Engineering Group of the Geological Society (EGGS) AGM ‘Developing the ground model’

Field guide to accompany fieldtrip on Saturday 13th April 2019

Aspects of the glacial and periglacial history of Southern Britain

Field trip leaders:

Simon Price (Arup)

Dave Giles (University of Portsmouth)

Acknowledgements

With thanks to Dr Andy Farrant (British Geological Survey) for advice on the stratigraphy and

structure of the Chalk in southern England.

Aim

The aim of this fieldtrip is to investigate the field evidence for the former presence, and

interaction between, glacier and ground ice in lowland Britain. The fieldtrip will include two

localities in Norfolk and Hertfordshire. Each locality will be investigated to evaluate the

geological and geomorphological evidence for the former presence of the British Ice Sheet

(BIS) and its interaction with bedrock of the Chalk Group in a British, lowland landscape.

The localities, and their relationship to Cambridge, are shown in Figure 1.

Figure 1 Localities and their geographical relationship to Cambridge

Introduction

The landscape of southern Britain has been shaped by multiple geological and

geomorphological processes that have acted throughout the Quaternary. The Quaternary

Period (2.6 Ma – present day) represents a geological interval of intensification in the

frequency and magnitude of high to mid latitude glaciation. Cyclical variations in the amount

and aspect of orbitally-controlled solar radiation reaching the Earth have been recognised

from atmospheric proxy records preserved in marine sediments and ice-cores e.g. (Shackleton

& Opdyke 1973; Shackleton & Pisias 1982; Lisiecki & Raymo 2005). The relationship between

isotope proxies and global changes in temperature and ice volume has enabled a

chronostratigraphical hierarchy for the Quaternary Period to be established (Table 1). The

Quaternary Period has been divided into Marine Isotope Stages (MIS) representing

alternating cold (glacial) and temperate (interglacial) events. During cold stages in mid and

high latitudes, ice sheets expanded along with corresponding fluctuations in the exposure of

terrestrial environments to periglacial activity (Benn & Evans 1998; Lowe & Walker 2015).

Period Epoch MIS Age

(Ma)

Northern Hemisphere Stages and substages Q

uat

ern

ary

NW Europe British Isles

Ho

loce

ne 1

0.017

Holocene Holocene

M

idd

le P

leis

toce

ne

2 ~0.02 Late Weichselian Late Devensian

3 ~0.05 Middle Weichselian Middle Devensian

4 ~0.07 Early Devensian

5a

5b

5c

5d ~0.12

Early Weichselian

5e ~0.13 Eemian Ipswichian

6

~0.39 Saal

ian

Warthe/Drenthe

Wo

lsto

nia

n

7 Schöningen,

Reinsdorf

Fuhne

11 ~0.40 Holsteinian Hoxnian

12 ~0.44 Elsterian Anglian

13-

21

0.79 Cro

mer

ian

Co

mp

lex

Interglacial IV

Cromerian

Glacial C

Interglacial III

Glacial B

Interglacial II

Glacial A

Interglacial I Beestonian

Table 1 Quaternary chronostratigraphical framework for Britain and northwest Europe. Chronostratigraphical units below the Cromerian stage not shown. After Lowe & Walker (2015) and Cohen & Gibbard (2011)

During Quaternary glaciations in Great Britain, the British Ice Sheet (BIS) extended into

southern Britain from ice accumulation centres in NW Scotland, the Lake District and North

Wales (Ehlers et al. 1991; Gibbard & Clark 2011; Lowe & Walker 2015). The influence of the

Scandinavian Ice Sheet in Eastern England, sourced from the North Sea region has also been

suggested (Ehlers & Gibbard 1991; Lunkka 1994; Clark et al. 2004; Ehlers et al. 2011) but

evidence for it has been questioned (Lee et al. 2002; Davies et al. 2011).

It is generally accepted that glaciation occurred on at least three occasions in Britain during

the Devensian (Weichselian), Wolstonian (Saalian) and Anglian (Elsterian) Stages. Although

evidence for glaciation between the Devensian and Anglian remains a topic of debate in Great

Britain, unequivocal evidence of glaciation at this time is widespread in mainland Europe. The

maximum terrestrial limits of glaciation during these stages is shown in Figure 2. Of these

terrestrial glacial events, glaciation during the Anglian Stage was the most extensive,

extending into southern England including present-day north London.

Evidence of glaciation in southern England beyond the southern extent of the inferred Anglian

glacial limit is recognised by sediments and landforms of possible glacial origin in the Isles of

Scilly (Hiemstra et al. 2006). Beyond the Anglian Stage glacial limit, lowland Britain was

exposed to multiple phases of non-glacial, cold-climate periglacial activity, including the

development of permafrost, during cold stages from the beginning of the Quaternary Period

(Ballantyne & Harris 1994).

Figure 2 Interpreted glacial limits in southern Britain modified after Gibbard & Clark (2011)

Differential weathering under cold and temperate Quaternary climates has resulted in

bedrock weakening and the production of deep zones of weathering in lowland southern

Britain (Murton & Ballantyne 2017). The extent of periglacial bedrock weathering generally

decreases from south to north because of sub-glacial erosion during subsequent glaciations.

Post-Anglian Pleistocene glacigenic sediments are similarly affected by periglacial weathering

and modification during subsequent glaciation. Where ground temperatures remain at 0°C

for two consecutive years or more, permafrost may form. The linkage between Mean Annual

Air temperature (MAAT), Mean Surface Temperature (MST) and ground thermal diffusivity

and conductivity provides a thermo-mechanical model on which to predict the potential

formation of permafrost (French 2007; Harris et al. 2009; Murton & Ballantyne 2017). The

interpreted former extents of permafrost in southern Britain derived from abiotic and biotic

temperature proxy evidence in the terrestrial periglacial record are shown in Figure 3.

Figure 3 Extent of permafrost and seasonally frozen ground in Great Britain during the Devensian (Weichselian) Stage. Note north-south climate gradient. A to E after Huijzer & Vandenberghe (1998), F after Isarin (1997) and following the style of Murton & Ballantyne (2017). Modelled permafrost thicknesses for East Anglia and Dartmoor estimated from ground thermal properties (diffusivity and conductivity) and mean annual air and surface temperatures from Busby et al. (2016). E) shows discrepancy between numerical methods of Busby et al. (2016) which estimate thick permafrost and biotic and abiotic climate proxies used by Huijzer & Vandenberghe (1998) which shows discontinuous or sporadic permafrost in SW England.

During the period 74 – 59 ka in Great Britain, it is estimated that the mean temperature of

the coldest month ranged between -32°C to -10°C (Huijzer & Vandenberghe 1998). The

presence of structures characteristic of permafrost, including involutions and ice-wedge casts

are associated with the regionally extensive ‘Barham Arctic Soil’ underlying Anglian-age

glacigenic deposits in East Anglia (Rose & Allen 1977; Kemp et al. 1993). This also provides

abiotic evidence for the presence of pre-Devensian permafrost. It is probable that this same

climatic zone controlling permafrost aggradation, also developed during each of the previous

glacial events in lowland Britain.

Locality 1

Thompson Common [TL94043 96588]

Thompson Common and the ‘Pingo Trial’ is an area of Breckland, northeast of Thetford Forest

in Norfolk. Its geological setting is summarised in Figure 4.

The site is underlain by the Upper Chalk (undifferentiated) which is in turn overlain by

glacigenic sediments including till and sand and gravel. Clasts, present as gravel are

dominated by chalk and flint and have been classified on the basis of lithostratigraphy as the

Lowestoft Formation. Conventionally, the Lowestoft Formation till has been associated with

deposition from a lobe of the British Ice Sheet (BIS) which advanced into southern England

from the North Sea region via the area of the present-day Wash. Glacigenic sediments are

overlain by cover sands, river terrace deposits, peat and head. The depth to geological

rockhead is interpreted to be ~4mbgl.

Thompson Common and nearby areas including Elveden show geomorphological and

geological for the past presence of permafrost and seasonal freezing and thawing of moist

soil. Locally, polygonal patterned ground is visible where aeolian cover sand overlies chalk

bedrock (Figure 5) where they form a series of features collectively referred to as frost

polygons. Geomorphological evidence of seasonal freezing and thawing is also present in the

Breckland landscape in the form of other types of patterned ground including stripes.

Frost (or cryogenic) mounds are features characteristic of cold climates within zones of

continuous and discontinuous permafrost. Evidence of their presence in the periglaciated

areas of southern Britain has important implications for the reconstruction of past climates

and for the understanding of changes in the mechanical behaviour of bedrock which are

affected by the past presence of permafrost. They are also important as relict frost mounds

in the form of pingos have been suggested to at least partially explain the sedimentology and

geometry of anomalous features below geological rockhead in central London (Berry 1979;

Hutchinson 1980; Banks et al. 2015).

Evidence for the former presence of permafrost is present in and around Thompson’s

Common. Present-day, almost circular ponds with partially ramparted margins have been

conventionally interpreted as relict frost mounds thought to represent the former presence

of pingos (Figure 6). In the Thompson Common area, the ramparted depressions are formed

within peat-rich sediments of the headwaters of the River Wissey which eventually flows

westwards to drain into the River Great Ouse.

The morphology and sedimentology of the mounds and depressions at Thompson Common

has recently been investigated by (Clay 2015). Clay (2015) used Ground Penetrating Radar

(GPR), electrical resistivity tomography (ERT) and coring to investigate the internal structure

of the mounds and interpret their likely mode of origin (Figure 7). At Thompson Common, in

the centre of the depressions, high resistivity material was proved to consist of 1.2m of dry

sand underlain by dry silty clay. This is further underlain to 2m by wet, chalk-rich silt. Medium

resistivity material is interpreted as gravel, potentially sourced from the partial collapse of

the surrounding ramparts. The ramparts comprise yellow, slightly gravelly, slightly silty,

medium- to coarse-sand.

The origin and classification of the relict frost mounds at Thompson Common remains the

subject of debate. The commonly used classes of frost mounds are shown in Tables XX to XX.

Open- and closed-system pingos are generally larger features that may form in coarse-grained

soils. Groundwater is commonly fed under high pressure, sufficient to overcome overburden

stress and to allow the growth of a central ice core causing doming and steepening of their

sides. Confined groundwater may be fed from sub-permafrost aquifers (open-system) or from

expulsion of porewater in closed-system pingos. Open-system pingos commonly form

adjacent to valley sides, on alluvial floodplains and on alluvial fans. Closed-system pingos

commonly form in the areas of former thermokarst lakes which have drained.

Frost mounds including lithalsas and palsas have a similar geomorphology to pingos. Their

mechanism of formation, host sediment and hydrogeological setting is different however.

They generally form in fine-grained, frost susceptible soils including clay, silt and moist organic

sediments. Consequently, as freezing of groundwater progresses, porewater is drawn to the

freezing front to form lenses of segregated ice. The accumulation of lenses of segregated ice

causes up-doming which is generally smaller in magnitude than pingos.

Clay conclusions……

Terrain Unit 3.11.5.3 Relict frost mounds / Relict ramparted ground-ice depressions: Pingos

Image

Fig 3.200a Lake within collapsed hydrostatic pingo, Hendrickson Island, Mackenzie Delta,

Canada (J. Murton).

Form /

Topography

Ramparted depression that marks former location of a frost mound. Ice-cored mounds or

hills developed in permafrost. Relict pingos and other ground-ice mounds may be indicated

by circular or ovate depressions, often surrounded by raised rampart-like rims. Hydrostatic

(or closed system) pingos occur in lowland settings within the continuous permafrost zone

and hydraulic (or open system) pingos are more common in valley bottom and footslope

localities in both discontinuous and continuous permafrost.

Landsystem Lowland Periglacial Terrain: Valley landsystem

Process of

Formation

Formed by injection of water into near-surface permafrost to form an ice core. Water under

sufficient pressure to overcome overburden stress. Pressure can develop in two ways:

hydrostatic pressure, where water is expelled from saturated coarse-grained sediments

during the refreezing of a talik (zone of unfrozen ground within permafrost) or hydraulic

pressure where artesian water pressures within a subpermafrost aquifer cause upward

injection and freezing of water.

Modern

Analogue

Fig 3.200b Hydraulic (open-system) pingo, Reindalen, Svalbard, Norway (C.K. Ballantyne).

Associated

Features

Solifluction landforms

Principal

Engineering

Significance

Variable particle sizes, Anisotropic permeability, Compressible soils, Anisotropic stiffness,

Anisotropic strength, Groundwater, Perched water tables, Voids

Principal

References

Ballantyne & Harris (1994), Ross (2013), Mackay (1998), Gurney (1998), Flemal (1976)

Hutchinson (1980, 1991, 1992), Banks et al. (2015), Long (1991)

Table 2 Classification of relict frost mounds, pingos (Giles et al. 2017). Full citations not provided in this handout. Refer to original publication.

Terrain Unit 3.11.5.4 Relict frost mounds / Relict ramparted ground-ice depressions: Palsas and lithalsas

Image

Fig 3.201a Relict lithalsa (ramparted depression), Llangurig, Powys, Wales (S. Gurney).

Form /

Topography

Low circular to oval permafrost mounds (palsas) up to 7m high, up to 30m in diameter. Can

cluster in high density groups (lithalsas). Occasionally surrounded by a low rim ridge.

Ramparts may contain overturned, deformed and tilted stratified sediments overlain by

mass movement deposit showing crude radially dipping stratification.

Landsystem Lowland Periglacial Terrain: Valley landsystem

Process of

Formation

Palsas develop in peaty mires in discontinuous or sporadic permafrost zones particularly

where precipitation is low. Result from differential winter frost penetration and frost heave.

Deeper penetration in areas of thinner snow cover promotes ice segregation and frost heave.

Eventually decay where erosion of the sides and loss of peat cover cause the core to melt

and collapse often forming ponds which can rapidly fill with peat. Lithalsas develop by ice

segregation in frost-susceptible (silty) soils that lack peat cover.

Modern

Analogue

Fig 3.201b Modern lithalsa (low tree-covered mounds), Fox Lake, Yukon Territory, Canada (J.

Murton).

Associated

Features

Principal

Engineering

Significance

Variable particle sizes, Anisotropic permeability, Compressible soils, Anisotropic stiffness,

Anisotropic strength, Groundwater, Perched water tables, Voids

Principal

References

Ballantyne & Harris (1994), Ross (2013), Seppälä (1988), Pissart (2002, 2003), Gurney (2000,

2001), Wolfe et al. (2014), Ross et al. (2011)

Banks et al. (2015)

Table 3 Classification of relict frost mounds, palsas and lithalsas (Giles et al. 2017). Full citations not provided in this handout. Refer to original publication.

Terrain Unit 3.11.5.7 Large relict thermokarst depressions

Image

Fig 3.202a Thermokarst depression (alas) containing a lake, thermokarst mounds behind,

Yakutsk, Siberia (J. Murton).

Form /

Topography

Near-circular depressions that may exceed 1km in diameter. Completely enclosed by higher

ground except for a single outlet up to 60m wide towards which the entire depression drains.

No constructional rampart. Depressions 6-7.5m deep and often contain a lake. Complex

sediment infill stratigraphy present.

Landsystem Lowland Periglacial Terrain: Valley landsystem

Process of

Formation

Formed by the degradation and thawing of ice-rich permafrost and thermal erosion causing

slumping and fall of the overlying ground.

Modern

Analogue

Fig 3.202b Large thermokarst depression (alas) developed in yedoma (ice complex), Yakutsk,

Siberia (J. Murton).

Associated

Features

Pingos, Ramparted depressions

Principal

Engineering

Significance

Variable particle sizes, Anisotropic permeability, Compressible soils, Anisotropic stiffness,

Anisotropic strength, Groundwater, Perched water tables, Voids

Principal

References

Ballantyne & Harris (1994), Burton (1987), Murton (1996b), West (1991), Burn (2013)

Hutchinson (1991), Banks et al. (2015)

Table 4 Classification of thermokarst depressions (Giles et al. 2017). Full citations not provided in this handout. Refer to original publication.

Locality 2

Barkway Chalk Pit [TL38138 36615]

Barkway Chalk Pit is a disused chalk pit ~8km south of Royston in north Hertfordshire. It’s

topographical and geological setting is shown in Figures 8 and 9.

The former chalk pit near the village of Barkway in Hertfordshire exposes inclined chalk

(formerly referred to as ‘Chalk Rock’ by Hopson et al., 1996) separated, at its northern edge,

by clay with gravel erratics, interpreted as till and classified as Lowestoft Formation. The Chalk

Rock is now given Member status and is part of the Southern Province Lewes Nodular Chalk

Formation (Figure 10). The base of the Chalk Rock Member was previously taken as the

boundary between the informal units of the Middle and Upper Chalk (Figure 10). The

exposure is described in references including Hopson et al. (1995) and Hopson et al. (1996),

from which much of the account described here is taken.

The area in and around Royston has long been known to exhibit anomalously steep deeps

within beds of the White Chalk Subgroup. Previously inferred to be the result of tectonic

‘flexure’, the association of till and repetition of stratigraphy with the steeply dipping chalk

led to the interpretation of the influence of pro- and sub-glacial glaciotectonic processes in

accounting for the anomalous dips (Hopson 1995).

Geological mapping and interpretation of aerial photographs enabled the distribution of chalk

rafts on the north-facing chalk escarpment south of Royston (Figure 11), to be identified

(Hopson et al. 1996). Regional mapping has demonstrated that the relative position and

elevation of the rafted Chalk Rock Member is higher in the landscape than it is in situ. The

extent and geometry of rafts of chalk can be inferred from aerial photographs where white

chalk is surrounded by reddish brown till of the Lowestoft Formation (Figure 12).

At least eight chalk rafts, stacked in an imbricate style and in places separated by till occur in

a zone approximately 6km and 1km wide. Individual rafts range between 100 and 800 m in

length and are at least 17.5m thick.

At Barkway Chalk Pit, the plana and lata biozones; above and below the Chalk Rock Member

respectively, are repeated in the northern face of the pit where they are separated by till

(Figure 13). A normal fault was formerly exposed in the western face of the pit, downthrowing

to the south. The Chalk Rock Member and the zones above and below comprise nodular chalk

with flint nodules and bands. Bored hardgrounds are present and coalesce. In places, the

Chalk is discoloured corresponding to zones of phosphatisation (generally brown) and zones

stained with glauconite (generally green). Marl seams are present stratigraphically beneath

the Chalk Rock Member. In the nearby Reed Chalk Pit, marl seams down to the Reed (Caburn)

Marl were previously exposed. The nodular form of the chalk is interpreted to be a primary

depositional feature reflecting winnowing of the chalky sediment at times of low rates of

sedimentation. In part, brecciation of the Chalk may indicate the former presence of

segregated ground ice. Dissolution features may be present and may in places be infilled with

loess.

The contact between the Chalk and till is sharp and inclined. The weathered light brown till

of the Lowestoft Formation comprises gravel of chalk and flint with less common quartz,

quartzite and Jurassic-derived shell fragments, within a clay and silt matrix. Its chalk content

has led to its classification with other chalk-rich facies till of southern England. It is believed

to have been deposited from a lobe of the British Ice Sheet (BIS) entering southern England

from the North Sea region and via the present-day region of the Wash.

The interpreted mechanism of emplacement is summarised in Figure 13. The mechanism of

emplacement implies that the Chalk escarpment had a similar expression to the present-day.

Further, the presence of the in situ Chalk Rock Member provided a low-angle bench in the

north-facing Chalk escarpment, which is not present to the east or west. A lobe of the BIS,

advancing into an existing, northward-facing embayment is envisaged which then encroached

on the feature-forming bench of the Chalk Rock Member.

The presence of marl seams below the Chalk Rock Member are interpreted to have acted as

zones of elevated groundwater pressure and decollement which enabled them to be

preferentially detached and incorporated into the basal zone of the advancing ice lobe. Initial

detachment may have taken place in a proglacial setting before incorporation into a subglacial

deforming wedge, analogous to the rafts of Chalk entrained within till in Overstrand in Norfolk

for example (Burke et al. 2009). The detachment of rafts of Chalk was subsequently facilitated

by reverse faulting forming fault-bounded imbricate thrust stacks. It is envisaged that the

Chalk escarpment was fully overridden by ice and that its retreat led to to slope instability

and the formation of normal faults, as exposed in the western face of the pit at Barkway.

Figure 4 Bedrock (a) and Quaternary (b) geology of the Thompson Common area from 1:50 000 scale DigMapGB50 data. Contains British Geological Survey materials © UKRI [2019]

Figure 5 Evidence of thermal contraction cracking (frost polygons or thermal contraction crack polygons) and patterned ground in the form of stripes near Elveden, Norfolk. Image from Google Earth, accessed April 2019

Figure 6 Perspective view of partially ramparted and water-filled depressions, Thompson Common, Norfolk. Image from Google Earth, accessed April 2019

Figure 7 GPR and ERT investigation of ramparts and depressions at Thompson Common from Clay (2015). a) location of features surveyed. b) results from ERT profile, feature B2. c) Results from GPR survey, feature B2

Figure 8 Topographical and geological setting of Barkway from Hopson et al. (1996). a) Topography. b) Bedrock geology (1:50 000 scale; note former stratigraphical terminology for the Chalk Group used). c) Selected physiographic regions

Figure 9 Bedrock (a) and Quaternary (b) geology of the Barkway area from 1:50 000 scale DigMapGB50 data. Contains British Geological Survey materials © UKRI [2019]

Figure 10 Comparison of former and current stratigraphical classification of the southern province Chalk Group (from Mortimore ‘London Basin Forum Chalk: An Atlas of Stratigraphy, Structure and Engineering Geology’, date unknown)

Figure 11 Aerial imagery derived from Google Earth showing the presence and style of interpreted chalk rafts between Barkway and Royston. a) View looking towards the east. b) View looking north. Barkway Chalk Pit shown by blue arrow in a) and b)

Figure 12 Features interpreted as rafts of chalk in the Royston area after Hopson et al. (1996). a) Mapped extent. b) Glacitectonic model

Figure 13 Cross-section and perspective sketch of the exposure at Barkway Chalk Pit (after Hopson 1995)

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