The formation of sub-glacial buried channels in the lower Gipping valley

T H E F O R M A T I O N O F S U B - G L A C I A L B U R I E D C H A N N E L S IN THE LOWER GIPPING VALLEY H . B . MOTTRAM & S . L . HUNTLEY*

Introduction DĂźring compilation of the Wartime Pamphlets on 'Water Supplies from Underground Sources' the presence of buried Channels became more evident than previously described by Boswell etc. and led to the comprehensive publication of their nature and occurrence by Woodland in 1970. Although Woodland plotted what he considered to be the courses of the main buried Channels in East Anglia it is quite common for the Suffolk County Laboratory to encounter previously unreported Channels.

Recent work carried out at Blood Hill (Bramford) proved the presence of a buried Channel and in order to ascertain its impact on local hydrogeology a review of data for the Lower Gipping Valley was undertaken by the Waste Disposal Division. T h e stratigraphic implications of some of the findings are presented in this paper.

Occurrence Buried Channels are normally fairly narrow and do not give rise to surface features so that earlier appraisals relied upon the chance striking of them being noted by well borers. More intensive grouping of boreholes, particularly for mineral and route planning, has helped to delineate sites of buried Channels in the Lower Gipping Valley. In some instances wells and other boreholes are too shallow to be definitive and the recording of a 20m thick raft of chalk among sands and gravels in the Stour Valley (Barker & H a r k e r , 1984) suggests that some reports of chalk may be misleading.

It has been a widely held belief that buried Channels tend to occur in the floors of the present day river Valleys in southern East Anglia. Rarely has information for the Gipping Valley north of Ipswich borne this out. In fact in this part of Suffolk the only significant buried Channel found is at a higher level. It originates in the western flanks of the Valley, and does not assume a more 'normal' position until below Sproughton. The course of the buried Channel is shown in Fig. 1.

Structure Occupying the flood piain in central Ipswich is a tract of low level gravelly deposits which rest on U p p e r Chalk or occasionally Lower Tertiaries * The authors are employed within the Highways Department of Suffolk County Council to provide specialist geological advice for the Authority's waste disposal and site investigation functions.

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L i t t l e Blakenham

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,

GreatlBlakenham

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Sproughton

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Blood Hill

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Broomvale Farm

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Sproughton Manor buried Channels proved

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conjectured course of buried Channels

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deepenings where data limited line of section in Fig 2

LOWER GIPPING VALLEY-LOCATION MAP

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(Eocene) at about Om O . D . Within this area is a zone less than 120m wide where gravels overlie chalk at distinctly deeper levels, sometimes reaching below —45m O . D . This is taken to represent a typical sub-glacial buried Channel. Its uneven base and configuration have been well described by Woodland. Towards the western outskirts of the town the deeper zone of gravels broadens out reaching a width of 1km but adjacent to Sproughton the Situation is more complex. The flood piain becomes restricted by a knoll at Sproughton Manor which appears to be largely composed of high level gravels above the U p p e r Chalk at + 5 m O . D . Immediately downstream of the knoll, Boulder Clay overlies the Chalk initially at a similar level but it soon steps down to about - 2 5 m O . D . with gravelly deposits overlying the Boulder Clay (See Fig. 2). The m a j o r development of the Channel attains a depth of below - 5 0 m O . D . through this section.

Between Sproughton and Blood Hill the existence of buried Channel deposits is known but data is relatively poor. The infilling strata include clays and granular materials but their disposition cannot be defined and nor can the base of the Channel which is known to exceed - 1 5 m O . D . It is possible that the Channel, up to 100m wide, is discontinuous through this section.

At Blood Hill the same materials are present. However, here it is known that Boulder Clay overlies U p p e r Chalk and that the clay is in turn entrenched by sands and gravels. This structure probably continues uninterrupted to Mason's Works at Great Blakenham but its base rises slightly to about + 5 m O . D . before truncation by a divergent course of the Channel which reaches to below - 1 0 m O . D . The base of the Channel has also been proved to be uneven at Mason's Works.

Development It is generally agreed that a sub-glacial stream of slurry, highly mobilised by a form of water pressure (the piezometric head), gouged out a variable course whilst under the ice sheet. Upon escaping the ice the heavy pebbles were soon deposited and the associated silts and clays were washed further away and laid down in the Orwell or at sea. The Channel in central Ipswich can be accounted for in this manner. It is tempting to link this with depressions in the Orwell but since the latter has been subjected to estuarine erosion it is likely that many of its depressions are younger and of a different origin to the deepenings under discussion.

The pressure below present day glaciers is known to vary and this p h e n o m e n o n would have caused uneven basal erosion and may also have produced a lateral effect. It is suggested that the broadening of the Channel at the western outskirts of Ipswich resulted from a lateral erosional effect. Of more significance is the fact that Boulder Clay occupies part of the Channel near Sproughton but has been cut out at the m a j o r Channel development.

This indicates that at least two phases of excavation occurred. Although this could have been caused by successive glaciations, most recent workers (Perrin et al. ,1979; Bristow & Cox, 1973) prefer to assign all glacial deposits to a single ice sheet. In such a case the sequence of events would have been that after the sub-glacial stream had eroded the broader area it became more

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restricted to the southern part as it deepened this section thus allowing Boulder Clay, possibly a melt out but probably a lodgement tili, to cover the abandoned part.

North of Sproughton the Channel is cut down into the Chalk of the lower flanks of the Gipping Valley from a level up to 30m above the Chalk of the present day flood piain. If a similar difference in levels occurred at the time of Channel formation then there appears to be a significant departure from the normal theory of a Valley bottom location for sub-glacial streams, but when the possible thickness of ice at this point is considered then the difference in levels becomes less significant. As melt water is produced throughout the base of ice, and since pressure distribution is complex, the development of sub-glacial streams above the lowest point is to be expected. Further local evidence of pressure fluctuations is found at Mason's Works. The established course of the buried Channel was truncated when the sub-glacial stream diverted towards Broomvale Farm. Despite recent extensive gravel extraction south of Broomvale Farm no further evidence of the newer course is known. It is possible that it lost impetus and died here while still beneath ice or that this was the limit of the retreating ice front.

The Boulder Clay lining the Channel between Blood Hill and Mason's Works most closely resembles the lodgement tili of the sequence described

by Allen (1982). This indicates either the activity of more than one ice sheet or that the Channel was abandoned sufficiently early for a significant thickness of lodgement tili to accumulate within it. Later outwash sands and gravels partly entrenched into the tili.

Conclusions Buried Channels are fairly common in East Anglia and may occur at previously unsuspected sites. Their formation was controlled by complex pressures below the ice and was largely independent of existing topography, although in southern East Anglia topography may have influenced the pressures and the easiest melt water escape routes. Evidence from the Lower Gipping Valley suggests that it required more than one phase of excavation to produce the buried Channel known here. The variable nature of the Controlling pressure is shown by such features as uneven basal erosion, divergence and possibly discontinuity of courses.

No attempt has been made to date the suggested events. In this context the relationship of the Boulder Clay in the Channel at Sproughton to the high level gravels of the curious knoll at Sproughton must be clarified.

Acknowledgements The authors are indebted to the County Surveyor, Suffolk County Council, for permission to publish.

References A . P . C . M . (1976). Planning application to Suffolk County Council No. MS655/76 - Mason's Works, Great Blakenham.

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THE FORMATION OF SUB-GLACIAL BURTED CHANNELS

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Allen, P. (Editor) (1982). Field Meeting Guide, Suffolk. Quaternary Research Association.

Barker, R. D . & H a r k e r , D . (1984). The location of the Stour buried tunnel-valley using geophysical methods. Q.J. Eng. Geol. (in press).

Boswell, P. G . H . (1914). O n t h e age of t h e Suffolk Valleys and t h e b u r i e d

Channels of drift. Q.J. Geol. Soc. London, 69,581.

Bristow, C. R . & Cox, F. C. (1973). The Gipping Till: A re-appraisal - East Anglian glacial stratigraphy../. Geol. Soc. London. 129,1.

Clarke, M. R. and A u t o n , C. A . (1982). The Pleistocenedepositional history of the Norfolk-Suffolk borderlands. Rep. Inst. Geol. Sei. No. 82/1, 23.

Lake, R . D . , Ellison, R. A. & Moorlock, B. S. P. (1979). Matters Arising: Middle Pleistocenestratigraphy in southern East Anglia. Nature, 265,663.

Perrin, R. M. S., Rose. J. & Davies, H . (1979). The distribution, Variation and origins of pre-Devension tills in eastern England. Phil. Trans. Roy Soc.(B), 287,535.

Rose, J., Allen, P. & Hey, R . W. (1976). Middle Pleistocene stratigraphy in East Anglia. Nature, 263, 492.

Woodland, A . W. (1970). T h e buried tunnel-valleys of East Anglia. Proc.

Yorks. Geol. Soc. 37, 521.

H. B. Mottram & S. L. Huntley, Highways D e p a r t m e n t , St. Edmunds House, Rope Walk, Ipswich.

A nest of the Norwegian Wasp A wasp nest, a good deal larger than a cricket ball, was made about 12 ft above the ground in a laburnum tree within 200 yds of the beach at the Bawdsey end of Felixstowe in 1982. The wasps were identified by Mr. M. C. Day of the British Museum (Natural History) in London as Norwegian wasps, Dolichovespula norwegica (Fabr.).

This record is in line with old records mentioned by Heathcote (1983) and indicate that the Suffolk coast is still a definite habitat of the species.

Reference Heathcote, G . D . (1983). Social wasps in Suffolk. Trans. Suffolk Nat. 19, 343.

Soc.,

W. E. Lewis, 7 Western Avenue, Felixstowe.

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