THE VALUE OF TEMPORARY SECTIONS: GRAVELS AT RUSHFORD, SUFFOLK

GRAVELS AT RUSHFORD, SUFFOLK

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THE VALUE OF TEMPORARY SECTIONS: GRAVELS AT RUSHFORD, SUFFOLK RICHARD WEST and RENDEL WILLIAMS Knowledge of the geology of Suffolk rests largely on the activities of the British Geological Survey over the last c.130 years, with the resultant maps and memoirs describing in detail the geology of particular areas. Added to this are descriptions by many Suffolk observers, in various depths of detail, describing particular sites or areas of the county. The matter does not stop once the ‘official’ maps and memoirs are published – one of the pleasures of geology is that new observations are continually possible because people are always digging holes for one reason or another. The result is that observations of permanent value may come to light by pure chance, so that is well worth noting, photographing and describing chance sections which are discovered on walks or journeys.

The following is a case in point, where sections were observed by Williams (1968) in the 1960s at Rushford, in the Little Ouse valley south-east of Thetford and immediately west of Knettishall Heath (Fig. 1). At the same time he recorded temporary sections at Redcastle Furze, on the south side of the Little Ouse valley at Thetford. Understanding of these sections became clearer in the light of later surveys of the Little Ouse valley (West, 2009). It then became clear that the observations allowed relative dating to be made of river incision in the Fenland during the cold Wolstonian Stage following the temperate Hoxnian Stage (interglacial), a conclusion significant for helping to solve the mysteries of the origin of a highly important geological feature of East Anglia, the Fenland basin.

Brandon Thet River

Thetford

Rushford

Little Ouse River Lopham Ford

The ck Bl a

eney Wav River

Bo urn

Figure 1. The valley of the Little Ouse River. The rectangle shows the position of the area described at Rushford.

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Williams (1968) described sections at Rushford as follows: “An interesting site is Rushford Heath (TL940807). A very slight valley crosses the Heath with slopes less than 2 degrees. Yet gravel workings show that about 12 feet of non-chalky sand and gravel underlie its floor, with ice-wedges at various horizons. Solifluction has almost filled in and obliterated a valley which formerly was much deeper. The excavations have not progressed far enough yet to give a complete cross section of the valley and its fill. The material in the centre of the valley is better sorted and bedded than the sludge at Thetford, but not as well as the river terrace there.”

Williams (loc. cit) compared the sediments at Rushford, including the sludge referred to above, with those on the valley slope at Redcastle Furze (on the south-west edge of Thetford) described as follows: “The lower part of the hillside is swathed in a sheet of sandy gravels of ‘cold’ character. Sorting and bedding are largely non-existent. The deposits generally have a mucky unwashed appearance. Pebbles and cobbles of flint are scattered in clayey sand. There are frequent changes of lithology but one feature which remains constant is the mixture of materials of different grade size. The deposit is a typical sludge of solifluctional origin. There are one or two ice wedges in it, but no molluscs. The sludge shows signs of washing in places, particularly towards the base of the hill, where it passes laterally into well-washed sands and gravels, forming a terrace a few feet above the level of the river. Ice wedges are abundant in the terrace and occur at many different horizons.

The transition from the sludge to the terrace could be seen in a number of trenches on different parts of the hill. Sometimes it took place over a distance as little as twenty feet; elsewhere it was more gradual.”

Figure 2 shows Rushford Heath’s local geology and the extent of the old gravel pit in which sections were observed. The 6-inch map of the this area (British Geological Survey, TL98SW), surveyed by T. E. Lawson (1979), is an excellent map showing the distribution of periglacial features as well as the geology in detail. The valley is cut in Upper Chalk. Patches of till (boulder clay) and glacial sand and gravel overly the Chalk, especially to the west of the valley; these are products of the Anglian glaciation. The sand and gravel is notable for the presence of brown quartzites.

The map shows clearly the extent of the gravel pit at Rushford, the limits of which are also seen in air photos taken in 1975, by which time the pit was worked out and levelled, apart from a small lake left in the part of the valley to the north of the road, adjacent to Rushford Heath. South from the road, the valley narrows and turns south-south-east. Here the valley contains active springs, now in part ponded. Evidently, spring action is responsible for the valley as we see it today. Damp gravelly swales (with quartzites) surround the area with springs, possibly the result of ground-ice formation in the past or of solution in the Chalk below.

Areas of patterned ground on the Chalk are seen in air photos and are noted on the 6 inch geological map and in Figure 2. Stripes fall into the valley on the slope to the east, with polygonal patterns on higher ground above the 25 m contour and also areas of anastomosing patterned ground, intermediate between polygons and stripes.

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Figure 2. The valley at Rushford, tributary to the Little Ouse River. Crosshatching indicates the position of the gravel pit in the valley. S marks the area with springs. Areas with periglacial patterns are shown (polygons, stripes and anastomosing patterns; not to scale).

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The presence of sand and gravel in the valley indicates aggradation in the valley following incision, the latter relating to lowering here of base level in the Little Ouse valley (to 15 m O.D. or less), and thus to a lowering in the Fenland basin into which the Little Ouse flows. The aggradation is punctuated by times of ice-wedge formation. Ice-wedges form under conditions of frozen ground, so the section indicates an alternation between active deposition of bedded sand and gravel, with water sources available, and times of frozen ground. Such variation indicates a degree of climatic change, but on what scale of time it is impossible to say. It would seem likely that the patterned ground on the valley slopes and higher ground is broadly associated with the period of aggradation in the valley, more particularly with the periods of icewedge formation, with periods of thaw resulting in movement of surface sediments into the valley. If so, the periglacial patterns were presumably initiated in an early part of the Wolstonian Stage, with the possibility that they were regenerated in later cold periods in the Wolstonian or in the last cold stage, the Devensian.

It is possible to relate the aggradation in the Rushford valley to the history of the Little Ouse valley. The gravels lie below the sands of Rushford Heath, the Lopham Sands (West, 2009). These are sediments of a lake formed by the damming of the Little Ouse by an ice advance into Fenland at c.160000 years (Gibbard et al., 2009), during the cold Wolstonian Stage, a lake which overflowed eastwards into the Waveney valley at the time. Therefore the time of incision of the Rushford valley, and so of the Fenland basin, and the subsequent aggradation of the sand and gravel belong to a time in the Wolstonian Stage before the ice advance into the Fenland.

Thus the description of the Rushford section, brief though it is, provides a view of processes of erosion and aggradation associated with the cold climates of the early part of the Wolstonian Stage, giving unique detail of a time of landscape evolution where little was known before, and illustrating the importance of recording temporary sections.

We thank the Rushford Estate for access permission.

References Gibbard,P. L., Pasanen, A. H., West, R. G., Lunkka, J. P., Boreham, S., Cohen, K. M. & Rolfe, C. (2009). Late Middle Pleistocene glaciation in East Anglia, England. Boreas 38: 504–528.

West, R. (2009). From Brandon to Bungay. An exploration of the landscape history and geology of the Little Ouse and Waveney Rivers. Ipswich, Suffolk Naturalists’ Society.

Williams, R. B. G. (1968). Periglacial climate and its relation to landforms.

Ph.D. Dissertation, University of Cambridge.

Richard West 3A Woollards Lane, Gt Shelford, Cambridge CB22 5LZ Rendel Williams Geography Laboratory, University of Sussex, Brighton BN1 9SJ

Trans. Suffolk Nat. Soc. 48 (2012)