59
THE STUDY OF INTERGLACIAL DEPOSITS IN By
R.
G.
SUFFOLK
WEST.
Sub-department of Quatemary Research, Botany School, Cambridge. THE Great Ice Age, which lasted about a million years, finished only 10,000 years ago, so that it is one of the most recent of geological events. It was characterised by great changes in climate, which produced alternately warm periods (interglacial periods) with a climate like that of the present day, and cold periods (glaciations) during which the ice caps of north-western Europe covered a much greater area than they do to-day. It is usually accepted that there were four major glaciations and between these were interglacial periods. During the glaciations and interglacials there were minor climatic oscillations which caused small advances and retreats of the ice sheets. T h e deposits formed by an ice sheet or glacier are called glacial deposits, and those formed during a glaciation but outside the ice sheet itself are called periglacial deposits. T h e most typical deposit of an ice sheet is boulder clay. This is formed by deposition of the material in the ice which has been picked up during its advance. T h u s if an ice sheet has advanced over an outcrop of Chalk, the boulder clay deposited by this ice will contain much material derived from the Chalk. It is therefore possible to trace the direction of movement of ice sheets by analysing the composition of the boulder clay. In Suffolk there are a two main types of boulder clay, a blue chalky boulder clay common on the high plateau of Suffolk, and a browner one which is best seen in the Ipswich area. One of the most common periglacial processes is solifluction— the movement of material down slopes by alternate freezing and thawing of the surface layers of the soil. Many periglacial structures, such as stone polygons and stone streams, can be seen to-day in Arctic latitudes. It is often very difficult to distinguish periglacial from glacial deposits, but it is necessary to do so in order to find the limits of ice sheets. It is not known how many ice sheets of the different glaciations covered Suffolk. Perhaps the two boulder clays represent different glaciations. But to make certain that they do so, it is necessary to find a deposit between them formed during an interglacial period. [Ed. : See M r . Spencer's recent discovery p. 53]. On the retreat of an ice sheet the surface of the drift (a collective term for glacial deposits) is usually very irregulär, and if it is impervious the hollows in the landscape become filled with water and form lakes. T h e remains of the fauna and flora associated with a warm climate may be found in the lake sediments of the
60
INTERGLACIAL DEPOSITS IN SUFFOLK
hollows, and the deposit can then be recognised as one formed during an interglacial period. No interglacial deposits have yet been found in Suffolk lying between two boulder clays, [Ed. : vide supra], but there are many lying in boulder clay hollows covered by sands and gravels. It is difficult to determine whether these covering deposits are glacial or periglacial, and their Identification as one or the other is a frequent source of controversy amongst geologists. Large and easily visible fossils, such as bones, leaves and seeds, are not usually very common in interglacial deposits, and it may be difficult to make use of these scanty remains to reconstruct interglacial conditions of life and climate. In many lake sediments, however, pollen grains (the male spores of the flowering plant, formed in the anthers) and the spores of lower plants are frequently preserved in great numbers. T h e walls of the pollen and spore cells are verv resistant to decay, and as their surface patterns and shapes vary considerably in the different species, it is possible to identify them. T h e sizes vary from a ten thousandth to a two hundred thousandth of a millimetre, so that a microscope must be used for the examination of the grains. Pollen is produced in large quantities by plants, especially those which are wind-pollinated. T h e wind-borne pollen falls as a rain on the ground, and if it falls on the surface of a lake, the g r a i n s will be i n c o r p o r a t e d i n t o the S e d i m e n t f o r m i n g at the bottom of the lake. T h e pollen rain falling in an area reflects the composition of the Vegetation of the area ; each species contributes an amount of pollen proportional to its frequency and its pollen productivity. The latter varies in different species, so that caution is necessary in relating the percentages of different pollen types to the Vegetation which contributed to the pollen rain. Nevertheless, given adequate care, by identifying the pollen grains contained in a sediment, and measuring the proportion of each present in a given sample, the Vegetation of the area at the time of the formation of the sediment can be reconstructed. By taking vertical samples in the deposit it is possible to find the changes in the Vegetation which took pace during the time period represented by the whole deposit. T h e climatic changes of the period can then be deduced from the evidence provided by the changes of the Vegetation. For example, Birch-Pine Vegetation indicates a climate similar to that of the present Birch-Pine zone of Vegetation of northern latitudes. A sample containing a mixture of pollen of warmth-loving deciduous trees, such as Oak, Elm and Lime, suggests a warm temperate climate. Using the technique of pollen analysis it is possible to trace the changes of Vegetation and climate which took place between the different glaciations. Different interglacial periods may be characterised by different vegetational changes, depending, for example, on the rates of migration of the different species into
Ăśl
INTERGLACIAL DEPOSITS IN SUFFOLK
the area after the retreat of the ice. Some species of plants may be present only in a particular interglacial period. This kind of evidence may make it possible to identify a given deposit as belonging to one particular interglacial, and consequently the deposits above and below it may be referred to known major glaciations. The work on the interglacial deposits of Suffolk is not sufficiently far advanced to make possible any conclusions about the number of interglacials and glaciations. Several interglacial deposits containing pollen are known in the county. At least two of them, at Hoxne and Saint Cross South Elmham, lie in hollows in a blue chalky boulder clay. Both of these deposits are being studied at the moment, and the pollen spectra from them suggest that they belong to the same interglacial period. The deposit covering the interglacial at Hoxne has for many years been a source of controversy; Reid Moir regarded it as a partially decalcified chalky boulder clay, but it may, in part, be a solifluction deposit. Thus it is not known for certain whether an ice sheet extended over the area after the Hoxne interglacial, but there is evidence that periglacial conditions were present at some period subsequent to the interglacial. The evidence at Hoxne, then, shows a sequence of glaciation—interglacial—glacial or periglacial. It is not possible at the moment to make any correlation of these deposits with those of other parts of Britain and the Continent. Further studies along these lines on other Suffolk interglacials may provide more evidence about the succession of events during the Great Ice Age and their correlation.
A NOTE ON FOSSIL WATER-FERNS IN
SUFFOLK
T w o genera of water-ferns, Azolla and Salvinia, have been recently discovered in interglacial deposits in Suffolk. Both these genera are new to the British Pleistocene Flora, but have been found in interglacial deposits on the Continent. Azolla was found in the deposits at the classic site of Hoxne, and Salvinia in the interglacial deposit recently discovered by Mr. Spencer at Bobbitt's Hole, near Ipswich. Only the male and female spores of the plants have been found so far. The spores of the fossil Azolla are very similar to those of the modern A. filiculoides, Lam., a species which was introduced into Britain towards the end of the nineteenth Century. The species of fossil Salvinia has not yet been determined. The botanical and geological significance of these interglacial fossils is being studied in detail. R. G.
WEST.