Part 2). One important difference between the two till members at Newney Green, however, is that their fabrics are generally at 9O.D.g; to one another, with that in the Great Waltham Till most closely in agreement with other evidence for ice flow direction, such as ^.the configuration of subglacial reen Member was orientated transverse to ice flow directions. He regarded this as suggestive of strong compressional forces within a highly deformed till, formed above a rough glacier bed.

Conclusions

There are four important elements to the geological interest at Newney Green. Firstly, the site shows gravels deposited by the early (pre-diversion) River Thames at a time when it flowed across Hertfordshire and East Anglia. Secondly, a temperate-climate soil was developed at the top of this gravel. Following this temperate phase, the climate deteriorated until permafrost conditions prevailed, leading to the formation of a periglacial soil with characteristic structures (large-scale structures include patterned ground and ice wedges, such as are formed at the present day in tundra regions through the growth and subsequent melting of ground-ice). This period of extreme cold culminated in the covering of the area by (Anglian) ice, which moved southwards, depositing till (boulder clay), the fourth element in the Newney Green sequence. It was at this time that the course of the Thames was blocked by the ice and the river adopted a new route through what is now the London area. The temperate-climate soil is of considerable importance, as it shows that the gravels were not deposited in the same cold episode as the overlying glacial deposits.

References