The Geology of the Wicken Vision Area, Lower Cam Valley, Cambridgeshire, UK
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Netherlands Journal of Geosciences — Geologie en Mijnbouw | 92 – 1 | 47-59 | 2013 The Geology of the Wicken Vision Area, lower Cam valley, Cambridgeshire, UK S. Boreham Quaternary Palaeoenvironments Group, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, United Kingdom Email: [email protected] Manuscript received: May 2011, accepted: September 2012 Abstract The geology of the Wicken Vision Area in the lower Cam valley is described with reference to a stacked series of ten cross-sections through Holocene and Pleistocene deposits, and Cretaceous/Jurassic bedrock across the region. A detailed cross-section through a Holocene palaeochannel, with radiocarbon dating and pollen analyses is also presented. The structural geology of the bedrock and the stratigraphic arrangement of the Pleistocene and Holocene deposits are used to describe landscape development for the area. This study offers a new understanding of the stratigraphy and context of the Holocene, Weichselian and earlier Pleistocene deposits of the lower Cam valley, and provides a unique view of sediment architecture in valley-fill sediments at the fen edge. Keywords: Holocene, Weichselian, Pleistocene, Cam valley, Wicken Vision Introduction simplistic description masks a wealth of geological complexity in the district. This paper aims to describe the lithostratigraphy and sediment The area included in this study stretches from North Fen, architecture of Holocene, Weichselian and earlier Pleistocene Upware and Wicken Fen in the north, to Reach, Swaffham deposits in the transitional area between the River Cam valley Bulbeck and Lode in the east, Fen Ditton to the south, and and southern Fenland by adding new geological observations Milton, Horningsea and Waterbeach to the west (Fig. 1). The by the author to archival borehole data. Understanding the study area encompasses Adventurers’ Fen, Swaffham Prior Fen geology of the lower Cam valley, known as the Wicken Vision and Bottisham Fen. The River Cam runs southwest to northeast Area, is crucial for articulating two disparate sedimentary across low lying fenland (<5 m O.D.) to the west of the study systems, where the deposits are sometimes cryptic, and area, and part of Adventurers’ Fen to the east is below Ordnance outcrops and topography are often mantled beneath an Datum. Several canalised tributary streams (lodes) including overgrowth of late Holocene peat. Bottisham Lode, Swaffham Bulbeck Lode, Reach Lode and The Wicken Vision is a hundred-year scheme by the National Burwell Lode drain northwest towards the River Cam from Trust, a British conservation and heritage charity, to create a new Chalk springs at the edge of the fenland area. 53 km2 nature reserve in eastern England between Cambridge and Wicken Fen (Fig. 1). Much of the Wicken Vision area is Geological setting privately-owned farmland, although the National Trust currently owns 808 ha (March 2013). Drainage and bedrock Situated at the point where the lower River Cam valley and the south-eastern part of Fenland meet, the area is characterised The River Cam is a tributary of the River Great Ouse, which by its flat, low-lying topography and by soils developed on flows for ca 50 km north of Cambridge through Fenland in a Holocene peat overlying clay-rich bedrock. However, such a largely artificial channel to enter the southern North Sea at 47 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 26 Sep 2021 at 17:18:52, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016774600000275 Netherlands Journal of Geosciences — Geologie en Mijnbouw 92 – 1 | 2013 Fig. 1. Map of the lower Cam valley and the Wicken Vision area showing the simplified geology taken from the Cambridge Sheet (188), and geological sections 1-10 shown in Figs 2-11. Note that Quaternary deposits are mapped only where >1 m in thickness. The location of the geological section at Reach Lode Bridge (Fig. 12) is also shown. The Ordnance Survey grid at 2 km intervals is included for reference. Reproduced with the permission of the British Geological Survey ©NERC. CP13/024 All rights Reserved. 1981. Cambridge Sheet. England and Wales Sheet 188 Solid and Drift Geology. 1 : 50,000. British Geological Survey. King’s Lynn on the Wash. At Cambridge the River Cam drains a aligned with the course of the modern river and disturbing the ca 1000 km2 catchment of low chalky hills to the south. The general dip of the bedrock strata to the southeast. The bedrock geology of the upper Cam valley has been described by in the Wicken Vision area comprises the Cretaceous West Melbury Boreham & Rolfe (2009). Northeast of Cambridge the River Cam (Marly) Chalk Formation (soft limestone), Gault Formation enters the Wicken Vision Area, a region of low-lying fen, before (mudstone), Woburn Sands Formation (sandstone) and the joining the Old West River (River Great Ouse) just south of Ely. Jurassic West Walton Formation (limestone) and Ampthill Clay The geology of the lower valley of the River Cam was first Formation (mudstone) (Moorlock et al., 2003), which are described in detail by Worssam & Taylor (1969), who recognised equivalent to the Lower Chalk, Gault Clay, Lower Greensand, north-south trending anticlinal and synclinal structures (the Upware Limestone and Ampthill Clay of the former classification Upware Axis) in the Cretaceous and Jurassic bedrock broadly (Matthews & Harvey, 1965; Forbes, 1965). 48 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 26 Sep 2021 at 17:18:52, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016774600000275 Netherlands Journal of Geosciences — Geologie en Mijnbouw 92 – 1 | 2013 Pleistocene and Holocene lithostratigraphy and beetle remains indicating an arctic environment. Later, Bell & depositional history Dickson (1971) reappraised the identifications of Chandler. The uncalibrated radiocarbon age calculated for plant material from Glacial till (boulder clay) was identified by Worssam & Taylor the site was 19,500±650 years BP (Godwin & Willis, 1964). This (1969) as one of the oldest Pleistocene deposits of the district. radiocarbon date calibrates using OxCal (Bronk Ramsey, 2009) The ‘valley gravels’ of the Cam system were separated by to 23,375±778 years calBP and places the 1st terrace into the elevation into numbered terraces (4th, 3rd, 2nd, 1st) using a late Weichselian (MIS 2). morphostratigraphic paradigm (Dixon, 1980; Clayton, 1981). The Holocene deposits of the lower Cam are represented by The oldest Pleistocene courses of the River Cam (3rd and 4th floodplain alluvium, palaeochannel silts, shell marls from cut- terraces and the Observatory gravels) pass through north off river meanders, fen peat (Nordelph Peat), and by intertidal Cambridge and continue northwest towards Willingham, the silts and clays (Barroway Drove Beds) from an early Bronze Age River Great Ouse and southern Fenland (Gao & Boreham, 2011). episode (ca 3,700 years calBP) (Waller & Hall, 1991) of marine The lithostratigraphy of the Pleistocene deposits in the valley inundation, which reached the northern part of the Wicken of the River Cam was described by Boreham in Bowen (1999). Vision Area. Several southern tributaries of the River Cam were Glacial till was attributed to the Barrington Works Member of diverted and canalised in post-Roman times to form artificial the glaciogenic Lowestoft Formation (LF) of presumed Elsterian ‘lodes’. Many of the lodes run in raised embankments above (MIS 12) age. Post-Elsterian fluvial sediments were placed the level of the surrounding fenland, and most have been in the fluviogenic Cam Valley Formation (CVF). Within this straightened or run in completely artificial courses, leaving formation, the 3rd Terrace early-Weichselian deposits of the older abandoned palaeochannels meandering across the fen, River Cam were correlated with the Arbury Member (CVF). such as the one encountered at Reach Lode Bridge. In mid-Weichselian times the River Cam avulsed to the east, Within the Cam Valley Formation, the 2nd Terrace mid- presumably into a pre-existing strike-aligned tributary valley Weichselian deposits of the River Cam are correlated with the also joining the River Great Ouse to the north, and formed a Sidgwick Avenue Member (CVF). The late-Weichselian 1st Terrace new ca 1 km wide braidplain indentified as the 2nd terrace. This deposits are correlated with the Barnwell Station Member (CVF) terrace feature runs to the west of the River Cam from Milton when exposed at or above the floodplain, and with the late- to Landbeach and Waterbeach (Fig. 1). The 2nd terrace is dated Glacial Midsummer Common Member (CVF) where present in a from the classic site at Sidgwick Avenue, Cambridge (Lambert buried channel-form beneath the floodplain surface. All et al., 1962) where plant macrofossils and molluscs from silty Holocene deposits, including peat and alluvium, were placed in and sandy bands within gravel indicate cool to cold climatic the Fenland Formation (FF) (Bowen, 1999). conditions. The author carried out radiocarbon dating on a skull In general the stratigraphy of the lower Cam valley of Bison priscus from the site, held at the Sedgwick Museum, corresponds with other parts of the Cam valley system. The University of Cambridge (Boreham, 2002). The uncalibrated stratigraphic units proposed in Bowen (1999) allow correlation radiocarbon (AMS) age for bone from the bison