The origin of relic cryogenic mounds at East Walton and Thompson Common, Norfolk, England

Proceedings of the Geologists’ Association 126 (2015) 522–535

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The origin of relic cryogenic mounds at East Walton and Thompson Common, Norfolk, England P. Clay * Richard Hale School, Earth Science, Hertford, Hertfordshire, SG138EN, United Kingdom

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 April 2015 Received in revised form 19 June 2015 Accepted 21 June 2015 Available online 4 September 2015

Contemporary pingo and lithalsa mounds are features which develop exclusively within cold climate environments in areas of continuous and discontinuous permafrost. The presence of pingo and lithalsa remnants as rampart enclosed ponds has been documented across temperate areas of the Northern Hemisphere and has been used to establish the extent of former permafrost. Two sites thought to be remnants of hydraulic pingo forms were investigated at East Walton and Thompson Common(s) in order to establish a precise origin. Through Ground Penetrating Radar and Electrical Resistivity Tomography the structure of ramparts and hollows has been investigated and interpreted. Linking this with physical and ground invasive techniques and a consideration of the hydrogeological setting it was apparent that the two sites had different origins. The topography, geology and hydrogeology of the East Walton area are a stereotypical setting for the development of hydraulic pingos. In contrast, the development of features within the Lowestoft Till and weathered Chalk Formations at Thompson Common implies that forms resulted from the heave of material through segregation ice lens growth. This has led to the creation of a model at East Walton which documents the development of remnant pingo ramparts and the strata which result. It is proposed that this model can be used in the study of similar landforms. ß 2015 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

Keywords: Pingo Lithalsa Periglacial Devensian Geophysical

1. Introduction During the Younger Dryas 12,800–11,500 years before present, large stretches of lowland Britain lay outside the limits of the wasting Devensian ice sheet within a vast periglacial domain. At the beginning of the Holocene, permafrost had receded northwards towards Scotland, to leave a suite of landforms across the south of the UK that were highly distinctive and characteristic of the former periglacial landscape. The ramparted ponds at East Walton, and Thompson Common(s) represent some of the most extensive and best preserved landscapes modified by the growth and decay of late Devensian perennial ground ice throughout the whole of N.W. Europe. Sites across Europe containing relict cryogenic mounds have been investigated and interpreted, in Belgium (Pissart, 1974 and 2002), South Wales (Watson and Watson, 1974), Ireland (Mitchell, 1971), Finland (Seppa¨la¨, 1972), and East Anglia (Sparks et al., 1972). The interpretation of these sites was linked to the early work of Mu¨ller (1959) and Mackay (1962) who, proposed that

Abbreviations: GPR, Ground Penetrating Radar; ERT, Electrical Resistivity Tomography; AOD, above ordnance datum. * Tel.: +07792415060. E-mail address: [email protected]

massive ice lens formation sourced from groundwater under hydrostatic ‘closed system’ or hydraulic ‘open system’ pressure, governed pingo origin. These early observations are still relevant and explain the genesis of most pingo forms. For those which were difficult to interpret or did not conform to the pressure mechanisms identified, Worsley and Gurney (1996) proposed the term ‘polygenetic’. Since the 1980s research has challenged the original interpretation of these ramparted ponds as relict hydraulic pingos and favoured instead a lithalsa origin (Seppa¨la¨, 1988; Mollard, 2000; Pissart, 2002; Gurney, 1995, 1998; Ross et al., 2005a,b). Although almost identical in morphology, a decayed lithalsa forms in fine grained silt sediments where capillary water is supplied to the ice body by cryosuction. In comparison hydraulic pingos generally form at the foot of high ground in coarse grained colluvial valley fill sediments where ground water is driven to the surface under an artesian head to form a pingo spring. Since the original study of East Anglian relic permafrost sites around East Walton, Norfolk by Sparks et al. (1972), new fieldwork techniques and improved knowledge of similar sites across the modern sub arctic and former temperate permafrost zones have resulted in the reinvestigation of many known European sites. The chosen study sites in Norfolk, East Anglia (East Walton and Thompson Common) are the best examples of rampart enclosed

http://dx.doi.org/10.1016/j.pgeola.2015.06.006 0016-7878/ß 2015 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

P. Clay / Proceedings of the Geologists’ Association 126 (2015) 522–535

depressions within eastern England and are sparsely documented (Prince, 1962, 1964; Sparks et al., 1972). Existing information on the East Anglian sites predates the key publications of modern active pingo research (Mackay et al., 1973; Mackay, 1988, 1998) and work on relic permafrost morphology (Flemal, 1970; De Gans, 1988; Pissart, 2002). For this reason, this study reinvestigated these sites using traditional fieldwork methods of core sample collection, field mapping and topographical surveying, employed by Sparks et al. (1972), with geophysical methods employed by Ross et al. (2005a), in Llanpumsaint, Wales. The data collected was used to improve the knowledge of distribution, morphology, structure and origin of the features at East Walton and Thompson Common. The study also investigated the geological and hydrogeological setting of the sites. By proposing an accurate origin for the relict cryogenic mounds at both sites, it is possible to address the outstanding issue relating to the extent of the former permafrost zone within East England. 1.1. Area of study The two study sites are located within Norfolk, England. East Walton Common, which can be located by National Grid Reference 316500E–574400N, is located within north Norfolk, approximately 10 km to the south east of Kings Lynn. Thompson Common, which can be located by National Grid Reference 593000N, 29600E, is

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located approximately 30 km to the southeast of East Walton, approximately 15 km north of Thetford (Fig. 1). A third location at Gayton Common, 2 km north of East Walton Common, was used to give spatial distribution of features in the fields surrounding East Walton Common, and has not been investigated in detail. The regional geology of East Anglia is divided in two. The western part of the region covering west Norfolk and north Cambridgeshire is the Fens, a low-lying area consisting of marine derived glacial clay and silt overlying Gault Clay, dissected by sandy, gravel River Terrace Deposits. Flandrian peat deposits cover the fens and support the regions agriculture. In contrast, the eastern part of the region covering east Norfolk, and Suffolk and within which the study sites lie is the chalk high ground which rises to 100 m AOD and dips steeply within the western reaches at the geological boundary with the Gault Clay of the Fens. The regional hydrology is dominated by the River Great Ouse and its tributaries the Little Ouse, Nar, Wissey and Lark. The middle and lower Chalk high ground within eastern East Anglia is the principle aquifer for Norfolk and west Suffolk, but contemporary groundwater levels are influenced by extraction pumping and thick Pleistocene Head and Till deposits that limit infiltration. 1.2. East Walton Common East Walton Common is situated 3 km north of the village of Gayton, National Grid Reference 319200E, 572700N on the A47

Fig. 1. Shows the solid geology of East Anglia which underlie both study sites (Geological Conservation Review, 2007, p. 2731).

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Kings Lynn to Swaffham. It is approximately 1.5 ha of flat land located at the foot of the south east face of the chalk escarpment on the geological boundary between the western limit of the Chalk and the Gault Clay of the Fens with Nar Valley and River Terrace Deposits overlying the Chalk within the south east of the Common. From East Walton (15 m AOD) the land rises eastwards to approximately 80 m AOD approximately 4 km east. The site comprises East Walton Common, a nature reserve of woodland, grassland and wetland habitats owned by English Nature. The features of East Walton Common consist of oval, horse shoe, and elongated ramparts up to 3 m high and enclosing water filled depressions. These ‘ramparted depressions’ range in size from 20 m to 150 m in diameter. Aerial photographs show that beyond the common within the neighbouring fields, ramparts have been levelled and ponds filled by ploughing with the exception of Gayton Common, 2 km north. Active springs and a high water table recharge the ponds all year round and supply water to the small streams that transect the common from east to west.

2. Materials and methods 2.1. Geomorphic mapping Geomorphic mapping was conducted at a 1:5000 scale from aerial photographs of East Walton, Gayton and Thompson, and proven by a walk over survey of the sites. Aerial photographs were scaled to 1:5000 OS base plans and divided into a grid, on which the size, shape, and location of each depression were superimposed. This made it possible to extract data for spatial analysis from all three sites so that these could be compared to sites of known origin. 2.2. Ground Penetrating Radar (GPR) GPR was chosen as a non-intrusive method to investigate the internal structure of ramparts and depressions. Similar work was successfully conducted by Ross et al. (2005a) on rampart enclosed depressions in Wales. Access to both commons prohibited the use of vehicles and large mechanical equipment, limiting the fieldwork to hand portable equipment.

1.3. Thompson Common 2.3. GPR and Topographical Survey Thompson Common is located approximately 20 km south east of East Walton, on the A1075 4 km to the south of Watton, National Grid Reference 300800E, 591700N. Thompson Common is a flat area of approximately 2.5 ha. The local highpoint is 56 m AOD at Rockland St. Peter 6 km to the north east. In contrast to East Walton Common, the site comprises a thin band of coarse sand gravel River Terrace Deposits overlying the Lowestoft Till Formation with chalk at depth. Thompson Common is a nature reserve consisting of woodland, and wetland which is managed by The Norfolk Wildlife Trust. A number of footpaths cross the common, most notably the Pedders Way which passes through the south east part of the common, and the Great Eastern Pingo Trail which loops around the common. The features of Thompson Common are similar to those found at East Walton Common. The depressions are water filled but few are enclosed within a complete rampart. Those ramparts that do exist are smaller and less well defined than those enclosing depressions at East Walton. Aerial photographs and a walkover of the area confirmed that these features once extended into the surrounding fields, but have been destroyed by ploughing.

The GPR survey was conducted using a PulseEKKO PE100 with a 200 MHz antenna with 1000-volt transmitter. A low frequency antenna was chosen to achieve a greater depth of penetration; however the depth of penetration was dependent upon the electrical conductivity of the substrata and the ground water table. Processing included Ekko trace Fix, dewow, AGC gain and migration using PulseEKKO software and a 2D F-K Migration package. Based on a CMP survey the average velocity of the electromagnetic waves was 0.08 m/ns at East Walton and 0.150 m/ ns at Thompson. A topographical survey was conducted using a Leica System 500 and 1200 to recreate 2D and 3D computer representations of the morphology of these features in which to insert the geophysical data. The survey was ‘static’ and ‘kinematic’ and collected spot heights from across the ramparts. At East Walton Common GPR data were collected from ‘Feature A1’ along 25 m profiles perpendicular to the ramparts with an antennae spacing of 0.5 m and a step size of 0.1 m (Fig. 2). The survey was limited to the inner and outer rampart walls due to water within the pond.

Fig. 2. Shows Features A1 and B2 and the location of GPR, ERT and intrusive surveys. Aerial photograph from Google Earth (2015).

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Fig. 3. Shows Features A2 and B2 and the location of the GPR, ERT and BH investigation. Aerial photograph adapted from Google Earth (2015).

At Thompson Common the GPR survey of ‘Feature A2’ was identical to ‘Feature A1’ at East Walton Common with four 25 m profiles set out perpendicular to the ramparts with an antennae spacing of 0.5 m and a step size of 0.1 m (Fig. 3). The GPR survey was limited to the inner and outer rampart walls due to water within the pond. This survey produced little information about rampart structure, and it was decided to extend the survey to a dry depression (Feature B2) within the south of the common. Data was collected along two 50 m transects spanning the rampart and the depression.

sample recovery was impeded by a high water table, and dense sand. Particle size analysis was conducted on a total of 10 samples from each site varying in location and depth, to identify the nature of the underlying geology. Samples were dried and weighed. Each sample was sieved in a stack ranging from 4 mm to 0.065 mm by shaker and then each fraction reweighed to determine the particle size distribution. 2.6. Spatial analysis

ERT was conducted at East Walton Common on the rampart and depression of Feature A1 and the dry depression of Feature B1. At Thompson Common, Feature B2 was surveyed to obtain structural data from and identify features which GPR could not. ERT was achieved using a Campus Tigre with 32 and 64 electrodes. Topographical data of electrode spacing was collected by Leica 1200 and imported into Microsoft Excel. The Leica data was edited to include the northings, eastings and height values only, and imported with the electrode data into RES2DINV for Windows XP for processing which included data inversion, extermination of bad points and topographical correction. At both sites the electrodes were spaced at 1.5 m intervals and set out along transects ranging from 50 m to 100 m. The ‘dry’ nature of the depressions of Features A1 and B1 at East Walton Common and B2 at Thompson Common allowed information to be collected from the rampart and hollow using a combination of ERT and GPR. These features were similar in form and therefore assumed to be similar in origin to the pond filled hollows found elsewhere across both sites.

Spatial analysis was used to apply Pissart’s (2002) theory that density can determine the origin of former ground ice landforms. Pissart (2002, p. 15) states that nowhere in the Arctic do ‘pingo forms have the high density seen at Hautes Fagne’, thus arguing against relic pingo interpretations at this site. Data for different types of pingo and lithalsa forms was collected from sites of known active and relic origin including the hydrostatic pingos of Tuktoyaktuk Peninsula (Mackay, 1962, 1977, 1998), the active hydraulic pingos of Greenland (Cruikshank and Colhoun, 1965; Worsley and Gurney, 1996), Alaska (Yoshikawa et al., 2003) and the relic lithalsas of Hautes Fagnes, Belgium (Pissart, 1974, 2000, 2002) and Wales (Gurney, 1995). The approximate shape and density of each feature at these comparison sites, East Walton (including the neighbouring fields at Gayton) and Thompson Common were determined from measurements obtained using Google Earth, and proven by field mapping at East Walton and Thompson Common. Pingo shape is variable with circular, oval, elongated hollows, surrounded by circular, and horseshoe shaped ramparts. In order to simplify the analysis of these features the widths and lengths of the ponds were measured as an approximate indication of hollow symmetry.

2.5. Hand auguring and particle size analysis

2.7. Site hydrogeology

A hand augur was used on shallow subsurface structures identified by geophysical methods to collect samples for particle size analysis to assess the suitability of the site geology for perennial ground ice formation. Holes were augured at 10 locations at each site (Figs. 2 and 3), up to 1.5 m below ground level within the rampart, and 2 m within the depression using a mild steel hand auger. A ‘Combination head’ was used to recover cohesive and granular samples from the rampart, and a ‘gouge head’ to collect soft cohesive layers of peat and clay from within the ponds. At shallow depths across both sites

Groundwater Contour Nets were constructed using mean groundwater elevation levels of boreholes on East Walton and Thompson Common(s), provided by the Environment Agency. Using 1:5000 Ordnance Survey maps, the borehole locations were plotted accurately to a scaled plan. Three on-site observation boreholes were used to relate groundwater level to a point above ordnance datum. Borehole locations together with the groundwater elevation OAD were added to produce a simplified groundwater contour map. The direction of groundwater flow was added at right angles to the contours.

2.4. Electrical Resistivity Tomography (ERT)

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3. Results 3.1. Ground Penetrating Radar At East Walton Common the GPR survey collected data from four profiles of Feature A1 achieving depths of 6 m. At Thompson Common the GPR survey of Features A2 and B2 achieved depths of 6–7 m. 3.1.1. East Walton Common Within profiles 1–4 of Feature A1 (Fig. 4) the data collected shows that ramparts are typically 3 m high and exceed 20 m in width. The core of the rampart consists of discontinuous concave reflections surrounded by steeply dipping stratigraphy. The solid (red) line proposes the original shape of the rampart, consisting of material which dipped steeply away from the centre of the rampart at angles greater than 35 degrees. Within the rampart core discontinuous reflections characterise its structure. Within the rampart wall (dashed red line) clinoforms dip at angles of less than 25 degrees. Typically these areas are 1–2 m thick at the rampart base, and thin towards the top of the rampart suggesting that the material derives from the top of the rampart. At the base of the inner rampart continuous horizontal reflections (coloured green) on lap the inner rampart wall. Profiles 2 and 3 on Fig. 4 exhibit shallow dipping clinoforms between 0 m and 14 m (highlighted in yellow) under the existing rampart structure. After further processing to migrate the image, and eradicate anomalies, the continuation of these reflections

suggests that the feature is real. Within profiles 1 and 4 at the base of the outer rampart between 0 and 2 m a mound structure approximately 0.5 m high and 4 m wide and consisting of convex reflections is truncated by the outer wall of the rampart. The water table seen as a strong horizontal reflection (dashed blue) cuts across profile 2 at 1 m depth. 3.1.2. Thompson Common The GPR results for Feature A2 show a series of discontinuous convex and concave reflections interrupted by hyperbolic reflections. Using Pulse Ekko software the data was migrated to remove the reflections thought to be caused by tree roots. After migration and the removal of the hyperbolic reflections very little information was obtained about the internal rampart structure (Fig. 5a and b). The GPR results for Feature B2 at Thompson Common include data collected from the ramparts and depression. These profiles suggest that the depression is approximately 2 m deep and 20 m  16 m in area (Fig. 6). The base of the depression is interpreted as the strong concave reflection highlighted red. In comparison to the ramparts at East Walton Common, Feature B2 at Thompson Common is considerably smaller at 2 m in height and 8 m in width. The rampart structure of Feature B2 profile orientated east to west, suggests a core of discontinuous reflections similar to that seen in profile 4, Fig. 4 at East Walton Common, however the ramparts surveyed show less evidence of post glacial modification and infilling of the depression witnessed at East Walton Common. This suggests that the once active forms of the features at

Fig. 4. Profiles 1–4 from Feature A1 at East Walton Common. Shows that all of the profiles have a similar structure. The key features are depicted above. The bold red line indicates the original steep shape of the rampart prior to modification. The dashed line indicates that modification processes have occurred on the flanks of the rampart to widen it, and reduce its height. The solid black line indicates possible sub-rampart structures which may represent an early structure at this location. The green horizontal reflections are interpreted as pond fill that occurred after the ice core has melted. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. (a and b) Shows profile 1 was migrated to collapse these points however very little structural information was obtained from (a).

Thompson Common are likely to have been lower and smaller than the higher, wider mounds at East Walton Common (Fig. 7). Filling the depression to depths of 3 m (lined green) are horizontal continuous reflections which on lap the inner rampart walls in a similar fashion to that seen at Feature A1 at East Walton Common. Strong horizontal reflections crosscut the profile at 6 m depth (highlighted by a blue dashed line) is interpreted as the Groundwater table which has reduced the strength of the GPR signal, making it harder to collect information below this depth. 3.2. Electrical Resistivity Tomography profiles 3.2.1. East Walton Common The Resistivity profile (Fig. 8) of feature shows that the ramparts extend from 0 to 48 m and 61 to 100 m marked by a

medium to very high resistivity, ranging from 149–260 Vm to 3 m depth. Between 6–24 m and 67.5–76.5 m there is an area of higher resistivity with a maximum value of 1168 Vm to depths of 1 mbgl. Ground water reduces resistivity below 1.5 m depth. A change from medium to low resistivity (a minimum of 32 Vm) between 48 and 67.5 m suggests the depression is up to 5 m deep. The profile from Feature B1, East Walton Common (Fig. 9) shows similar apparent resistivity to that from Feature A1. The highest resistivity of 462–242 Vm to depths of 1 mbgl can be seen along a tracked area (between 3 and 13.5 m). Between 0–18 m and 27–36 m the rampart has medium resistivity with a maximum of 242 Vm at the surface, and a minimum of 127 Vm at 2 mbgl. The depression is characterised by medium resistivity ranging from 127–176 Vm to depths of 5 mbgl. Below 1 m the areas of low resistivity are interpreted as saturated strata.

Fig. 6. Profile 1 of Feature B2 aligned north to south. The outline of the depression is highlighted red, and the subsequent fill material shaded green. The water table is highlighted by the blue dashed line. On the northern side of the feature there is no pronounced rampart. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 7. Profile 2 of Feature B2 aligned west to east. The outline of the depression and rampart is highlighted red, and the subsequent fill material shaded green. The water table is highlighted by the blue dashed line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. ERT profile of Feature A1 at East Walton.

3.2.2. Thompson Common The profile from Feature B2 (Fig. 10) between 21 and 34.5 m suggests that the depression is concave and up to 6 m deep. The base of the depression is characterised at the boundary of medium resistivity shown in green with a maximum of 150 Vm and the low resistivity with a minimum resistivity of 31 Vm. This contrast suggests a change in subsurface material. A similar drop from medium to low resistivity between 4.5–12 m and 36–45 m suggests that a second and third depression could be present to either side of the surveyed depression. At the near surface to a depth of 3 m, high resistivity rampart material with a maximum of 485 Vm overlies the depression.

3.3. Hand auguring 3.3.1. Ground proofing geophysical data Using the ERT profiles, shallow hand augered hollows were sunk to prove the geophysical interpretation (Table 2). Above the water table the ramparts are shown to have medium to high resistivity which auguring to depths of 1 m revealed to be dry, coarse sand (Figs. 11–13 sample holes S1, 2, 4, 5, 9, 11). The water filled depression of Feature A1 at East Walton Common has low resistivity at the near surface which auguring revealed as saturated uncompact fibrous peat (Fig. 11 sample hole S3). Underlying the peat is grey chalk silt containing soft chalk

Fig. 9. ERT profile of Feature B1 at East Walton.

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Fig. 10. ERT profile of Feature B2 at Thompson.

Fig. 11. Profile of Feature A1 at East Walton showing the areas recorded for auguring.

rubble is represented by a zone of higher resistivity. This zone of higher resistivity suggests that the depression extends to 8 m and is cone shaped. In profile B1 at East Walton Common, the high resistivity along a tracked area (3–13.5 m) that run across the feature is considered to be due to the recent modification of the rampart by vehicle and cattle. Auguring within this zone at sample hole S6 revealed that the material was no different to the rest of the rampart. The medium resistivity of the hollow fill below 2 m was revealed by hand auguring to be a zone consisting of gravelly coarse sand likely to be collapsed rampart material (Fig. 12). At Thompson Common the areas of high resistivity shown in Feature B2 sample holes S10 and S11 were augured to depths of 2 m. The material overlying the depression to depths of 1.2 m in sample hole S10 is considered to represent a secondary collapse of rampart material into the depression. The material below this coarse sand is dry silty clay. The area of low resistivity between survey points 24 and 34.5 m sampled in sample hole S11 at 2.1 m

depth suggests a change in substrate. Auguring sample hole S10 penetrated this zone to reveal a wet chalk silt characteristic of weathered chalk bedrock at 1.95 m. Auguring could not penetrate deeper than 2.1 m however the ERT profile suggests that the chalk silt fills the hollow (Fig. 13). The areas of medium resistivity between 12–20 m and 30–39 m at a depth of 2.5 m are interpreted as the continuation of coarse sandy rampart material. 3.3.2. Rampart profiles Particle size distribution (PSD) results for samples recovered from East Walton Common prove to depths of 1 m that the ramparts sampled at S1, 2, 4, 5, consist of an orange slightly gravelly (5%) medium (50–60%) to coarse (20–25%) sand with less than 5% fine grained material. Below 1 m the samples recovered show a coarsening of material to 10–15% coarse sand. The gravel is predominately fine subangular flint with the occasional coarse flint fragment.

Fig. 12. Profile of Feature B1 at East Walton showing the areas recorded for auguring.

Fig. 13. Profile of Feature B2 at Thompson showing the areas recorded for auguring.

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Particle size distribution results for samples recovered from sample holes S6, 9, 11, at Thompson Common prove that the rampart is constructed of yellow slightly gravely (5%) slightly silty (10%) medium (55–75%) to coarse (10–15%) sand. The gravel is predominately fine sub-angular to sub-rounded flint and soft chalk fragments. There is no obvious change in particle size distribution with depth however the sand is light grey in colour below 1 m. Ground water was encountered at approximately 0.7 m outside of the rampart. No samples were recovered below 1.75 m from the ramparts due to the dense nature of the material. Samples taken from the ramparts of Feature A1 at East Walton Common indicate that the sand has a greater coarse fraction (25%) in comparison to sand recovered from Feature B2 at Thompson Common which is predominantly medium grained sand. 3.3.3. Pond fill The pond stratigraphy of Feature A1 at East Walton Common and Feature A2 at Thompson Common shown in Figs. 10 and 11,

sample holes S3 and S7 is dominated by loosely compacted fibrous Peat. Within the deepest sections of both ponds, a dark dense silty organic (Gyttija) was recovered at depths below 1 m. An organic rich light grey silty clay recovered as a 100 mm layer within the auger head lines each pond (Table 2) is believed to be the base of the pond. In contrast the depression fill of Feature B1 at East Walton common shown in Fig. 12 sample hole S10 consists of 1.2 m thick slightly clayey coarse sand above a thin deposit (0.4 m) of soft brown sandy organic clay. Between 1–1.2 m the sand is saturated. Within the base of the depression below 1.6 m is coarse sand with occasional fine angular chalk and flint gravel resembling rampart material. Similarly the fill stratigraphy of Feature B2 at Thompson consists of thin slightly clayey organic sand topsoil above a 1 m thick deposit of slightly clayey slightly gravelly sand similar to rampart material. Below 1.2 m dark grey organic rich silt with occasional quartz sand fills the depression to depths of 2 m (Table 2). Recovered below 2 m white creamy silt with soft

Table 2 Showing a representation of the lithology augured from the ramparts and depressions of Feature A1 at East Walton Common and Feature B2 at Thompson Common. Similar stratigraphic sequences were found through either site. Location

Depth (cm)

Stratigraphy

(S2) Rampart Feature A1 (Walton Common)

0–150

Sand Topsoil merging into Medium grained SAND with occasional fine to medium gravel of flint and chalk (River Terrace Deposits)

(S11) Rampart Feature B2 (Thompson Common)

0–150

Sand Topsoil merging into Slightly clayey medium SAND with occasional medium gravel of flint (River Terrace Deposit)

(S3) Pond Feature A1 (East Walton Common)

0–50

Spagnum PEAT with wood fragments

(S10) Pond Feature B2 (Thompson Common)

100–200

Gyttja with micro shell

200–210

Pale Grey silty CLAY with organic fragments

0–120

Coarse clayey SAND with medium gravel of flint and wood fragments (Reworked River Terrace Deposit)

120–195

Dark grey silty CLAY with occasional sand (Lowestoft Till)

195–210

Cream SILT putty chalk (Grey Chalk).

Photograph

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fractured chalk nodules is interpreted as a zone of weathered chalk bedrock (Table 2). 3.3.4. Chalk bedrock On behalf of the Environment Agency, boreholes were sunk by Soil Mechanics in 1995 within the western part of the common and show superficial deposits of slightly gravelly sandy River Terrace Deposits overlie the chalk to a depth of 5 m. Visual inspections of the ground within the eastern part of East Walton Common indicate that chalk bedrock outcrops at the surface with a thin topsoil cover. In contrast at Thompson Common, BGS plan no.160 Swaffham (Barrow et al., 1999) indicates that the Lowestoft Till formation underlies the site to depths of +10 m. However Soil Mechanics logs from boreholes sunk on the Common show the chalk bedrock at 4 m depth, covered by a layer of slightly gravelly clayey sand and a 2 m think band of grey clay of the Lowestoft Till. This latter interpretation was confirmed by auguring within the depression of Feature B2, which revealed this grey clay as a 0.75 m thick layer above a white silt putty chalk likely to represent the top of the white chalk bedrock at 2 m at approximately 28 m AOD (Table 2).

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Table 1 The mean, mode and median of hollow areas. Location

Mean

Mode

Median

Range

East Walton Gayton Thompson Tuktoyaktuk

776 895 620 1804

300 150 150 300

462 100 400 1500

50–3600 50–3250 100–2400 225–5000

Table 1 shows that East Walton, and Thompson Common have many smaller ponds (50–150 m2), and almost no large ponds which exceed 3500 m2. In contrast, the Tuktoyaktuk Peninsula hydrostatic pingos and Hautes Fagne Plateau relic lithalsas have several features with areas which exceed 3000 m2, which suggests no relationship between contemporary hydrostatic pingo and relic lithalsas, because these features are generally much larger. The data was plotted in log-space to transform them to approximately normal distribution. Further analysis comparing the pond dimensions (width to length) of the features across the five sites showed that there is no detectable difference between the dimensions (length and width) of all of the features as shown in Fig. 15.

3.4. Spatial analysis Fig. 14 shows that the most common pond areas across all five sites range from 100 m2 to 400 m2. This suggests that the East Anglian and Canadian sites are similarly dominated by smaller features. The ‘Mean pond area’ for East Walton Common and Thompson Common are very similar ranging from 620 m2 to 895 m2 which suggests that the relationship between the East Anglian sites is strong. In comparison, the Tuktoyaktuk Peninsula and Hautes Fagne Plateau sites have much greater ‘mean pond areas’ ranging from 1761 m2 to 1804 m2. The hydrostatic pingos of Toytukyatak Peninsula grow much larger than active hydraulic pingo sites in Greenland and Alaska, suggesting a relationship between the East Anglian sites and those sites observed in Greenland and Alaska but no relationship between any of the relic or contemporary hydraulic pingo sites and the hydrostatic pingos of the Tuktoyaktuk Peninsula.

3.4.1. Pond density The density of contemporary and relict perennial ice mound morphology was compared across a number of sites using aerial photographs from Google Earth and field maps from the published literature (Wu et al., 2005; Allard and Rosseau, 1999; Pissart, 1988). Each study site was divided into 1 km2, and the features within each were totalled to give a density per km2. The greatest density of landforms was achieved at an active lithalsa site at Nunvak, Quebec and the lowest at a hydraulic pingo site in Alaska (Table 3). The East Anglian sites have similar density to each other, with East Walton Common having the marginally greater density of 38/km2, compared to Thompson common (37/ km2). With the exception of the Quebec site, East Walton Common and Thompson Common have a considerably higher density of features per kilometre than the other comparison sites, but have a similar density to those observed in Trail Island, East Greenland by Worsley and Gurney (1996).

Fig. 14. Shows that the most common range of features is 100–500 m2. The East Anglian study sites include East Walton Common and neighbouring village of Gayton and Thompson Common.

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Fig. 15. Compares the dimensions of pond width against lengths for all the study sites (in Log-1).

Table 3 Density of features per 1 km2. Taken from aerial photographs, and field maps obtained by the literature review.. Form

Locality

Approximate density/km2

Unknown Unknown Unknown Active lithalsa Active hydraulic pingo Relic lithalsa Active hydraulic pingo Active hydrostatic pingo Relic lithalsa Active hydraulic pingo

East Walton Common, Norfolk (aerial photograph and field mapping) Gayton, Norfolk (aerial photograph and field mapping) Thompson Common, Norfolk (aerial photograph and field mapping) Northern Quebec (Allard and Rosseau, 1999) East Greenland, Trail Island (Worsley et al., 1995) Hautes Fagnes Plateau, along the Monshau–Eupen road, Germany (Pissart, 2000) Tibetan Plateau (Wu et al., 2005) Tuktoyaktuk Peninsula, Canada (Mackay, 1998) and aerial photograph Llanpumsaint South Wales (aerial photograph) Alaska (Pissart, 1988)

38 15 37 +67 33 20 20 17 10 1–2

Further analysis of the study sites using 1 km2 grids of the aerial photographs, allowed each site to be converted into a Poisson distributed variable with an approximate 95% confidence interval at 2  sqrt(N) – Table 4. The results determined below show that East Walton and Thompson have a marginally higher density than the other sites. The exception is the active lithalsa site of Northern Quebec, where high densities make this site significantly different to all of the other locations. The Hydrostatic Pingos of the Tuktoyaktuk Peninsula have the lowest densities. In general with the exception of the lithalsa site in Quebec, there is little difference between the surface densities of hydrostatic, hydraulic pingos and lithalsa forms.

Table 4 Poisson distributed variable analysis of surface density for site to site comparisons of landform density. Locality

N

2  sqrt(N)

Northern Quebec East Walton Thompson Trail Island Hautes Fagnes Tibetan plateau Tuktoyaktuk Llanpumsaint

67 38 37 33 20 20 17 10

16.37 12.32 12.16 11.48 8.94 8.94 8.2 6.34

3.5. Hydrogeology Groundwater flow follows the chalk bedrock which underlies both sites at relatively shallow depth. The groundwater contour maps were constructed using mean averages of yearly ground water elevations from data provided by the Environment Agency monitoring programme. The reliability of the ground water elevation models are restricted by missing data or relatively short monitoring periods. Although details of the aquifer underlying each site remains unknown, it has been assumed that each common is influenced by one aquifer, and that the mean ground water elevation and direction of flow proposed for each site relate to single aquifer dominance. Around East Walton the chalk aquifer is not confined. Groundwater was encountered within the sandy River Terrace Deposits at 0.5 m below ground level in holes augered at the base of the outer rampart wall. From the Environment Agency’s groundwater monitoring programme, mean ground water elevation was highest within the north east of the common at the base of the chalk escarpment (16.5 m AOD). Groundwater elevation is shown to drop with distance away from the chalk high ground across the common to 13 m AOD at Walton Wood approximately 1.5 km southwest of the chalk escarpment. A south westerly groundwater flow is therefore the result of the Chalk high ground in the north east.

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On Thompson Common groundwater was also encountered at relatively shallow depth during hand auguring and could be representative of a perched water table in localised sand deposits. Groundwater elevation is greatest at Caston Common (38 m AOD). Groundwater elevation drops south westerly across the site to 28.5 m AOD at Breckles Heath. This shows that a south westerly groundwater flow is imposed on the common from the Chalk high ground to the north east. The Lowerstoft Till which overlies the underlying Chalk will act as a barrier to confine the Chalk Aquifer, reduces the occurrence of spring activity, which would be essential for the production of hydraulic pingos. 4. Discussion 4.1. Characteristics of rampart structure using GPR The overturned, deformed, stratified, tilted and radically dipping strata described by Ross et al. (2005a) of relic ramparted depressions in Wales is present within the internal structures of ramparts at Thompson and East Walton Commons. The interpretation of GPR profiles from East Walton and Thompson Common suggest that the original rampart was symmetrical in shape as shown in Fig. 16 and formed over two phases. It is proposed that the first phase of rampart development marks the end of ice core growth. The substantial uplift of the ground would cause the overburden to fail laterally and slide off of the ice core to accumulate around the base. This initial phase of deposition is characterised within the outer rampart wall by strata which dip continuously away from the depression. Within the rampart core a series of chaotic discontinuous reflections are interpreted as overturned and deformed strata created as the overburden material was deposited. The second phase of development interprets the steep inwardly dipping stratum seen within the inner rampart wall, as over steepened loose granular material that has collapsed inwards as the ice core supporting it melts. A third phase of development which was not observed at Thompson Common results as the steep unvegetated, granular ramparts are weathered causing rapid episodic failure. The gentle dipping concave and convex reflections of the near surface strata on the rampart flanks (highlighted by red dashed lines in Fig. 4) are interpreted as failed rampart material which has collected at the base of the rampart and within the depression. The convex and

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concaved reflections imply that failure is through slumping or solifluction as the ice core melts and the unstable ramparts collapse inwards without the support of the ice. Following on from this, the ponds surveyed at East Walton Common and Thompson Common fill with later Holocene peat deposits and at East Walton Common, with material eroded from the rampart. Sparks et al. (1972) uses the geomorphology of the ramparts and auguring evidence to propose that the formation of features at East Walton was cyclic. Profiles 2 and 3 taken from Feature A1, at East Walton Common (Fig. 4), display gently dipping clinoforms underneath the existing rampart which are considered to predate the existing rampart and could represent the development of earlier features. These structures were not evident at Thompson Common indicating no evidence here for cycles of growth and decay often seen with hydraulic pingo formation. 4.2. Characteristics of hollow structure using GPR The depression is created by a shortage of overburden material to refill the void left by the melting of the ice core. The GPR profiles of Feature A1 at East Walton Common suggest that the partial collapse of the inner rampart wall caused it to encroach into the hollow to form a cone or concaved basin. The horizontal reflections of the hollows at East Walton and Thompson are post glacial fill material consisting of weathered rampart debris, Holocene organics intersected by thin silt deposits. 4.3. Geological influence At both East Walton Common and Thompson Common the ramparts consist of reworked River Terrace Deposits. These deposits consist of non-frost susceptible gravely medium to coarse sands. The presence of subangular fine to medium chalk gravel within the River Terrace Deposit reflects the frost shattering of the chalk high ground to the immediate east. Thompson Common has Lowestoft Till underlying the River Terrace deposits, but East Walton Common does not. The stratigraphy of the hollow is characterised by deposition. The first phase of deposition is the collapse of the sandy rampart material to create a bowl or cone shaped hollow. This may occur on a small scale on several occasions as the ramparts are weathered to create a stratigraphy which has several thin layers of sand cutting through organic sediments. The second phase of deposition is

Fig. 16. The proposed three stages of rampart formation evident from GPR profiles at East Walton Common. Both sites showed clear evidence of phases 1 and 2 as the inner and outer rampart is formed, however there is little to no evidence of rampart modification at Thompson Common.

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represented by a thin layer of organic rich silty chalk likely to represent the settling out of fine material held in suspension within the pond water. The third phase of deposition occurs as the pond is colonised by vegetation to create a succession of Holocene organic fill, which includes the early dark compacted Gyttja at the base of the hollow and the recent fibrous sphagnum peat at the surface. At Thompson the depression fill of Feature B2 is different to that of Feature A2. The dry hollow consists of collapsed clayey, sandy rampart material in the upper layer overlying dry grey silty clay. The chalk silt augered at the base of the clay is considered to be the weathered chalk bedrock. The grey silty clay of the Lowestoft Till Formation acts as an aquitard to the rise of groundwater, causing the depression to remain dry. The absence of organic material within the fill sequence supports the conclusion that this hollow has never supported a deep pond. Unlike the ponds within the River Terrace Deposits which continue to be fed by springs, Feature B2 could have developed as a result of the frost susceptible nature of the underlying geology proposing a lithalsa origin. The saturated nature of the environment and ‘frost susceptible properties’ (Pissart, 2002) of the fine grained Lowestoft Till, could lead to the formation of raised ice mounds by segregated ice lens growth through ‘cryosuction’ (Worsley et al., 1995). 4.4. Proposed origin of features Two sites that were thought to represent the remnants of late Devensian hydraulic pingos have been investigated at East Walton Common and Thompson Common, England using geophysical methods, a limited intrusive ground investigation and desktop study. Visual interpretations of both sites suggest that rampart and depressions were of similar origin, the data collected suggests that the features found in East Walton Common are relic hydraulic pingos, whilst the features at Thompson Common are likely to represent relic lithalsa forms. East Walton Common is a model example of a relic hydraulic pingo site. The high ground to the north east of the common is responsible for creating the hydraulic system that would have fuelled pingo growth. The geology and hydrogeology of the site is archetypal of contemporary pingo lithology. Thompson Common is also located at the foot of Chalk high ground to the north east, but the presence of the Lowestoft Till overlying the site makes the area less suitable for pingo spring formation. The high density of features at East Walton Common appears to be a unique characteristic which is not seen at any other contemporary or relic hydraulic pingo site in the northern hemisphere. The geology, hydrogeology and structure of the features at Thompson Common do not fit the same model of pingo growth found at East Walton Common. It is unlikely that the features that occur at Thompson Common would have been fuelled by pingo springs activity given the presence of the overlying impermeable Lowestoft Till, which would impeded the movement of ground water to the surface. This substrate is frost susceptible and it would be possible for segregated ice lens development to occur to create lithalsa like structures. Today, the passage and rise of groundwater at the site continues to be restricted and leaves many of the hollows dry, supporting the idea that the features at Thompson Common are not pingos. Structurally the ramparts at Thompson Common are smaller and do not contain a third phase of rampart modification that is observed at East Walton Common. This suggests that the features of Thompson Common were lower forms and subsequently had less available material for rampart development as the features

formed from sediment heave of the Lowestoft Till, rather than sediment uplift during pingo formation. This research has highlighted the need to develop the current understanding of the decay sequence of active pingos, and the landforms that result. For late Devensian forms that decayed many thousands of years ago, this can be achieved through the investigation of the internal structure of ramparts, using geophysical and intrusive methods. This will provide some evidence of the processes that were responsible for their construction and development. The interpretation of rampart structures at East Walton Common can be used as a model to explain rampart development for all active and relic pingo morphologies. If the ramparts of a particular site under investigation occur in coarse sand and have the characteristic outwardly and inwardly dipping tilted strata and mass wasting structures evident on the rampart sides, then it is probable that the rampart developed in a similar fashion to those at East Walton Common. 4.5. Permafrost within East England Gurney (2000) states that ‘true’ hydraulic pingos are ‘perennial’ and form within ‘permafrost environments’ which rely on ‘a pressure system’ to support pingo ice formation. The current hydrogeology of the site, where springs are driven by water moving towards East Walton Common from the chalk escarpment implies that hydraulic pressure significantly influences ground water in this area. In colder phases, it is likely that permafrost conditions similar to East Greenland or Alaska would exist to great depths and impede groundwater movement, however the work by (Yoshikawa et al., 2003) on the pingo hydrology in discontinuous permafrost regions in Alaska, finds permafrost dominates valley bottoms of north facing slopes, but is largely absent on south facing slopes. The mean annual ground temperature of south facing slopes in permafrost areas can be up to 2.5 8C (Yoshikawa et al., 2003), which accommodates contemporary mean pingo spring temperatures observed in Alaska. Seasonal variations in ground temperatures in the shaded valleys range from 0.5 8C to 3 8C, making it possible for the existing spring networks of the chalk escarpment at East Walton Common to be active within summer months. Yoshikawa et al. (2003) also find that contemporary Alaskan hydraulic pingos freeze only in the winter months due to quick temperature loss of the upper ground layers, from a reduction in air temperature, with lower ground depths unaffected by freezing. The presence of patterned stripes on the chalk escarpment next to the site documented by Sparks et al. (1972) further supports the theory of a deep active layer or groundwater table unhindered by permafrost. The discontinuous permafrost setting found in Alaska and Greenland today, is the proposed scenario for the relict cryogenic features scattered across East Anglia. This would postulate an age towards the end of the Younger Dryas Stadial, as permafrost thaw and ameliorating temperatures would give the most satisfactory explanation for the presence of pingo spring activity in this location. 5. Conclusion The purpose of this research was to reinvestigate the true origin of probable late Devensian relic ice mounds located on East Walton Common and Thompson Common, Norfolk using current information and modern fieldwork techniques including geophysical methods. From this research, it has been possible to conclude that the landforms found at both sites are of periglacial origin; are similar in appearance, containing rampart enclosed depressions

P. Clay / Proceedings of the Geologists’ Association 126 (2015) 522–535

clustered together in high densities and located in a similar setting in East Anglia just 30 km apart. However detailed analysis of the geology and hydrogeology of the areas that surround both sites and which are likely to control the mechanisms for ice mound growth, add weight to the suggestion that it is unlikely that the features of East Walton Common which are likely to be hydraulic pingo forms are of similar origin to those found on Thompson Common which are more likely to be lithalsas. This is supported by evidence obtained from differences in the subsurface structure of the ramparts and depressions of features found at each location and could explain the smaller, more subdued appearance of the features at Thompson Common, in comparison to the much larger depressions and higher, steeper ramparts at East Walton Common. The successful application of geophysical surveying to interpret the origin of unknown ice mound morphology has also proposed a non-invasive fieldwork method which protects the structure of the landform from damage, and provides reliable evidence for interpreting the formation of the feature by using the characteristic subsurface strata of the ramparts at East Walton Common. This now makes it possible to determine with greater certainty the origin of relict landforms similar to those found at East Walton Common. Acknowledgements This research would not have been possible without the generous help from others. The author would like to give special thanks to Dr. Charlie Bristow, Earth and Planetary Sciences, Birkbeck, University of London who gave considerable time and advice throughout the course of this work and Mr. Michael Clay who contributed to complete the fieldwork. Dr. Rebecca Briant, Geography, Environment and Development Studies, Birkbeck, University of London for her support and advice in writing this article. I would also like to thank Dr. Jamie Pringle at Keele University, Dr. Marek Ziebart at the Department of Civil, Environmental and Geomatic Engineering, University College London, Dr. Karen Hudson Edwards, Earth Sciences Department, Birkbeck College, Mr. M Ramscar at the Environment Agency, and Mr. D Stevens at Norfolk Wild Life Trust. References Allard, M., Rosseau, L., 1999. The internal structure of a palsas and a peat plateau in the Reivere Boniface Region, Quebec. Interferences on the formation of segregation ice. Ge´ographie Physique et Quaternaire 53 (3), 373–387. Barrow, G., Bennett, F.J., Blake, J.H., Cameron, A.G., Jukes-Brown, A.J., Hawkins, C.E., Reid, C., Skertchley, S.B.J., Whitaker, W., 1999. British Geological Survey plan no.160 Swaffham 1:50,0000. Geological Survey of England and Wales 1:63,360/ 1:50,000 geological map series, New Series Crown copyright and database rights. Ordnance Survey. Cruikshank, J.G., Colhoun, E.A., 1965. Observations on Pingos and other landforms in Schuchertdal, Northeast Greenland. Geografiska Annaler Series A, Physical Geography 47 (4), 224–236.

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