Milton Formation

Substantial spreads of gravel and sands, underlying Anglian-age glacial deposits, have been described from the Northamptonshire district, most recently by Belshaw (Belshaw, Reference Belshaw1989, Reference Belshaw2007, unpublished; Belshaw et al., Reference Belshaw, Hackney, Smith, Langford and Briant2004, Reference Belshaw, Hackney and Smith2005, Reference Belshaw, Hackney and Smith2006; Barron et al., Reference Barron, Morigi and Reeves2006; Figs 1 and 3) and previously by Castleden (Reference Castleden1980), Clarke & Moczarski (Reference Clarke and Moczarski1982), Davey (Reference Davey1991) and Smith (Reference Smith1999). Their significance has not been fully considered in regional pre-glacial drainage reconstructions, as Belshaw (unpublished) stressed. The details presented herein incorporate his views, supplemented by his unpublished notes.

Fig. 3. Milton Formation sands and gravels at Hill Farm Quarry (for location see Fig. 1): a. looking towards the northeast; b. adjacent view towards the east; c. adjacent view towards the southeast–south (photographs by R.K. Belshaw, private collection).

The pebbly sands of Northamptonshire occur in a series of spreads that extend from the Watford Gap southeastwards to south of Northampton. They have been typically exposed in the village of Milton Malsor (National Grid Reference (NGR): SP 7355), and were investigated and described from workings at Hill Farm (NGR: SP703577: Fig. 3) by Belshaw and colleagues (2004, 2005), where they were up to 10 m thick, although they generally reach 5–7 m (Belshaw, Reference Belshaw1989). They continue to the south of Bedford, where they are mapped near Cardington (Barron et al., Reference Barron, Sumbler, Morigi, Reeves, Benham, Entwisle and Gale2010). This spread has been traced a distance of over 30 km. A second spread from beneath Rockingham Forest is also well-known (Judd, Reference Judd1875) and has been described in detail by the same authors. It was worked at Brigstock (NGR: SP558850) and appear to fill a buried network of drainage channels reported by Hollingworth and Taylor (Reference Hollingworth and Taylor1951) and Kellaway and Taylor (Reference Kellaway and Taylor1952).

The similarity of these pebbly sands (the clasts comprising local Jurassic lithologies, but the sands attributed to a Triassic source) has resulted in them being grouped together as the Milton Formation (Sinclair & Smith, in Maddy, Reference Maddy and Bowen1999). Their sedimentary structures indicate that they were laid down by braided streams flowing in a southeasterly direction predominantly in cold climates.

Exposures in the Milton Formation deposits are scarce, therefore a new cross-section along the A43 road south of the junction with the M1 motorway (Fig. 1; reconstructed from boreholes assembled from the BGS website: http://mapapps.bgs.ac.uk/GeoRecords/GeoRecords.html) provided a rare opportunity to examine the form of the sediment body. This section (Fig. 4) shows Milton Formation sands and gravel resting on Lower Jurassic (Lias) clay and underlying glacial diamicton (Lowestoft and possibly potentially the Wolston Formation), confirming previous authors' observations. Here the Milton Formation (Milton Malsor river) sequence fills a channel wider than 1 km, clearly trending in a southeastwards direction. The deposits are locally incised by a small stream, alluvial deposits filling a small cross-cutting valley.

Fig. 4. Cross-section along the A43 trunk road, south of the junction with the M1 motorway (Fig. 1), reconstructed from borehole records (source: http://mapapps.bgs.ac.uk/GeoRecords/GeoRecords.html), showing the channel-like distribution of the Milton Formation and its relation to the bedrock and overlying deposits.

The Milton Formation deposits of the Milton Malsor and Brigstock streams consistently indicate two sub-parallel drainage lines, aligned from WNW to ESE across Northamptonshire from close to the M1 motorway towards the southeast (Belshaw et al., 2004, 2005, 2006; Belshaw, Reference Belshaw2007, unpublished). The local lithology of the gravel-sized clasts contrasts with the long-distance origin of the sand fraction. Belshaw (2007, unpublished) was convinced that the lack of quartz and quartzite pebbles was significant in indicating the provenance of these deposits. However, the absence of these clasts could reflect that the rivers were reworking the sand facies of the Sherwood Sandstone Formation (Triassic) since this represents the closest source of the quartz-bearing sands. This observation, together with their altitudinal distribution, convinced Belshaw et al. (Reference Belshaw, Hackney, Smith, Langford and Briant2004, Reference Belshaw, Hackney and Smith2005, Reference Belshaw, Hackney and Smith2006) and Belshaw (2007, unpublished) that this indicates that the rivers must predate the initiation of the perpendicularly aligned proto-Soar/'Bytham river' in the Midlands region. Subsequently, the Milton Formation deposits were eroded during emplacement of overlying Lowestoft Formation (and potentially Wolston Formation) glacial deposits (Barron et al., Reference Barron, Sumbler, Morigi, Reeves, Benham, Entwisle and Gale2010).

In general there are limited indications of the age of these spreads, apart from them being sandwiched between the Jurassic bedrock beneath and the Lowestoft Formation till (Anglian Stage) above, implying that they are of pre-Anglian age. The only vertebrate fossils recovered include a few fragments of horse bone, a tooth and ‘three rotted tusks’ from Hill Farm; ‘none . . . allow an age determination’ (Belshaw, Reference Belshaw1989, p. 43), but the occurrence of horse, a steppe animal, supports the view that the river gravels and sands were laid down, as elsewhere in southern Britain, under cool-climate conditions.

In the 1990s, however, an exposure at Courteenhall Grange Farm, near Collingtree (NGR: SP7555: Figs 1 and 5), revealed fossiliferous fine-grained sediments up to 12 m thick filling a channel within the pebbly sands. Although this Courteenhall Member is thought to represent a separate Brigstock stream, its general setting and lithological similarity mean that it has been grouped with the Milton Formation. The fossil assemblages recovered include plant macrofossils of fruits, seeds and wood, and ostracods (Smith, Reference Smith1999; Smith et al., Reference Smith, Gillmore and Sinclair2000), which together indicate a fluvial interglacial floodplain channel–fill complex, accumulated under a temperate climate. Based on the fossil assemblages, Smith (Reference Smith1999) and Smith et al. (Reference Smith, Gillmore and Sinclair2000) correlated the interglacial deposits with the Cromerian Stage s.l. (i.e. early Middle Pleistocene). Assuming this age is correct, it confirms that the stream was flowing before the Anglian-Stage glaciation.

Fig. 5. Sediment sequences exposed at Courteenhall Grange Farm, near Collingtree, Northamptonshire (for location see Fig. 1), modified from Smith et al. (Reference Smith, Gillmore and Sinclair2000), showing the Milton Formation fluvial gravel and sands (including intercalated finer-grained Cromerian-age interglacial fluvial channel-fill units), unconformably overlain by Lowestoft Formation glacial diamicton (till).

Complex discussion of local details led Belshaw (Reference Belshaw207) to conclude that the Milton and Brigstock rivers originated during the early Pleistocene but continued into the early Middle Pleistocene based on drainage line relationships in the region, a view restated by Barron et al. (Reference Barron, Morigi and Reeves2006, Reference Barron, Sumbler, Morigi, Reeves, Benham, Entwisle and Gale2010). The principal reason for suggesting this age was the presence of the proto-Soar 'Bytham river' in the immediate pre-Anglian, as suggested by Rose and colleagues (Rose, Reference Rose1987, Reference Rose1994, Reference Rose2009; Lee et al., Reference Lee, Rose, Hamblin and Moorlock2004a,Reference Lee, Booth, Hamblin, Jarrow, Kessler, Moorlock, Morigi, Palmer, Pawley, Riding and Roseb; cf. above, Fig. 2). However, since it is now known that the latter cannot have existed until after the Anglian-Stage glaciation, based principally on the litho- and biostratigraphy in the Midlands and East Anglia, the history of the drainage alignments in the region is greatly simplified. It appears that the immediately pre-Anglian period, i.e. the local pre-glacial Pleistocene, was characterised by a drainage system that is aligned WNW to ESE, the rivers of the south Midlands flowing southeastwards potentially to join the Thames as left-bank tributaries, as discussed below. Upstream the Milton course appears to be aligned towards the Watford Gap, northwest of Northampton (Barron et al., Reference Barron, Morigi and Reeves2006; Fig. 1).

Letchworth Formation

A minor find of critical importance is the Letchworth Gravel (Figs 1 and 6). A small spread of sandy gravels (facies Gms) and pebbly sands was discovered capping a hill beneath Fairfield Hospital (now a residential development) immediately northwest of Letchworth in Hertfordshire (NGR: TL 205353) at c. 70–77 m OD during geological mapping by Hopson et al. (Reference Hopson, Aldiss and Smith1996). The gravels here are 3.5 m thick, but reach as much as 9 m thick further south. Their pebble assemblage includes abundant quartz and quartzite pebbles, together with some sandstone and local flint. This isolated outcrop rests on chalk bedrock and is overlain by the local Anglian-age Lowestoft Formation diamicton. It is therefore interpreted to represent an immediately pre-Anglian or possible early Anglian Thames left-bank tributary stream derived from the Midlands (the Midlands Triassic rocks being the source of the quartz and quartzites exotic to the Hertfordshire region; Hopson et al., Reference Hopson, Aldiss and Smith1996; Smith & Rose, Reference Smith and Rose1997; Fig. 2). Not unreasonably, the stream was assumed to be a Thames Kesgrave Formation equivalent (Hopson et al., Reference Hopson, Aldiss and Smith1996; Smith & Rose Reference Smith and Rose1997), derived from the area west of Leicester, judging from the broadly northwest to southeast sub-parallel alignment of regional pre-glacial drainage in southern Britain (Gibbard, Reference Gibbard1988). Whilst it is possible that it would broadly be aligned with the Milton Formation Brigstock course, it is more likely that it is the downstream equivalent of the Milton Malsor course (Fig. 7). Although this explanation is certainly reasonable given the lithologies and the unit's setting, Belshaw et al. (Reference Belshaw, Hackney and Smith2005) state emphatically that no upstream equivalent is known in Northamptonshire, so this spread partially remains enigmatic.

Fig. 6. Letchworth Formation: A. distribution map; B. cross-section along the north–south line shown in a, modified from Smith & Rose (Reference Smith and Rose1997).

Fig. 7. Reconstructed drainage alignments in southern central England during the pre-Anglian Middle Pleistocene, based on the interpretations presented in the text.

However, to have sourced the quartz and quartzite-bearing Triassic bedrock (Fig. 2), this stream would almost certainly have crossed, or at least come very close to, the proto-Soar/’Bytham river’ valley had the two rivers existed at the same time. Moreover, although the gradient of the Letchworth stream cannot be reliably determined, it would have crossed the proto-Soar valley at a level higher than that at which the Baginton Formation deposits occur in the Leicester area (c. 60 m OD: Rice, Reference Rice1968). The inevitable conclusion is that the Baginton Formation river could not have coexisted with the Letchworth stream, and therefore the former, since it is the larger river and potentially occurs at a lower level, must be younger – a conclusion also reached by Smith and Rose (Reference Smith and Rose1997). These authors attempt to explain this apparent conflict of evidence by invoking later beheading of the Letchworth Formation stream by a younger ‘Bytham river’ aligned from the southwest. Whilst this is possible, there is no evidence to support this assertion (since the latter has no long history in the Midlands area; Gibbard et al., Reference Gibbard, West, Boreham and Rolfe2012b; Belshaw, unpublished) and therefore it cannot be tested. In summary, this single Letchworth spread reinforces the view that the Midlands ‘Bytham river’ cannot have existed before the Anglian-Stage glaciation and indicates that the drainage in the Midlands region was aligned perpendicular to the later proto-Soar course.

The Letchworth Formation spread occurs immediately east of the Hitchin–Stevenage tunnel valley, a major landform that originated as an Anglian-age glacial meltwater conduit (Woodland, Reference Woodland1970; Gibbard, Reference Gibbard1974, Reference Gibbard1977; Hopson et al., Reference Hopson, Aldiss and Smith1996: Fig. 1). Such substantial valleys are generally regarded as having been superimposed on the landscape without necessarily requiring pre-existing valley alignments. However, the occurrence of the Letchworth deposits implies that a depression through the Chiltern Hills (Fig. 1) must have occurred along the broad NNW–SSE alignment to have allowed the river to continue towards its confluence with the River Thames in Hertfordshire (cf. Smith & Rose, Reference Smith and Rose1997). Indeed these authors concluded that altitudinal correlation of the Letchworth sediments with the youngest member (Westmill Lower Gravel) of the Thames Kesgrave Formation via this route to Ware implied that the unit was deposited just prior to the Anglian-Stage glaciation.

Palaeogeographical implications

In a recent compilation of the Cenozoic drainage history of the region, Gibbard and Lewin (Reference Gibbard and Lewin2003) concluded that the major river systems have remained broadly consistent in their alignments over much of the era. This interpretation, based on fluvial sedimentation and provenance, unequivocally demonstrated that the major elements (the Thames, Solent, Irish Sea river and possibly the early Trent river) existed throughout much of the Tertiary, i.e. for at least 55 Ma, and continued to flow throughout the pre-glacial Pleistocene (Gibbard, Reference Gibbard1988). This evidence indicates that the precursors of the major drainage lines of southern England were initiated on a southeast to eastwards-sloping land surface resulting from uplift in the west and northwest that began at the end of the Cretaceous and has continued intermittently throughout the Cenozoic (e.g. Cope, Reference Cope1994, Reference Cope1995). This gave rise to a predominant northwest to southeast alignment of major streams, i.e. parallel to the regional dip of the Mesozoic and Tertiary strata (Gibbard & Allen, Reference Gibbard and Allen1994; Boreham & Langford, Reference Boreham and Langford2006). In the light of this observation, it is striking that the ‘Bytham river’ (proto-Soar), as originally proposed, was aligned southwest to northeastwards across the Midlands, i.e. perpendicular to the regional dip, or parallel to the strike of the Mesozoic strata. The alignment of the 'Bytham river' perpendicular to the regional topographic slope is problematic for capture of the Thames headwaters, as Rose (Reference Rose1987, Reference Rose and Keen1989a, Reference Rose1994) has advocated. Indeed, the establishment of the proto-Soar following beheading and redirecting of the Thames waters (cf. Maddy, Reference Maddy and Bowen1999) as a consequence of substantial glaciation of the region is a more credible explanation.

The proto-Soar/'Bytham river' is much younger than the Anglian glaciation and this explains the apparent conflict of evidence for a pre-existing (i.e. pre-Anglian) drainage alignment of both major and minor streams, such as those represented by the Milton and Letchworth formation deposits. Judging from their projected courses, these streams were aligned broadly northwest to southeast from the south central Midlands to join the Thames as left-bank tributaries in Hertfordshire and Essex. Their courses would therefore have been aligned sub-parallel to the Ingham stream (Trent) in the east and the Thames' own headwaters in the west (Fig. 7).

The alignment of the Milton and Letchworth formation streams implies that their courses towards the Thames were adjusted to the regional geological dip on the Chalkland Chiltern Hills, their courses appearing to have been exploited by the later Anglian-age Hitchin–Stevenage and Cam–Stort tunnel valleys.

Additional evidence of a drainage pattern that evolved from ‘consequent’ streams to ‘subsequent’ streams is provided by the six other major valleys that cross the Chiltern cuesta in the extra-glacial area to the southwest of the Hitchin–Stevenage valley, at Luton, Dunstable, Dagnall, Tring, Wendover and Princes Risborough. Five of these ‘gaps’ are dry (‘wind gaps’), whereas the sixth, the easternmost one, is occupied by the River Lea, southeast of Luton (Fig. 1). The floors of the gaps decline in elevation southeast towards the Vale of St Albans and the London Basin lowlands (Jones, Reference Jones1981, pp. 34–35) and gravels occur within some of them, for example the Princes Risborough Sand and Gravel (Horton et al., Reference Horton, Sumbler, Cox and Ambrose1995). These authors attributed such gravels to deposition by rivers that flowed southeastwards through the Chalk cuesta. According to Sumbler (Reference Sumbler1995), these former ‘consequent’ rivers flowed down the Chalk dipslope towards the Thames; their valleys, now truncated by the Chiltern scarp, are higher than, and therefore pre-date, the drainage system of the River Thame, a ‘subsequent’ river which extends into the Vale of Aylesbury. This realignment, resulting from lowering of river base levels throughout the Thames' system catchment, could have occurred in response to river diversion following the opening of the Dover Strait. This evolution closely parallels the coeval response reported in the upper Scheldt basin in Flanders by Vandenberghe and De Smedt (Reference Vandenberghe and De Smedt1979).

As Belshaw et al. (Reference Belshaw, Hackney, Smith, Langford and Briant2004) noted, it is clear from the lithological composition of the Milton gravels (which lack exotic lithologies especially the quartz and quartzite assemblages that typify rivers that drained the Midlands' Triassic rocks) that the stream extended through the Watford Gap, potentially to the Rugby area. The stream must therefore have been reworking predominantly sand. Judging from the same criteria, however, the occurrence of frequent quartz pebbles in the Letchworth deposits, in contrast to the Milton Formation units, could reflect the fact that the stream was exploiting Triassic pebble-bed lithologies that had been unroofed in the Rugby area later than the spreads present in the Northampton district. If this is accepted, this implies that the Letchworth deposits are younger that those further to the north-west. The interglacial deposits identified at Courteenhall Farm indicate that the Brigstock stream was certainly flowing in the early Middle Pleistocene.

The implication of these conclusions therefore is that the main drainage alignment in southern England was dominated by northwest to southeast-aligned consequent streams until the Middle Pleistocene Anglian glaciation. This pattern continued that already established during the preceding late Tertiary (cf. Gibbard, Reference Gibbard1988; Gibbard & Allen, Reference Gibbard and Allen1994; Gibbard & Lewin, Reference Gibbard and Lewin2003), i.e. parallel to the regional tilting of the British landmass, as later concluded by Belshaw et al. (Reference Belshaw, Hackney and Smith2005).

This river system was effectively destroyed by regional (Anglian) glaciation in the Middle Pleistocene, a new system being established on the deglaciated terrains. The alignments of this new system were dictated, in part, by the glacial topography, as noted by Rose et al. (Reference Rose, Allen, Kemp, Whiteman, Owen and Boardman1985) in East Anglia. Subsequent topographic evolution has been controlled across the region by weathering, mass movement and fluvial erosion principally under periglacial and permafrost conditions. Under these conditions strike-orientated rivers, aligned along the vales of the region underlain by frost-susceptible Mesozoic clays, such as the recent Soar and Ouse, came to dominate the landscape following deglaciation (Belshaw, Reference Belshaw2007, unpublished; Murton & Belshaw, Reference Murton and Belshaw2011; Boreham & Langford, Reference Boreham and Langford2006). As elsewhere in lowland eastern England this must have begun forming in the latest, post-glacial Wolstonian (i.e. = Warthe Stadial) times (e.g. Gibbard et al., Reference Gibbard, West, Boreham and Rolfe2012a), judging from the topographic situation in which the last interglacial (Ipswichian/Eemian Stage) fluvial deposits occur and assuming that the Wolstonian glaciation is the equivalent of that during the Drenthe Stadial on the near Continent, as these authors demonstrate. The development continued throughout the cold phases of the Devensian (= Weichselian) Stage.