DIFERENTES TIPOS DE COMPRESIÓN

The following section contains a description o f the revised British Quaternary stratigraphie scheme applied during this project, which is based on evidence Ifom the Oxygen Isotope Record and Thames terrace system. Distinctions between site groupings assigned to different climatic stages are mainly based on bio stratigraphie evidence.

Earlv Middle Pleistocene

A much greater complexity is now recognised in British early Middle

Pleistocene deposits in comparison to the single Cromerian Interglacial o f the previous

scheme (Preece & Parfitt 2000). The potential for additional interglacial stages was first brought to light when attempts at correlation between the British and the Dutch

sequences were made. In the Netherlands four separate interglacials have been recognised within the ‘Cromerian Complex’ (Zagwijn 1996). To which o f these interglacials does the Cromerian Interglacial relate or is the British Cromerian a composite grouping o f a number o f separate interglacial stages?

Interglacial I, the first o f the Dutch ‘Cromerian’ sequence displays reversed magnetic polarity. All British sites assigned to the Cromerian Interglacial have normal polarity, so correlation with Interglacial I is ruled out. However, this still leaves three temperate stages to account for in the British record. Initial ideas suggested that the Cromerian immediately preceded the Anglian and correlated with Dutch Interglacial IV (West 1980). The additional Dutch interglacials would therefore be placed in the major depositional hiatus in the British sequence between the Beestonian and the Cromerian (fig.2.1). However, evidence fi'om mammalian bio stratigraphy has thrown this

interpretation into doubt.

A mammalian biostratigraphic event crucial to the interpretation o f early Middle Pleistocene deposits is the transition fi'om the water vole Mimomys savini to

Arvicola terrestris cantiana (Koenigswald & Kolfschoten 1996). This is recognised by

an evolutionary change fi'om rooted to un-rooted, permanently growing cheek teeth. The majority o f sites traditionally assigned to the Cromerian Interglacial are

characterised by the early form Mimomys, while Arvicola is recorded fi'om Ostend, Norfolk in deposits assigned to the closing stages o f the Cromerian. The transition between the two forms was therefore initially thought to have occurred during the later part o f the Cromerian (Stuart & West 1976). However, the discovery o f two new British early Middle Pleistocene sites led to a reinterpretation o f this conclusion.

At Westbury-sub-Mendip, Somerset (Bishop 1982) and Boxgrove, West Sussex (Robert & Parfitt 1999), Arvicola is recorded in association with early Middle

Pleistocene species, but in full interglacial conditions. This suggests the existence o f separate early Middle Pleistocene interglacials, differentiated by the presence o f

Mimomys or Arvicola. At the type-site o f Dutch Interglacial IV at Noordbergum,

Arvicola is recorded (Koenigswald & Kolfschoten 1996), suggesting correlation o f the

British Mimomys faunas with earlier ‘Cromerian Complex’ interglacials. The situation is however further complicated by additional biostratigraphic evidence, which suggests that both the Mimomys and Arvicola sites in the British Isles represent multiple

The Mimomys sites o f West Runton and Pakefield/Kessingland can be divided using bio stratigraphy and palaeoenvironmental evidence (Stuart & Lister 2001). The deposits at West Runton are characterised by an absence o f thermophilous plant taxa and molluscs indicating continental conditions. Thermophiles such as Salvinia natans

are present at Pakefield/Kessingland, along with Palaeoloxodon antiquus.

Hippopotamus sp. and Megaloceros dawkinsi not recorded fi'om West Runton. P.

antiquus is a species generally associated with the late Middle Pleistocene and M.

dawkinsi is recorded fi'om later early Middle Pleistocene sites e.g. Boxgrove, which

would suggest a later date than West Runton. A separate temperate stage, slightly later than West Runton but still in the earlier part o f the Cromerian Complex is indicated.

Evidence is also emerging that at least two separate early Middle Pleistocene interglacials may contain Arvicola. This is based on sites fi'om Germany associated with the homblende-augite transition, Arvicola being recorded fi'om sites e.g. Karlich G and Miesenheim on both side o f the boundary (Koenigswald & Kolfschoten 1996; Kolfschoten & Turner 1996). This would indicate that Interglacials III and IV o f the Dutch sequence both belong in the Arvicola zone (Preece & Parfitt 2000). A second evolutionary transition in the narrow-skulled voles {Microtus gregaloides - Microtus

gregalis) can also be used to separate the two Arvicola temperate episodes

(Koenigswald & Kolfschoten 1996; Kolfschoten & Turner 1996). The ancestral form

M. gregaloides is recorded throughout the sequence at Westbury-sub-Mendip, while M.

gregalis occurs at Boxgrove (Preece & Parfitt 2000). This is a strong indication that the

Boxgrove fauna post-dates the entire Westbury sequence, and would tentatively suggest a correlation o f Westbury with Interglacial III and Boxgrove with Interglacial IV. Although, a correlation o f units 15/2+4 at Westbury with Boxgrove unit 4c has also been proposed (Schreve et al. 1999).

When all o f the evidence concerning the stratigraphie division o f the British early Middle Pleistocene is summarised a highly complex picture emerges (fig.2.4). There is sufficient climatic variability present in the Oxygen Isotope Record to account for this (Bassinot et al. 1994). N ot aU o f the identified groups may represent separate 01 Stages but perhaps some may be sub-stages o f the same numbered divisions (Stuart & Lister 2001). Divisions identified within the Westbury-sub-Mendip assemblage may relate to climatic variation at the interglacial sub-stage level (Schreve et al. 1999) Further evidence and testing is required to support the theories that have been set up (Preece & Parfitt 2000), but a reasonably clear stratigraphie picture emerges for the early Middle Pleistocene sites included in this study (section 2.2).

Augite Hornblende Arvicola Mimomys savini Brunhes + Matuyama NETHERLANDS Cromer IV (Nordbergum) + Cromer III (Rosmalen) + Cromer II (Westerhoven) + Cromer I (Waardenberg) GERMANY Miesenheim 1 Karlich G ENGLAND Anglian Glaciation Boxgrove + Westbury Cave +? Pakefield/Kessingland

West Runton UFWB +

I

I

I

Late Middle Pleistocene

A major turnover in the mammalian fauna o f the British Isles appears to have been associated with the Anglian glaciation. This results in late Middle Pleistocene faunas having a composition very different from the early Middle Pleistocene sites discussed. In terms o f the large mammal species examined here; C. lycanoides, U. deningeri, H. latidens, E. altidens, S. hundsheimensis, M. verticornis, M. savini. M.

dawkinsi and B. schoetensacki are restricted to the early Middle Pleistocene, while U.

arctos, S. kirchbergensis, S. hemitoechus, M. giganteus and B. primigenius first appear

in the British fauna after the Anglian. Important body size differences may also mark this transition (Bishop 1982; Parfitt 1999), and will be investigated further in this project.

As indicated by the Oxygen Isotope Record (section 2.1.2) and river terrace formations (section 2.1.3), three interglacial periods appear to have occurred during the late Middle Pleistocene. But only one interglacial dating to this period, the Hoxnian, has until recently been recognised in the British sequence. To which Isotope Stage does this interglacial relate (GI Stage 11, 9 or 7) or is it a composite o f multiple temperate periods? The type locality o f the Hoxnian at Hoxne, Suffolk is a kettle hole deposit in Anglian till (Gladfelter 1993), and no depositional break appears to occur between the tills and lake-fiU deposits. This would suggest that the Hoxnian represents the first post-Anglian interglacial in 01 Stage 11. The base o f the sequence at Swanscombe is also considered to represent the Anglian (Conway 1996). But more importantly the interglacial deposits from this site and Clacton are within the first post-diversion terrace o f the Thames-Medway (Bridgland 1994).

A correlation o f the Hoxnian with 01 Stage 11, the first post-Anglian interglacial would therefore seem secure, but evidence from the amino-acid geochronology dating method suggests an alternative interpretation (Bowen et al.

1989). Amino-acid molecules can exist as two stereo-isomers: D and L forms. In living tissues all proteins contain the L-form, but after death racemization and épimérisation occur until equilibrium between the two forms is reached. The D/L ratio o f biological material, especially foraminiferal tests and mollusc shells, can therefore be used to indicate the relative ages o f deposits by representing the time elapsed since death. Problems exist with the interpretation o f results from this method as diagenesis is sensitive to temperature and pH and may vary in rate between species (Sykes 1991).

A range o f British Pleistocene sites have been analysed by this method, same- age groupings have been produced and related to the stages o f the Oxygen Isotope

Record (Bowen et al. 1989). D-aUoisoleucine/L-isoleucine ratios indicate a division between Hoxnian sites. Swanscombe and Clacton correlate with 01 Stage 11, but lower ratios are produced from Hoxne indicating a correlation with the Stage 9 interglacial. This finding, if correct, has important consequences for a wide range o f stratigraphical interpretations, such as the unity o f the Hoxnian group o f sites and the positioning o f the Anglian glaciation in relation to the Oxygen Isotope Record. However, there is no biostratigraphical (Schreve 1997) or body size (section 5.7.1) evidence to support a division between the Swanscombe, Clacton and Hoxne sites.

The conclusions from the Bowen et al. (1989) study are frirther questioned by the finding that the method has produced apparently inaccurate results for this time range (> 200 kyr B.P.) (Bridgland 1994). The dating o f the Boxgrove fauna by amino- acid geochronology is also problematic (Roberts & Parfitt 1999). Amino-acid ratios indicate a Stage 11 age for the interglacial deposits, the same as Swanscombe and Clacton, but the Boxgrove fauna is unquestionably pre-Anglian in character and very distinct from Swanscombe and Clacton.

I f all Hoxnian sites can be assigned to Stage 11, is there any evidence o f the interglacials relating to 01 stages 9 and 7 in the British Isles? I f amino-acid analysis is unable to distinguish sites o f Stage 9 age accurately, then evidence from river terrace stratigraphy may provide evidence o f interglacial sites o f this age. In the Lower Thames system, the second post-diversion terrace (Corbets Tey (Taplow) Gravel Formation) can be considered to contain deposits o f the Stage 9 interglacial (Bridgland

1994). This would indicate that the mammalian fauna from Grays Thurrock, Essex is Stage 9 in age (Bridgland 1994). Biostratigraphical analysis has indicated that Stage 9 faunas can be differentiated from the preceeding Stage 11 interglacial using small mammal species (Schreve 1997). Among the larger mammals U. arctos replaces U.

spelaeus in Stage 9, while C. crocuta is recorded, which was absent from the Stage 11

interglacial.

A much larger body o f evidence is available concerning the existence o f an interglacial relating to 01 Stage 7 in the British Isles. The suggestion o f an unidentified interglacial pre-dating the Ipswichian was first put forward by Sutcliffe (1975, 1985, 1995a) on the basis o f mammalian biostratigraphic evidence. Two very different faunal groupings can be identified in post-Hoxnian temperate contexts and were initially placed into the Ipswichian Interglacial (Stuart 1982). The first grouping is the characteristic ‘hippo fauna’ which lacks evidence o f humans and E. ferus, and is

dififerent mammalian fauna, containing E. feru s and M primigenius o f ‘Ilford type’ but with no records o f D. dama, was considered to represent the later stages o f the

Ipswichian Interglacial.

This interpretation is however called into question by the fact that deposits containing the Mammuthus-Equus fauna have never been found directly overlying a hippo assemblage. Additionally the feasibility o f a major community turnover during an interglacial period has been questioned, a lowering o f sea level to allow immigration o f the mammoth fauna or Scottish réfugia having to be invoked. New evidence from river terrace stratigraphy (Bridgland 1994) and amino-acid dating techniques (Bowen et al. 1989) has resulted in a reassessment o f the age o f Ipswichian deposits.

Sites where the Mammuthus-Equus fauna is recorded, such as Ilford, Aveley, and West Thurrock, Essex, and Crayford and Northfleet, Kent are now interpreted to lie in a stratigraphically higher Thames terrace (Taplow/Mucking Gravel) than the hippo fauna o f Trafalgar Square (Bridgland 1994). Additionally, amino-acid ratios are consistently higher from sites containing a Mammuthus-Equus fauna e.g. Stanton Harcourt, Aveley, Stutton, Selsey, Crayford (mean D/L = 0.16), in comparison to hippo sites (mean D/L value 0.09) (Bowen et al. 1989). These ratios are correlated with Stage 7 o f the Oxygen Isotope Record. Therefore, two separate interglacials appear to be present in the traditional concept o f the Ipswichian, the mammoth-horse faunas representing an earlier interglacial period, which can be considered to be GI Stage 7.

Final support for this reinterpretation o f Ipswichian faunas comes from the site o f Marsworth, Buckinghamshire (Green et al. 1984). Here the Lower Channel fauna is o f characteristic Mammuthus-Equus type and is below further interglacial deposits containing hippopotamus; the two interglacials are separated by deposits indicating periglacial conditions. Travertine clasts from the M arsworth Lower Channel,

containing leaf impressions o f interglacial tree species, have been dated to 200-140 kyr B.P. using uranium-series methods (Green et al. 1984). This corresponds closely to the estimated dates for GI Stage 7 o f ca. 245-186 kyr B.P. (Martinson et al. 1987). A large number o f sites containing the characteristic Mammuthus-Equus fauna, from both open and cave depositional contexts, have now been correlated with GI Stage 7 on

biostratigraphic grounds (Schreve 1997).

Good evidence therefore exists for three late Middle Pleistocene interglacials in the British Isles, relating to GI Stages 1 1 ,9 and 7. But what evidence is there for the corresponding glacial periods, GI Stages 10, 8 and 6? A single glaciation, the

scheme o f Mitchell et al. (1973) (section 2.1.1). Reinterpretation o f the sites on which this glaciation was based now indicates that they may be Anglian in age (Rose 1987,

1991). Very little evidence o f late Middle Pleistocene glaciation is preserved in Britain, as ice extents appear to have been less than occurred in the Devensian, resulting in the destruction o f the majority o f the deposits (Schreve 1997).

The most reliable evidence appears to relate to the Stage 6 glaciation. Signs o f climatic deterioration are recorded above interglacial deposits newly assigned to Stage 7 e.g. Marsworth Lower Channel, Buckinghamshire (Green et al. 1984), Brighton- N orton raised beach (Keen 1995) and below Sub-Stage 5e raised beaches e.g. Bacon Hole, Gower (Stringer et al. 1986). A number o f mammalian biostratigraphic indicators o f 01 Stage 6 have been identified. These include small body size in E. fe ru s (Parfitt et al. 1988; Bates et al. 2000) and a large form o f northern vole Microtus oeconomus

(Sutcliffe et al. 1987; Schreve 1997; Bates et al. 2000). Stage 6 M. oeconomus also displays a distinct molar morphology (Green et al. 1984; Sutcliffe et al. 1987). By 01 Stage 6, the normal M. primigenius tooth morphology o f the Late Pleistocene has replaced the Tfiord type’ form o f the Stage 7 interglacial (Lister & Sher 2001).

Late Pleistocene

Radiometric dating o f speleothem deposits associated with hippo faunas; which include H. amphibius and D. dama but lack evidence o f E. ferus, M. primigenius and the mollusc Corbicula flum inalis and the activity o f humans (Gascoyne et al. 1981; McFarlane & Ford 1998), has securely correlated sites originally assigned to the beginning o f the Ipswichian with 01 Stage 5e. However, faunas previously placed at the end o f the Ipswichian have now been moved to an earlier stratigraphie position in 01 Stage 7. Can any deposits be assigned to the later sub-stages o f Stage 5? Good evidence for deposits relating to this time period comes fi'om the relatively long sequences recorded in the Gower Caves o f Bacon Hole and Minchin Hole.

Both o f these caves contain beach deposits that have been assigned to the high sea level stand o f Sub-Stage 5e (Sutcliffe et al. 1987). Above these ancient beaches interglacial faunas are recovered which can be considered to post-date 01 Stage 5e, but are overlain by speleothems dated to ca. 80 kyr B.P. (Stringer et al. 1986; Sutcliffe et al. 1987; Proctor 1994). This places the deposits in the time range o f Sub-Stages 5c-a. The faunas from post-Stage 5e contexts appear to lack H. amphibius and contain evidence

o f M. primigenius not present in Sub-Stage 5e (Stringer et al. 1986; Sutcliffe et al.

by the presence o f D. dama and absence o f E. feru s associated with the additional absence o f H. amphibius are Brentford, Hoe Grange and the Hyaena Stratum at Tom ew ton Cave.

The Oxygen Isotope Record reveals additional complexity in the Devensian glaciation, which spans three Isotope Stages. Biostratigraphical studies o f faunas pre­ dating the last glacial maximum (01 Stage 2) suggest the presence o f two groupings, which may relate to 01 Stages 4 and 3 (Currant & Jacobi 1997, 2001). The first fauna is low in diversity and is dominated by B. priscus and R. tarandus with a very large form

o f U. arctos; no evidence o f human activity can be found in these deposits. The best

example o f this type o f fauna can be found in the large collection o f material fi'om Banwell Cave, Somerset (Currant & Jacobi 1997, 2001).

This faunal grouping is considered to date to 01 Stage 4, an initial cold episode o f the Devensian, which included extensive glacial development (Lowe & Walker

1997). Uranium-series studies fi’om Stump Cross Cave, N orth Yorkshire, date the Banwell-type fauna to 83 ± 6 kyr B.P. (Sutcliffe et al. 1985). This would appear to place the apparently cold-adapted fauna in the later part o f 01 Stage 5, but an early Stage 4 age is within the error range o f the date (Currant & Jacobi 1997).

The second Devensian fauna is more diverse and is based on Coygan Cave, Dyfed or the Pin Hole Lower Fauna (Currant & Jacobi 1997, 2001). However, it is possible that this diversity results fi'om the combination o f distinct faunas, that relate to extremely rapid climatic fluctuations (A. Currant pers. comm.) A mix o f temperate and cold climate elements occurs, and a number o f species, including humans, absent fi'om the Stage 4 grouping, are recorded at these sites (Currant & Jacobi 1997, 2001). The Coygan/Pin Hole-type faunas are considered to represent 01 Stage 3, a relatively temperate period o f the Devensian.

GI Stage 3 covers the time period fi'om ca. 59-24 kyr B.P. (Martinson et al. 1987), so a large proportion o f faunas fi'om this time period would therefore be expected to be at the effective limits o f radiocarbon dating (five half-lives o f '"*C) (Lowe & Walker 1997). A combination o f radiocarbon and uranium-series dating suggests that Coygan Cave was occupied as a hyaena den fi’om 40 (if not 60) until 24 kyr B.P. (Aldhouse-Green et al. 1995), closely corresponding with 01 Stage 3. This finding is supported by dating o f the lower fauna fi’om Pin Hole, Derbyshire, which is also assigned to the Coygan/Pin Hole-type grouping (Currant & Jacobi 1997, 2001). The fauna appears to have accumulated fi’om 50-38 kyr B.P. (Jacobi et al. 1998). All

radiocarbon ages are quoted as ’"‘C years before present; a 10 kyr age is