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doi:10.1016/j.qu

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Earth Sciences,

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Quaternary Science Reviews 25 (2006) 739–762

Palaeoenvironmental interpretation of an ice-contact glaciallake succession: an example from the late Devensian

of southwest Wales, UK

James L. Etiennea,b,�,1, Krister N. Janssona,c,1, Neil F. Glassera, Michael J. Hambreya,Jeremy R. Daviesd, Richard A. Watersd, Alex J. Maltmana, Philip R. Wilbyd

aCentre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigion SY23 3DB, UKbGeological Institute, Department of Earth Sciences, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zurich, SwitzerlandcDepartment of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden

dBritish Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

Received 22 July 2004; accepted 23 March 2005

Abstract

During the late Devensian (late Weichselian) glaciation, a number of large proglacial lakes developed in dammed river valleys

along the southwest coast of Wales, U.K. This paper presents sedimentological data, together with a Digital Terrain Model, to

establish the sedimentation history, dynamics and evolution of the largest lake, glacial Llyn (Lake) Teifi. Buried valley-fill sequences

within the margins of the former lake basin reveal a thick succession of glaciolacustrine muds which coarsen upward into, or are

locally abruptly overlain by, proximal deltaic, subglacial and glaciofluvial deposits. Sediment delivery pathways represented in the

lacustrine succession include gravity flows, suspension settling, deltaic aggradation and iceberg rafting, the latter indicating ice-

contact conditions. The lacustrine muds are variably deformed, with a range of syn- and post-depositional structures, some of which

indicate subglacial deformation associated with overriding of the lacustrine succession. Syn-depositional structures indicate high

sedimentation rates, which may explain an absence of bioturbation structures. The overall coarsening-upward succession and cap of

subglacial and/or glaciofluvial deposits support recent theories suggesting that glacial Llyn Teifi formed during glacial advance.

There is no evidence to support glaciomarine conditions of sedimentation in this area of the Irish Sea basin.

r 2005 Elsevier Ltd. All rights reserved.

1. Introduction

The Quaternary geology of southwest Wales hasattracted considerable attention since Charlesworth(1929) suggested that a series of proglacial lakesdeveloped in the region during the last deglaciation(late Devensian/late Weichselian). The largest of these,

e front matter r 2005 Elsevier Ltd. All rights reserved.

ascirev.2005.03.019

ing author. Geological Institute, Department of Earth

Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzer-

1 632 6951; fax: +41 1 632 1080.

ess: james.etienne@erdw.ethz.ch (J.L. Etienne).

ress: Centre for Glaciology, Institute of Geography and

University of Wales, Aberystwyth, Ceredigion SY23

glacial Llyn Teifi, is thought to have developed in theTeifi valley (Fig. 1), fed by meltwater from the Irish Seaglacier which dammed the estuary. Meltwater alsoentered from highland-derived Welsh ice, and overspillfrom another lake, glacial Llyn Aeron, to the northeast(Charlesworth, 1929). Similar lakes are thought to havedeveloped to the north and southwest of the Teifi.Charlesworth (1929) considered channel networks cross-ing modern watersheds to have been incised duringperiods of lake overspill. Jones (1965) hypothesised thatLlyn Teifi developed during sequential stages as colswere exposed by a receding ice margin, the mostimportant of which cross watersheds at Pedran [SN256325 (UK national grid reference)], Llantood [SN 154

ARTICLE IN PRESS

Fig. 1. Location maps. Elevation data derived from the Land-Form PANORAMATM Digital Terrain Model, sourced from EDINA Digimap.

(a) Major topographical features in the Teifi valley region of southwest Wales. Location of glacial meltwater channels based on Glasser et al. (2004),

and unpublished manuscript maps (Jansson, K.N.). (b) Surface distribution of glaciolacustrine clay, Welsh glacial till and Irish Sea glacial deposits,

derived from 1:50,000 scale BGS Digital Data for the Cardigan (193) sheet and Afon Teifi Catchment Survey (Waters et al., 1997) under Licence

2003/116 British Geological Survey.rNERC. (c) The distribution of buried valleys in the lower Teifi region, modified from Glasser et al. (2004) and

Waters et al. (1997).

J.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762740

ARTICLE IN PRESSJ.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762 741

417] and Cippyn [SN 135 480; Fig. 1]. However, manyauthors have suggested that the morphology of thesechannels is more consistent with subglacial meltwatererosion than drainage in a subaerial environment(Bowen and Gregory, 1965; Bowen, 1967, 1971; John,1970; Bowen and Lear, 1982; Glasser et al., 2004).Recent advances in the understanding of the regionalQuaternary stratigraphy support the former existence ofglacial Llyn Teifi (Waters et al., 1997; Fletcher andSiddle, 1998; Wilby, 1998; Hambrey et al., 2001);however, the stratigraphic succession indicates lakeimpoundment during advance of the Irish Sea glacier,not recession as originally proposed by Charlesworth(1929).

Drillcores from a number of buried valleys (Fig. 1c) inthe region provide an opportunity to examine thedepositional record of a thick glacial lake successionformed in a proglacial environment. This paper exam-ines in detail the sedimentation history of glacial LlynTeifi, and advances a palaeoenvironmental interpreta-tion for the lacustrine succession. We also present a two-dimensional model of glacial lake evolution in southwestWales based on the sedimentological record andgeomorphological features, which are coupled to ahigh-resolution Digital Terrain Model (DTM) toreconstruct glacial lake outlines and depths.

2. Geological and topographical setting

The Afon (River) Teifi (Fig. 1a) rises in the uplands ofmid-west Wales, flowing south, and then west throughLampeter, Llandysul, Newcastle Emlyn and Cardigan,before entering Cardigan Bay (Fig. 1a). Northeast ofLlandysul, the river flows across Llandovery (Silurian)mudstones and sandstones; to the west, the Teifi isunderlain by a thick sequence of Caradoc and Ashgill(Upper Ordovician) turbiditic mudstones and sand-stones. These are tightly folded and cut by numerousCaledonoid (NE–SW) trending faults (Davies et al.,2003).

The broad form of the Teifi valley is considered to bea product of Tertiary uplift and erosion (Brown, 1960;Dobson and Whittington, 1987), initiated in theMesozoic, and thought to be related to continentalrifting and formation of the Atlantic Ocean. Duringthese periods of uplift, the proto-Teifi developed,supplied by headwaters draining the southern flank ofPumlumon Fawr (Jones and Pugh, 1935; Jones, 1946;Coster and Gerrard, 1947).

The bedrock floor of the Teifi is deeply incised(Blundell et al., 1969), to at least �60m O.D. [metresrelative to ordnance datum] in the estuary mouth (Lear,1986), and is consistent with low relative sea-level at thetime of incision (Fletcher, 1994). At this time much ofCardigan Bay would have been subaerially exposed.

During interglacial periods, eustatic recovery probablyresulted in the drowning of parts of the lower Teifivalley, accompanied by sedimentation on the valleyfloor (Jones, 1965). Subsequent stadials resulted in theTeifi either exhuming these parts of the valley, or locallycutting new bedrock courses (Jones, 1965). At somestage, the rivers Rheidol and Ystwyth captured theTeifi’s northern headwaters.

Evidence to support the complex evolution of theTeifi valley system occurs in the form of deeply incised,sediment-filled, abandoned courses (Table 1 and Fig. 1).A number of these have been identified betweenLlanybydder [SN 525 440] and the coast, which, despitebeing largely subsurface, can still be traced across themodern landscape. The subdrift form of parts of theseearlier courses has been investigated using geophysicalmethods (Allen, 1960; Francis, 1964; Nunn and Boztas,1977; Carruthers et al., 1997). Some of the tributaryvalleys of the Teifi exhibit comparable styles of incision,abandonment and infill (Waters et al., 1997; Fletcherand Siddle, 1998; Hambrey et al., 2001).

Two distinct sets of buried valleys occur, both in closegeographical association with the modern river (Fig. 1c).In some areas, the valleys take the form of broadmeanders that are bypassed by parts of the present rivercourse. Elsewhere, the present river follows the course ofthe buried valleys. Those which are bypassed by theAfon Teifi are typically between 3 and 7 km in length,and 0.5–0.8 km in width (Glasser et al., 2004). Goodexamples occur to the north and south of Cenarth(Nawmor [SN 270 425] and Cwm-cou courses [SN 270407], respectively), between Castell Malgwyn, nearLlechryd [SN 215 435] and Pen-y-Bryn [SN 179 429],and at Llwynpiod Farm [SN 178 475; Fig. 1c]. Cross-sectional profiles of the bedrock floors based onBouguer gravity anomaly data and resistivity traversesshow bench-like incisions, which may indicate severaldiscrete stages of incision rather than a single period ofdownward erosion and therefore a progressive but non-linear fall in relative sea level, with several stages ofrejuvenation (Carruthers et al., 1997; British GeologicalSurvey, 2003).

Geophysical investigations of the valley-fill succes-sions show marked boundaries resulting from densityvariations, which in turn reflect differences in thecharacter of the sediment fill. Stratigraphic subdivisionof the valley-fill sediments is limited by the availablegeophysical data; however, a generalised sequence ofhigher density clay overlain by lower density silt andsand may be inferred (Carruthers et al., 1997). Ofparticular interest is the nature of the contact betweenthe units which is often irregular, with possible incision-and-fill bodies in the upper parts of the buried channelfill (e.g. British Geological Survey, 2003). A thick fill ofglacigenic sediment supports at least a pre-late Deven-sian timing for the incision of the buried valleys. This

ARTICLE IN PRESS

Table 2

Borehole cores referred to in this paper

Core name Drilling site Altitude

(m OD)

Depth to

bedrock (m)

Table 1

Buried preglacial courses and tributaries of the Afon Teifi

Buried valley Location Bedrock altitude

(m OD)

Source Interpretation

Teifi estuary Around estuary mouth �55 Davies et al. (2003),

Lear (1986)

Preglacial courses of the

Afon Teifi�SN 150 500 �60

Llwynpiod Llwynpiod Farm SN

1768 4764

�43.4 Hambrey et al. (2001)

Pen-y-Bryn Pen-y-Bryn �31.8 Hambrey et al. (2001)

SN 1761 4285

Castell Malgwyn

(near Llechryd)

SN 215 435 �22.85 Allen (1960)

Cenarth north (Nawmor) Coed-y-cwm SN 268 427 �9.15 Francis (1964)

Mouth of Nawmor

stream SN 266 417

12.2 Jones (1965)

Cenarth south (Cwm-cou) Around Cenarth SN 269

415

9.15 Jones (1965)

Near Cwm-cou SN 293

421

Between 27.5 and 33.5

Llandudoch SN 1585 4549 61 Hambrey et al. (2001) Preglacial tributary

valleys

Penparc SN 2012 4844 50.75 (Penparc 1 core) Hambrey et al. (2001)

46.74 (Penparc 2 core)

J.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762742

paper examines the sedimentological record preserved inburied valley-fills for two abandoned preglacial coursesof the River Teifi (at Pen-y-Bryn and Llwynpiod), andtwo buried tributary systems (at Penparc and Llandu-doch; Fig. 1a).

Llwynpiod Llwynpiod Farm,

SN 1768 4764

40 83.4

Pen-y-Bryn Pen-y-Bryn,

SN 1761 4285

41 72.8

Llandudoch

(SDL39)

Llandudoch,

SN 1585 4549

103 42

Penparc 1 Sand and gravel quarry

near Banc-y-Warren,

SN 2012 4844

93 42.25

Penparc 2 Sand and gravel quarry

near Banc-y-Warren,

SN 2012 4844

93 46.26

3. Methods

3.1. Core analysis

Sedimentological analyses were undertaken on fiverotary-drilled cores (Table 2, Fig. 2) from buried valleysin the lower Afon Teifi region, supplemented by fielddescriptions of natural and man-made exposures.Lithofacies are described according to particle size,shape and lithology, texture, sorting, depositionalstructures, bedding relationships, gravel percentageand colour. Deformed sediments are described in termsof macro- and microstructural features. Gravel clastroundness was evaluated according to the six-point scaleof Powers (1953).

3.2. Geographical Information Systems (GIS)

reconstruction

Two-dimensional reconstructions are presented forpossible configurations of ice-dammed glacial lakes inthe region. These were generated in a GIS environmentand are based on the distribution of glacial deposits,meltwater channels and glacial lineations. The initialstage of the glacial lake formation is based on ice-

marginal positions for the Irish Sea glacier which allowdamming of the topography while leaving the Teifivalley ice-free. The direction of glacier flow is con-strained by observations on striae at the mouth of theTeifi valley (Hambrey et al., 2001). Reconstructions ofthe subsequent evolution of the lake are based on theelevation of topographic cols, which set the outline ofthe glacial lake perimeters (cf. Jansson, 2003) and on theregional distribution of Irish Sea and Welsh glacialdeposits (Waters et al., 1997; Fig. 1b). Glacial meltwaterfeatures and cols which dissect present day watershedsare deemed important topographic controls on potentialconfigurations for lake altitude and lateral extent underice-dammed conditions. The configuration of the WelshIce Cap and the outline of its ice margin during the Late

ARTICLE IN PRESSJ.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762 743

Devensian are poorly understood; however, glaciallineations south–southeast of the Teifi valley (Fig. 1a)give some indication of the direction of ice flow.Meltwater channels and glacial lineations were mappedusing 1:50,000 and 1:63,000 scale vertical stereopairs ofblack and white aerial photographs.

The Land-Form PANORAMATM DTM, sourcedfrom EDINA Digimap, was used for the reconstructionof the glacial lake system, and has a horizontalresolution of 50m and a vertical (elevation) resolutionof less than 3m. No correction has been performed fornon-uniform isostatic depression in the glacial lakereconstructions due to a lack of data on isostaticrebound at the time of glacial lake formation.

4. Lithofacies analysis

Detailed stratigraphic logging of drillcore from buriedvalley-fill sequences in the lower Afon Teifi regionreveals a variety of distinctive lithofacies including clay,silty clay, silt, sand, gravel and diamicton. Fourlithofacies associations are recognised: (i) coarse-graineddeposits, (ii) mud and coarse-grained debris, (iii) silt andsand, and (iv) diamicton, silt, sand and gravel (Table 3).It should be noted at the outset that an in situ fauna orflora is lacking, with evidence for multiphase allochtho-nous reworking in both microfossil assemblages (Wilk-inson, 1997a, b). There is no evidence for bioturbationanywhere within the buried valley-fill successions. Thesefeatures compound the limitations of core examination(limited exposure), such that three-dimensional geome-try and palaeontological evidence (body-fossils andichnofauna) commonly used for process-based inter-pretations are unavailable.

4.1. Coarse-grained lithofacies association (LA1)

4.1.1. Muddy angular gravel

At the base of the buried valley successions there is apoorly sorted, unconsolidated muddy angular gravel(Table 3, Figs. 2 and 3a). Gravel clasts are subangular toangular, typically comprising over 60% volume, butlocally as much as 95% (e.g. Llandudoch core). Raresubrounded clasts also occur (Penparc 2 core). Thematrix is a poorly sorted sandy-mud or sandy-silt, andlocally infiltrates cracks in the underlying Ordovicianbedrock palaeosurface.

4.1.2. Sandy to muddy gravel

The muddy angular gravel lithofacies passes grada-tionally upwards into a variable sandy to muddy gravel(Table 3), comprising 65–75% subangular to well-rounded clasts in a medium- to coarse-grained sandy-mud matrix. Subrounded to rounded gravel clasts aredominant, all of which are derived from local Ordovi-

cian mudstones and sandstones. In the Llwynpiod core,this lithofacies locally comprises less than 50% clasts,and is therefore better texturally described as clast-richmuddy diamicton.

4.1.3. Pebble conglomerate

This lithofacies is a variably cemented silty-sandstoneconglomerate (Table 3, Fig. 3b). Clasts are angular towell-rounded, although the modal range is subangularto subrounded. Thin-section analysis reveals a calciumcarbonate cement with secondary patchy iron oxidecement. The conglomerate is polymictic, with locallyderived Ordovician mudstone and sandstone clasts. Thematrix is silt to fine sand. This lithofacies is interbeddedwith clay and silty clay laminite, and, in the loweststratigraphic occurrence, overlies the muddy angulargravel lithofacies (Fig. 2). The conglomerate only occursin Penparc 1 core.

4.1.4. Interpretation of coarse-grained lithofacies

association (LA1)

Muddy angular gravel is interpreted as a product ofperiglacial weathering. Angular gravel clasts, localprovenance and transitional base into weathered bed-rock supports formation as a result of weathering,possibly resulting from frost heave activity (cf. French,1996). The pre-existing fracture characteristics of theparent material are considered important (French,1996), and initial outcrop weathering is likely to havebeen aided by the cleaved nature of the LowerPalaeozoic bedrock. In this interpretation, the upwardstransition from shattered mudstone bedrock intoangular pebble-sized fragments with a muddy matrixrelates to progressive gelifraction during multiple freeze-thaw cycles. Texturally, this lithofacies is similar tosupraglacial debris recorded from modern glacialenvironments (e.g. Glasser and Hambrey, 2001); how-ever, the transitional contact on weathered bedrock, andan absence of associated glacigenic deposits are incon-sistent with a supraglacial origin for the debris.

The upwards transition from muddy angular gravelinto variable sandy to muddy gravel together with theincrease in abundance of rounded gravel clasts aresuggestive of reworking. The roundness of the gravelclasts, and the occupation of this lithofacies at the baseof buried preglacial courses and tributaries of the Teifiare consistent with fluviatile deposition. This supportsthe earlier interpretation of Hambrey et al. (2001). Noerratics or clast surface features such as striae and facetsare present to suggest a glacial origin for any of thismaterial.

The origin of the conglomerates at the base ofPenparc 1 core is less certain. The alignment of gravelclast long-axes dipping at relatively low angles (o401)may indicate imbrication or slope-parallel sedimenta-tion. It is possible that the conglomerates comprise in

ARTICLE IN PRESS

Table

3

Buried

valley-filldepositsin

thelower

Teifiregion;characteristics

andinterpretationsforlithofacies

associations1(coarse-grained

debris),2(m

udandcoarse-grained

debris),3(siltandsand)and4(diamicton,silt,

sandandgravel)

Lithofacies

Locality/

borehole

Clast/m

atrix-

supported

Degreeof

sorting

Gravel

Matrix

Characteristics

Interpretation

%Particle-size

range

Roundness

Lithology

Texture

Colour

Lithofacies

association

1:

Co

ars

e-g

rain

edd

epo

sits

Muddyangular

gravel

Llwynpiod

Variable

clast-to

matrix-supported

Poor

60+

Granulesto

pebbles

SA-A

rare

SR

Lower

Palaeozoic

mudstoneandsandstone

Sandy-m

udor

sandysilt

Grey-yellowishgrey

Transitionallybasedon

weathered

bedrock

Fines

upwards

Massive

Product

ofperiglacialweathering.

Insitu

frost-shattering/gelifraction

ofOrdovicianbedrock

Pen-y-Bryn

Penparc

1

Penparc

2

SDL39

Sandyto

muddygravel

Llwynpiod

Matrix

-supported

Moderate

to

poor

65–75

Granulesto

pebbles

SA-W

RLower

Palaeozoic

mudstoneandsandstone

Veinquartz

Medium

to

coarsesandy

mud

Grey

Fluvialdeposits

ofthepre-glacial

AfonTeifiincludingreworked

periglacialmaterial

Pen-y-Bryn

Penparc

1

Penparc

2

SDL39

Pebble

conglomerate

Penparc

1Variable

clast-to

matrix-supported

Moderately

well-sorted

30–60

Granulesto

pebbles

A-W

RModal

range:

SA–SR

Lower

Palaeozoic

mudstoneandsandstone

Silty-sand

Yellow

Clast

long(a-)axes

aligned,

dippingo401

Erratics

ofIrishSea

glacial

provenance

Slopeparallel

sedim

entationalong

lakemargin—possibly

accumulationoffluvial,beach

or

primary

slopedeposits

2:

Mu

da

nd

coa

rse-

gra

ined

deb

ris

Laminite

Llwynpiod

Matrix-supported

Well-sorted

to

verywell-sorted

o5

Granulesto

pebbles

R-V

A,Modal

range:

SA–SR

Lower

Palaeozoic

mudstoneandsandstone

Coarsesandstone

conglomerate

Veinquartz

Yellow

sandstone

Permo-Triassic

sandstone

Chert

Tertiary

Lignite

Shellfragments

Clots

ofsiltorcoarsesand

ClaySilty-clay

Clayey-siltSilt

Clay:purplish-brownto

brown,grey-green

Silty

clay,clayey

siltandsilt:

white,

greyish-w

hite,

creamyyellow,

yellowish-grey

yellowish-brown,

reddish-browngreenish-

grey,pale

pink

Norm

allymicrograded

laminae

(locallyonsub-m

illimetre

scale)

Starved

ripples

Erosivelybasedlaminae

Wispyandconvolute

laminae

On-lappinganddown-lapping

contacts,

Lateralmicrograding

Loaded

basalcontactsbeneath

dropstones

Soft-sedim

entclasts/

pseudonodules

Abundantnorm

almicrofaults

Rare

reverse

faults

Anti-form

al,syn-form

al,

recumbentandtightto

isoclinal

foldsincludingchevronand

slumpfolds

Shearstructures

Liquefactionstructuresand

dew

ateringpipes

Erratics

ofIrishSea

glacial

provenance

Fine-grained

turbidites,suspension

settlingfrom

interflowsand

underflows,subaqueousslumps.

Rhythmitelithofacies

mayindicate

aseasonalcontrolonsedim

ent

deliveryto

thebasin,possible

varves?

Pen-y-Bryn

Penparc

1

Penparc

2

SDL39

Rhythmite

Pen-y-Bryn

Penparc

1

SDL39

Massiveclay

andsiltyclay

Llwynpiod

Moderately

well-sorted

Pen-y-Bryn

Penparc

1

Penparc

2

SDL39

Diamicton

Llwynpiod

Matrix-supported

Poor

o20

Granulesto

pebbles

A-SR

Modal

range:

SA–SR

Lower

Palaeozoic

mudstoneandsandstone

Chert

Permo-Triassic

sandstone

Shellfragments

Mudorsandy

mud

Greyorbrownishgrey

Gradationalupper

andlower

contacts

Underlyingmuddylithofacies

bearhigher

percentageof

lonestones

Striatedgravel

clasts

Erratics

ofIrishSea

glacial

provenance

Ice-rafted

debrisfrom

icebergs

calvingofftheadvancingmargin

of

theIrishSea

glacier

Pen-y-Bryn

Penparc

1

Penparc

2

SDL39

ARTICLE IN PRESSSandandgravel

Pen-y-Bryn

Variable

clast-to

matrix-supported

Well-sorted

to

moderately

well-sorted

475

Granulesto

pebbles

A-W

RModal

range:

SA–SR

Lower

Palaeozoic

mudstoneandsandstone

Chert

Fine-coarse

sandand/silt

Grey

Fining-upward

andcoarsening-

upwardscycles

Norm

ally-graded

sand

Ripple-cross

lamination

Erosivelybased

Erratics

ofIrishSea

glacial

provenance

Coarse-grained

turbidites

generated

bysudden

dropsin

lakebase

level

3:

Sil

ta

nd

sand

Silt,siltysand

andsand

Llwynpiod

Matrix-supported

Well-sorted

to

verywell-sorted

o2

Granulesto

pebbles

SA–SR

Lower

Palaeozoic

mudstoneandsandstone

Jasper

Silt,silty-sand

orsand

Verypaleyellowto

light

grey,brightyellow,

dark

greyorbrownish

grey

Massive

Bedded

orlaminated,planar,

horizontalto

subhorizontal,

locallyinclined

Composedofquartz,

white

micaandblack

lignite

fragments,rare

shellfragments

Fining-andcoarseningupward

cycles

Norm

alfaults,locally

abundant,planar.Rare

graben-

likestructuresandconjugate

faultsets

Small-scale

folds,tightto

isoclinal,recumbent

Dew

ateringpipes

ErraticclastsofIrishSea

glacialprovenance

Deltaic

aggradationunder

more

proxim

alconditions

Pen-y-Bryn

SDL39

4:

Dia

mic

ton

,si

lt,

san

da

nd

gra

vel

Diamicton

Llwynpiod

Matrix-supported

Poor

15–30

Granulesto

cobbles

VA-R

Modal

range:

SA–SR

Lower

Palaeozoic

mudstoneandsandstone,

coarsesandstone

conglomerate

Veinquartz

Chert

Permo-Triassic

sandstone

Yellow

siltstoneand

sandstone

Lignite

Chalk

Granite

Shellfragments

Mudorsand

Grey,purplish-greyor

mottledyellowish-

brown

Striatedgravel

clasts

Erratics

ofIrishSea

glacial

provenance

Massive

Containspurpleto

orangesoft-

sedim

entclastsofdeform

ed

laminite(Pen-y-Bryn)and

greenish-greyto

reddish-brown

clay,whitesiltyclay,silt,fine-

medium

sandandwhite

micaceousparticles

SubglacialtillofIrishSea

glacial

provenance

Pen-y-Bryn

SDL39

Planar-

laminatedsand

andsilt

Penparc

1Matrix-supported

Well-sorted

to

verywell-sorted

o1

Granulesto

pebbles

SR-R

Lower

Palaeozoic

mudstoneandsandstone

Cem

entedsandnodules

Silt,silty-sand

orsand

Yellow

orpale

yellowish-grey

Composedofdetritalquartz,

feldspar,whitemicaandblack

lignitefragments,shell

fragments

Planarlaminated,occasionally

thinning-upwardsand

becomingmore

rhythmic-

upwards

Bed

dipsshallowly

southwards

Contain

cementedsandnodules

Norm

alandreverse

faults,

planarandlocallyanastomising

Glaciofluvialsedim

entationduring

glacier

recession,including

depositionofsheetsandsunder

equilibrium

flow

conditions,

unidirectionalmigratingsand

waves,asboth

channelised

bodies

andsheets

andlarger-scale

migratory

dunes

under

fluctuating

dischargeregim

es.Channelised

coarse-gravelly

sandandsandy

gravel

bodiesindicativeofrelatively

highdischargedistributaries

with

gravelbed-loadsdrainingtheglacier

Penparc

2

Penparc

Quarry

Complex

ARTICLE IN PRESSTable

3(c

on

tin

ued

)

Lithofacies

Locality/

borehole

Clast/m

atrix-

supported

Degreeof

sorting

Gravel

Matrix

Characteristics

Interpretation

%Particle-size

range

Roundness

Lithology

Texture

Colour

margin,probably

attheonsetofthe

meltseason.

Soft-sedim

entclastswithin

the

latter

deposits

indicativeofpartially

frozenconditions(cf.Allen,1982)

suggestingform

ationofperennial

groundiceunder

periglacial

conditions.

Ripple-cross-

laminatedsand

andsilt

Penparc

1Matrix-supported

Well-sorted

to

verywell-sorted

o1

Granulesto

pebbles

SR-R

Lower

Palaeozoic

mudstoneandsandstone

Tertiary

lignite

Silt,silty-sand

orsand

Yellow

orpale

yellowish-grey

Thinly

bedded

(o0.1m)

Composedofdetritalquartz,

feldspar,whitemicaandblack

lignitefragments,shell

fragments

Subcritical,criticalandrare

supercriticalclim

bingripples

Straightcrestedripple

planform

s

Unidirectionalpalaeoflow

towardsS/SW

Ripple

amplitudes

o80mm,

wavelength

o0.3m

Planarlaminaefillripple

‘sand

term

inate

clim

bingcycles

Climbingpatterns:I(A

-B,

A-

B-

S)andIV

(A-

B-

A,

A-

B-

A-

B-

A)common,

asreported

byAllen

(1980)

Erratics

ofIrishSea

glacial

provenance

Penparc

2Penparc

Quarry

Complex

Tabularplanar

cross-bedded

sand

Penparc

Matrix-supported

Verywell-

sorted

o5

Granulesto

pebbles

SA-R

Lower

Palaeozoic

mudstoneandsandstone

Carboniferouslimestone

Acidigneousrocks

Medium-coarse

sand

Reddish-orangeor

orange

Large-scale

(metres)

tabular

cross-beds

Granule

andpebble

stringers

Erratics

ofIrishSea

glacial

provenance

quarry

complex

Sandygravel

SDL39

Variable

clast-to

matrix-supported

Well-sorted

to

moderately

well-sorted

65–75

Granulesto

boulders

SA–SR

Lower

Palaeozoic

mudstoneandsandstone

Coarsesandstoneand

granularsandstone

conglomerate

Permo-Triassic

sandstone,

siltstoneandconglomerate

Chert

‘Sugary’sandstone

Bioclastic

Carboniferous

limestone

Varietyofigneousrocks

includingAilsa

Craig

riebeckitemicrogranite

Shellfragments

Tertiary

lignite

Soft-sedim

entclasts

Fine-coarse

sandandsilt

Yellow

orpale

yellowish-grey

Massiveorfining-upwards

Erosionally-based

Horizontalto

sub-horizontal

sheets,lenticularchannelised

bodies,large-scale

foresetbeds

Contain

soft-sedim

entclastsof

laminatedsiltandsilty-sand,

andarm

ouredmudballs

Erratics

ofIrishSea

glacial

provenance

Penparc

1

Penparc

2

Penparc

quarry

complex

Coarsegravelly

sand

Penparc

Matrix-supported

Verywell-

sorted

towell-

sorted

50–70

Granulesto

pebbles

SA–SR

Medium-coarse

orverycoarse

sand

Reddish-orange,

orange

oryellow

Fining-upwards

Laminated

Horizontalto

sub-horizontal

bedform

s

Planarcross-bedding

Large-scale

tangentiallybased

trough-cross

beds

Ferric-ironoxidestained

norm

alandreverse

faults

(planar)

Erratics

ofIrishSea

glacial

provenance

quarry

complex

Abbreviationsin

roundnesscolumn:VA¼

veryangular,A¼

angular,SA¼

subangular,SR¼

subrounded,R¼

rounded,WR¼

wellrounded.

ARTICLE IN PRESS

Fig. 2. Stratigraphic logs and interpretative correlation for buried tributary valleys and preglacial courses of the Afon Teifi.

J.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762 747

ARTICLE IN PRESSJ.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762748

ARTICLE IN PRESSJ.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762 749

part periglacial deposits such as talus; however, therounded nature of some of the clasts suggests that thismaterial may have been fluvially worked or derivedfrom earlier glacial deposits. The interbedded nature ofthe conglomerates with mud laminite (Fig. 2) isconsistent with either a fluvial or beach provenance forthe conglomerates. The latter is considered most likely.The cementation of this lithofacies is likely to be aneffect of early diagenesis, as gravel-sized clasts of theconglomerate are reworked into the overlying mudlaminite (see below).

4.2. Mud and coarse-grained lithofacies association

(LA2)

LA2 dominates large parts of the buried valleysuccessions and includes massive and laminated clay,silty clay, clayey silt and silt (Table 3). These depositssharply overlie the muddy gravel of the coarse-grainedLA1 (Fig. 2). In the Penparc 1 core, these lithofaciessharply overlie, and are interbedded with, pebbleconglomerate (Fig. 2). Associated with the muddysediments are a range of coarse-grained lithofaciesincluding muddy and sandy gravel, and diamicton.

4.2.1. Laminites

The laminite lithofacies is a silt-laminated mud(Table 3) which comprises millimetre to submillimetreclay, silty clay, clayey silt and silt laminae. Silt-laminated mud forms beds from a few tens ofcentimetres to several metres in thickness. Laminaetypically fine upward and in places display lateralmicrograding. Bundles of silt laminae comprise units�10–15mm thick (Fig. 3c) and are commonly calcar-eous, have erosive bases and lobate and flame-likestructures on their upper surfaces. Some silt and clayeysilt laminae contain coarse sand- to granule-sizedparticles, the latter being underlain by downwarpedlaminae, and overlain by thin clay drapes. Laminae arehorizontal to subhorizontal, but are locally laterallydiscontinuous or convolute.

Where clay rich, the laminite contains sandy silt, siltand silty clay soft-sediment clasts, or pseudonodules.These are coarse sand- to pebble-size and are sub-rounded to rounded, globular in shape or indistinctdisseminated clots (Fig. 3c). In the Llwynpiod andPenparc 1 cores, the soft-sediment clasts are veryangular to subrounded, and have laminae inclined up

Fig. 3. Buried valley-fill deposits in the lower Teifi region; lithofacies associat

(a) Muddy angular gravel at the base of the Llandudoch core, (b) pebble co

long-axes, (c) normally faulted laminite in Pen-y-Bryn core. Note the silty

rhythmites in the Pen-y-Bryn core, (e) massive clay lithofacies, (f) dropstone

laminite towards the top of the muddy succession in Penparc 1, (h) ice-rafted

depth in core.

to 451. In the Penparc 1 core, some soft-sediment clastsare internally faulted, and are bisected by normalmicrofaults. Towards the base of Penparc 1, laminitescontain partially disaggregated conglomeratic clasts,identical in character to the underlying conglomerates(see basal deposits; LA1).

4.2.2. Rhythmites

In the Llandudoch core (SDL39), rhythmicallydeveloped alternations of silt, silty clay and clay occur(Table 2). Rare examples also occur in the Pen-y-Brynand Penparc 1 cores (Fig. 3d). Unlike the laminitelithofacies, the rhythmites are silt dominated and formunits from tens of centimetres to several metres inthickness (SDL39). Individual rhythmites are 15–40mmthick and comprise three distinct units. The lower unit istypically erosively based, finely laminated silty clay orsilt, with several tens of normally micrograded submilli-metre scale clayey silt laminae. This grades upwards intoa middle unit of more coarsely inter-laminated silt, siltyclay and clay. Occasionally dispersed fine- to medium-grained sand particles occur within the lower and middledivisions. The upper unit is a homogeneous steel-grey topurplish brown or dark brown clay, which contains raresubmillimetre inter-laminae of silty clay. Rare examplesof globular soft-sediment pseudonodules of silt and siltyclay occur in some clay laminae. In the Llandudochcore, tripartite rhythmites sometimes gradationallyoverlie a medium or coarse sand lamina containingsubangular to subrounded granule-sized mudstoneclasts. Some rhythmites are incomplete, with noapparent deposition (or preservation) of the lower ormiddle units. In some cases, the rhythmites areinterrupted by a thin (o10mm) lamina of poorly sortedgritty silty clay. These divisions are similar to thosedescribed by Fletcher and Siddle (1998). The rhythmitesare horizontal to subhorizontal, but are locally inclinedin the Llandudoch core by as much as 301.

4.2.3. Massive clay and silty clay

The laminite and rhythmite lithofacies are inter-bedded with thick units (up to 5m) of massive clayand silty clay (Fig. 3e). These contain rare (o5%)laminae, which are typically laterally discontinuous,subhorizontal, inclined or convolute in form. Flame-likestructures are apparent in some silty clay and siltlaminae. Pseudonodules of silt and silty clay locallyoccur, globular in morphology, as disseminated

ions 1 (basal deposits; a, b) and 2 (mud and coarse-grained debris; c–h);

nglomerate at the base of Penparc 1 core. Note the alignment of clast

pseudonodules towards the bottom of the photograph, (d) tripartite

and associated coarse debris, Penparc 1, (g) intensely deformed mud

diamicton in Llwynpiod core. Units marked on photographs relate to

ARTICLE IN PRESSJ.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762750

specks, and as larger granule- to small pebble-sized soft-sediment clasts. Some beds contain fine-grained sand,particularly where associated with diamicton (seebelow).

4.2.4. Features common to laminite, rhythmite and

massive lithofacies

The silt-laminated mud, rhythmites and massivedeposits described above contain scattered very coarsesand- to granule-sized clots of silt or sand, or granule- topebble-sized clasts (lonestones and dropstones; Table 2,Fig. 3f). Laminae beneath pebble-sized clasts areoccasionally down-warped or faulted. Gravel-sizedclasts are capped by clay or silty-clay drapes. Massivemud lithofacies commonly display a marked increase inlonestone abundance where overlain by diamicton (seebelow). Sporadically distributed and finely comminutedshell fragments occur.

A variety of deformation structures affect lithofaciesof association 2 (Table 3). Normal microfaults areabundant, whilst reverse faults are relatively rare (inPenparc 1). The former are locally intensively devel-oped, especially where associated with tight fold closuresand often give rise to wholesale internal brecciation.Small-scale folds are also common, and include anti-formal, synformal and recumbent, tight and isoclinalstructures. In Penparc 1, a shear zone characterises thetop of the mud and coarse-grained debris lithofaciesassociation. This shear zone is �3m thick and containsflat-lying shear planes displacing soft-sediment clastswithin a brecciated matrix (Fig. 3g). Thin-sectionanalysis shows the breccias to be polymictic, with colourand particle-size variations between different laminiteclasts. High-angle kink bands are well developedthroughout the shear zone, while small normal andreverse faults dissect laminite clasts. Liquefactionstructures occur in all cores.

4.2.5. Diamicton

The laminite, rhythmite, massive clay and silty claylithofacies are occasionally punctuated by thin(�50mm–2m) beds of diamicton or muddy gravel(Fig. 3h). These deposits display sharp or gradationallower and upper contacts, and typically overlie massivesediments containing a high abundance of lonestonesand dropstones. Erratic and striated clasts occur withinthese coarse-grained deposits, and include chert, Permo-Triassic sandstone and shell fragments. Gravel clastpercentages vary widely, but are typically less than 20%by volume. Muddy laminite overlying diamicton bedsoften drape gravel-sized clasts. Diamicton beds occur ata number of elevations in the Llwynpiod core, but aregenerally absent in the Pen-y-Bryn core. In theLlandudoch core, diamicton beds are typically thin(o100mm), and have higher gravel percentages(450%).

4.2.6. Sand and gravel

In the Pen-y-Bryn core, sand and gravel depositsinterrupt an otherwise thick mud sequence (Fig. 2). At�11.67m O.D. is a thin (0.25m), coarsening-upwardcycle of interlaminated silty clay, clayey silt and fine-grained sand, which grades upwards into silty sandcomprising three beds, two massive and erosionallybased, and one ripple-cross laminated. These sedimentsare overlain by 0.85m of clast-rich sandy to muddygravel (65–75% gravel) or massive sandy to muddydiamicton (25–30% gravel).

At �9.5m O.D. a 0.5m thick bed of erosionallybased, coarse gravelly sand forms the base of a fining-upward succession (Fig. 4a). This well-sorted medium-to coarse-grained sand contains granule- to pebble-sizedclasts, typically subrounded to well-rounded and of localderivation.

At �8.3 and �7.5m O.D. two further erosively basedgravel beds form fining-upward cycles into well-sortedfine- to medium-grained sand and silt. The gravels arestructureless, variably sandy to muddy and comprise60–75% gravel. The gravel clast assemblage is domi-nated by locally derived material; however, a single chertclast was observed in the lower of the two beds.

4.2.7. Interpretation of mud and coarse-grained debris

lithofacies association (LA2)

Silt-laminated muds (laminite) may be deposited by arange of different processes, including rainout ofparticulate matter (interflows and overflows), sedimen-tation from turbidity currents (underflows), or sedimentredistribution resulting from subaqueous slumping.Structures observed here such as discontinuous laminae,erosional basal contacts, convolute bedding, microfaultsand soft-sediment rip-up clasts are characteristic of fine-grained mud turbidites (Stow and Piper, 1984). How-ever, identification of individual flow units is oftenchallenging, and it is difficult to establish whetherlaminae bundles accumulated under variable flowvelocities with periods of waxing and waning, or as aresult of background suspension fallout from interflows.Although features such as sole and tool marks were notidentified, and an in situ deep water fauna is absent, thegross palaeoenvironmental setting is amenable toturbidite deposition. The silt-laminated mud (laminitelithofacies) is thus interpreted as a combination of fine-grained suspension fallout and mud-turbidite deposits.It is possible that some of these deposits may representprodeltaic sedimentation. The occurrence of pebble-sized lonestones and dropstones supports meltout oficeberg-rafted debris (e.g. Grobe, 1987; Andrews et al.,1997; Smith and Andrews, 2000) indicating an ice-contact environment.

The rhythmites share many characteristics withvarves; however, varve-like sediments may also bedeposited by a variety of other mechanisms (Moncrieff,

ARTICLE IN PRESS

Fig.4.Buried

valley-filldepositsin

thelower

Teifiregion;lithofacies

associations2(m

udandcoarse-grained

debris;a),3(siltandsand;b,c),and4(diamicton,silt,sandandgravel;d–h).(a)Sandy

gravel

interruptingthemuddysuccessionin

Pen-y-Bryn,(b)silty-sandlaminites

inLlwynpiodcore.Note

theblack

laminaecomposedprimarily

oflignitefragments.Downwarped

edges

dueto

drillingdisturbance,(c)dew

ateredsiltandmud,Llwynpiodcore,(d)muddydiamictonin

theLlandudoch

core,(e)planar-laminatedsiltysandpassingupinto

clim

bing-rippledbeds(Penparc

quarry),trowel

forscale

(circled),(f)subcriticalto

criticalclim

bingrippledsiltysand,Penparc

quarry,trowel

forscale

(circled),(g)boulder-sized

clastsofunconsolidatedlaminatedsiltandsand

within

achannelised

sandygravelbody,Penparc

quarry,(h)tangentiallybasedtroughcross-bedded

coarsegravelly

sands,trowelforscale(circled).Unitsmarked

onphotographsa–drelate

todepth

incore.

J.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762 751

ARTICLE IN PRESSJ.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762752

1988), including suspension settling from sediment-laden interflows, distal low concentration turbiditycurrents (Pickering et al., 1986), or continuously fedturbidity currents (e.g. Lambert and Hsu, 1979), whichneed not have an overriding seasonal control. Distin-guishing turbidites from varves is further complicated,in that underflow deposits may form an intrinsic part ofa varve couplet (see De Geer, 1912). The coarse lowerunit of the rhythmites observed here is often erosive,which may support an underflow origin; however, aswith the silt-laminated muds, background fallout fromsuspension is considered important, especially in thegeneration of the uppermost clay lamina of therhythmites unit. Given an absence of dates to constrainrhythmite counts, a varve interpretation is consideredtenuous.

The diamicton is interpreted as iceberg-rafted detrituson the basis of its association with lonestone anddropstone-bearing muds, the occurrence of striated anderratic clasts of Irish Sea glacial provenance, andvariability in clast roundness (Boulton, 1978; Benn andEvans, 1998). Conformable contacts at the top ofdiamicton beds, particularly where laminated muddrapes gravel clasts, are consistent with continuoussubaqueous sedimentation. The deformation style be-neath the beds is consistent with loading of the sedimentpile and localised expulsion of pore waters, which maybe explained by deposition from grounded icebergs, orby dumping of ice-rafted debris. The absence of obviouspercussion features or erosional basal contacts (e.g.from iceberg keel drag) suggests that the latterinterpretation is more satisfactory. In addition, there isno evidence for shearing-induced structures within theunderlying sediments to suggest subglacial deformation.The absence of ‘flow’ folds (see Evenson et al., 1977),flutes, sole marks and erosional contacts precludes asubaqueous flow genesis (e.g. Gibbard, 1980). Somematerial may have been delivered by adfreezing in ice-proximal littoral zones by perennial lake ice, a processthat has been recognised in some glacial lake successions(e.g. Smith, 2000). Comminuted shell debris throughoutthese sediments is considered to indicate glacial rework-ing of marine sediments from Cardigan Bay.

Ripple-cross-laminated sand associated with othersand and gravel deposits in the Pen-y-Bryn core may bea product of quasi-steady turbidity currents generatedby hyperpycnal inflow (Mulder and Alexander, 2001).This is supported by the coarsening upward trend, andfining-upward cap relating to waxing and waning flowconditions (see Mulder and Alexander, 2001). Wilby(1998) noted that the upper two gravel beds have sharpbases and fine-upward, their gross form and thicknessbeing similar to high-density turbidites described frommodern glacial lake settings by Sturm and Matter(1978). However, the lack of evidence for channeliseddeposition or close association to a delta led Wilby

(1998) to suggest that the deposits are gravel-rich densityflows, triggered as a result of sudden, relatively minordrops in lake level. The gravel beds indicate asubstantial, possibly rapid change in hydrologicalconditions and probably reflect either a drop in baselevel giving rise to fluvial input, or high-energy subaqu-eous flows initiated by overspill from adjacent lakebodies. Correlative lithofacies appear to be absent in theLlwynpiod core at similar altitudes, where more muddysediments persist, so if accounted for by a drop inbase level, it is unlikely that complete lake drainageoccurred.

Reverse-sense kink bands and other deformationstructures observed in thin-sections from the shear zonein Penparc 1 core are consistent with a compressivetectonic regime. The kink bands indicate a transitionalductile to brittle-strain deformation style; deformationwhen water-saturated is thus considered very unlikelyand indicates a post-depositional stage of deformation.

4.3. Silt and sand lithofacies association (LA3)

A thick (up to 20m) and monotonous sequence of silt,silty sand and sand lithofacies gradationally overlies themud and coarse-grained debris of LA2, demonstratingan overall coarsening upwards in the stratigraphicsuccession. This sequence includes massive (possiblydrilling disturbed), bedded and laminated sediments.The silt and sand lithofacies are texturally well sorted;however, some beds contain outsize granule to pebble-sized lignite clasts. Where bedded or laminated, thesediments typically fine-upward from fine-grained sandinto silt. Millimetre-scale laminae characterise the finer-grained parts of the succession where mud-laminatedsilts occur. These are common in the upper part of thePen-y-Bryn borehole, where they are rhythmicallybedded on a 30–50mm scale. Drapes of purplish brownclay and silty clay laminae occasionally cap fining-upward units. Elsewhere, coarsening-upward units ofsilt into fine sand occur. Examples of convolute laminaeand laminae composed of lignite fragments are common(Fig. 4b).

Throughout the silt and sand lithofacies, rare gravelclasts occur, derived predominantly from local Ordovi-cian rocks. Granule-sized angular to well-rounded soft-sediment clasts of homogeneous brown to reddishbrown mud also occur. Massive and normally gradeddeposits are commonly erosionally based and comprisebeds 0.1–0.5m thick. Brown-coloured, poorly sortedclayey sand locally caps this lithofacies association, andis transitional upwards into diamicton (diamicton, silt,sand and gravel LA4).

Silt and sand lithofacies of association 3 are locallydeformed by faults, folds, convolute bedforms, chaoti-cally bedded units and dewatering structures (Table 3).Faults are locally abundant and planar showing normal

ARTICLE IN PRESSJ.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762 753

millimetre-scale throws. Folds are of small-scale, andare tight to isoclinal and locally recumbent. Dewateringstructures occur in the Llwynpiod core (Fig. 4c), andmay reflect sediment liquefaction followed by expulsionof pore-water, although some examples have resultedfrom drilling disturbance.

4.3.1. Interpretation of silt and sand lithofacies

association (LA3)

The silt and sand lithofacies comprise part of anoverall coarsening-upward motif in the buried valley-fillsuccessions reflecting a change in the availability ofaccommodation space. Potential mechanisms for redu-cing the availability of accommodation space include (i)a drop in base level, (ii) a progressive increase in sourceproximity, and (iii) high rates of sediment supply,exceeding the rate at which accommodation space isgenerated.

The silt and sand lithofacies show a variety ofstructures including graded bedding, microfaults, con-volute lamination, ripple-cross lamination and erosivebasal contacts which are consistent with deposition fromsediment-laden turbidity currents, although, as withlithofacies of association 2, it is difficult to distinguishindividual turbidites from background sedimentationdeposits resulting from plume fallout and tractiondeposition. Graded bedding may relate to diurnalvariation in sediment discharge, or sediment remobilisa-tion by density underflows and subaqueous slumps.Indeed, chaotically bedded deposits may reflect syn- orpost-depositional mixing as a result of slumping,although some are due to partial homogenisationresulting from drilling disturbance. Local small-scalerecumbent folds may also indicate slumping. As thesedeposits are transitional upwards from those of the mudand coarse-grained debris LA2, two possible interpreta-tions may be suggested.

The first interpretation is that the silt and sandaccumulated under deltaic conditions; the second, thatthe sediments represent growth of ice-marginal subaqu-eous fans (grounding-line fan deposits). However,accumulation as part of a grounding-line fan system isconsidered unlikely. The deposits lack a significantcoarse-grained debris component, as would be expectedfrom sediment delivery during calve dumping (Powell,1990), or direct meltout of subglacial debris at the ice-margin (Benn and Evans, 1998). Also, with theexception of isolated gravel clasts, little sedimentologicalevidence exists to support extensive iceberg rafting ofdebris. Although grounding-line fans can be dominatedby subaqueous efflux of meltwater (Powell, 1990), withthe exception of small-scale folds, large-scale re-organi-sation of the sediment pile does not appear to havetaken place. Beds remain horizontal to subhorizontal,and are unlikely to have accumulated under ice-pushconditions (e.g. Powell, 1981).

Downslope subaqueous gravitational reworking ofdebris and deposition from density underflows (turbiditycurrents) under variable discharge can account for partof the observed lithofacies and probably representforeset and prodelta deposits. Fine-grained mud-lami-nated silts are interpreted as suspension settling depositsin prodeltaic environments. Other background sedimen-tation deposits probably occur; however, it is difficult todistinguish these from turbidite facies. The lack ofassociated coarse-grained debris (with the exception ofrare lonestones) that could be attributed to ice-marginalprocesses suggests that a deltaic origin is most likely forthe silt and sand LA3.

4.4. Diamicton, silt, sand and gravel lithofacies

association (LA4)

LA4 comprises diamicton, silt, sand and gravel whichlie erosionally upon deposits of the mud and coarse-grained LA2 (Penparc 1 core) or the silt and sand LA3(Llwynpiod, Pen-y-Bryn and Llandudoch cores). Thedescriptions below include field observations from sandand gravel quarries near Banc-y-Warren (Penparc, Fig.1a), which are vertically contiguous with the silt, sandand gravel recovered in the Penparc boreholes. Thesedeposits are stratigraphically above the mud and coarse-grained debris of LA2, the upper part of which isintensively deformed (Fig. 3g).

4.4.1. Diamicton

This lithofacies is a massive, clast-rich (15–30%)muddy to sandy diamicton (Fig. 4d) which caps the siltand sand LA3 (Table 3). Bed thicknesses vary fromcentimetres to several metres. Clasts include locallyderived mudstones, sandstones, conglomerates andquartz, and erratics including red sandstone, chert,yellow siltstone and sandstone, lignite, and a variety ofigneous rocks. In some examples, small pebble-sized(�5mm) fragments of broken mollusc shells occur.Some clasts (o5%) bear striations.

The diamicton locally contains purple to orange soft-sediment clasts of deformed laminite, greenish grey toreddish brown clay, white silty clay, silt, fine- tomedium-grained sand and particles of white mica. Inthe Pen-y-Bryn core, the diamicton overlies an intenselydeformed brecciated sequence of unconsolidated sandand mud (Fig. 5).

4.4.2. Planar-laminated sand and silt

Planar-laminated deposits occur as horizontallybedded or shallow dipping, fining-upward units ofwell-sorted pale yellowish grey medium- to coarse-grained sand, medium to fine sand, and sandy silt(Table 3). Fining-upward units are typically less than40mm thick and are frequently capped by a concentra-tion of lignite fragments. In places, nodules of cemented

ARTICLE IN PRESS

Fig. 5. Deformed lithofacies at the top of lithofacies association 3 (silt

and sand) in the Pen-y-Bryn core. Units marked relate to depth in core.

Fig. 6. (a) Palaeocurrent directions based on climbing-ripple-cross-

laminated lithofacies exposed at Penparc, (b) distribution of faults

displacing silt, sand and gravel deposits of lithofacies association 4 at

Penparc.

J.L. Etienne et al. / Quaternary Science Reviews 25 (2006) 739–762754

sand occur, typically subspherical or platy in shape; thelatter tend to lie subparallel to bedding. Fining-upwardunits recur for up to 2–3m and locally become morerhythmic and thin up the succession (Fig. 4e). At thislevel, lignite-dominated fine sand and silt laminae occuron a 4–5mm scale, but are typically laterally discontin-uous after 2–3m. These beds dip o201 towards thesouth. Planar-laminated and bedded lithofacies dom-inate the silt and sand succession in the Penparc 1 and 2cores.

4.4.3. Ripple-cross-laminated sand and silt

Climbing ripple-cross-laminated fine- to medium-grained sand, fine sand and silt occur in the quarry asthinly bedded (o0.1m) climbing rippled units (Fig. 4f,Table 3). Ripple-cross laminae are planar, consistentwith straight-crested ripple planforms, supported by thelocal preservation of straight-crested ripples withincemented beds. Thin (o10mm) drapes of clayey-siltoccasionally cap rippled units. Palaeoflow directions areconsistently oriented towards the south/southwest(Fig. 6a), as previously observed by Helm and Roberts(1975) and Allen (1982). Planar ripple-cross-laminateddeposits also occur in the upper part of the succession inthe Penparc 1 and 2 cores.

4.4.4. Tabular planar cross-bedded sand

Metre-scale tabular cross-bedded sands occur rarely(Table 3). This lithofacies is characteristically coarser-grained than the other silt and sand lithofacies describedabove, and bears clast stringers. The sands comprisingthese tabular beds are strikingly orange in colour.

4.4.5. Sandy gravel

Sandy gravel (Table 3) occurs as normally graded,erosionally based, subhorizontal to horizontal sheets, asfining-upward lenticular channelised bodies, and aslarge-scale foreset beds. Within the lower parts of thequarried succession, sheet-like and lenticular gravelbodies predominate, typically as 1–2m thick beddedunits, erosionally based on the planar-laminated andripple-cross-laminated deposits described above. Large-scale foreset beds only occur in the Pant-y-Dwr quarry,towards the top of the stratigraphic succession. Locallyderived rocks are common; however, Permo-Triassicsandstone, chert, sandstone, bioclastic limestones(Carboniferous), yellow siltstone and a variety of acidigneous rocks, including Ailsa Craig riebeckite micro-granite also occur. Broken shell debris and lignitefragments are common. Many gravel clasts bearstriations, particularly the softer sandstone and lime-stone lithologies.

Where normally graded, thin sand laminae occurtowards the top of fining-upward units. Lenticularchannelised bodies display markedly undercut channelmargins, and commonly contain a lag of pebble- toboulder-sized clasts of unconsolidated laminated siltysand (Fig. 4g). Laminae within these clasts are typicallyinclined by 20–301, but are locally over 801 relative tothe bedding plane orientation. Elsewhere, small pebble-sized soft-sediment clasts of structureless red-stained,medium-grained sand and pebble-sized mudballs occur.The latter are composed of a heterogeneous sandy-silty-clay, with an armour of coarse-grained sand andgranule-sized gravel clasts; where observed, these oftenappear elongate in the bedding plane orientation.

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4.4.6. Coarse gravelly sand

Coarse gravelly sand comprises fining-upward unitsof laminated very coarse or coarse to medium sandsupporting subangular to subrounded clasts (Table 3).Bed thicknesses are typically less than a metre.Laminated units comprise horizontal to subhorizontal,low-angle planar cross-bedded foresets or large-scale,tangentially based trough-cross-bedded units (Fig. 4h).This lithofacies is interbedded with the planar-laminatedand ripple-cross-laminated deposits. The gravelly sand islocally capped by thin laminae (5–40mm) of clayey-siltor homogeneous clay.

4.4.7. Deformation structures

The deposits at Penparc are dissected by a pervasivenetwork of normal and reverse faults. Normal faults aredominant and occur as conjugate sets, typically planar,or locally anastomosing and strike northeast to south-west (Fig. 6b). Good examples of graben-style normalfault structures occur. Reverse faults are rare, andlocally show similar strike orientations, but with throwsmore consistently directed towards the north–northeast(Fig. 6b). Displacements along these faults are small,with throws usually less than 0.5m. However, largerthrows 41m displace some beds. Evidence for at leasttwo phases of faulting exists, illustrated where faultscross-cut one another. In most cases, exposures showthe faults to stand proud of the deposits they dissect,with a range of different styles of cementation. Mostfaults appear to be slightly higher in silt content, whilstothers are firmly cemented either by calcium carbonateor iron oxide. In other examples, the faults are notcemented, but are picked out either by a red staining, ordisplacement of laminae. Where poorly cemented, faultdrag of adjacent laminated sediment is demonstrable.

4.5. Interpretation of diamicton, silt, sand and gravel

lithofacies association (LA4)

In the diamicton, sedimentological features (Table 3and Fig. 2) including texture (massive, poorly sorted),clast roundness (subangular to subrounded), and thepresence of striated and erratic clasts suggest that thismaterial has undergone basal glacial transport (Boulton,1978; Benn and Evans, 1998). The diamicton isinterpreted as the product of primary subglacialsedimentation, associated with overriding of the under-lying succession. Incorporation of deformed soft-sedi-ment clasts, notably laminites, massive clay, silt andsand supports localised reworking of the upper part ofthe succession. This is further supported by locallyintense deformation of the underlying lithofacies, forexample in the Pen-y-Bryn core, with evidence forbrittle-ductile deformation (Fig. 5). A similar diamictonoccurs at the drill-site at Llwynpiod. Both this, and thePen-y-Bryn deposits are laterally contiguous with a

widespread Irish Sea basal till recently mapped through-out the area (British Geological Survey, 1997, 2003).Contrary to diamicton lithofacies preserved in the lowerparts of the cores, these facies do not have gradationalcontacts on lonestone- and dropstone-bearing deposits.

The silt, sand and gravel lithofacies are interpreted aspart of a sand-dominated glaciofluvial outwash plain;such deposits are typical in modern proglacial outwashenvironments (e.g. Williams and Rust, 1969; Rust, 1972,1975; Bluck, 1974; Smith, 1974, 1985; Miall, 1978).Channelised gravel bodies are interpreted as theproducts of rapid discharge under upper flow regimes.Frozen ground conditions during early melt season inthe proglacial area provides a mechanism to explain thepreservation of soft-sediment clasts within the gravels,and undercut channel margin stability during channelincision (cf. Allen, 1982). Where soft-sediment clasts arepreserved, it is likely that either (i) the clasts were rapidlyburied, and/or (ii) the flows were not significantlyprolonged to melt and disaggregate the sediment. Floodconditions are supported by the preservation ofarmoured mudballs within gravel beds (e.g. Krzyszton,1984), the elongate nature of which suggests somedegree of post-depositional compaction.

Laterally extensive gravel bodies are likely to havedeveloped during sheet floods, facilitated by therelatively low relief of the outwash plain (e.g. Rustand Koster, 1984). Infiltration of silt and sand, anddeposition of laminae capping fining-upward gravelunits is indicative of falling stage conditions (e.g. Bluck,1974; Steel and Thompson, 1983). Coarse gravelly sandprovides evidence for channelised flow, with large-scalegrouped trough-cross beds interpreted as migratingdunes (e.g. Miall, 1978; Maizels, 1995). The fining-upward cap of mud may indicate the final stages ofwaning-flow deposition, back-ponding or channel aban-donment and overbank sedimentation. Tabular cross-bedded sands may also correspond to migrating dune-forms.

Planar-laminated silt and sand are interpreted aslaterally extensive sheet-flood deposits (cf. Miall, 1978).The fining-upward nature may represent waning flow,supported by the deposition of low-density woodytissues (lignite) which cap fining-upward units. Repeti-tive fining-upward units and thinning-upwards mayrepresent diurnal variations in discharge (e.g. SambrookSmith, 2000), or progressive flow reduction duringpulsed discharge towards the end of the melt-season.

Climbing-rippled silt and sand are indicative of highrates of bed aggradation (e.g. Stear, 1985), and representmigration of straight-crested ripple bedforms. Transi-tions from subcritical and critical climbing ripples intosupercritical climbing ripples and draped-laminationcaps are also indicative of waning-flow conditions.However, climbing-ripple-cross lamination can developin several environments, common in glaciolacustrine

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deltas (Ashley, 1995), alluvial regimes (e.g. Collinson,1996), glaciofluvial environments (e.g. Maizels, 1995),and even englacial cavity flow systems such as eskers(e.g. Allen, 1971). Given the combination of associatedlithofacies, the sedimentary relationships exhibitedbetween channelised gravel bodies, the sediments theyincise and the local inclusion of soft-sediment silt andsand clasts, a glaciofluvial interpretation for thissuccession is considered most likely. A glaciofluvialinterpretation is also likely given the shear zonedeformation in Penparc 1 core. The contractionaltectonic style and kink bands indicative of transitionalductile to brittle deformation conditions are consistentwith processes of subglacial deformation. The contactwith the overlying sand and gravel is interpreted as adisconformity that correlates with subglacial till exposedat similar elevations adjacent to the buried tributaryvalley. These tills are locally overlain by the sand andgravel in the Penparc area. The disconformity thereforerepresents a depositional hiatus between lacustrinesedimentation and deposition of the overlying sandand gravel complex, and precludes an origin by deltaic

Fig. 7. Schematic model illustrating the range of depo

progradation as suggested by Helm and Roberts (1975)and Eyles and McCabe (1989).

5. Sedimentation history and dynamics of glacial Llyn

Teifi

Based on the stratigraphic and sedimentologicalevidence, the following depositional model is suggestedfor glacial Llyn Teifi (Fig. 7).

1.

sitio

Lake formation during advance of the late DevensianIrish Sea glacier. This is based on (i) an absence ofrecognisable subglacial deposits at the base of thestratigraphic succession, (ii) the overall coarsening-upward nature of the succession, indicative ofincreasingly more proximal and/ shallow-water de-position, (iii) subglacially deformed sediments at thetop of the lacustrine successions in the Pen-y-Brynand Penparc 1 cores, and (iv) the preservation ofsubglacial and glaciofluvial sediments stratigraphi-cally above the lacustrine sequence. A late Devensian

nal processes operating in glacial Llyn Teifi.

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timing is supported by the youngest microfaunain assemblages from Pen-y-Bryn and Llwynpiod(Wilkinson, 1997a, b); precise dates are yet to beestablished for this stratigraphic succession.

2.

Probable rapid establishment of deep-water sedimen-tation as meltwater inundated the Teifi valley,indicated by abrupt contacts between lacustrine muds(LA2) and older deposits at the base of the valley-fillsuccessions (LA1), and by the absence, or preserva-tion of only thin transgressive deposits. Sedimenta-tion from suspension settling, hyperpycnal flowgenerating interflows and underflows and resultinggravity flows were probably the dominant deposi-tional processes (mud and coarse-grained debrisLA2). These sediments represent a complex interplayof often inseparable processes, particularly withregard to distinguishing individual flow units depos-ited as a result of turbidity current activity. Lone-stones and dropstones throughout these sediments,and locally thick interbeds of shelly diamicton ofIrish Sea glacial provenance, indicate deposition fromicebergs calving off the advancing damming marginof the Irish Sea glacier (Fig. 7).The absence of bioturbation structures, low concen-tration of organic debris, a lack of autochthonousmacrofauna and abundant evidence for allochtho-nous reworking throughout micropalaeontologicalassemblages (Wilkinson, 1997a, b) all support anoligotrophic (low productivity) lacustrine system.High rates of sedimentation are envisaged, withevidence for slumping and resulting load-induceddeformation resulting in sediment liquefaction. Apermanent ice cover is considered unlikely, given theevidence for ice-rafting of debris throughout thesuccession, although perennial lake ice may haveformed. Autochthonous debris may have beendelivered as a result of erosion within the basincatchment, or Welsh glacial activity to the east. Ice-contact glacial lakes are typically polymictic andundergo density stratification (as a result of increasedsuspended sediment concentrations with depth)rather than thermal stratification (Ashley, 1995;Miller, 1996). Although some of the siltier laminaein lithofacies of association 2 are calcareous, asignificant authigenic mineralogical component ap-pears to be lacking, and is not supportive of chemicalstratification. A turbiditic origin for much of themuddy lithofacies of LA2 suggests that mixing mayhave occurred during subaqueous gravity flow events.

3.

In the Llwynpiod and Pen-y-Bryn cores, the dom-inance of silt and sand throughout the upper part ofthe succession indicates a reduction in accommoda-tion space and deltaic progradation. The occurrenceof Irish Sea glacial deposits including subglacial tillsstratigraphically above the lacustrine sequence sup-ports overriding of the lacustrine sequence, rework-

ing, and deformation of the underlying sediments.Outwash sediments overlying subglacially deformedlacustrine muds (Penparc 1) record the final stages ofice-marginal sedimentation during glacier recession.There is little evidence to suggest that an extensivelake body developed in the Teifi valley during therecession of the Irish Sea glacier from this region.Further data are required to establish whether this istrue of other glacial lakes in the region.

4.

This succession indicates deposition in a terrestrialglacial environment and does not support thecontroversial glaciomarine depositional model pro-posed by Eyles and McCabe (1989) for this part ofthe Irish Sea basin.

6. A model for ice margin configuration and glacial lake

evolution

In Fig. 8, we present a model for the evolution of theglacial lake system. Ice-flow directions are constrainedby the orientation of glacial lineations and striations;maximum ice limits are based on the regional distribu-tion of Irish Sea and Welsh glacial deposits (Figs. 1b and8; British Geological Survey, 1997, 2003). The glaciallake evolution is modelled during advance of the IrishSea glacier, as evidenced by the vertical lithostratigraphyand deformation history of the Llyn Teifi basin fill(Fletcher and Siddle, 1998; Hambrey et al., 2001; thisstudy). For Llyn Teifi, the glacial lake outline isconstrained in part by the observed areal and altitudinaldistribution of glaciolacustrine deposits and by topo-graphy (Waters et al., 1997). Although modified duringthe late Devensian, detailed studies of the geomorphol-ogy and sedimentary fill of glacial meltwater channelsacross the region demonstrate that many were incisedduring a period of pre-late Devensian glaciation(Glasser et al., 2004), and thus acted as importantcontrols on glacial lake development. The water bodiesidentified are glacial lakes Aeron, Aberporth, Teifi andNevern.

The initial stage of meltwater damming and glaciallake formation occurred when advance of the Irish Seaglacier impeded westward drainage, possibly by closingthe natural drainage pathway around Strumble Head(Fig. 8a). At this time, a shallow (up to 100m depth)lake could have developed offshore of the present daycoastline against the advancing ice margin, withinundation of deeply incised river basins at altitudesbelow modern sea level. At this stage lacustrinesedimentation probably started in drowned preglacialcourses of the Teifi, notably the basal parts of the valley-fill successions at Llwynpiod and Pen-y-Bryn.

With continued advance, isolated lake bodies beganto develop in the Aeron valley (up to an altitude of129m O.D.), the basin around Aberporth (120m O.D.),

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Fig. 8. Shaded colour relief maps illustrating the evolution of late Devensian glacial lakes along the southwest coast of Wales; arbitrary time slices

(a–f) show different stages of lake development. Lake reconstructions are based on maximum altitude defined by potential cols or spillways. Ice-

marginal positions are constrained by the surface distribution of Irish Sea and Welsh glacial deposits, derived from 1:50,000 scale BGS Digital Data

under Licence 2003/116 British Geological Survey. rNERC. Ice-flow directions are based on distribution of glacial sediments, glacial striae, till

lineations and drumlins. Location of glacial meltwater features based on Glasser et al. (2004) and this study. Relief maps were generated using the

Land-Form PANORAMATM Digital Terrain Model sourced from EDINA Digimap. (a) Initiation of glacial lakes along the southwest coast of

Wales. Early lake development offshore of the present coastline and at low altitudes within coastal river basins. Lacustrine sedimentation begins in

parts of the lower Teifi valley. (b) Isolated glacial lakes begin to develop in the Aeron, Aberporth, Teifi and Nevern areas, with maximum lake-level

altitudes defined by cols at 129, 120, 100 and 70m O.D., respectively. (c) Southerly advance of the Irish Sea glacier results in near-closure of the

glacial Llyn Aberporth. Cols in coastal regions are overridden, with new maximum lake-level altitudes defined for glacial Lake Aeron and Teifi at 160

and 129m O.D., respectively. (d) Further advance sees a reduction in the extent of glacial Llyn Nevern, and the glacial Llyn Aberporth is completely

overridden. Glacial Llyn Aeron begins to spill over its eastern margin into the upper reaches of the Teifi valley. (e) Closure of the Llantood overspill

allows glacial Llyn Teifi to fill up to 170m O.D., confluencing with glacial Llyn Aeron in the north. Possible confluence of Irish Sea and Welsh glacial

masses at this time. (f) Advance of the Irish Sea glacier towards its maximum position, glacial Llyn Teifi divides into two basins, the westernmost

confluencing with glacial Llyn Nevern. Both basins drain southwards, the remnant Nevern lake discharging via a channel at Crymych (220m O.D.),

the eastern Teifi basin discharging via a col at Pedran (190m O.D.).

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the Teifi valley (100m O.D.) and the Nevern area (70mO.D.; Fig. 8b). During this time, tributary valleys of thelower Teifi valley started to flood. Cols controllingmaximum lake-level altitudes at this stage were locatednear the coast, and may have been overridden beforebeing activated as subaerial drainage routes; the Cippynchannel (100m O.D.; SN 135 480) and the Cross Innchannel may have never actively drained glacial LlynTeifi or Aeron, respectively.

As the Irish Sea glacier advanced farther south, moredistally located cols became the primary controls onlake-level altitude (Fig. 8c). Glacial Llyn Aberporthbecame reduced in size, and discharged westwards intoglacial Llyn Teifi. At this stage, glacial Llyn Aeron(160m O.D.) spilled over its eastern boundary, throughthe Pen-lan channel into the upper reaches of the Teifivalley and glacial Llyn Teifi spilled over the Llantoodcol at 129m O.D [SN 154 417] into glacial Llyn Nevern(Fig. 8c). The maximum altitude of glacial Llyn Nevernat this stage of ice-sheet advance was governed by a colat 70m O.D. Meltwater flowing over this col conse-quently drained into the Gwaun-Jordanstown channelsystem.

During the further advance of the Irish Sea glacier,the col at 160m O.D. of the glacial Llyn Aeron wasclosed and the lake surface altitude raised to 170m O.D.(Fig. 8d). Soon after, glacial Llyn Aberporth reached itshighest level at 192m O.D. (Fig. 8d). During this time,glacial Llyn Teifi continued to drain through theLlantood channel, but the advancing ice-margin hadnow closed the col at 70m O.D. north of the Gwaun-Jordanstown channel system. This closure allowedglacial Llyn Nevern to expand up to 115m O.D.governed by a col southeast of Mynydd Carningli(Fig. 8d).

Later (Fig. 8e), the watershed between lakes Aeronand Teifi was fully breached, leading to their confluenceup to a possible altitude of 170m O.D., followingclosure of the Llantood overspill. At this stage, glacialLlyn Aeron and Teifi drained through a channel nearBlaenffos into Llyn Nevern, which in turn spilled overthe col at 115m O.D. into the Gwaun-Jordanstownchannel system.

As the Irish Sea glacier advanced towards itsmaximum position, glacial Llyn Teifi divided into twobasins, the westernmost of which coalesced with glacialLlyn Nevern (Fig. 8f). Both basins drained southwards,the remnant Nevern lake discharging via the Rhosdduchannel (220m O.D.), the eastern Teifi lake basindischarging via a col at Pedran (190m O.D.).

7. Implications of lake palaeogeography

Traditionally, the level of glacial Llyn Teifi isconsidered to have been regulated by the surrounding

topography, with major outflows suggested via spillwayscentred around Cippyn, Pontgarreg (Llantood), andPedran (e.g. Charlesworth, 1929; Jones, 1965). Themodel presented in Fig. 8 presents maximum potentiallake surface altitudes based on the available geomor-phological and sedimentological evidence, but also onthe assumptions that (a) the ice dam remained coherentand (b) sufficient water was supplied to the basin toflood the topography. With meltwater sources from theadvancing Irish Sea glacier, outlet glaciers of the WelshIce Cap, and overspill from adjacent lake bodies, watersupply is not considered problematic. Yet coarse-grained sediments interbedded with finer lithofacies atthe base of the Penparc cores and in the Pen-y-Brynsuccession do not readily correspond in altitude with theproposed topographic controls, and indicate that lakesurface altitudes may have been moderated by otherprocesses.

In a recent review, Tweed and Russell (1999)identified a number of potential drainage initiation ortrigger mechanisms, some of which need to be con-sidered as alternative controls on the extent of LlynTeifi. Mechanisms include subaerial drainage over theice dam and ice-dam stability. Whether Llyn Teifiunderwent subaerial drainage is difficult to establish,as the preservation potential of such an event in thegeological record is extremely low. Given the accumula-tion of glaciofluvial outwash sediments at Penparcduring deglaciation at altitudes of 146m O.D., thesurface of the Irish Sea glacier must have exceeded150m O.D at this time. The altitude of glaciolacustrinedeposits in the Llandudoch core also requires aconsiderably thick ice mass during advance. Given theseproxies, we consider it unlikely that subaerial lakedrainage occurred.

Other processes which may have been more importantinclude frictional melt-widening in response to highhydrostatic pressures allowing lake drainage through theice dam (see Glen, 1954; Liestøl, 1956; Higgins, 1970;Sturm and Benson, 1985), ice-dam flotation, or siphon-ing (Tweed and Russell, 1999). Siphoning (resultingfrom connectivity with the subglacial and englacialdrainage system of the glacier) may have occurred,although a strong seasonal imprint throughout thelacustrine succession is absent.

Given the evidence for glacial overriding and dis-placement of Llyn Teifi it is unlikely that the lakedevelopment impacted significantly upon the flowdynamics of the Irish Sea glacier over regional scales;however, it is possible that advance of Welsh outletglaciers draining through the Teifi valley was retarded.This may explain the predominance of Welsh glacialdeposits only in the upper reaches of the Teifi valleysystem. Palaeogeographical reconstructions of glacialLlyn Teifi and adjacent ice-dammed lake bodies indicatethat much of the glacial meltwater in the region was

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re-routed towards the southwest via the Gwaun-Jordanstown channel system, and at a later stage tothe south via the Rhosddu and Pedran channels. This issignificant, since the geomorphological development ofdrainage networks in these areas is in part a result ofmeltwater routing beyond both glacial limits and thetopographic constraints of the drainage basins.

8. Conclusions

1.

During the advance of the late Devensian Irish Seaglacier onto the mainland of southwest Wales, glaciallakes Aeron, Aberporth, Teifi and Nevern formedbetween the ice margin and higher terrain towards theeast. The altitudes of these glacial lakes increased assuccessive cols were overridden at lower elevationclose to present day coastline. The general trend ofthe drainage from these glacial lakes was towards thesouthwest, parallel to the ice margin. Cols at 70 and115m O.D. west and southeast of Mynydd Carninglihad a profound control on the outline and depth ofthe whole glacial lake system, which to a large extentdrained through the Gwaun-Jordanstown channelsystem in Pembrokeshire.

2.

Sedimentological and stratigraphical investigations ofburied valley-fill sequences in the Teifi region indicatefairly rapid inundation following damming by theIrish Sea glacier. High sedimentation rates areenvisaged, with turbiditic flows and suspensionsettling being the dominant sedimentation processes.The sedimentological evidence indicates an oligo-trophic, density-stratified lacustrine system whichmay have developed a perennial ice cover. Minordrops in base level probably occurred duringincomplete drainage events. The succession coarsensupward into deltaic deposits indicating a reduction inavailable accommodation space and more proximalsedimentation. Continued advance of the Irish Seaglacier displaced the lake, while a widespread basaltill sheet was deposited on top of the lacustrinesequence which was locally deformed by subglacialprocesses. The final phase of sedimentation relates tothe accumulation of ice-marginal glaciofluvial depos-its during glacial recession. The sedimentological andstratigraphical evidence indicates terrestrial processesof deposition, with no evidence for glaciomarineenvironments.

Acknowledgements

The authors wish to thank all the staff at the BritishGeological Survey NGDC core store at Keyworth,Nottingham. J.L. Etienne acknowledges funding underN.E.R.C. [CASE] studentship No. NER/S/A/2000/

03690 in association with the British Geological Survey.Krister Jansson acknowledges funding from the RoyalPhysiographic Society in Lund. N.F. Glasser and M.J.Hambrey acknowledge funding through British Geolo-gical Survey (Natural Environment Research Council)Research Agreement GA/98E/14. J.R. Davies, R.A.Waters and P.R. Wilby publish with the permission ofthe Director, British Geological Survey (N.E.R.C.). Wealso wish to thank David Oates for thin-sectionpreparation, Jon Merritt, Brian Moorlock and refereesDave Evans and Danny McCarroll for their construc-tive comments on the manuscript.

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