C3.1      General landscape assessment
   C3.2      Geology
   C3.3      Geomorphology
   C3.4      Soils
   C3.5      Climate
   C3.6      Hydrology

C3.1 General Landscape Assessment

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Wicken Fen lies in a part of England not generally noted for the beauty of its landscape. The region is both flat and low-lying and is also intensively cultivated. However, the size of the Fen, and its layout, with long broad droves through fen, scrub, and woodland, isolates it from the surrounding farmland so that it provides a place where one can walk in country approaching about as close to wilderness as is possible inland in East Anglia. The surrounding country, although farmed, is fairly free from buildings and power lines, so that there are attractive vistas from the edges of the fen - which is, because it is undrained, raised above its surroundings. The tower hide provides even more extensive views.

The Fen's habitats, their traditional management, the Trust's buildings in Lode Lane, and the setting of the Fen in arable farmland, all contribute to its landscape value. These, and the peaceful quietude of the Fen are to be conserved.

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C3.2.1 Underlying Rocks

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Over most of the property, surface peat deposits overlie Gault Clay. The stratum below this, the Lower Greensand, lies directly beneath the peat near the north-west edge of Verrall's Fen (around GR TL 549 707). Likewise, the Upware Limestone (Corallian) may similarly lie beneath the peat at the extreme north-west corner of Verrall's Fen at GR TL 549 708. The Lower Greensand was exposed by deep ditching outside Howe's Bank in 1988. The Borrow Pit on Adventurers' Fen, dug in 1989, passed through about 10 m. of Gault Clay, which was highly fossiliferous in places, and just reached the Lower Greensand.

The Lower Greensand is a water-bearing stratum with a positive pressure of 0.5 - 1.0 m A.O.D. (above ordinance datum) immediately beneath the Gault Clay; it is thus a possible source of uncontaminated water for irrigation of the Fen should this be deemed desirable and feasible.

C3.2.2 Drift and alluvium

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The car park and Warden's House stand on Chalky Boulder Clay. Glacial outflow channels may have been responsible for the sand, gravel and other alluvial material which have been found below the peat in parts of Adventurers' Fen, and which are likely to occur elsewhere on the property.

The former courses of rivers across the Fen are indicated by roddons (a local term denoting a silt-rich deposit which aerial survey often shows to mark the former course of rivers).


C3.2.3 Peat

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Peat deposits cover the whole of the property except for marginal areas along the edge of the Wicken Ridge. Peat depth increases with distance south-westward from the Wicken Ridge, ranging from zero on Wicken Breed Fen to c. 4m near Howe's Bank, and from zero to c. 3m on St.Edmund's Fen. In Adventurers' Fen, the Charles Raven Reserve has one metre depth of peat or less in most places, increasing to 2 m. or so in Evans' Fen. The arable land in Adventurers' Fen probably has less than 60 cm of peat, and clay is being ploughed up in some areas.

The peat that now covers the property began to form about 4500 years ago. Radiocarbon dates of 4380 + 140 and 4605 + 110 B.P. have been reported (Radiocarbon, 1, 60-76 (1961)) for oak trees growing on the surface of the Gault Clay at GR TL 559 701 and buried by the onset of peat formation. Recently a date of 4200 B.P. has been obtained for bog oaks from north of the Fen. A radiocarbon date of 3670 + 90 B.P. has been proposed for the basal peat at a site on Christy's Drove (Stout, 1971), but Godwin expressed some doubts about this date.

The only published peat profile (Godwin, 1940), for a site near Drainers' Dyke, indicated Gault Clay at 385 cm below the surface, with wood peat containing abundant Alder (Alnus glutinosa) pollen from 385 to 362 cm. The interval 49-12 cm consisted of shell marl and the remainder of sedge peat, with freshwater shells abundant in the zone 100-49 cm. Subsequent studies have shown that shell marl is widespread at this level, presumably as a relic of a period when large parts of the Fen were flooded during the summer. Another profile has recently been studied at the Botany School, University of Cambridge. The preservation of virtually undisturbed profiles of fenland peat is a feature now almost unique to Wicken Fen. Over most of the rest of Fenland much of the peat has wasted away with drainage, so that the sequence which remains at Wicken is potentially of great significance in the study of the history of vegetation and climate. Much work along these lines has already been carried out (Godwin, 1940; 1978).

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Wicken Fen lies at the edge of the Fen Basin and abuts onto the Wicken Ridge. Originally it had a more natural boundary to the north formed by the Upware Ridge (Corallian Limestone), but a strip between the ridge and the Fen was reclaimed in the 19th century (Rowell, 1983). It is drained by Wicken Lode which has a sinuous course, suggesting that it was dug along the line of a natural watercourse. Drainage is westwards into the River Cam, which is itself a tributary of the Great Ouse. Wicken Lode flows into Reach Lode, which is pumped into the Cam at Upware.

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A monograph of the Soil Survey of Great Britain includes coverage of the Wicken area (Hodge & Seale, 1966). The soils of Adventurers' Fen, save for Evan's Fen, are classified as Adventurers' Series (Shallow phase) and the peat soils over the remainder of the property as Adventurers' Series (Deep phase). Marginal soils on Gault Clay along the edge of the Wicken Ridge are classified as Peacock Series (peaty or humose gleys) and Wicken Series (clay loam with gleying). "Gley" is a term describing the red and grey mottling which is characteristic of poorly aerated, seasonally waterlogged soils. The soil map included in the monograph indicates the presence of shell marl over most of St Edmund's Fen and much of the Sedge Fen, with minor areas in Adventurers' Fen. The monograph includes profile descriptions and analyses. The results of further chemical analyses carried out by the former Department of Applied Biology, University of Cambridge, are in the archives, as are maps of shell marl distribution over the Sedge Fen.

Surface pH values of the peat soils were previously considered to be typically alkaline or neutral, but in recent years surface acidification has been well documented and can probably be explained in terms of increasing dryness. (H. Johnson, 1983-84; m.s. in archives). The soil survey monograph also mentions non-calcareous profiles and describes examples.

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Weather recording at the Fen started in January 1985. Maximum and minimum temperature and rainfall are measured once daily at 16.00, and notes are made on cloud cover and other features of interest. Observations were previously made at Fordham and are currently made at Swaffham Prior, about 6 km south of the Fen. Rainfall is also measured at Upware and at Stretham Pumping Station. The summary below is based on the account in Perring et al. (l964), with additional information from Anon (1963).

The climate is markedly continental by British standards. Notable features include:

Rainfall which is among the lowest in the British Isles, with an annual mean of 525 mm (21"). About 52% of precipitation falls between April 1 and September 30; July to November are the wettest months.

Annual average potential transpiration is about 20", and the average summer water balance (precipitation minus potential evaporation in the period April to Sepember) is about minus 4". The number of days with snow lying is low. The number of dry days ( less than 0.01" precipitation) is high, at about 210. Agricultural drought is likely in 8 years out of ten.

Relative humidity is low compared with other areas, especially in April and May. The annual average is around 75- 80%. Local variations within the Fen in summer can, however, be extreme, depending on the nature of the surrounding vegetation.

Temperatures are similar to most of inland SE England: monthly means range from about 40 F (5 C) in January to about 62 F (16 C) in July. Late frosts are frequent.

Sunshine is intermediate in comparison with the rest of Britain, with about 1500 hours of bright sunshine per year.

Westerly winds predominate; westerly and south-westerly winds are the most common, and south-easterlies the rarest. Mean annual wind speed is about 8 m.p.h. (3.6 m/sec)

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C3.6.1 The drainage system

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The drainage of Wicken Fen is to the west, into the River Cam and thence into the Great Ouse. Wicken Lode flows westwards; most of its flow derives from Monks' Lode (the New River), which arises from springs on the margins of the chalk uplands near Newmarket. Drainers' Dyke is used as a drainage channel from agricultural land to the north of the Fen into Wicken Lode; drainage into Drainers' Dyke is effected with a diesel pump situated on Spinney Bank and operated by the neighbouring landowners. Directions of flow are shown in Figure 26.

There is some possible confusion about the use of various terms describing watercourses. They are used here in the following senses:

Lode - an artificial watercourse, often navigable, and flowing by virtue of carrying water from the chalk uplands on the fen margins to the main drainage channels;

Drain - a large artificial watercourse, usually flowing;

Dyke - an artificial watercourse, usually narrow, and without perceptible flow;

Ditch - a small artificial watercourse, rarely flowing and sometimes only intermittently wet.

C3.6.2 Hydrological Surveys

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In the 1980s, the Committee became concerned about the apparent increasing dryness of the surface peat on the Sedge Fen, and the possible effects of the re-opening of old ditches and of peat-digging. A survey of the hydrology of the Sedge Fen was commissioned and was carried out by the Institute of Hydrology, Staylittle, Llanbrynmair. Their interim report was submitted in December 1985, and the final report December 1988. Their work generally confirmed pre-existing ideas about the hydrology of the Fen (see C3.6.4 below).

Following the waterproofing of the boundary banks on the Sedge Fen and on Adventurers' Fen, the hydrological state of the Fen has apparently changed. This is being investigated in 1992 in a renewed survey of the IOH boreholes on the Sedge Fen, and a new survey of land and water levels and water movement on Adventurers' Fen.

C3.6.3 Water levels in the lodes

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Water levels in Wicken Lode are currently maintained at 1.829 m A.O.D. during the summer and at 1.676 m A.O.D. during
the winter (Rowell, 1983). These levels are controlled by an automatic sluice at Upware, and Anglian Water (formerly the Great Ouse River Authority) and its successor, the National Rivers Authority, have undertaken to maintain these levels.

The level at which water can be maintained in the lode system, and hence in the Fen, is determined by the height of the embankments enclosing the waterways. Concern about the state of some of these embankments led Anglian Water (NRA) to commission consultants to study the problem and make recommendations.

The consultants' report recommended that the part of the Trust property on Adventurers' Fen could be used as an emergency flood storage reservoir. Engineering works would be carried out to prepare the site to receive floodwater under extreme conditions: a flood of a magnitude that may occur only once in 20 years, combined with a 12-hour breakdown of the Upware pumps.

This scheme has now been implemented (1990). Limited bank strengthening has been carried out, and haul roads made along the embankments of Burwell Lode. This involved the remaking of Commissioners' Drain and the excavation of clay from the existing clay pond on Adventurers' Fen; the borrow pit has been retained and is now a large, deep water-body.

A level grassed spillway was built from Wicken Lode onto Evans' Fen; this represents the lowest point in the entire Reach - Burwell - Wicken Lode system at 2.15 m A.O.D. The area which would be flooded in emergency is shown on Map 27.

C3.6.4 Water table, flooding and control

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In the Sedge Fen and St Edmund's Fen, soil water table is related to the water level in Wicken Lode. It was shown in the 1930s (Godwin, 1932) that, during the winter, the soil water table surface is convex, being close to the soil surface in the centre of the Fen, and at lode level near the edges; in the summer, conversely, it is concave and lowest at the points furthest from the lodes. Thus, during wet winters substantial surface flooding may occur, and during dry summers the water table may be well below the surface. These conclusions are supported by the Final Report from the Institute of Hydrology.

In the 1970s, water was pumped onto some of the sedge fields prior to harvesting but this practice has now ceased.

In 1986-87 and 1988-89 Howe's Bank and Spinney Bank, which together form the northern boundary of the reserve, were waterproofed by the insertion of a polythene membrane to a depth of 3-4m. As a result of these works, seepage losses through these banks would appear to have been greatly reduced, and water levels during the summer on the north side of the Fen are much higher than formerly. Some areas of scrub, and some trees, have died as a result.

Water levels in the Charles Raven Reserve are normally dependent upon the water levels in Commissioners' Drain, which lies lower than the rest of the Fen, and leads to a discharge point into the still lower ditch system of the Internal Drainage Board. At present, water can be fed into the ditch surrounding the Mere from Wicken Lode through a 4" diameter pipe. This water can be tranferred to the Reed Field by gravity and this is done from May to August. From September onwards the Reed Field is drained into Commissioners' Drain (see Section A2.4).

In 1985, a culvert under Burwell Lode was installed. A sluice at the southern end potentially allows control of the water level in Commissioners' Drain, and incorporates a meter which measures water discharge into the IDB system. The sluice is under the control of IDB staff.

As part of the NRA Scheme (see C3.6.3 above), a further sluice was installed half-way along Commissioners' Drain. This would potentially allow different water levels to be maintained in the two halves: higher in the part beside the reed field; lower in that beside Rothschild's Lapwing, an area of rough grazing affected by rush invasion. As a result of the engineering scheme, Commissioners' Drain has been widened and deepened. The Borrow Pit is at present an isolated water-body, but will eventually be linked to the ditch along the waterproof bank.


C3.6.5 Trends and changes

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Historical records suggest that flooding of the Sedge Fen surface in the early years of the present century was more regular and deeper than at present (Rowell, 1983). Comparison of past and present records suggests that there has been no change in the level of the surface peat since the 1930s (Gowing, 1977). In spite of this, there is a strong general impression from, for instance, the greater exposure of tree roots on the surface, and the appearance of communities which suggest peat decomposition, that the peat has shrunk in recent years.

In the recent past, the peat had begun to dry to a greater depth in dry summers than it had previously (Gowing, 1977). This can only be attributed to greater evaporative loss from the vegetation, to a decline in inflow, or an increase in outflow through the banks (Gowing, 1977; Rowell, 1983). Godwin (1931) found that loss by evapotranspiration was up to five times greater from grass-dominated vegetation than from scrub. Since the 1930s, scrub has increased greatly at the expense of grassy vegetation; on the basis of Godwin's work, this would be expected to have reduced losses by evaporation and transpiration and thus led to higher water levels. This does not seem to have occurred, so it seems likely that there has been a real drop in water levels on the Sedge Fen, perhaps as a result of the installation of better pumps at Upware in 1942 (Rowell, 1983). However, Lode levels were becoming lower up to about 1949; the installation of automatic pump control at Upware has reduced fluctuations in levels.

These trends would appear to have been at least partially reversed by the waterproofing of Howe's and Spinney Banks mentioned above. The surface peat is consistently quite wet in summer in spite of continued low rainfall since 1988. A new survey of water levels on the Fen is urgently needed.

3.6.6 Eutrophication

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A disturbing trend in recent years has been the increase in nitrogen and phosphorus content of the waters of the Fen (Lunel, 1984; Lee, 1988). The waters are not isolated from those around; Wicken Lode receives, from Monks' Lode, water draining from agricultural land. Water is pumped into Drainers' Dyke from the fields of Spinney Abbey Farm and New Barn Farm. The ditch which eventually flows into the brickpits was found in 1984 to be heavily enriched with phosphorus and nitrogen, originating from an overflowing slurry pit at Spinney Abbey Farm. This has now been corrected, and the improvements to Spinney Bank Ditch in 1988-89 effectively isolated this inflow from the Brickpits area. Finally, faults in the sewage pumping station in Lode Lane can lead to episodes of spillage of raw sewage around the William Thorpe Building and into nearby water-courses. Pressure should be applied to ensure that Anglian Water keeps the pump in good order.

Eutrophication has important management implications for the fen vegetation and for the ditch communities. Interior ditches should, where practicable, be isolated from those continuous with outside watercourses; this may be achieved with peat banks, through which a pipe that gives control over water movement may pass. Water should not generally be conducted into the interior ditches, except in consultation with the Secretary.