A NOTE ON THE INTERGLACIAL DEPOSIT AT BOBBITSHOLE, NEAR IPSWICH B. W . SPARKS and
R . G . WEST
THE interglacial deposit in the Valley of the Belstead Brook at Bobbitshole has beert intensively studied from a number of points of view (Davis 1955, West 1957, Sparks 1957) but certain questions have always remained open. One of these was whether there was any trace of a marine invasion corresponding with the Eemian transgression of the Low Countries. Recent reopening of a section at Bobbitshole (January, 1967) revealed a bed which was not included in the stratigraphy described by West in 1957. The bed in question is a contorted shelly sand, some 20-30 cm. thick, which lies between the top of West's bed B and the overlying roughly sorted sandy gravel, bed A; that is between the top of the Interglacial deposits and the overlying gravels which are attributed to cold conditions in the Last Glaciation. In the field this shelly sand could be seen to contain fragments of marine bivalves including the cockle, Cardium sp. Accordingly a sample of this material was washed down and examined. Under the microscope it was apparent that the fragments of marine shells were very worn indeed. Some of the fragments have the characteristic ribbing of cockles, while others appear to be very worn fragments of the thicker parts of marine shells, especially the hinge plates. They are all obviously derived. They may be coupled with the well rounded and polished sand grains, for these also appear to be of marine origin. But it is most likely that both the marine shell fragments and the sand grains have been derived from the Red Crag mapped as outcropping on the sides of the Belstead Brook Valley. The sand, however, does contain an indigenous fauna, but this is one of land and freshwater Mollusca, which are rather scarce and usually broken, but among which the following species have been recognised. Valvata cristata Müller Valvata piscinalis (Müller) Bithynia tentaculata (Linne) Bithynia inflata (Hansen) Carychium sp Lymnaea truncatula (Müller) Lymnaea palustris (Müller) Lymnaea peregra (Müller) Planorbis leucostoma Millet Succinea sp Cochlicopa lubrica (Müller) Truncatellina cylindrica (Ferussac) Vertigo antivertigo (Draparnaud) Vertigo pygmaea (Draparnaud)
4 7 6 1 1 9 3 1 2 1 2 1 1 1
391
INTERGLACIAL DEPOSIT AT BOBBITSHOLE
Vertigo angustior Jeffreys Vallonia costata (M端ller) Vallonia pulchella (M端ller) Hygromia hispida (Linne) Helicella itala (Linne) Limax sp. Agriolimax cf. agrestis (Linne) Agriolimax cf. reticulatus (M端ller) Pisidium amnicum (M端ller) Pisidium subtruncatum Malm Pisidium henslowanum (Sheppard) Pisidium nitidum Jenyns
1 1 18 2 1 1 2 1 1 1 5 3 TOTAL
77
Of these identifications a number are somewhat doubtful. Bithynia inflata has not been recorded from this deposit and the identification is based solely on an operculum, which, however, seems far too rounded for B. tentaculata. The Lymnaea species are very broken and the numbers must be treated with reserve. The Vertigo angustior is only the worn top half of a shell but it does not appear to be pusilla. T h e species of the slug, Agriolimax, are based solely on the shell shape in accordance with those photographed before from Bobbitshole (Sparks 1957). In this list there are a number of new records for Bobbitshole: Bithynia inflata, if it is correctly identified, is not uncommon in other Last Interglacial deposits; Vertigo pygmaea and Helicella itala are apparently both new; while it is interesting to have confirmation of the southern xerophilous snail, Truncatellina cylindrica, as only one specimen of this had previously been recorded (Davis 1955) among all the thousands identified by Davis and Sparks. There is not the slightest influence in this bed of any brackish let alone salt water conditions. Rather it seems that the succession from open water to marsh of the Interglacial deposit was followed by a phase of renewed erosion of the valley sides and stream deposition. The freshwater species in the above list, notably the Pisidia, are characteristic of moving water, while the large proportion of land Mollusca from a variety of habitats is also more characteristic of a stream than of a pool or a marsh. Species such as Vallonia costata, Truncatellina cylindrica, and Helicella itala are found in dry habitats: Cochlicopa lubrica, Vertigo antivertigo, and V. angustior, Vallonia pulchella, and Agriolimax spp. inhabit much damper areas, some of them being marsh dwellers. At the top of the succession from Bobbitshole described earlier (Sparks 1957) land species reached a level of 50% of the total, while in this section they are not far short of that. From the general nature of the Mollusca it would seem that this thin sand succeeded the interglacial beds previously described fairly quickly, for it has a similar type of fauna.
392 Transactions of the Suffolk Naturalists',
Vol. 13, Part 6
Although it is only negative evidence there is then no trace of interglacial marine influence at Bobbitshole. On the south coast of Britain sea level rose above its present level in Zone / o f the Last Interglacial (West and Sparks 1960) and probably reached a mean level about twenty-five feet above the modern level. If the Eem sea never reached the Bobbitshole beds it must mean, making an allowance for tidal ränge, that the land then stood at least some thirty feet higher than at present relative to the south coast, i.e., this part of East Anglia has been downwarped not less than about thirty feet since the middle of the Last Interglacial. This is a similar conclusion to the one we reached in studying the Stutton brickearth (Sparks and West 1963). In these deposits there is a trace of brackish influence which must have been at about the time of the maximum sea level of the Last Interglacial. Yet it is found at two feet O.D. and, allowing for tidal ränge, cannot represent a sea level above a b o u t - f o u r feet O.D. This again is about thirty feet lower than that of the south coast. It is perhaps better to put this rate of warping into perspective by saying that it represents a rate of about one millimetre every five or ten years on the average. Once again we are indebted to Mr. H. E. P. Spencer for calling our attention to the exposure. REFERENCES
Davis, A. G . (1955). T h e Mollusca of Bobbitshole Interglacial Beds, Ipswich. Trans. S u f f . Nat. Soc. 9, 110-4. Sparks, B. W . (1957). T h e non-marine Mollusca of the interglacial deposits at Bobbitshole, Ipswich. Phil. Trans. Royal Soc. Lond. B, 241, 33-44. Sparks, B. W . and West, R. G. (1963). T h e interglacial deposit at Stutton, Suffolk. Proc. Geol. Assoc. 74, 419-432. West, R. G . (1957). Interglacial deposits at Bobbitshole, Ipswich. Phil. Trans. Royal Soc. Lond. B, 241, 1-31. West, R . G . and Sparks, B. W . (1960). Coastal interglacial deposits of the English Channel. Phil. Trans. Royal Soc. Lond. B, 243, 95-133.