Erratic palynomorphs from some British tills

The palynological investigation of tills (boulder clays) may provide considerable information about the direction of ice movement in lowland areas during the Pleistocene. Tills from Dyfed, Cheshire, Devon, Suffolk and Lincolnshire were subjected to palynological investigation. The palynomorphs recovered enabled some of the formations traversed by the ice-sheets to be identified, and thus an estimate of the direction of ice movement to be made. In most cases these estimates agreed with estimates based upon other methods of determining the direction of ice movement.


INTRODUCTION
During the Pleistocene glaciations, ice-sheets spread over large lowland areas in Europe and North America. The behaviour of these ice-sheetsis not yet well-known; it has been studied by examining: 1. the morphology of the deposits and erosive features 2. the orientation of clasts in glacial deposits; 3. the provenance of older deposits entrained by the ice, using either petrological or heavy-mineral analysis. Palynology offers another method of analysing the derivation of glacial deposits. Clasts from tills (boulder clays) were examined palynologically by early workers such as Eisenack and Wetzel. Palynomorphs were first recovered from the matrix of tills by Iversen (1936) and subsequently by, among others, Hyyppa ( 1936), Brander (1941), Heinonen (1957), Van Gijzel, Overweel & Veenstra (1960), Norris (1962), Cushing (1964), Andersen (19659, Miller & Benninghoff (1 969) and Hansen (1979). For various reasons (e.g. insufficient biostratigraphic and systematic information or unfavourable geology) none of these authors was able to state the direction of movement of the ice-sheets that deposited the tills. The potential of till palynology is, however, well-illustrated by the work of Hansen (1979) who demonstrated that the matrix of Danish tills came from within 20 km of its point of deposition.
This study consists of a palynological investigation of six British tills from five localities: Aberaeron, Dyfed ; Chelford, Cheshire; Fremington, Devon ; Great Blakenham, Suffolk and Tattershall Thorpe, Lincolnshire. left by the ice; MET H 0 DS Standard palynological techniques were used in sample preparation. The samples were mounted in 'cellosize' and Canada Balsam. The slides are lodged in the collections of the Palynology Laboratory in the Department of Geology, University of Sheffield. Palynomorphs were identified to specific level, whenver possible. Their ranges were determined by reference to the palynological literature. The outcrop from which the palynomorphs could have come was established by reference to Geological Survey maps of the relevant areas. It was assumed that the ice-sheets passed over all the formations represented in the palynomorph assemblages. LOCALITIES 1. Aberaeron. Blue-grey Devensian till of Irish Sea origin is exposed on the foreshore and is overlain by about 6 m of head which forms a low cliff. The sample came from SN464634, about lkm northeast of the mouth of the Afon Aeron. The glacial deposits at Aberaeron were described by Williams (1927), and subsequently by Mitchell (1962) and Watson & Watson (1967).
2. Chelford. Two tills of Devensian age are known from the sand pits of Chelford, Cheshire (SJ824720). The Lower Till (of uncertain age) overlies 'Ipswichian' shelly clays and is itself overlain by the Chelford Sands and Interstadial Deposit (60,800 BP). The Upper Till overlies these deposits. It contains erratics from the Pennines, Lake District and Irish Sea (Simpson & West, 1958;Coope, 1959). Mr. T. Good supplied the author with samples from the deposits at Chelford.
3. Fremington. Isolated remnants of till at Fremington, near Barnstaple, north Devon, described by Maw (1864), Stephens (1966), Wood (1970), Edmonds (1972) and Kidson & Wood (1974), are older than the local raised beaches (Kidson & Wood, 1974), which are of Ipswichian age (Andrews, Bowen & Kidson, 1979;Keen, Harmon & Andrews, 1981). The Fremington Till is thus Wolstonian or older. It contains rare 'giant' erratics of Scottish origin (Dewey, 1910;Taylor, 1956), and foraminifera and heavy minerals derived from the Irish Sea (Wood, Hunt 1970 (Rose, 1977;Rose, Allen & Hey, 1976). West & Donner (1956) and West (1963) show the Anglian ice-sheet entering East Anglia from the west and northwest using data from measurements of clast orientations. Perrin, Rose & Davies (1979), working with the distribution of heavy minerals, show the Anglian ice-sheet entering East Anglia and radiating out from the Wash. The sample of Lowestoft Till from Great Blakenham was given to the author by Mr. I. Bryant.
5. Tattershall Thorpe. Wragby Till (the chalky Boulder Clay of Wood, 1880 andJukes-Brown, 1885) at Tattershall Thorpe (TF225599) is overlain by Ipswichian interglacial deposits (I. Bryant, pers. comm., 1981) and Devensian sands and gravels. Straw (1979) states that the Wragby Till is of Wolstonian age, whereas Perrin, Rose & Davies (1979) suggest that it is of Anglian age. Perrin, Rose & Davies (1979) also challenge the other author's assumption that the Wragby Till was deposited by a southwards-moving ice-sheet. They show that heavy-mineral distribution in the Wragby Till is more likely to be the result of an ice-sheet moving northwest from the Wash into Lincolnshire. Mr. I. Bryant supplied the author with a sample of Wragby Till.

RESULTS
Palynomorphs of many different ages were present in all samples. In most cases forms derived from the local bedrock were very common. Each till is discussed separately below.
1. Aberaeron. Palynomorphs of Tremadoc-Ashgill, Silurian, Carboniferous, Permo-Triassic, Jurassic, Cretaceous, Tertiary and Quaternary age are present. The Silurian taxa are likely to be derived from the local bedrock (Llandovery); other forms must have been brought into the area by Irish Sea ice. Rocks of Permo-Triassic age (represented by striate bisaccates), Jurassic age (represented by taxa such as Classopollis, Cycadopifes, Callialasporites and rare spores and dinocysts), Tertiary age (represented by forms such as Wetzeliella and much pollen) and Quaternary age (represented by pollen, and dinocysts such as Spiniferites septentrionalis Harland and Leptodinium multiplexum Wall & Dale) are known from Cardigan Bay (Garrard, 1977;Wilkinson & Halliwell, 1979 (1979) or Garrard (1977). The nearest Cretaceous outcroup to the north of Aberaeron is probably that of Northern Ireland (Hancock, 1961). The ice-sheet that deposited the Aberaeron Till may therefore have travelled past the Cretaceous outcrop in Northern Ireland before crossing the Lleyn Peninsula and moving south across Cardigan Bay to Aberaeron. This is similar to the direction of ice movement in Cardigan Bay suggested from the distribution of erratic boulders by Garrard (1977).
2. Chelford Lower Till. The Lower Till at Chelford is dominated by Quaternary pollen and freshwater algal spores. It also contains a few Permo-Triassic forms, mostly striate bisaccates, and some Carboniferous spores. The Quaternary species are mostly derived from the underlying Ipswichian shelly clays (two samples from the Ipswichian deposits had markedly similar pollen spectra) ; the Permo-Triassic species are likely to be derived from the local bedrock and the Carboniferous species are probably from the Pennines. The ice-sheet that deposited the Lower Till thus probably originated in the Pennines.  (Evans & Thompson, 1979;Wilkinson & Halliwell, 1979). Rocks of Rhaetic age (represented by forms such as Ovalipollis ovalis Krutzsch) and Jurassic age (represented by forms such as Valensiella ovuta (Deflandre) Eisenack, Dinopterigium absidatum Drugg, and Ctenidodinium panneum (Norris) Lentin & Williams) outcrop over much of the length of the Bristol Channel, therefore these suites of palynornorphs offer little directional information. The Tertiary (represented by angiosperm pollen) offers more information, since all the Tertiary outcrops in the Bristol Channel and southern Irish Sea are to the west and northwest of Fremington.  Halliwell, 1979). The Fremington glacier must therefore have come from a direction of between 275" and 300°, and have been travelling nearly due east; a conclusion which agrees well with the geomorphological results of Edmonds ( 1972) and Kidson & Wood (1974).

Great Blakenham. The Lowestoft Till from Great
Blakenham contains Carboniferous, Permo-Triassic, Rhaetic, Jurassic, Cretaceous and Pleistocene (pollen only) palynornorphs. The presence of Carboniferous spores such as Densosporites spp., Lycospora spp., Cristatisporites sp., suggests that some of the Anglian ice may have originated in the Pennines and moved in a general southeasterly direction towards Great Blakenham. This probable northwesterly origin for the Lowestoft Till agrees with both the clast-orientation data of West & Donner (1956) and the heavy-mineral analyses of Perrin, Rose & Davies (1979); however, the suggestion by Perrin, Rose & Davies (1979) that the Lowestoft ice-sheet came into East Anglia from the North Sea cannot be confirmed on palynological grounds, since no Pleistocene dinocysts (which would be expected if the Lowestoft Till had a North Sea origin) were encountered in the sample.
6. Tattershall Thorpe. Palynomorphs of Rhaetic, Jurassic (mostly Upper), Lower and Upper Cretaceous and Quaternary age are present in the Wragby Till. The Quaternary species are all pollen and freshwater algal spores. The Upper Jurassic palynomorphs are likely to be derived from the local bedrock. The Rhaetic outcrops to the west and northwest of Tattershall Thorpe on land, and to the east and north in the North Sea, and is thus of little use in suggesting a direction of ice movement. The occurrence of Cretaceous forms suggests an origin from the sector from northwest through north to southeast, while the total absence of Carboniferous taxa suggests that the ice-sheet never passed over rocks of that age either in the Pennines or in Northumberland. The lack of Pleistocene marine dinocysts suggests that the Wragby ice-sheet had not come directly from the North Sea. It is therefore probable that the Wragby Till at Tattershall Thorpe was ernplaced by an ice-sheet moving in a general southerly direction along the strike of the Mesozoic outcrop in Lincolnshire. There is no strong evidence, however, to decide whether this was the case; the assertion by Perrin, Rose & Davies (1979) that the Wragby Till was emplaced by an ice-sheet moving northwestward from the Wash is almost equally probable on the palynological evidence, except for the absence of Pleistocene dinocycsts.

DISCUSSION
In this paper it has been assumed that the ice-sheets carried with them identifiable palynomorphs from all the formations they traversed. This assumption may not always be valid, for instance the Fremington Till contains material derived from Permo-Triassic rocks (Wood, 1970), but no unequivocal Permo-Triassic species were found in the samples from Frernington.
The resolution of the direction of ice-movement is to a great extent dependent upon the accuracy of the resolution of the age of the derived palynomorphs and thus of the formations passed over by the glacier. It is also dependent upon the degree of detail of the geological maps available, especially in offshore areas.
The resolution of the 'age' of the derived palynomorphs may, in some instances, be complicated by earlier reworking, such as that reported by Muir (1967) and Phillips (1974). This reworking is impossible to detect, unless it has left palynomorphs in a very degraded state.
It is suggested, therefore, that although till palynology is capable of providing considerable information about the formations that an ice-sheet has traversed, it should be used in conjunction with the other techniques outlined in the Introduction, rather than in isolation.