Little Heath

[TL 017 083]

D.R. Bridgland

Highlights

A controversial site revealing deposits of shallow marine or fluviatile origin, Little Heath is critical for the understanding of the Late Pliocene/Early Pleistocene evolution of the London Basin and the Middle Thames catchment.

Introduction

The existence at Little Heath of high-level superficial deposits of considerable antiquity was first noted by Prestwich (1890b). Sediments at this site, attributed to a Pliocene marine phase, were subsequently described in detail by Gilbert (1919a). Since that time there has been considerable controversy about the age and origin of unfossiliferous deposits of this type (Barrow, 1919a, 1919b; Sherlock, 1919, 1922, 1924, 1929; Wooldridge, 1927a, 1957, 1960; Wooldridge and Linton, 1939, 1955). They occupy similar topographical positions to both the fossiliferous Pliocene outliers of the North Downs (Stebbing, 1900; Davies, 1917; Chatwin, 1927; Dines and Edmunds, 1929; John and Fisher, 1984) and Chilterns (Dines and Chatwin, 1930) and the earliest fluvial deposits, the Pebble Gravel, which may be of Late Pliocene or Early Pleistocene age (Whitaker, 1864, 1889; Hughes, 1868; Prestwich, 1881, 1890a, 1890b; White, 1895, 1897; Salter, 1896, 1898; Wooldridge, 1927a, 1957, 1960; Wooldridge and Linton, 1939, 1955; see above, Introduction to Part 1). It remains unclear, furthermore, whether deposits such as those at Little Heath are of shallow-marine/ littoral or of fluviatile origin. These enigmatic, degraded remnants provide the only indication of the palaeoenvironment in Britain during the Pliocene/earliest Pleistocene.

(Table 3.1) Correlation of tributary and main Thames formations within the Pebble Gravel Group and other pre-diversion gravels in the Middle Thames and Vale of St Albans regions.

Prestwich (1890b, p. 139) referred to a Tertiary outlier at Little Heath, preserved in a large depression in the Chalk some 550 ft (168 m) above O.D., associated with an indistinct development of his Wesdeton Shingle'. Barrow (1919a) considered this to represent Pebble Gravel of a comparable composition to the Reading Beds outlier at Lane End, which he also regarded as Pliocene (see above). Wooldridge (1927a) considered the Little Heath Gravel to be a lateral extension of his 'High-level (500 ft) Pebble Gravel' of North London, which he correlated with the Pliocene marine transgression of the North Downs. Recently the deposits at Little Heath have been central to a reappraisal of the evidence for a Pliocene/Lower Pleistocene marine platform on the Chilterns (Moffat, 1980; Moffat and Catt, 1983, 1986a), much of the evidence for which has been questioned in recent years (Pinchemel, 1954; Catt and Hodgson, 1976).

Description

The earliest detailed record of the deposits at Little Heath was by Gilbert (1919a, 1919b), who described a sequence of clay, gravel, sand and 'pebbly clay' overlying the Chalk. The site received little subsequent attention until a new section some 10 m deep was excavated as part of a recent reappraisal of the evidence for Neogene marine deposits on the Chilterns (Moffat, 1980; Moffat and Catt, 1983). The following summary of the sequence at Little Heath is derived from the descriptions by Gilbert and Moffat:

In the recent re-excavation, Chalk was proved at 159 m O.D. in the base of the excavation; it and the brown clay were reached only by angering. Mineralogical and mechanical analyses suggest that beds 2–4 belong to the Reading Beds (Moffat, 1980, 1986; Moffat and Catt, 1983). The remainder of the sequence at Little Heath appears to be of later, probably Pliocene or earliest Pleistocene age. Some evidence of disturbance, probably resulting from solution of the underlying Chalk, was noticed by Moffat (1980).

Interpretation

Gilbert (1919a) based his interpretation of the gravel and loathy sand (beds 6 and 7) at LittleHeath as marine deposits on a number of observations. Firstly, these deposits fine upwards, a feature that he likened to Cambrian marine conglomerates, but which would now be regarded as equally typical of fluvial sedimentation. They possess a grain-supported structure, which Gilbert interpreted as a feature of marine, rather than fluviatile or glacial gravels. There is a gradation between unworn and well-rounded pebbles, which he considered similar to that found on modern beaches in which the materials are 'immediately derived'. Gilbert noted that large clasts showed evidence of beach hammering and that the surface morphology of the gravel body was suggestive of a beach. Finally, laminations and evidence for periodic exposure (sun-cracks, rain-spots, etc.) in the sand (bed 7) were interpreted by Gilbert as evidence of an intertidal environment.

According to Gilbert, the Little Heath gravels were not confined to a depression in the Chalk, as Prestwich (1890b) had claimed, but also capped the Chalk plateau as a horizontal undisturbed sheet. Gilbert compared them to the high-level deposits capping the North Downs in Surrey, attributing both to a Pliocene marine episode. Barrow (1919a) agreed with this assessment, likening the Little Heath gravel to the Pebble Gravel of the area north of London, which contains only local materials plus quartz and 'lydite' pebbles from the Lower Greensand.

Barrow (1919b) divided the Pebble Gravel into two types: one composed of smaller pebbles and occupying a platform at 400 ft, the 'Upper Plain' of Hughes (1868), and the other, a higher, coarser type, typically occupying land at c. 500 ft (152 m) O.D. in the Stanmore area. It was with the latter type that Barrow (1919a) classified the Little Heath gravel, noting that it lay even higher, 50 ft (15 m) above the Stanmore deposit, an increase in elevation that was accompanied by a further increase in pebble size. Barrow believed that the coarser, higher (500 ft) Stanmore Pebble Gravel was a beach deposit associated with the same marine episode as the 400 ft (Northaw) Pebble Gravel.

Sherlock (1919) was not convinced of the Pliocene age of the Little Heath deposits. He pointed to the occurrence of gravel in the Reading Beds at Lane End, Buckinghamshire (White, 1906), of similar composition to that at Little Heath, containing quartz, 'lydite' and subangular flints. Sherlock suggested that the various high-level gravels of the Chilterns might also be in situ Reading Beds; indeed, in the Aylesbury Geological Survey memoir (Sherlock, 1922), he included all the stratified deposits at Little Heath in the Reading Beds. In an extensive review he argued strongly against the existence of Barrow's '400 ft Platform' and against there having been a submergence in Pliocene times (Sherlock, 1924). He interpreted the Pebble Gravel (sensu lato) as 'a Glacial Drift derived from local materials' (Sherlock, 1924, p. 9).

Of considerable significance to this dispute was the allied controversy over White's (1906, 1908a) interpretation of the quartzose gravel at Lane End, Buckinghamshire, as in situ Reading Beds (see above, Introduction to Part 1). Although White had interpreted a deposit overlying these gravels as basal London Clay, Barrow (1919a) did not accept that they were Palaeogene. The similarity of the gravels at Lane End and Little Heath, cited by Barrow as evidence that the former was of Pliocene age, was equally indicative to Sherlock (1919, 1922, 1924) that the Little Heath deposits were part of the Reading Beds. However, the Lane End deposits were reinvestigated by Wooldridge and Gill (1925), who fully confirmed White's findings.

Wooldridge (1927a) recognized numerous outliers at high levels on the North Downs and South Downs, all of which he regarded as Diestian (Pliocene) in age, on the basis of their heavy-mineral content. A characteristic heavy-mineral suite, with an abundance of garnet and a relative abundance of monazite and coarse andalusite, had been recognized in the Lenham, Netley Heath and Headley Heath Beds of the North Downs by Davies (1915, 1917). Wooldridge attributed these deposits, together with a number of isolated outliers of high-level gravel on the North Downs and a few in the Chilterns, to a Diestian marine transgression, proposing that they be collectively referred to as Lenham Beds. According to Wooldridge (1927a, p. 80) 'the only locality at which the Lenham Beds are well-developed on the Chiltern plateau is at Littleheath near Berkhampstead'. He claimed that the heavy-mineral assemblage from Little Heath was of Diestian character and quite different from Reading Beds mineral suites (Wooldridge, 1927a, 1927b, in Sherlock, 1929). He also pointed out that the Little Heath sands, being marine, are finer grained and better sorted than the typical (fluviatile) Reading Beds sands of the district. Wooldridge distinguished the various Diestian outliers from the true Pebble Gravel, although he regarded the latter as little if any younger than the Lenham Beds, and interpreted the highest Pebble Gravel of the Stanmore area as a 'true Diestian shingle'.

The correlation, based on heavy minerals, between the Netley Heath Beds and the Lenham Beds was disproved by the discovery of Late Pliocene (Red Crag) fossils in the former (Chatwin, 1927), indicating that they post-date the Lenham Beds. As a result, Sherlock (1929) argued strongly against the stratigraphical usefulness of heavy-mineral analysis and remained unconvinced that the Little Heath Gravel was of Pliocene age. Red Crag fossils were later discovered in sandstone blocks within high-level drift at Rothamsted, apparently confirming the existence of a Pliocene marine episode in Hertfordshire (Dines and Chatwin, 1930).

Wooldridge and Ewing (1935) carried out a reappraisal of the Palaeocene and alleged Pliocene deposits of the Chilterns, including a detailed petrographical examination. As a result, they concluded that the Little Heath gravel, together with many other patches of high-level deposits in the area, were of Pliocene age. Wooldridge and Linton (1939, p. 55) emphasized the relation of the Little Heath outlier to 'sharply rising ground behind the escarpment edge at Ivinghoe Beacon'. They claimed this to be a fossil shoreline, more conspicuous in the Chiltern area than on the North Downs, being traceable from near the Goring Gap to beyond Luton.

Doubts about this type of geomorphological evidence have been expressed in more recent accounts, however, both in the Chilterns (Pinchemel, 1954; Jones, 1974; Catt and Hodgson, 1976; Moffat, 1980; Moffat and Catt, 1986a, 1986b; Moffat et al., 1986) and south of the Thames (Docherty, 1967; John, 1980; Fisher, 1982). In particular, Moffat (1980; Moffat et al., 1986) noted that the alleged marine 'bench' on the Chiltern dip slope corresponds closely with monoclinal folding of the Chalk and suggested that the feature results from this geological structure rather than from marine erosion. Similar explanations of alleged Pliocene/Early Pleistocene 'benches' in north-west Kent (Docherty, 1967, 1971) and Surrey (John, 1980) have been put forward in recent years.

Moffat (1980; Moffat and Catt, 1983) concluded, on mineralogical grounds, that the lowest three units (beds 2–4) at Little Heath were in situ Reading Beds, differentiating them from the overlying gravel and sands on the same basis. He interpreted the upper gravel at Little Heath (bed 6) as a beach deposit, principally on sedimentary grounds, although the presence of c. 25% glauconite in the lens of sand within this unit (see above) provides strong evidence for a marine influence, as this mineral is formed only in marine conditions (Porrenga, 1967) and there is no obvious source from which it could be derived secondarily in such quantities. Moffat also considered the overlying sand to be of marine origin, the main evidence being the presence in it of c. 5% glauconite. He found no evidence of a glacial origin for the uppermost unbedded gravel (bed 8); the matrix of this unit was found to closely resemble the Reading Beds and to differ from the underlying sand and gravel (beds 5–7). Moffat therefore concluded that the deposit was a colluvial accumulation of bedrock and drift material, presumably derived from upslope areas now removed or modified by erosion.

Although concluding that the geomorphological case for a Pliocene shoreline on the Chilterns was unfounded, Moffat (1980; Moffat and Catt, 1983) supported Wooldridge's (1927a) interpretation of the Little Heath deposits as being of marine origin. He cited the similarity between the particle-size properties of this deposit and the Headley Formation of Surrey as supporting evidence, in addition to similarities in fine-sand mineralogy between both of the above and the Rothamsted Red Crag (Moffat, 1980; Moffat and Catt, 1983, 1986a).

Only one other high-level outlier was accepted by Moffat and Catt (1986a) as an in situ Pliocene or Early Pleistocene marine deposit. This is a remnant of Pebble Gravel (sensu lato) at Lane End [SU 817 917], c. 203 m O.D., above the quartzose Reading Beds and basal London Clay (White, 1906; Wooldridge and Gill, 1925; see above, Introduction to Part 1). This remnant was found to be similar to the Little Heath gravel in both its clast and heavy-mineral composition. It lies 28 km to the south-west of the Little Heath outlier and over 30 m higher. Moffat and Catt considered this difference in altitude to be the result of differential tectonic movement in south-eastern Britain since the

Pliocene (see below). Moffat (1986) demonstrated that the clast-size distribution of quartz pebbles in the two widely separated outliers was distinct from those in other high-level gravels, but similar to that from quartzose Reading Beds, suggesting derivation solely from that source (see Harrow Weald Common). Other gravel remnants at Gustardwood [TL 175 160] and Cowcroft [SP 983 017] were interpreted as possibly derived, at least in part, from earlier gravels of the Little Heath type (Moffat, 1980). Moffat considered the matrix of the gravel at Gustardwood to include Late Pleistocene loessic material, implying that the deposit in its present form originated comparatively recently, although in situ pedogenic mixing may have caused the introduction of much later (Upper Pleistocene) loess into a Pliocene or Lower Pleistocene gravel.

Moffat and Catt (1986a) demonstrated the degree of post-Pliocene tectonic activity using a projection of the basal surface of the Red Crag from ordnance datum in north-east Essex to almost 200 m O.D. in Surrey and south Buckinghamshire (Figure 3.5). They attributed the inclination of this surface to subsidence of the north-eastern part of the London Basin during the Pleistocene. It seems likely, however, that uplift of the western area may also have occurred, perhaps as an isostatic response to erosion (Chapter 1), as well as subsidence of the North Sea Basin. The outliers on the North Downs at Netley Heath and Headley Heath have been used to reconstruct this surface, as well as the residual blocks of fossiliferous Red Crag sand at Rothamsted (Figure 3.5). The Little Heath and Lane End deposits are at suitable elevations to suggest a correlation on this basis with the Red Crag, despite their lack of fossils (Moffat and Catt, 1986a).

However, as (Figure 3.6) shows, the relations between these two outliers, if they are contemporaneous, imply a north-eastward gradient comparable to that observed in the highest Thames terrace deposits. In the case of the latter, the gradient at least partly reflects an original downstream slope. A fluvial interpretation of the Lane End and Little Heath deposits, which would make them the earliest Neogene river deposits to have been preserved, must therefore be considered. As in the various types of Pebble Gravel, sublittoral/littoral characteristics such as clast shape may reflect derivation from older Palaeogene or Neogene strata. The clast-lithological composition of the gravel at Little Heath (above and (Table 3.2)) is very much what might be expected for the earliest fluviatile gravels of the Thames system. Thus interpretation as a fluvial aggradation would satisfy the limited evidence that exists for the origin of these deposits, with the possible exception of their high glauconite content at Little Heath. This suggests that it might be premature to consider the littoral origin of the gravel at Little Heath to be settled. The question of whether such remnants truly result from a marine episode is likely to remain controversial for the foreseeable future.

The Little Heath outlier thus represents one of only two gravel remnants that can be interpreted as littoral deposits of Late Pliocene or Early Pleistocene age, out of many such outliers once recognized. It is possible that this deposit broadly correlates with the Red Crag of East Anglia. The evolution of the London Basin at the time of the Pliocene/Lower Pleistocene transition is poorly understood, there being little remaining evidence from which to reconstruct the contemporary coastal or drainage lines. The very existence of high-level littoral deposits in situ is a subject of controversy. The GCR site at Little Heath provides an important facility for the study of this enigmatic phase in the early development of the Thames catchment.

Conclusions

The sequence of clay, gravel, sand and pebbly clay deposits overlying the Chalk at Little Heath has long been controversial; even today there is much uncertainty over the age and origin of these sediments. An early interpretation, formerly widely accepted and still with some advocates, held that the Little Heath deposits were part of a complex spread of sands and gravels deposited by a Pliocene sea that covered the London Basin and extended on to the North Downs and the Chiltems (around and before 2 million years ago). This view was supported by mineralogical analyses, which showed that comparable deposits extended over quite large areas. Others have suggested correlation with the Reading Beds (c. 60 million years old) or the marine Pliocene Red Crag of East Anglia. Recent work suggests that deposition by a river during the Pliocene or early Pleistocene is a further possibility, which would make the sediments at Little Heath the earliest known Neogene deposits of this type. The marine origin so widely suggested, however, cannot yet be discounted, so it is certain that the Little Heath site will remain controversial in the future.

(Table 3.2) Clast-lithological data (in percentage of total count) from the Middle Thames and Vale of St Albans (compiled from various sources). The data concentrates on key sites, GCR sites and localities mentioned in the text. Note that many different size ranges are included and that these yield strikingly different data (this can be observed where results from different fractions from the same deposits have been analysed). As in (Table 4.2), (Table 5.1) and (Table 5.3), the igneous category includes metamorphic rocks (very rarely encountered) and the quartzite category includes durable sandstones. The Tertiary flint category comprises rounded pebbles (sometimes subsequently broken) reworked from the Palaeogene (see glossary with (Table 4.2)).

References