- Email: [email protected]
Stratigraphy and environmental significance of Pleistocene deposits at Fisherton, near Salisbury, Wiltshire
Stratigraphy and environmental significance of Pleistocene deposits at Fisherton, near Salisbury, Wiltshire
c.
P. Green, D. H. Keen, D. F. M. McGregor, J. E. Robinson and R. B. G. Williams GREEN, C. P., D. H. KEEN, D. F. M. McGREGOR, J. E. ROBINSON & R. B. G. WILLIAMS. 1983. Stratigraphy and environmental significance of Pleistocene deposits at Fisherton, near Salisbury, Wiltshire. Proc. Geoi. Ass. 94 (1), 17-22. Sediments and an associated molluscan and ostracod fauna were exposed in 1974 in a temporary excavation at Fisherton, near Salisbury. The sections showed a chalk rubble at the base, overlain by terrace deposits of the River Nadder, comprising fluvial gravels below, and sandy clays with colluvial affinities above, from which the fauna was collected. Both the sediments and the fauna suggest a cool climate origin for the Fisherton deposits. An age in the early part of the Devensian glacial stage is proposed. C. P. G. & D. F. M. M., Department of Geography, Bedford College, University of London, Regent's Park, London NWl 4NS D. H. K., Department of Geography, Coventry (Lanchester) Polytechnic, Priory Street, Coventry CVl5FB J. E. R., Department of Geology, University College, Gower Street, London WClE 6BT R. B. G. W., The Geography Laboratory, University of Sussex, Falmer, Brighton, Sussex BNl 9QN
1. INTRODUCTION The Fisherton locality yielded rich mammalian and molluscan faunas in the 19th century (Lyell, 1827; Prestwich & Brown, 1855; Blackmore, 1864; Evans, 1864; Blackmore, 1867; Tylor, 1869; Blackmore & Alston, 1874; Kennard & Woodward, 1901; Reid, 1903). Early investigations of the site are reviewed by Delair & Shackley (1979). The importance of the Fisherton site arose from the large amount of mammalian fossil material found there, and, as Reid (1903) observed, from the distinctly 'Arctic' character of the mammal fauna. Reid also recognised, however, that despite these 'Arctic' characteristics, 'the associated land and fresh-water mollusca ... are all living British forms'. The workings at Fisherton were abandoned by 1900 and the deposits remained largely inaccessible until 1974, when exposures described in this paper were visible in road works at the junction of the Devizes and Wilton roads (SU 137303). Figures 1 and 2 show the main relief features. To the south of Salisbury Railway Station, the ground rises from the floodplain of the Nadder, at 47 m, to the level of the Station, in a bluff 4 to 5 metres in height. To the east of the Station this bluff is dissected and less steep. The Station stands on a terrace that rises northward between 51 m and 53 m. This feature is evidently a facet of Reid's (1903) 'low terrace', and is referred to here as the Fisherton Terrace. The 1974 site lies between 50 m and 51 m on the degraded eastern margin of this terrace. To the north the terrace is bounded by a bluff about 2 m 17
Fig. 1. The Fisherton area. 1. Site of pit in 1878 O.S. plan - ?Futcher's. 2. Site of pit on 1878 O.S. plan?Harding's. 3. Approximate site of Baker's pit. 4. Approximate site of gaol (Lyell, 1827). 5. Old Manor Hospital and grounds (former Lunatic Asylum). 6. 1974 site. 7. Approximate site of cutting at Railway Station (Prestwich & Brown, 1855). 8. Railway Station. A,B,C: lines of profiles in Figure 2. Contours at one metre intervals.
high. Above this the ground rises gently between 54 m and 58 m. Several of the 19th century brickpits were on the upper part of this slope (Fig. 1).
c.
18
P. GREEN ET AL.
..............
a
.~
60
A""
,
100
50
I
",-'-,
I
Metres <, ..... _~
55
--, --
-''''---'---'''''''-c::..-:--J:: B
...................... <,
~-._=-.,.-~~=.::::;;.;-r----_ ~3: ~ ----i~::t~
!'_-. ·-·.. .
50
~
".
~
..
~
2
'._f
,,:
,~
8
--"",~
· · ·-·-·-A
'
R. Nadder <, _ -
1,........._-
45m.a.o.d.
Fig. 2. Profiles across the Fisherton area. 1. Approximate configuration of Harding's pit in the second half of the nineteenth century. 2. 1974 site. 3. Approximate position of cutting at Railway Station (Prestwich and Brown, 1855).
m
m
D
Brown clay
I'
:.! BuffsiltYClay lU/-,·i)i IGravel b :::.; ;;::·J Gre'tfgreen sands
g~9J,jShell y sand
Fig. 3. Parts of the 1974 sections, drawn from field sketches and photographs.
~W:':t Chalk rubble
19
PLEISTOCENE DEPOSITS AT FISHERTON
2. SEDIMENTS
(d) Stony clay
The deposits examined are divided here into four groups which are described in their inferred order of succession upwards. (a) Chalk mbble This deposit comprises poorly sorted angular and subangular chalk fragments, and broken but unrolled flints in a paste of chalk debris. In places this material occurs in crude beds alternating with beds of less compact chalk rubble containing sand and rolled flint. In the lower part of the rubble, thin seams of fine chalk gravel occur. The upper surface of the rubble is uneven so that chalk rubble may occupy most of the exposure, or elsewhere may disappear beneath the foot of the section (Fig. 3). (b) Gravel This is a coarse, iron-stained river gravel, composed largely of flint. In places it penetrates the chalk rubble in steep-sided pipes, but it is itself penetrated by the overlying sediments so that its thickness is variable. The contact with the chalk rubble is sharp but that with the overlying deposits is variable, and locally gradual. (c) Sands and loams These occur either on the gravel, or as isolated masses within it. Their texture is variable. The most common sediments are greenish-gray sands which may be succeeded upward by buff-coloured silty clays. Bedding is often preserved but its attitude is usually disturbed. Calcareous material is locally abundant. In sample Fl calcium carbonate was present as shell material and as continuous sheets on bedding planes, with tubular structures perpendicular to these sheets. Shells in sample F1 often occurred in small clusters. The larger Oxylomas were frequently found in twos and threes with a few small pebbles. Samples F1, F2 and F3 came from sediments on top of the gravels (Fig. 3). The F1 mass was coarse sand below, passing up into fine sand. The F2 and F3 masses resemble one another closely. They lack the high silt content associated with brickearth deposits, but could represent a mixture of alluvial sand, with silt and clay derived from backwater deposition or slopewash (Table 1).
The uppermost part of the sections was commonly occupied by a dark reddish-brown clay containing bleached angular flint fragments. This clay has a sharp lower boundary and is, in a few places, piped into the underlying sediments.
3. FAUNA (a) Mollusca The Mollusca were separated from the sediments using the procedure of Sparks (1961). Many shells are broken but there is no indication that they are reworked from pre-existing sediments. Some of the shells are so fragile that they are unlikely to have been transported for any great distance. The fauna (Table 2) is dominated by Pupilla muscorum (Linne), Oxyloma pfeifferi (Rossmassler) and TABLE 2. Fisherton: List of Mollusca Species Valvata piscinalis (Muller) Bithynia tentaculata (Linne) Lymnaea truncatula (Muller) Lymnaea peregra (Muller) Anisus leucostoma (Millet) Gyraulus laevis (Alder) Ancylus fiuviatilis Muller Oxyloma pfeifferi (Rossmassler) Cochlicopa lubrica (Muller) Vertigo pygmaea (Draparnaud) Pupilla muscorum (Linne) Vallonia costata (Muller) Vallonia pulchella (Muller) Vallonia excentrica Sterki Vallonia sp. Punctum pygmaeum (Draparnaud) Arion sp. Nesovitrea hammon is (Strom) Milax sp. Limax sp. Trichia hispida (Linne) Arianta arbustorum (Linne) d. Sphaerium corneum (Linne) Pisidium casertanum (Poli) Pisidium henslowanum (Sheppard) Pisidium nitidum Jenyns Pisidium moitessierianum Paladilhe Pisidium sp.
Stratum F1 6 2 2 6 12 5 2 232 0 0 505 5 2 1 13 2 granules 0 1 1 83 1 f 2 1 1 1 7
F2 0 0 0 0 34 0 0 24 0 1 15 0 0 0 0 0 0 0 0 0 5 0 2f
F3 2 2 1 0 8 0 1 31 5 5 410 1 6 20 0 17 0 4 8 24 107 2 2f 1 1
2
6
TABLE 1. Sediment texture (%) fine medium coarse clay silt sand sand sand sample 0-2 urn 2-63 ~m63-250 ~m 250-700 ~m 700-2000 ~m F2 F3
22.4 23.4
27.7 26.3
44.3 44.6
4.5 4.1
1.5 1.2
f = non-umbonal fragments; other bivalve counts are of umbos
The gastropod identifications for Fl are by R. B. G. Williams; those for F2 and F3 are by D. H. Keen. The bivalve identifications are all by D. T. Holyoak.
20
c.
P . GREEN E T A L.
Trichia hispida (Linne) which form 86 per cent of the Mollusca counted . These species are common in Pleistocene periglacial deposits in calcareou s areas , but are not found living together at the present time . Pupilla muscorum is a noted xerophile, while Trichia hispida favours comparatively moist habitat s such as damp meadows and marshes. Oxyloma pfeiff eri is normally a marshland and waterside species. The form present at Fisherton is the cold-climate variety known as O. pfeif eri schumacheri (see Spark s in Shotton, 1968, for a discussion). The association of the thre e species in periglacial deposits is often explained by the assumption that the Pupillas lived on the driet valley sides and that their shells were transported by slope processes onto the wetter valley floor, where they were incorporated in deposits containing the shells of species living there. Alternatively, it has been suggested (Kerney, Brown & Chandler , 1964) that in the Pleistocene P. muscorum may have been tolerant of marshy conditions. Vallon ia costata (Miiller) and V. excentrica Sterki are two other species at Fisherton that are now xerophiles. Both are common in dry , open habitats such as shortgrass downland and it is doubtful whether the y were ever tolerant of marshy conditions. The presence of relatively moist habitats is however indicated not only by Trichia hispida but also by Vallonia pulchella (Miiller), Punctum pygmaeum (Draparnaud), and Arianta arbustorum (Linne) . The fauna provides evidence of open countr y since the Vallonia species and Pupilla generally avoid any form of shading and Trichia, Punctum and A rianta often occur in open country, although they also frequent woods. The existence of aquatic habitats is indicated by twelve species. Lymnaea truncatula (Muller), A nisus leucostoma (Millet) and Pisidium casertanum (Poli) are in Sparks' (1961) 'slum' group and are cap able of living in muddy , poorly oxygenated pools prone to drying. Valvata piscinalis (Miiller), Bithynia tentaculata (Linne) , Ancylus fluviatilis Miiller , Gyraulus laevis (Alder) , Pisidium henslowanum (Shepp ard) and P. moitessierianum Paladilhe are in Sparks' moving-water group . The remaining three species L. peregra (Muller) , Spha erium corneum (Linne) , and Pisidium nitidum Jenyns inhabit all types of aquatic habitats except the worst slums. The slum group comprises 3.7 per cent of the total Mollusca (and 55.4 per cent of the aquatic species). The moving water group comprises only 1.4 per cent of the total Mollusca (and 20.9 per cent of the aquatic species). Interpretation of the slum group presents few problems. These species could have lived in ephemeral ponds on the floodplain during the accumulation of the sands and loams. High values in sample F2 for the typical slum species A. leucosto ma , the absence in this sample of moving-water species, and the relative scarcity of P. mus corum , are particularly suggestive of a small, short-lived and isolated water-body.
In view of the predominance of land Mollusca in samples F1 and F3, the presen ce of moving-water species requ ires explanation . Although the Mollusca come from valley floor sediments , the 1974 site is close to the valley side and may have been away from the main river channel at most times. The faun a may therefore be largely colluvial in origin. The preservation of the terrestrial shells in small cluster s suggests that they were carried onto the floodplain by surface wash. The presence of the moving-water species may indicate reworking of the colluvial depos its by river water during times of flood. The rarity of shells of these species suggests that flooding was infrequent. Alth ough the Fisherton fauna is interpreted as periglacial , all the taxa present are found today in lowland Engl and, with the exception of Oxyloma pfeifferi schumacheri. Six species (Gyraulus laevis, Ancylus fluviatilis, Vertigo pygmaea (Draparnaud), Vallon ia excentrica, Pisidium henslowanum and P. moitessierianum) do not extend north of the Arctic Circle in Europe , and Bithynia tentaculata and Trichia hispida scarcely do so . Present European distributions , however, are a notoriously imperfect guide to climatic tolerances. Gyraulus laevis, Pisidium henslowanum and Bithyn ia tentaculata are found in the Soviet Arctic (Zhadin, 1952) and are plainly very hard y species. Vertigo pygma ea and Pisidium moitesserianum occur in Siberia (Likharev and Rammel'meier , 1952; Zhadin, 1952) and are also quite hard y. Vallonia excentrica does not have a particularly northern or arctic range at the present day, but it was a common Late-glacial species in Britain and can be presumed to tolerate at least a moderate degree of cold. The presence of Trichia hispida at Fisherton may indicate that the climate was not fully glacial in character and was perhaps subject to some maritime influence. Kerney (1971) points out that the northern limit of Trichia in Europe appears to be climatically controll ed and its numbers in Devensian molluscan assemblages vary seemingly in accordanc e with the climatic conditions. In the Late Devensian it was a common taxon in Zones II and III, but it was mostly absent from Zone I when the climate was colder. If the evidence of Trichia is reliable , the climate at Fisherton was not one of extreme cold and may possibly have been interstadial in char acter. (b) Ostracoda
Samples Fl and F3 yielded a small number of ostracod valves. F1 contained : Ilyocypris gibba (Ramdohr) and juvenile moults of Candona . F3 yielded one specimen of Ilyo cypris and two valves each of Prionocypris zenkeri (Chyzer) , Tonnac ypris loessica Diebel & Pietrzeniuk and Amplocypris tonnensis Diebel & Pietrzeniuk . This assemblage indicates a cool or cold climate . I. gibba and P. ze nke ri are modern species which prefer cool temp erate water. T. loessica and A . tonne nsis are
PLEISTOCENE DEPOSITS AT FISHERTON
species (Diebel & Pietrzeniuk, 1975, 1978) recorded only once before, from an early Weichselian colluvial deposit at Burgtonna in Saxony. The colluvium there comprises reworked loess indicative of cool climatic conditions. At Burgtonna, I. gibba was also recorded, giving a further measure of correlation with the Fisherton assemblage. In terms of local ecology, I. gibba, a swimming species, suggests a slow-moving water body with some growth of weed. Candona has a preference for burrowing within the substrate, and its presence suggests soft bottom conditions. P. zenkeri is an active, swimming ostracod, often found now in shallow pools or ponds. As Tonnacypris and Amplocypris are only known as fossils, precise ecological data cannot be derived. For Burgtonna however, Diebel & Pietrzeniuk visualised 'shallow standing water or small ponds, with muddy bottom conditions, which may from time to time have dried out, or been enlarged' (1978, p. 211). The ecological conclusion from the 1974 faunas is that the sands and loams were deposited under cool climatic conditions, on open ground, probably a floodplain surface, perhaps with a few trees or shrubs but no woodland. Although the deposits are related to nearby fluvial action, the probable habitat for the aquatic species is small pools of standing water, or minor channels carrying surface wash from neighbouring slopes onto the floodplain. 4. DISCUSSION (a) Stratigraphy Figures 1 and 2 show that the 1974 site exposed sediments underlying the Fisherton Terrace. Prestwich & Brown (1855) described similar sediments resting directly on the Chalk near the Railway Station. Their account and the sediments in the 1974 site show that the deposits underlying the Fisherton Terrace are different from those recorded in the brickpits. All the latter records indicate regular bedding inclined downslope broadly in sympathy with the ground surface. Tylor (1869) shows the bedding steepening near the valley side, and both he and Blackmore (n.d.) confirm that the deposits reach their maximum thickness (approximately 9.1 m) in that situation. Figure 2 shows that Harding's pit extended downward to a level approximately 5 metres below the surface of the Fisherton Terrace, and that the regularly-bedded deposits recorded in the pit occur at the same level as the disturbed sediments in the Fisherton Terrace. (b) Early Faunal Records Tylor (1869) and Blackmore (Delair & Shackley, 1979) both make it clear that two shelly horizons were
21
present in the brickpits at Fisherton. The term 'Pupa bed' is used for the upper horizon, and Tylor recorded only seven species from it. He described the lower level as 'the great shell bed' and mentions the recovery of 31 species by Prestwich & Brown. Delair & Shackley (1979) present a summary of the early faunal lists. Only three species recorded by them as having been plentiful or numerous were not well represented in the present study. One, 'Helix arbustorum' (Arianta arbustorum) was found in small quantities, while 'Helix nemoralis' (Cepaea nemoralis Linne» and '20nites fulvus' (Euconulus [ulvus (Miiller)) were absent. These larger species may have been over-represented in early records by the selection of single shells from open sections. Although nothing precise can be learned from the early faunal lists, there is no indication in them of environmental conditions different from those suggested by the 1974 material. (c) Dating The age of the Fisherton fauna cannot be demonstrated conclusively. However, the similarity of the ostracod assemblage to that of Diebel & Pietrzeniuk (1978) in an early Weichselian deposit suggests an early Devensian age. The occurrence of the mollusc Trichia hispida suggests that the climate was not fully glacial and might even have been interstadial in character, while the lack of such typical Late Glacial molluscs as Abida secale (Draparnaud) and Helicella itala (Linne) suggests that the Fisherton deposits are not of that age. Devensian Mollusca are however poorly known before the Late Glacial (Kerney, 1977) and it is impossible at present to refer the site on molluscan grounds to a particular period within the early or middle Devensian.
5. CONCLUSION The 1974 sections and the fauna from them clarify the stratigraphy of the Fisherton locality. The deposits form part of a river terrace. The chalk rubble and coarse river gravel in the lower part of the succession suggest climatic conditions of periglacial severity. The overlying sandy deposits and their fauna suggest an amelioration of climate towards cool interstadial conditions. A date early in the Devensian seems most likely for the fauna. The relative positions of the terrace deposits and the Fisherton brickearths suggest that they form parts of a single depositional accumulation. In addition, the similarity of the 1974 fauna to faunas reported from the brickearths in the 19th century suggests that the two deposits are of similar age.
22
c.
P. GREEN ET AL.
References BLACKMORE, H. P. 1864. Remains of birds' eggs found at Fisherton, Near Salisbury. Edinburgh New Philosophical Journal. N.S. 19, 74-5. - - . 1867. On the recent discovery of flint implements in the drift of the valley of the Avon. Wilts. Archaeol. Nat. Hist. Mag., 10,221-33. - - & E. R. ALSTON. 1874. On fossil Arvicolidae. Proc. Zool. Soc. Lond., 460--71. - - , n.d. 'Locked notebook'. MS in Salisbury Museum. DELAIR, J. B. & M. L. SHACKLEY. 1979. The Fisherton brickpits; their stratigraphy and fossil contents. Wilts. Archaeol. Nat. Hist. Mag., 74, 1-16. DIEBEL, K. & E. PIETRZENIUK. 1975. Neue Ostrakoden aus dem Pleistozan von Burgtonna (Bezirk Erfurt). Zeit. geol. wiss. Berlin., 3, 87-97. - - & - - . 1978. Die Ostrakodenfauna aus dem Jungpleistozanen (Weichseklaltzeitlichen) Deckschichten von Burgtonna in Thiiringen. Quatiirpalaontologie, 3, 207-21. EVANS, J. 1864. On some recent discoveries of flint implements in drift deposits in Rants and Wilts. Q. JI geol. Soc. Lond., 20,188-99. KENNARD, A. S. & B. B. WOODWARD. 1901. The post-Pliocene non-marine mollusca of the south of England. Proc. Geol. Ass., 17, 213-60. KERNEY, M. P. 1971. A Middle Weichselian deposit at Railing, Kent. Proc. Geol. Ass., 82, 1-11. - - . 1977. British Quaternary non-marine mollusca: a brief review. In (ed., F. W. Shotton), British Quaternary studies: recent advances, Oxford, 31--42.
lusca: a brief review. In (ed., F. W. Shotton), British Quaternary studies: recent advances, Oxford. 31-42. - , E. H. BROWN & T. J. CHANDLER. 1964. The Late-Glacial and Post-Glacial history of the Chalk escarpment near Brook, Kent. Phil. Trans. R. Soc. Lond., B248, 135-204. LIKHAREV, 1. M. and E. S. RAMMEL' MEIER, 1952. Terrestrial mollusks of the fauna of the U.S.S.R. Translated from Russian. Israel Program for Scientific Translativns, Jerusalem, 1962. LYELL, C. 1827. On some fossil bones of the elephant and other animals, found near Salisbury. Proc. Geol. Soc. Lond., 1, 25-26. PRESTWICH, J. & J. BROWN. 1855. On a fossiliferous drift near Salisbury. Q. JI geol. Soc. Lond., 11, 101-107. REID, C. 1903. The geology of the country around Salisbury. Mem. Geol. Surv. G.B. SHOTTON, F. W. 1968. The Pleistocene succession around Brandon, Warwickshire. Phil. Trans. R. Soc. Lond., B254, 387--400. SPARKS, B. W. 1961. The ecological interpretation of Quaternary non-marine mollusca. Proc. Linn. Soc. Lond., 172,71-80. TYLOR, A. 1869. On Quaternary gravels. Q. JI geol. Soc. Lond., 25, 57-100. ZHADIN, V. 1. 1952. Mollusks of fresh and brackish waters of the U.S.S.R. Translated from Russian. Israel Program from Scientific Translations, Jerusalem, 1965.
c.
P. Green, D. H. Keen, D. F. M. McGregor, J. E. Robinson and R. B. G. Williams GREEN, C. P., D. H. KEEN, D. F. M. McGREGOR, J. E. ROBINSON & R. B. G. WILLIAMS. 1983. Stratigraphy and environmental significance of Pleistocene deposits at Fisherton, near Salisbury, Wiltshire. Proc. Geoi. Ass. 94 (1), 17-22. Sediments and an associated molluscan and ostracod fauna were exposed in 1974 in a temporary excavation at Fisherton, near Salisbury. The sections showed a chalk rubble at the base, overlain by terrace deposits of the River Nadder, comprising fluvial gravels below, and sandy clays with colluvial affinities above, from which the fauna was collected. Both the sediments and the fauna suggest a cool climate origin for the Fisherton deposits. An age in the early part of the Devensian glacial stage is proposed. C. P. G. & D. F. M. M., Department of Geography, Bedford College, University of London, Regent's Park, London NWl 4NS D. H. K., Department of Geography, Coventry (Lanchester) Polytechnic, Priory Street, Coventry CVl5FB J. E. R., Department of Geology, University College, Gower Street, London WClE 6BT R. B. G. W., The Geography Laboratory, University of Sussex, Falmer, Brighton, Sussex BNl 9QN
1. INTRODUCTION The Fisherton locality yielded rich mammalian and molluscan faunas in the 19th century (Lyell, 1827; Prestwich & Brown, 1855; Blackmore, 1864; Evans, 1864; Blackmore, 1867; Tylor, 1869; Blackmore & Alston, 1874; Kennard & Woodward, 1901; Reid, 1903). Early investigations of the site are reviewed by Delair & Shackley (1979). The importance of the Fisherton site arose from the large amount of mammalian fossil material found there, and, as Reid (1903) observed, from the distinctly 'Arctic' character of the mammal fauna. Reid also recognised, however, that despite these 'Arctic' characteristics, 'the associated land and fresh-water mollusca ... are all living British forms'. The workings at Fisherton were abandoned by 1900 and the deposits remained largely inaccessible until 1974, when exposures described in this paper were visible in road works at the junction of the Devizes and Wilton roads (SU 137303). Figures 1 and 2 show the main relief features. To the south of Salisbury Railway Station, the ground rises from the floodplain of the Nadder, at 47 m, to the level of the Station, in a bluff 4 to 5 metres in height. To the east of the Station this bluff is dissected and less steep. The Station stands on a terrace that rises northward between 51 m and 53 m. This feature is evidently a facet of Reid's (1903) 'low terrace', and is referred to here as the Fisherton Terrace. The 1974 site lies between 50 m and 51 m on the degraded eastern margin of this terrace. To the north the terrace is bounded by a bluff about 2 m 17
Fig. 1. The Fisherton area. 1. Site of pit in 1878 O.S. plan - ?Futcher's. 2. Site of pit on 1878 O.S. plan?Harding's. 3. Approximate site of Baker's pit. 4. Approximate site of gaol (Lyell, 1827). 5. Old Manor Hospital and grounds (former Lunatic Asylum). 6. 1974 site. 7. Approximate site of cutting at Railway Station (Prestwich & Brown, 1855). 8. Railway Station. A,B,C: lines of profiles in Figure 2. Contours at one metre intervals.
high. Above this the ground rises gently between 54 m and 58 m. Several of the 19th century brickpits were on the upper part of this slope (Fig. 1).
c.
18
P. GREEN ET AL.
..............
a
.~
60
A""
,
100
50
I
",-'-,
I
Metres <, ..... _~
55
--, --
-''''---'---'''''''-c::..-:--J:: B
...................... <,
~-._=-.,.-~~=.::::;;.;-r----_ ~3: ~ ----i~::t~
!'_-. ·-·.. .
50
~
".
~
..
~
2
'._f
,,:
,~
8
--"",~
· · ·-·-·-A
'
R. Nadder <, _ -
1,........._-
45m.a.o.d.
Fig. 2. Profiles across the Fisherton area. 1. Approximate configuration of Harding's pit in the second half of the nineteenth century. 2. 1974 site. 3. Approximate position of cutting at Railway Station (Prestwich and Brown, 1855).
m
m
D
Brown clay
I'
:.! BuffsiltYClay lU/-,·i)i IGravel b :::.; ;;::·J Gre'tfgreen sands
g~9J,jShell y sand
Fig. 3. Parts of the 1974 sections, drawn from field sketches and photographs.
~W:':t Chalk rubble
19
PLEISTOCENE DEPOSITS AT FISHERTON
2. SEDIMENTS
(d) Stony clay
The deposits examined are divided here into four groups which are described in their inferred order of succession upwards. (a) Chalk mbble This deposit comprises poorly sorted angular and subangular chalk fragments, and broken but unrolled flints in a paste of chalk debris. In places this material occurs in crude beds alternating with beds of less compact chalk rubble containing sand and rolled flint. In the lower part of the rubble, thin seams of fine chalk gravel occur. The upper surface of the rubble is uneven so that chalk rubble may occupy most of the exposure, or elsewhere may disappear beneath the foot of the section (Fig. 3). (b) Gravel This is a coarse, iron-stained river gravel, composed largely of flint. In places it penetrates the chalk rubble in steep-sided pipes, but it is itself penetrated by the overlying sediments so that its thickness is variable. The contact with the chalk rubble is sharp but that with the overlying deposits is variable, and locally gradual. (c) Sands and loams These occur either on the gravel, or as isolated masses within it. Their texture is variable. The most common sediments are greenish-gray sands which may be succeeded upward by buff-coloured silty clays. Bedding is often preserved but its attitude is usually disturbed. Calcareous material is locally abundant. In sample Fl calcium carbonate was present as shell material and as continuous sheets on bedding planes, with tubular structures perpendicular to these sheets. Shells in sample F1 often occurred in small clusters. The larger Oxylomas were frequently found in twos and threes with a few small pebbles. Samples F1, F2 and F3 came from sediments on top of the gravels (Fig. 3). The F1 mass was coarse sand below, passing up into fine sand. The F2 and F3 masses resemble one another closely. They lack the high silt content associated with brickearth deposits, but could represent a mixture of alluvial sand, with silt and clay derived from backwater deposition or slopewash (Table 1).
The uppermost part of the sections was commonly occupied by a dark reddish-brown clay containing bleached angular flint fragments. This clay has a sharp lower boundary and is, in a few places, piped into the underlying sediments.
3. FAUNA (a) Mollusca The Mollusca were separated from the sediments using the procedure of Sparks (1961). Many shells are broken but there is no indication that they are reworked from pre-existing sediments. Some of the shells are so fragile that they are unlikely to have been transported for any great distance. The fauna (Table 2) is dominated by Pupilla muscorum (Linne), Oxyloma pfeifferi (Rossmassler) and TABLE 2. Fisherton: List of Mollusca Species Valvata piscinalis (Muller) Bithynia tentaculata (Linne) Lymnaea truncatula (Muller) Lymnaea peregra (Muller) Anisus leucostoma (Millet) Gyraulus laevis (Alder) Ancylus fiuviatilis Muller Oxyloma pfeifferi (Rossmassler) Cochlicopa lubrica (Muller) Vertigo pygmaea (Draparnaud) Pupilla muscorum (Linne) Vallonia costata (Muller) Vallonia pulchella (Muller) Vallonia excentrica Sterki Vallonia sp. Punctum pygmaeum (Draparnaud) Arion sp. Nesovitrea hammon is (Strom) Milax sp. Limax sp. Trichia hispida (Linne) Arianta arbustorum (Linne) d. Sphaerium corneum (Linne) Pisidium casertanum (Poli) Pisidium henslowanum (Sheppard) Pisidium nitidum Jenyns Pisidium moitessierianum Paladilhe Pisidium sp.
Stratum F1 6 2 2 6 12 5 2 232 0 0 505 5 2 1 13 2 granules 0 1 1 83 1 f 2 1 1 1 7
F2 0 0 0 0 34 0 0 24 0 1 15 0 0 0 0 0 0 0 0 0 5 0 2f
F3 2 2 1 0 8 0 1 31 5 5 410 1 6 20 0 17 0 4 8 24 107 2 2f 1 1
2
6
TABLE 1. Sediment texture (%) fine medium coarse clay silt sand sand sand sample 0-2 urn 2-63 ~m63-250 ~m 250-700 ~m 700-2000 ~m F2 F3
22.4 23.4
27.7 26.3
44.3 44.6
4.5 4.1
1.5 1.2
f = non-umbonal fragments; other bivalve counts are of umbos
The gastropod identifications for Fl are by R. B. G. Williams; those for F2 and F3 are by D. H. Keen. The bivalve identifications are all by D. T. Holyoak.
20
c.
P . GREEN E T A L.
Trichia hispida (Linne) which form 86 per cent of the Mollusca counted . These species are common in Pleistocene periglacial deposits in calcareou s areas , but are not found living together at the present time . Pupilla muscorum is a noted xerophile, while Trichia hispida favours comparatively moist habitat s such as damp meadows and marshes. Oxyloma pfeiff eri is normally a marshland and waterside species. The form present at Fisherton is the cold-climate variety known as O. pfeif eri schumacheri (see Spark s in Shotton, 1968, for a discussion). The association of the thre e species in periglacial deposits is often explained by the assumption that the Pupillas lived on the driet valley sides and that their shells were transported by slope processes onto the wetter valley floor, where they were incorporated in deposits containing the shells of species living there. Alternatively, it has been suggested (Kerney, Brown & Chandler , 1964) that in the Pleistocene P. muscorum may have been tolerant of marshy conditions. Vallon ia costata (Miiller) and V. excentrica Sterki are two other species at Fisherton that are now xerophiles. Both are common in dry , open habitats such as shortgrass downland and it is doubtful whether the y were ever tolerant of marshy conditions. The presence of relatively moist habitats is however indicated not only by Trichia hispida but also by Vallonia pulchella (Miiller), Punctum pygmaeum (Draparnaud), and Arianta arbustorum (Linne) . The fauna provides evidence of open countr y since the Vallonia species and Pupilla generally avoid any form of shading and Trichia, Punctum and A rianta often occur in open country, although they also frequent woods. The existence of aquatic habitats is indicated by twelve species. Lymnaea truncatula (Muller), A nisus leucostoma (Millet) and Pisidium casertanum (Poli) are in Sparks' (1961) 'slum' group and are cap able of living in muddy , poorly oxygenated pools prone to drying. Valvata piscinalis (Miiller), Bithynia tentaculata (Linne) , Ancylus fluviatilis Miiller , Gyraulus laevis (Alder) , Pisidium henslowanum (Shepp ard) and P. moitessierianum Paladilhe are in Sparks' moving-water group . The remaining three species L. peregra (Muller) , Spha erium corneum (Linne) , and Pisidium nitidum Jenyns inhabit all types of aquatic habitats except the worst slums. The slum group comprises 3.7 per cent of the total Mollusca (and 55.4 per cent of the aquatic species). The moving water group comprises only 1.4 per cent of the total Mollusca (and 20.9 per cent of the aquatic species). Interpretation of the slum group presents few problems. These species could have lived in ephemeral ponds on the floodplain during the accumulation of the sands and loams. High values in sample F2 for the typical slum species A. leucosto ma , the absence in this sample of moving-water species, and the relative scarcity of P. mus corum , are particularly suggestive of a small, short-lived and isolated water-body.
In view of the predominance of land Mollusca in samples F1 and F3, the presen ce of moving-water species requ ires explanation . Although the Mollusca come from valley floor sediments , the 1974 site is close to the valley side and may have been away from the main river channel at most times. The faun a may therefore be largely colluvial in origin. The preservation of the terrestrial shells in small cluster s suggests that they were carried onto the floodplain by surface wash. The presence of the moving-water species may indicate reworking of the colluvial depos its by river water during times of flood. The rarity of shells of these species suggests that flooding was infrequent. Alth ough the Fisherton fauna is interpreted as periglacial , all the taxa present are found today in lowland Engl and, with the exception of Oxyloma pfeifferi schumacheri. Six species (Gyraulus laevis, Ancylus fluviatilis, Vertigo pygmaea (Draparnaud), Vallon ia excentrica, Pisidium henslowanum and P. moitessierianum) do not extend north of the Arctic Circle in Europe , and Bithynia tentaculata and Trichia hispida scarcely do so . Present European distributions , however, are a notoriously imperfect guide to climatic tolerances. Gyraulus laevis, Pisidium henslowanum and Bithyn ia tentaculata are found in the Soviet Arctic (Zhadin, 1952) and are plainly very hard y species. Vertigo pygma ea and Pisidium moitesserianum occur in Siberia (Likharev and Rammel'meier , 1952; Zhadin, 1952) and are also quite hard y. Vallonia excentrica does not have a particularly northern or arctic range at the present day, but it was a common Late-glacial species in Britain and can be presumed to tolerate at least a moderate degree of cold. The presence of Trichia hispida at Fisherton may indicate that the climate was not fully glacial in character and was perhaps subject to some maritime influence. Kerney (1971) points out that the northern limit of Trichia in Europe appears to be climatically controll ed and its numbers in Devensian molluscan assemblages vary seemingly in accordanc e with the climatic conditions. In the Late Devensian it was a common taxon in Zones II and III, but it was mostly absent from Zone I when the climate was colder. If the evidence of Trichia is reliable , the climate at Fisherton was not one of extreme cold and may possibly have been interstadial in char acter. (b) Ostracoda
Samples Fl and F3 yielded a small number of ostracod valves. F1 contained : Ilyocypris gibba (Ramdohr) and juvenile moults of Candona . F3 yielded one specimen of Ilyo cypris and two valves each of Prionocypris zenkeri (Chyzer) , Tonnac ypris loessica Diebel & Pietrzeniuk and Amplocypris tonnensis Diebel & Pietrzeniuk . This assemblage indicates a cool or cold climate . I. gibba and P. ze nke ri are modern species which prefer cool temp erate water. T. loessica and A . tonne nsis are
PLEISTOCENE DEPOSITS AT FISHERTON
species (Diebel & Pietrzeniuk, 1975, 1978) recorded only once before, from an early Weichselian colluvial deposit at Burgtonna in Saxony. The colluvium there comprises reworked loess indicative of cool climatic conditions. At Burgtonna, I. gibba was also recorded, giving a further measure of correlation with the Fisherton assemblage. In terms of local ecology, I. gibba, a swimming species, suggests a slow-moving water body with some growth of weed. Candona has a preference for burrowing within the substrate, and its presence suggests soft bottom conditions. P. zenkeri is an active, swimming ostracod, often found now in shallow pools or ponds. As Tonnacypris and Amplocypris are only known as fossils, precise ecological data cannot be derived. For Burgtonna however, Diebel & Pietrzeniuk visualised 'shallow standing water or small ponds, with muddy bottom conditions, which may from time to time have dried out, or been enlarged' (1978, p. 211). The ecological conclusion from the 1974 faunas is that the sands and loams were deposited under cool climatic conditions, on open ground, probably a floodplain surface, perhaps with a few trees or shrubs but no woodland. Although the deposits are related to nearby fluvial action, the probable habitat for the aquatic species is small pools of standing water, or minor channels carrying surface wash from neighbouring slopes onto the floodplain. 4. DISCUSSION (a) Stratigraphy Figures 1 and 2 show that the 1974 site exposed sediments underlying the Fisherton Terrace. Prestwich & Brown (1855) described similar sediments resting directly on the Chalk near the Railway Station. Their account and the sediments in the 1974 site show that the deposits underlying the Fisherton Terrace are different from those recorded in the brickpits. All the latter records indicate regular bedding inclined downslope broadly in sympathy with the ground surface. Tylor (1869) shows the bedding steepening near the valley side, and both he and Blackmore (n.d.) confirm that the deposits reach their maximum thickness (approximately 9.1 m) in that situation. Figure 2 shows that Harding's pit extended downward to a level approximately 5 metres below the surface of the Fisherton Terrace, and that the regularly-bedded deposits recorded in the pit occur at the same level as the disturbed sediments in the Fisherton Terrace. (b) Early Faunal Records Tylor (1869) and Blackmore (Delair & Shackley, 1979) both make it clear that two shelly horizons were
21
present in the brickpits at Fisherton. The term 'Pupa bed' is used for the upper horizon, and Tylor recorded only seven species from it. He described the lower level as 'the great shell bed' and mentions the recovery of 31 species by Prestwich & Brown. Delair & Shackley (1979) present a summary of the early faunal lists. Only three species recorded by them as having been plentiful or numerous were not well represented in the present study. One, 'Helix arbustorum' (Arianta arbustorum) was found in small quantities, while 'Helix nemoralis' (Cepaea nemoralis Linne» and '20nites fulvus' (Euconulus [ulvus (Miiller)) were absent. These larger species may have been over-represented in early records by the selection of single shells from open sections. Although nothing precise can be learned from the early faunal lists, there is no indication in them of environmental conditions different from those suggested by the 1974 material. (c) Dating The age of the Fisherton fauna cannot be demonstrated conclusively. However, the similarity of the ostracod assemblage to that of Diebel & Pietrzeniuk (1978) in an early Weichselian deposit suggests an early Devensian age. The occurrence of the mollusc Trichia hispida suggests that the climate was not fully glacial and might even have been interstadial in character, while the lack of such typical Late Glacial molluscs as Abida secale (Draparnaud) and Helicella itala (Linne) suggests that the Fisherton deposits are not of that age. Devensian Mollusca are however poorly known before the Late Glacial (Kerney, 1977) and it is impossible at present to refer the site on molluscan grounds to a particular period within the early or middle Devensian.
5. CONCLUSION The 1974 sections and the fauna from them clarify the stratigraphy of the Fisherton locality. The deposits form part of a river terrace. The chalk rubble and coarse river gravel in the lower part of the succession suggest climatic conditions of periglacial severity. The overlying sandy deposits and their fauna suggest an amelioration of climate towards cool interstadial conditions. A date early in the Devensian seems most likely for the fauna. The relative positions of the terrace deposits and the Fisherton brickearths suggest that they form parts of a single depositional accumulation. In addition, the similarity of the 1974 fauna to faunas reported from the brickearths in the 19th century suggests that the two deposits are of similar age.
22
c.
P. GREEN ET AL.
References BLACKMORE, H. P. 1864. Remains of birds' eggs found at Fisherton, Near Salisbury. Edinburgh New Philosophical Journal. N.S. 19, 74-5. - - . 1867. On the recent discovery of flint implements in the drift of the valley of the Avon. Wilts. Archaeol. Nat. Hist. Mag., 10,221-33. - - & E. R. ALSTON. 1874. On fossil Arvicolidae. Proc. Zool. Soc. Lond., 460--71. - - , n.d. 'Locked notebook'. MS in Salisbury Museum. DELAIR, J. B. & M. L. SHACKLEY. 1979. The Fisherton brickpits; their stratigraphy and fossil contents. Wilts. Archaeol. Nat. Hist. Mag., 74, 1-16. DIEBEL, K. & E. PIETRZENIUK. 1975. Neue Ostrakoden aus dem Pleistozan von Burgtonna (Bezirk Erfurt). Zeit. geol. wiss. Berlin., 3, 87-97. - - & - - . 1978. Die Ostrakodenfauna aus dem Jungpleistozanen (Weichseklaltzeitlichen) Deckschichten von Burgtonna in Thiiringen. Quatiirpalaontologie, 3, 207-21. EVANS, J. 1864. On some recent discoveries of flint implements in drift deposits in Rants and Wilts. Q. JI geol. Soc. Lond., 20,188-99. KENNARD, A. S. & B. B. WOODWARD. 1901. The post-Pliocene non-marine mollusca of the south of England. Proc. Geol. Ass., 17, 213-60. KERNEY, M. P. 1971. A Middle Weichselian deposit at Railing, Kent. Proc. Geol. Ass., 82, 1-11. - - . 1977. British Quaternary non-marine mollusca: a brief review. In (ed., F. W. Shotton), British Quaternary studies: recent advances, Oxford, 31--42.
lusca: a brief review. In (ed., F. W. Shotton), British Quaternary studies: recent advances, Oxford. 31-42. - , E. H. BROWN & T. J. CHANDLER. 1964. The Late-Glacial and Post-Glacial history of the Chalk escarpment near Brook, Kent. Phil. Trans. R. Soc. Lond., B248, 135-204. LIKHAREV, 1. M. and E. S. RAMMEL' MEIER, 1952. Terrestrial mollusks of the fauna of the U.S.S.R. Translated from Russian. Israel Program for Scientific Translativns, Jerusalem, 1962. LYELL, C. 1827. On some fossil bones of the elephant and other animals, found near Salisbury. Proc. Geol. Soc. Lond., 1, 25-26. PRESTWICH, J. & J. BROWN. 1855. On a fossiliferous drift near Salisbury. Q. JI geol. Soc. Lond., 11, 101-107. REID, C. 1903. The geology of the country around Salisbury. Mem. Geol. Surv. G.B. SHOTTON, F. W. 1968. The Pleistocene succession around Brandon, Warwickshire. Phil. Trans. R. Soc. Lond., B254, 387--400. SPARKS, B. W. 1961. The ecological interpretation of Quaternary non-marine mollusca. Proc. Linn. Soc. Lond., 172,71-80. TYLOR, A. 1869. On Quaternary gravels. Q. JI geol. Soc. Lond., 25, 57-100. ZHADIN, V. 1. 1952. Mollusks of fresh and brackish waters of the U.S.S.R. Translated from Russian. Israel Program from Scientific Translations, Jerusalem, 1965.