Archaeomagnetic Dating and Palaeochannels Sediments

Journal of Archaeological Science (1998) 25, 149–163

Archaeomagnetic Dating and Palaeochannels Sediments: DataFrom the Mediaeval Channel Fills at Hemington, Leicestershire

C. Ellis

AOC (Scotland) Ltd, Edinburgh, U.K.

A. G. Brown

Department of Geography, University of Exeter, U.K.

(Received 28 May 1996, revised manuscript accepted 28 February 1997)

This paper illustrates how archaeomagnetic dating can be used for palaeochannel fills within floodplain sequences. Thesites investigated are palaeochannels associated with the mediaeval braided/anastomosing channels of an unstable reachof the River Trent at Hemington, north-west Leicestershire. This reach has revealed abundant archaeological remainsincluding fish weirs, anchor stones, a Norman mill and three mediaeval bridges. The studies used here show goodagreement between archaeomagnetic dating and other evidence from archaeology, dendrochronology and radiocarbondating. Fabric and mineral analyses of the sediments are used to identify the source of the remanent magnetization andit is believed that this is predominantly the result of the biogenic synthesis of magnetic minerals in high pH but anoxicconditions. This study illustrates the type of alluvial sediments that are most suitable for archaeomagnetic dating.

? 1998 Academic Press Limited

Keywords: ARCHAEOMAGNETISM, ALLUVIUM, PALAEOCHANNELS, ANISOTROPY OFSUSCEPTIBILITY, PYRITE.

Introduction

B uried landscapes within floodplain sedimentarysequences can give unique insights into humanactivity, whether this is long-term or seasonal

occupation, industrial exploitation, pastoral or arablecultivation (Brown & Keough, 1992; Brown, 1997).Rivers and their valleys have always attracted humanattention and exploitation primarily because of theproximity of water but also because of fertile alluvialsoils, potential food resources and the relative ease oftransport. The study of fluvial and alluvial sedimentin association with archaeological material can pro-vide information concerning the specific site setting,the local habitat, the regional environment andthe processes of site preservation and destruction. Theabsolute dating of alluvial units is essential for thedevelopment of an accurate alluvial chronologywhich will facilitate the exploration of the relation-ships between human activity, climatic conditions,vegetation and geomorphology.The varying direction of the geomagnetic field over

the last 2000 years has been broadly established forBritain (Tarling & Dobson, 1995). The British archaeo-magnetic reference curve has been constructed from466 archaeological sites in Britain and in north-eastFrance (Clark, Tarling & Noel, 1988). However, in

1490305–4403/98/020149+15 $25.00/0/as970203

addition some fine-grained sediment has been found torecord the earth’s magnetic field (Clark, Tarling &Noel, 1988; Batt & Noel, 1991; Clark, 1990, 1992; Batt,1992). Most of the data used in the construction ofthe British geomagnetic secular variation curve of theearth’s field were derived from in situ fired archaeologi-cal material and to compensate for magnetic distortionall inclinations from floors were increased by 2·4) and1·2) from mixed walls and floors, with only a smallproportion obtained from sediment (Clark, Tarling &Noel, 1988). Archaeomagnetic directional dating isachieved through the measurement of the mean anglesof dip (inclination) and declination of magnetic min-erals in samples. The natural remanent magnetization(NRM) is then matched against the reference curve toobtain a date either of deposition (which is referred toas detrital remanence magnetization (DRM)), or post-deposition (referred to as post-depositional remanencemagnetization (PdRM)). Unfortunately the calibrationcurve contains a number of tight crossovers which canresult in ambiguity and potential alternative dates(Clark, Tarling & Noel, 1988). The calibration curvealso changes at varying rates and so there are periodsof good and poor dating resolution. The density ofdata points is extremely good for the second half of the13th century , whereas the 1000–100 curve isconstructed largely from a three-point running mean of

? 1998 Academic Press Limited

150 C. Ellis and A. G. Brown

combined lake data of Turner & Thompson (1981).The dark ages are again a period of little culturaldifferentiation and development in Britain and hencethere is a lack of independent data points. The loop of 600–900 was observed in lake sediments (Clark,Tarling & Noel, 1988) and there is a lack of primarydated material for this period. However, despite theseproblems it has been suggested that a dating of &20years (68% confidence level) is possible when using theBritish archaeomagnetic reference curve from 850to present because the curve is relatively open, with onecrossover, and is of relatively good resolution (Clark,Tarling & Noel, 1988). It is also suggested that a dateof &10 years is possible after 1600.Anisotropy of magnetic susceptibility (AMS) is a

method of three-dimensional petrofabric analysis(Hamilton, 1967; Hamilton & Rees, 1970; Ellwood,1984). The purpose of studying AMS is to obtaininformation on the depositional processes of all thegrains, although the magnetic content is a minuteproportion of the whole sample (Hrouda, 1982; Johns& Jackson, 1991). The technique is used in the estima-tion of preferred orientation of grains in sediments(Rees, 1961). There are two types of microfabric ele-ments; the first is a planar structure due to the effectsof gravity and the second is linear, due to the actionof fluid forces, for example hydrodynamic forces(Hamilton, 1967). It is also important to determine theextent of AMS in specimens because of the potentialdistorting effect on NRM.This paper presents data from the archaeological

site of Hemington Fields, north Leicestershire (SK461 307). The site is a working gravel quarry, located atthe confluence of the River Derwent and River Trent(Figure 1). The floodplain at this point is some 4 kmwide; on the east side of the floodplain lies the conflu-ence of the River Sour. The site provided an excellentopportunity to extract oriented samples of fine-grainedsediment and to establish whether this material carrieda natural remanent magnetization (NRM). A numberof analytical techniques were used to investigate depo-sitional and post-depositional processes and aid in thereconstruction of the sedimentary history of the site.

Field and Laboratory ProceduresEight vertical sections through palaeochannels werecreated during gravel extraction (Figure 2). Sampledsediment was fine grained, in situ and physically undis-turbed. The sections were cleaned, then described andrecorded in the form of sedimentary logs. Undisturbed,orientated samples were taken using aluminium mono-lith tins (54#11#9 cm) where the tops of the tinswere horizontal and the length vertical. The monolithtins were oriented to true north using a magneticcompass. The samples were wrapped in foil and plasticsheet to prevent loss of soil moisture, inhibit bioturba-tion and give protection when transported. At sections

1 and 2 orientated perspex cubes (of approximatedimensions of 2·2#2·2#2·2 cm) were inserted into theface by hand. The advantage of using monolith tinswas that a relatively large sample could be taken andtherefore the bulk of the sample was protected fromphysical distortion (Gravenor, Symons & Coyle, 1984)and loss of moisture (Noel, 1980). Bulk sampleswere also taken from the main clay, silt and sandstratigraphical horizons of each section.Using a stainless steel blade and knife, orientated

cubic specimens were cut from the undisturbed mono-lith samples. Each cut specimen was transferred to acubic specimen holder, the end cleaned and the speci-men capped and sealed. Volume susceptibility (ê) wasmeasured using a balanced a.c. bridge circuit. Mag-netic fabric/anisotropy of susceptibility (AMS) wasmeasured using a computer linked Molspin Minisepanisotropy delineator instrument. Various anisotropyparameters were also calculated. Natural remanentmagnetization (NRM) measurements were made in aMinispin slow speed computerized fluxgate magnetom-eter. Mean directions were calculated using an inter-active palaeomagnetic data analysis software package,IAPD (Torsvik, 1992). These were corrected toMeriden (52·43)N:1·62)W), following the methods ofIrving (1964) and Noel & Batt (1990). The correctedmean directions and their associated cone of con-fidence were plotted onto a digitized version of theBritish archaeomagnetic reference curve, taken fromClark, Tarling & Noel (1988). To establish the stabilityof magnetization stepwise, alternating field demagnet-ization was carried out on two specimens from section1, eleven from section 2 and eight from section 7.Specimens were subjected to progressively increasedfields up to 60 mT.Particle size analysis was carried out using standard

dry sieving methods (B.S.I., 1967). The silt- and clay-size fraction was dispersed in 0·1% calgon, placed in anultrasonic bath for 10 min and subsequently analysedusing a Microtechnics model 5000ET SediGraph.X-ray diffraction and X-ray fluorescence analysis werecarried out by A. Smith, University of Leicester. Thewater content and loss on ignition were determinedfollowing the methods of Bascombe (1974). Threethin sections, one from unit 2 and two from unit 4,were prepared by Mr K. Schrapel of Queens College,University of London. Freeze-dried specimens fromunit 4 were analysed in a scanning electron micro-scope (SEM) and by energy-dispersive X-ray analysis(EDXA).

Site DescriptionsArchaeology

During gravel extraction an abundance of mediaevalarchaeological artefacts and structures have been ex-posed at Hemington Fields. The plethora of archaeo-logical material is a consequence of the site’s location

Archaeomagnetic Dating and Palaeochannels Sediments 151

Derby

Nottingham

HemingtonFields

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11th century bridge

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Figure 1. Location of Hemington Fields and a plan of the site of Hemington Fields. Shown are the locations of the sampled palaeochannelsand their extrapolated course, the old county/parish boundary and the location of the three excavated bridges.

and its physical nature. The location was used as acrossing point of the Trent between Leicester andDerby/Nottingham and the site was probably chosenbecause of the existence of a number of shallowchannels. However, the reach was unstable and struc-tures were repeatedly destroyed and buried by gravel(Brown & Salisbury, in press). Wooden posts, timbers

and anchor stones have been recovered from thesurface of the Mediaeval gravel and in some palaeo-channels. These artefacts have been interpreted as theremains of fish weirs and possibly anchor stones, net orbasket weights (Salisbury, 1987, 1988, 1995). In 1985, a12th century mill and mill dam were rapidly excavatedby the Leicestershire Archaeological Unit (Clay &


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Figure2.Sedimentarylogsfortheeightsampledsections.FollowingthemethodofTucker(1989).Cischarcoal.


Salisbury, 1990). Excavation also revealed a series of10 palaeochannels dated between the 7th and 20thcenturies .In 1993–1994 three mediaeval bridges were identified

(Cooper, Ripper & Clay, 1994). The earliest bridge waspreserved as two box-like caissons constructed frommassive timbers; the timbers were felled between c. 1066 and 1096 (Cooper, Ripper & Clay, 1995). Thesecond bridge lay some 50 m south (upstream) of the11th century one; this bridge was constructed of pile-driven posts and dendrochronology revealed it to dateto 1216 (Salisbury, 1995). The last bridge lay some20 m further upstream and dendrochronology pro-duced a felling date of between 1230 and 1240 fortwo oak timbers from its stone and timber starlings(Cooper, Ripper & Clay, 1995). A track known as thePortway was aligned with this bridge, confirming thehypothesis that this area was an important crossingpoint of the River Trent (Cooper, Ripper & Clay,1994). Quarrying began at the end of 1993 on the eastside of an ox-bow lake; this channel once formed theold county and parish boundary (Figure 1). The chan-nel was abandoned before 1830, when avulsionchanged the course of the River Trent to that followedat present (Clay & Salisbury, 1990). Three largepalaeochannels have been uncovered in this area; as yetno archaeology has been found in relation to these.Palaeohydrological analysis of the bridges and associ-ated channels has shown how the flow of the Trent wasprogressively switched from the western to the easternside of the floodplain during the 12th–13th centuries(Brown, 1996; Brown & Salisbury, in press).

GeomorphologyThe simplified stratigraphy at Hemington Fields con-sists of Mercia mudstone bedrock. Lying unconform-ably above the irregular bedrock surface wereapproximately 2–2·5 m of tabular and shallow cross-bedded travels. Organic sediment from an erodedpalaeochannel cut into the base of the gravels and intobedrock has provided two radiocarbon dates of 12,530 (uncal. beta-85499) and 11,730 (uncal. beta-85448). This, with the common occurrence of ice-wedge casts in the gravels, indicates that the gravelswere deposits during the Late Glacial Stadial (YoungerDryas) between c. 11,500 and 10,500 (Brown &Salisbury, in press). These gravels were capped byapproximately 2–2·5 m of Mediaeval gravel. Cut intothe Mediaeval gravel are a complex series of palaeo-channels. The Holocene floodplain is blanketed byc. 1–2 m of silt/clay overbank sediment. The contactbetween the Devensian and Mediaeval gravels can beidentified, in some locations by a subtle colour differ-ence from a pink/reddish to a yellow/brown hue, anerosional contact made obvious by truncated icewedges and the occurrence of subangular to sub-rounded boulder sized rocks, interpreted as anchorweights (Salisbury, pers. comm.).

During the mediaeval period the river graduallyevolved through a braided/anastomosing phase into ameandering, single channel form. The palaeochannelswere a consequence of this change in the system, theyare characterized by a basal infill of black silty sand,10 1·7/1. The sediment contained a limited quantityof small detrital organic material. When freshlyexposed the sediment was black, homogeneous andvery moist. Contact with air caused a rapid colourchange to brownish black, 10 2/3, and the formationof coarse prismatic peds. Ancient rootlets occurredin both the overbank and palaeochannel sediment; inthe latter these were made visible by the presence ofbrownish black aureoles, the result of oxidation. Asummary description of the sedimentary units is givenin Table 1 and Figure 2.

Table 1. Description of the main sedimentary units at HemingtonFields. The units described are shown in the sedimentary logs ofFigure 2

Unit 1. Clayey silt. 7·5 2/3, very dark brown. Mottled, 5 3/6,dark reddish brown. Coarse blocky peds. Circa. 5% rootlets.

Unit 2. Clayey silt. 10 4/1, dark grey to brown 10 4/4. Mottled,5 3/4, dark reddish brown, especially along root channels andlaminae; iron concretions. Occasional manganese nodules. Someamorphous organic fragments.

Unit 7. Bright brown clayey silt.

Unit 2/3. Clayey silt. 10 4/3, greyish yellow brown.

Unit 3. Clayey silt. 10 4/2, greyish yellow brown. Occasionalvivianite. Occasional visible laminae, manganese flecks and ironnodules.

Unit 4. Silty sand. 10 1·7/1, black and 10 3/2, brownishblack zones. Vivianite largely associated with this unit. Occasionallaminae. Rare sandstone clasts. Some amorphous organics,occasional dead rootlets and fibrous organic materials.

Unit 9. Coarse to fine, well sorted sand. 10 4/4, dark yellowishbrown, 10 3/1, very dark grey. Top of bands often stained black.Occasional grit-sized material.

Unit 10. 1 cm thick silt clay unit (section 11). 10 3/1. Vivianite.

Unit 5. Clayey silt. 10 4/2, greyish yellow brown to 10 3/2,brownish black.

Unit 11. Coarse sand with pebbles (section 11).

Unit 6. Gravel. At boundary with upper, younger unit generallymatrix supported, coarsening downwards to clast supported.

ResultsFabric data

A total of 675 specimens were measured for AMS. Asummary of the results and illustrative examples fromthe major units of specific sections are given below (seeEllis, 1995 for a detailed discussion of the results).

Section 7 The magnetic fabric mean direction of unit 2,section 7 (Table 2) lay on a north-east–south-west axis,


Table 2. Mean directions of the principal axis of maximum susceptibility (Kmax) for unit 2, section 7

Sample Depth (cm) N Dec Inc R á95 ê Ro ä

102–110 251–253·5 10 47 21 8·6 20·7 6·43 5·03 30·6888–101 253–257·5 14 53 18 11·5 19·2 5·27 5·98 34·5359–87 258–267 29 31 20 23·2 13·6 4·86 8·65 36·7234–43 268·5–278·5 10 47 16 8·3 23·3 5·26 5·03 34·01

N is the number of specimens; Dec is the mean declination; Inc is the mean inclination, R is the resultant vector (Butler, 1992); á95 is the coneof confidence (Fisher, 1953), ê is the precision parameter (Fisher, 1953), Ro is the significant point (Irving, 1964) and ä the angular standarddeviation (Butler, 1992).

Table 3. Parameters of anisotropy of susceptibility for unit 2, section 7

SampleMeanK ó

MeanA ó

MeanL ó

MeanF ó % F

Meanq ó

102–110 4·62 0·40 0·013 0·060 0·059 0·045 0·076 0·031 70·0 0·52 0·2988–101 4·19 0·27 0·015 0·003 0·068 0·035 0·080 0·035 71·4 0·65 0·3659–87 4·88 1·32 0·015 0·040 0·075 0·040 0·070 0·033 51·7 0·74 0·4034–43 7·31 2·47 0·020 0·006 0·105 0·50 0·090 0·059 60·0 0·82 0·45

Mean K is the mean of the three principal axes (Kmax, Kint and Kmin) (Tarling, 1983); Mean A is the mean total anisotropy (#100) (Owens,1974); Mean L is the mean degree of lineation (#100), the intensity of linear-parallel orientation (Khan, 1962); Mean F is the mean degree offoliation (#100), the intensity of the planar–parallel orientation (Khan, 1962); % F is the percentage of foliation; Mean q is the mean shapeof the ellipsoids (Granar, 1958) and ó is the standard deviation.

270 90

180

0258–267 cm

Figure 3. Magnetic fabric of unit 2, sample 59–87, where Kmax (.)is the principal axis of maximum susceptibility, Kint (0) is theintermediate axis of susceptibility and Kmin (,) the minimum axis ofsusceptibility. Declination is shown as an angle measured in aclockwise direction from north (90)) and inclination as an angle fromthe central vertical point (90)). +=Mean.

perpendicular to the major axis of the channel at thislocation (Figure 1). The upper two samples weredominated by foliar fabric and the lower two by alineated one (Table 3). The fine-grained nature of thesediment, coupled with the anisotropy of susceptibilitydata, shows that deposition was under low velocityconditions. However, hydrodynamic forces dominatedgrain alignment in the lower portion of the unit andgravity the upper portion. The samples exhibitedscattered Kmin axes of susceptibility and dippingKmax (Figure 3); this pattern is indicative of post-depositional root disturbance of the original planarfabric. All four samples had low mean intensities ofsusceptibility, and post-depositional dissolution andreprecipitation of Fe-rich minerals was also indicatedby the occurrence of mottles and iron concretions.Similar depositional and post-depositional processesare suggested for unit 2.There was no indication of post-depositional distur-

bance in unit 2/3, section 7. The domination of foli-ation showed that the unit was deposited in still water.The eight samples from unit 3, section 7 had a similarsequence of deposition to unit 2; a reduction in theinfluence of water currents with time. There is noindication of post-depositional disturbance.The magnetic fabric of unit 4 was again predomi-

nantly foliar, with no indication of physical post-depositional disturbance (this was also the case for unit4 in sections 1, 2, 4, 9 and 11). Occasional samples weredominated by a linear fabric. No discernible preferredorientation of grains in unit 3 (sections 7 and 10) orunit 4 (sections 9 and 11) was observed under the SEM;

this was a consequence of the fine-grained nature of thesediment and the domination of a platy, face-to-face,foliar structure. Horizontal microlaminae, highlightedby organic matter, were observed in thin sectionderived from section 4, unit 4.


Section 9 Similar conditions of deposition and post-deposition are proposed for unit 2 and unit 4 as thosedescribed above for the same units of section 7. How-ever, rounded clasts of unit 4 were observed in unit 9;the latter was a well-sorted sand.

Section 10 A similar scenario to that given for unit 2,section 7 is suggested for unit 2, section 10. However,the unit 2 of this section was dominated by lineation,indicating a slightly higher flow regime. Unit 3, section10 was dominated by a foliar fabric. The basal sampleof unit 3 exhibited dipping Kmax and Kmin axes ofsusceptibility, indicative of a disturbed fabric. Thesample was derived from 2 cm above a sandstone blockwhich formed part of the 13th century bridge pier. It isprobable that the fabric mirrored the undulating bed ofsandstone blocks.

Archaeomagnetic dataAlternating-field stepwise demagnetization demon-strated that all the samples of unit 3 and unit 4 (allsections) carried a stable NRM (Figure 4). The mediandestructive fields fell in the range of 30–40 mT. Thefinal archaeomagnetic results for each section are givenin Table 4. The remanence directions of all thesamples, except that of section 4, were tightly clustered.The intensity of NRM varied significantly with lithol-ogy and the colour of the sediment (Table 5). It wasfound that on oxidation in the laboratory there was asignificant loss of intensity of NRM and magneticsusceptibility and a change in the remanence directionsin units 3 and 4 (Tables 6 & 7).

H9

270 ,Up

180 , 180

Exp: 3

Sample: H9 North

Mo/M

0 800°C, 0e400200 600

(b)

(a)

Figure 4(a) and (b). Alternating field demagnetization of specimen9, section 7. (a) The points represent orthogonal projection of thesuccessive positions of the resultant magnetization vector withincreasing peak demagnetizing field. (b) Variation of the totalintensity and successive directions of magnetization with peakdemagnetizing field.

Mineralogical dataThe basic mineral components of unit 4 (sections 1, 2,4, 7, 9 and 11) were largely derived from reworkedDevensian alluvium. X-ray diffraction revealed that thedominant minerals were quartz and potassium feld-spar, with illite/muscovite, chlorite and kaolinite clayminerals. It is significant that no other major iron-bearing minerals, apart from paramagnetic chlorite,were detected. X-ray fluorescence analysis established7·22% Fe and 0·96% TiO2 content; vital componentsof the titanomagnetites group (Butler, 1992). Thesediment was dominated by 67·75% silica, 16·28%aluminium.Framboids (spherical mineral aggregates) were ob-

served from unit 4 (section 4) in thin section and units3 and 4 (sections 4, 7, 9 and 10) under the scanningelectron microscope. The framboids were most com-mon in organic-lined voids and root channels. How-ever, some were observed in the matrix and these wereregularly associated with vivianite (Fe2(PO4)2.8H2O),a secondary phosphate occurring in oxidized zonescontaining pyrite and organic fragments (Hamilton,Woolley & Bishop, 1990). The framboids were con-structed from individual octahedron crystals arranged

in spheres of between 10 and 50 ìm in diameter (Figure5). The framboids were elementally analysed usingEDXA, which identified the major elements as Feand S. This is believed to be the first time such mono-sulphuric sediments have been recorded in a Holocenefluvial environment of this kind (see later discussion ofsulphur mineralogy).Diatoms of Campylodiscus noricus, Nitzschia species

and Navicula species and other small unidentified frag-ments were observed in unit 4, section 4. The meanorganic content of unit 4 (all sampled channels) laybetween 6 and 14%; most of the organic content wasamorphous under the microscope and probably origi-nated from algae. The mean soil moisture content forunit 1 was 8·5%, unit 2 between 9 and 25%, unit 3between 11 and 40% and unit 4 between 7·5 and 40%.


The characteristic grain size of the units is given inTable 7. During grain size analysis magnetic particlesbecame attached to a magnetic bean (used to maintainslit and clay suspension). However, no magnetic par-ticles were noted when the sediment had been treatedwith hydrogen peroxide to remove organic matter.

DiscussionDepositional environments

The generally planar, foliated nature of unit 4 (in allthe sampled channels) demonstrated that most of the

Table 4. Archaeomagnetic data for palaeochannel sediment, Hemington Fields, North Leicestershire. Hemington Fields, latitude 52·87 andlongitude "1·317. Dec and Inc are normalized to Meriden, latitude 52·34, longitude "1·62

HemingtonFields Sample N

MeanmAm"1 ó

Depth(cm) Dec Inc Dec Inc á95 ê

Calendar date (68% confidence)

Section 1, unit 4 1 pushed 11 51 30 79–110 "14 77 "13·6 76·8 3·1 217 1725&50Unit 4 2 13 124 44 118–124 "13 76 "12·7 75·7 2·9 209 1725&50Unit 5 3 8 92 92 124·5–127 "5 75 "4·9 72·7 3·5 526 1675&50

Section 2, unit 4 1 7 143 65 180–187 "7 62 "7·0 61·8 4·0 225 c. 1300–1350Unit 4 2 14 148 88 200–211 25 63 24·7 62·6 2·0 378 c. 1100

Section 3, unit 3 1 14 22 19 185–193 6 58 5·9 57·5 2·2 333 1400&50

Section 4, unit 4 1 7 35 5 5 69 4·9 68·6 4·7 169 c. 1650–1450/400–600Section 7, unit 2 (1) 59–87 21 5 4 258–267 14 65 14·8 64·6 2·0 254 1150&50Unit 2 (1) 33–58 26 167 15 256·5–273 19 66 18·8 65·6 1·9 213 1000&50Unit 2/3 (1) 1–13 13 190 95 276·5–278·5 13 74 12·9 73·7 1·4 829 c. 700Unit 2/3 (1) 15–25 12 178 110 279–281 20 72 19·8 72·6 1·2 1272 c. 900Unit 3 (1) 26–32 7 443 158 281·5–283·5 16 70 15·8 69·6 1·4 1974 c. 900Unit 3 (2) 1–17 16 51 147 278·5–283 "1 73 "0·9 72·7 2·4 237 1650&50Unit 3 (2) 18–26 9 467 198 285·5–287·5 "16 77 "15·6 76·7 1·4 1397 c. 1750&50Unit 3 (2) 27–35 9 145 169 288–290 "7 76 "6·8 75·7 2·1 603 1700&50Unit 3 (2) 36–53 18 200 143 291–295·5 17 69 16·8 68·6 1·6 476 c. 900/1500Unit 3 (2) 54–71 18 103 70 296·5–301 31 68 30·6 67·6 1·4 642 1000&50Unit 4 (2) 72–88 16 90 46 302·5–306·5 26 72 25·7 71·6 1·0 1238 c. 900&50Unit 4 (2) 89–120 30 206 101 307–316·5 31 68 30·6 67·6 0·6 1707 1000&50Unit 4 (2) 121–137 18 70 52 318–322 28 69 27·7 68·6 1·4 635 1000&50Unit 4 (2) 138–143 6 31 14 322·8–324·8 19 72 18·8 71·6 2·1 1004 900&50

Section 9, Unit 4 1–63 65 415 168 231–248 10 75 9·9 74·7 0·6 840 750&50Unit 4 64–117 53 1323 332 248–260 20 74 19·8 73·7 0·5 1289 ?

Section 10, unit 3 10, 11, 12 27 76 44 26–33 0 58 "0·1 57·5 1·5 359 1400&50Unit 3 13, 14 17 75 22 33–39 5 57 4·7 56·6 2·1 286 1400&50Unit 3 16, 17 18 78 25 42–46 2 56 1·9 55·5 2·2 257 1375&50

Section 11, unit 4 1–35 35 142 103 453–463·5 14 60 13·8 59·5 1·0 589 1225&50

(1) and (2) are the monolith numbers from a section 7; N is the number of specimens in a sample; Mean mAm"1 is the mean intensity ofremanence; ó is the standard deviation of the intensity of remanence; á95 is the cone of confidence at the 95% confidence level and ê is theprecision parameter. Section 1, sample 1 is the only sample where cubic specimen holders were pushed directly into the field section, otherspecimens were obtained by subsampling from monolith tins.

Table 5. The change in intensity of remanence within and betweendifferent coloured lithologies

Unit Colour mAm"1

Unit 2 Dark grey and brown 0·63–167·37Unit 2/3 Dark grey and brown 25·7–190Unit 3 Greyish yellow brown 14·36–467Unit 4 Black 31·3–1624·33

mAm"1 is the intensity of remanence.

sediment settled out from suspension and that gravita-tion was the major force acting upon the grains andfloccules. The palaeochannels can be envisaged assedimentary sinks, where trapped flood water and algalgrowth resulted in stagnant, anoxic bottom-water con-ditions. Allochthonous and autochthonous organicmatter was readily preserved. The occasional lineationof unit 4 was a result of rare and short-lived influxesof sediment-laden flood waters. A sudden increase invelocity was apparent from the sedimentary log ofsection 9; where unit 9 contained rounded lenses ofunit 4, thought to be scoured out as the floodwaterentered the palaeochannel. The deviation of somemean magnetic fabric directions from the major axis ofa given palaeochannel is explained, firstly by the domi-nation of foliation and secondly by the measurement oftwo or more flow directions. These flow directions areprobably relics of occasional multidirectional currents,caused by wind action or secondary events within thepalaeochannels.Grain size, organic content and water content data

indicate that the depositional environment of unit 3,section 7 was not significantly different from that ofunit 4, section 7. However, the units are characterized


Table 6. Mean loss of intensity of remanence and volume susceptibility and mean remanence direction change onoxidation for units 3 and 4, Hemington Fields

Section and unitMean % lossmAm"1 (dry)

Mean loss vol êSI (dry)

Mean Decdifference

Mean Incdifference

Section 9, unit 4 99·90 ó 0·05 96·71 ó 0·61 52·14 ó 27·36 37·17 ó 25·89Section 10, unit 3 98·84 ó 0·72 87·46 ó 4·23 46·91 ó 76·48 18·16 ó 16·41

mAm"1 is the intensity of remanence; vol ê is the volume susceptibility; Dec is the declination; Inc the inclinationand ó the standard deviation.

Table 7. Characteristic grain size for the major sedimentary units,Hemington Fields

Unit Grain size

Unit 1 Silty clayUnit 2 Silty sandUnit 3 Silty sandUnit 4 Sandy silt

Figure 5. Pyrite framboids in a degraded root channel, unit 3,section 10. Scale in ìm.

by different magnitudes of magnetic mineral content.Unit 3, section 10 was deposited in a deep poolenvironment (Pitts, 1995), where the water at thesediment/water interface was still; however, Berner(1980) notes that a minimal quantity of oxygen isneeded for the formation of pyrite framboids, whichalso only form under pH 7–8 (Marnette et al., 1993).

The avulsion and abandonment of the channel wasprobably a consequence of a flood which destroyed the13th century bridge and resulted in the deposition ofunit 3. Field observations and magnetic fabric analysisindicate that unit 2/3 of section 10 had experiencedpost-depositional turbation and the dissolution andoxidation of iron compounds. However, the archaeo-magnetic data indicate that the post-depositional pro-cesses were not of a significant magnitude to distort ordestroy the NRM.The presence of Campylodiscus noricus diatoms is

indicative of alkaline conditions of pH 7 or greater andthey are common in standing or nutrient-rich water.The Nitzschia species are a large group and are mostcommon in eutrophic water. The Navicula species are avaried group of diatoms and are generally found inrunning water with a high nutrient content and a widerange of pH; however, some do prefer alkaline orneutral pH. The diatoms confirm the alkaline nature ofthe sediment and water.

ArchaeomagnetismThe cyclic nature of the British archaeomagnetic refer-ence curve has resulted in a number of tight curves andloops, which can result in alternative dates for oneindependent sample. A second drawback of the refer-ence curve is that the distance between each halfcentury marker is not uniform and so when a datapoint falls onto the interpolated curve and the á95 coneof confidence is drawn, the & in years are difficult toestimate. The amount of change in secular variationis different in specific periods, for example the linebetween 1500 and 1000 is rather large, while thatbetween 600 and 700 is tiny and therefore theassignment of a more precise date is more likely in theformer than in the latter. Coupled with this problem isthe resolution of the curve; some of the curve isconstructed from many data points (independentlydated) while others have very few anchor points. Forexample Batt (1992) noted that there were 50 pointsbetween 0 and 250 but only four points between 500 and 750; again this will affect the reliability ofdating independent samples. The archaeomagnetic re-sults from Hemington Fields are given below, andbecause of the lack of sequence dated levels coupledwith the problems of curve, data errors are broadlyestimated to &50 years at the 68% confidence level.


Section 1 The upper two samples derived from unit 4,section 1, have been dated to 1725&50. Unit 5,stratigraphically lower than unit 4, has been dated to 1675&50 years. It can thus be concluded from thearchaeomagnetic evidence that the channel was aban-doned shortly before 1675&50. It is interesting tonote at this stage that the samples comprising field-pushed cubic specimens have a lower mean intensity ofremanence than those specimens cut from an undis-turbed column. The lower mean moment was causedby a loss of moisture over a period of 2 weeks beforemeasurement and the higher mean intensity was due toa more effective retention of moisture within the largerundisturbed sample and the measurement of the speci-mens directly after subsampling. However, the methodof sampling, pushing cubic specimen holders directlyinto the sediment versus subsampling from a monolithtin, did not affect the archaeomagnetic data.

Section 2 The two sample mean directions are signifi-cantly different at the 95% confidence level. Sample 1has a high á95 level and lies at the limit of acceptabledata. Neither sample with the confidence limits plottedintersected the curve, preventing the assignmentof&years. The two archaeomagnetic dates for unit 4,section 2 may lie some 175–325 years apart, over amaximum of 13 cm. Thus it is possible that remanencewithin this unit has been acquired gradually andsequentially.

Section 3 Due to a crossover in the reference curvethere are two possible dates for unit 3, section 3: 1400&50 and 1275&50. The stratigraphy of thesection is characteristic of the top of a point bar(Figure 2), where unit 3 and unit 7 are covered byoverbank alluvium (unit 1). It is likely that the pointbar is a relic of the westwardly meandering 13thcentury palaeochannel (Figure 2). If so, the earlier dateof 1275&50 is more probable. This date fits withtentative dating on stratigraphical and archaeologicalgrounds (Salisbury, pers. comm.).

Section 4 The archaeomagnetic results from section 4were inconclusive. The sampled unit was covered by c.13th century gravels and silts and so a date of c. 1450 can be rejected. Mapping indicated that section 4and section 7 were the same channel (Salisbury &Brown, pers. comm.). Assuming this to be correct, thena date of c. 400–600, although rather broad, is inagreement with the sedimentary and geomorphologicaldata. This date is also in reasonable agreement with aradiocarbon date on brushwood with bark from thebase of the fill (cal 605, 1 sigma cal 550–645Beta-87291).

Section 7 The base of unit 4, section 7 has been datedto 900&50, rising to 1000&50 at a depth of301 cm, some 21 cm above the lowermost sample. Thesediment acquired a remanence over a maximum

40

50

80–30

Declination

Incl

inat

ion

–10

55

60

65

70

75

–20 0 10 20 30

1950 AD

600 AD

1200 AD

Figure 6. The archaeomagnetic data from section 7 plotted onto theBritish archaeomagnetic reference curve (digitized from Clark,Tarling & Noel, 1988), where each 100 years is marked by a solidsquare. The error bars are calculated from the á95 and show the coneof confidence at the 95% level. /, —, Clark, Tarling & Noel, 1988;+, section 7.

period of 150 years; unfortunately the evidence ofsequential acquisition is minimal. Sample 54–71, unit3, yielded a date of 1000&50 and has the samevalues of declination and inclination as sample 89–120,unit 4. This suggests that the remanence acquisition inunit 4 and at the base of unit 3 was simultaneous. Thedata point of sample 72–88 does not fall onto or crossthe interpolated reference curve (Figure 6). It could beargued that the sediment shows a systematic inclina-tion error (systematic because the á95 of the sample isvery small). However, when viewed within the contextof the unit, this sample is not perceived as problematic.The base of unit 2 is dated to 1000&50. Anarchaeomagnetic anomaly within the sequence ispresent in unit 3 (with the exception of the lowersample) and unit 2/3. The dates for the central threesamples of unit 3 are similar and therefore remanenceacquisition was concurrent. However, the samplesbelow, as well as those above, are apparently youngerand so there is no stratigraphical progression of thedates (Table 4). Stepwise AF demagnetization wasperformed on eight specimens from units 2/3 and 3.The sediment carried a very stable NRM (0–40/50 mT),with no indication of a viscous or weaker secondarychemical remanence. Of interest is the intensity ofremanence, generally higher in units 2/3 and 3 than inunits 2 or 4. It is thought that prior to c. 1500 mostof unit 3 and unit 2/3 experienced a post-depositional,geochemical transformation, resulting in the produc-tion of a strong, secondary chemical remanence. Unit 2carries a stable and credible remanent magnetization.The action of post-depositional processes, such asseasonal wetting and drying and root penetration, areattested by the presence of mottles, iron concretionsand a disturbed magnetic fabric; but these processes


have not destroyed the magnetic remanence. The lowintensity of remanence of the top sample may be theresult of oxidation and destruction of the magneticcontent, a reflection of the environment (which maynot have favoured magnetotactic bacteria), or even aconsequence of a detrital source of magnetic grains.

Section 9 Initial data analysis revealed that the 12samples could be grouped into two populations (Table4). However, two samples, sample 46–54 (sampledfrom a depth of 243–245 cm) and sample 108–117(sampled from a depth of 258–260 cm), had similarvalues of declination (0·16) difference) and inclination(0·69) difference). This indicates that the undated lowersample (undated because the remanence data did notintersect the reference curve), despite a declinationerror, is of a similar date to the top sample, 750&50. The reason for the apparent declination erroris unclear. One possible explanation is the incorpor-ation of sand lenses from unit 9; the alignment of theseby hydrodynamic forces could have physically dis-torted the remanence. However, AMS is carried by thelarger multi-domain grains and the NRM by thesmaller multi-domain and single-domain grains andthe sample is dominated by foliar ellipsoids. Also,given the low degree of anisotropy (0·007), it does notseem possible that coarser sand grains are masking ordistorting the remanence. The mean magnetic momentof the lower original samples was much greater thanthe upper samples, ranging between a minimum of315 mAm"1 and maximum of 1309 mAm"1 higher. Itis suggested the declination error may be related to thehigher intensity of remanence phenomenon. Thiswould indicate that there was a magnetic and perhapsgeochemical difference within the unit of a post-depositional origin.

Section 10 The archaeomagnetic curve is rather com-plex around 1250–1450. Data points of varying agesare clustered (see Clark, Tarling & Noel, 1988). Theuse of the reference curve, without consideration of thearchaeological data, would yield a tentative calendardate for unit 3, section 10, of c. 1250–1300. How-ever, a dendrochronological felling date for timberused in the construction of the bridge is the late 1230s.Therefore, it is reasonable to assume that the dateabove is too close to that of the construction of thebridge. This date would allow only a minimum of 60years of use; without taking into consideration the timefor the construction of the bridge, its destruction, thedeposition of unit 3 and the acquisition of remanence.A large scale flood was documented to have affectedthis area in 1401 and is proposed as the onethat destroyed the bridge (Brown, 1996; Brown &Salisbury, in press). A date of 1400&50 correlateswell with the archaeological and historical data. Unit 3must have been deposited fairly rapidly and theremanence, if not detrital, was acquired soon after.

Sulphide mineralogy

Determination of the magnetic mineralogy is necessarybefore drawing conclusions regarding the origin of thenatural remanent magnetization (NRM). Studies ofrecent sediment have shown that generally detritalmagnetite (titanomagnetites and titanomaghaemites)is the principal magnetic phase present and the carrierof depositional (DRM) or post-depositional (PdRM)remanent magnetization (Batt, 1992; Butler, 1992).The potential ferromagnetic and ferrimagnetic contentof the source rocks for the Hemington palaeochannelsediment is low and the fine-grained detritus from theMercia mudstone has a low mean susceptibility, domi-nated by haematite. However, the volume suscepti-bilities and intensities of NRM of units 3 and 4 wereexceptionally high and so another, non-detrital, mag-netic source has to be sought. Other main remanencecarriers in recent sediments are iron oxides (maghae-mite and haematite), the iron–manganese oxyhydrox-ides and certain iron sulphides, for exampleferrimagnetic greigite (Fe3S4), which are associatedwith in situ authigenic or diagenetic chemical changesand so will carry a chemical remanence. Because of thesimilar grain size of magnetite and greigite and verysimilar AF demagnetization behaviour, medium de-structive field >38 and 35 mT, respectively (Snowball& Thompson, 1990; Fassinbinder & Stanjek, 1994;Reynolds et al., 1994), it is very difficult to distinguishbetween the two minerals.It is postulated that the brown colour of unit 2 is due

to hydroxides and the black colour of unit 4 is due tothe presence of iron monosulphide minerals. Theformer can be classified as the hydroxide zone and thelatter as the monosulphuric zone, as defined by Love(1967). Unit 3, the grey sediment, is analogous tothe bisulphuric or pyrite zone (Love, 1967) which isnotable for the presence of pyrite framboids (Al-aghaet al., 1995).Sulphide formation is primarily a biological process,

requiring a supply of dissolved sulphate, organicmatter and iron (iron oxide and hydroxide) (Vaughan& Lennie, 1991; Sawlowicz, 1993); these conditionswere fulfilled in units 3 and 4. The breakdown oforganic matter is achieved by the bacterial reductionof sulphate, via bacterial anaerobic decompositionof organic matter and the production of hydrogensulphide, H2S. The H2S reacts with iron minerals toproduce black monosulphides (Berner, 1970).Anoxic sedimentary conditions most often occur in

the upper few millimetres of a sediment (Leslie, Lund& Hammond, 1990) and especially in oxygen-depletedwaters (Karlin, 1990). Geochemical processes of or-ganic matter degradation and sulphur reduction havebeen shown to affect the magnetic mineral compositionof sediments in two ways. The first is the dissolution ofmagnetic grains and the second is the production ofauthigenic magnetic minerals which can actually in-crease the ferromagnetic/ferrimagnetic concentration,


increasing the intensity of magnetization and the pro-duction of a CRM (Snowball, 1991). Conditions formonosulphide production can be envisaged for theHemington palaeochannels, where trapped floodwaters in an organic-rich environment resulted in thedeposition of an organic-rich, silty sand under reducingconditions.None of the monosulphides are thermodynamically

stable (Berner, 1970). A phenomenon often noted inlake sediments is a loss in the intensity of NRM andmagnetic susceptibility. This is thought to be due to theoxidation of labile greigite and biological oxidation ofpyrite (Hilton et al., 1986; Hilton, 1990; Snowball &Thompson, 1990; Crockford & Willet, 1995). On airdrying there was dramatic loss of intensity of NRMand volume susceptibility in both units 3 and 4 and adramatic change in remanence direction (Table 6).However, a loss of susceptibility and a change inremanence direction during oxidation should not besignificant if single-domain magnetite oxidizes tomaghaemite (Henshaw & Merrill, 1979). Hilton (1990)shows that a loss in susceptibility and intensity in ananoxic lake sediment is caused partially by the micro-bial degradation of magnetite under oxidizing con-ditions and the oxidation of authigenically formedgreigite to a less strongly magnetic form. Hilton (1990)observed a large intensity loss when the sedimentwas treated with hydrogen peroxide and subsequentlydried (this was also observed in unit 4). This phenom-enon was deemed consistent with the presence ofgreigite. XRD analysis of the anoxic Esthwaite lakesediment revealed no evidence of a magnetic material(as was the case in unit 4), but it was suggested that thiswas due to the total Fe content being lower than the5% needed to register a reasonable signal (Hilton,1990). However, extraction of the magnetic fractionfrom freeze-dried samples by Hilton (1990) andSnowball & Thompson (1990) revealed the presenceof greigite.The formation of greigite demands strong reducing

conditions, but not totally without oxygen (Al-aghaet al., 1995). The production or formation of greigitecan firstly be of an inorganic origin, biologically medi-ated by sulphate-reducing bacteria and, secondly, theresult of the diagenesis of bacterial magnetite (Mannet al., 1990; Hoffman, 1992). Single domain magnet-ites, <0·1 ìm, pyrrhotite (FeS1+x), greigite and pyriteare produced by magnetotactic bacteria (see Chang &Kirschvink, 1989; Heywood et al., 1990; Stolz, Lovley& Haggerty, 1990; Bazylinski et al., 1991; Moskowitz,Frankel & Bazylinski, 1993; Bazylinski, Frankel &Garratt-Reed, 1994). Most of these magnetotactic bac-teria require molecular oxygen and produce magneticminerals intracellularly in the form of a chain within amagnetosome (Stolz, Lovley & Haggerty, 1990). Otherforms of magnetotactic bacteria containing some formof iron sulphide have been identified in hydrogensulphide-rich, anoxic sediments (Bazylinski et al.,1991). Magnetotactic bacteria have also been found

to produce ferrous phosphate, vivianite, throughbiologically induced mineralization. The vivianite isthought to be a by-product of the export of ferrousions (Bazylinski et al., 1991). The magnetic mineralsproduced by magnetotactic bacteria are used for orien-tation. Many of the bacteria are located in the upperfew millimetres of sediment. On death they fall outof suspension and may align with the ambient geo-magnetic field to produce a DRM. Both anaerobicmagnetotactic bacteria and sulphate-reducing bacteria,such as GS-15, are proposed as primary producers ofauthigenic iron sulphides and magnetite, respectively,in anoxic sediments (Stolz, Lovley & Haggerty, 1990;Bazylinski et al., 1991).Post-depositional greigite has been observed in

Cretaceous strata (Reynolds et al., 1994), within recentanoxic marine sediments (Leslie, Lund & Hammond,1990) and recent lacustrine deposits (Hilton, 1990;Snowball & Thompson, 1990; Snowball, 1991).Hoffman (1992) argues that greigite, rather than beinga relatively rare mineral, is in fact common in anoxic,sulphide-rich sediments associated with organic matterand is often observed with other non-magnetic sul-phides such as pyrite. Pyrite is cubic in form andparamagnetic at room temperature. The presence ofpyrite framboids in old root channels and fissuresshowed that they formed after root penetration andwere therefore authigenic or diagenetic in origin. Pyriteframboids form indirectly from or via iron monosul-phides (Sawlowicz, 1993). Sweeney & Kaplan (1973)and Vaughan & Lennie (1991) suggest that framboidsdevelop when the initial iron sulphide precipitate istransformed to greigite. Berner (1970) suggests pyriteonly occurs when there is an excess of sulphur and thesediment is characteristically grey (e.g. unit 3), but itwill not form when there is no excess of sulphur andthen the sediment is characteristically black and anoxic(e.g. unit 4). The consensus is that pyrite forms at theinterface between oxic/anoxic zones and that vitalferrous and sulphide ions are supplied by the reducingsediment and the oxidizing component from theoxidized zone. It is perhaps these processes that can beenvisaged in unit 3 at Hemington.The remanence carrier in units 3 and 4 has not been

formally identified, but preliminary results indicatedthat an iron monosulphide, probably greigite, is re-sponsible for the exceptional quality of the archaeo-magnetic data. The authigenic or diagenetic formationof greigite necessitates that the remanence is chemical;however, the timing of this remanence is difficult todetermine. Hallam & Maher (1994) suggest thatgreigite was formed during early diagenesis and so theyregard the remanence as primary, rather than a second-ary overprint. The archaeomagnetic date of unit 3,section 10, is some 100 years after the construction ofthe bridge, so it seems reasonable to presume that ifgreigite is the remanence carrier, its authigenic ordiagenetic production took place soon after sedimentdeposition.


Conclusions

(1) Relic features of depositional and post-depositional disturbance (mostly in units 2 and3, sections 1, 2, 7 and 10 in the form of rootletpenetration and chemical alteration) were ident-ified using AMS and thin section analysis. Unit 4(sections 1, 2, 4, 7, 9 and 11) was generally charac-terized by a foliated, still water, depositional fabric.However, in some instances wind or secondarycurrents are thought to have affected the orienta-tion of the larger mineral grains and may accountfor the general lack of correlation between theorientation of channel banks and the mean AMSdirections. Low velocity conditions dominatedsediment deposition in the palaeochannels. Thepresence of water in the palaeochannels was main-tained by seepage from the main channel andoccasional overbank flooding.

(2) The use of a variety of techniques has led to theconclusion that unit 4 (sections 1, 2, 4, 7, 9 and 11)was monosulphuric. Anaerobic depositional andpost-depositional conditions prevailed with theauthigenic/diagenetic production of monosulphideminerals, including limited pyrite formation. It hasbeen postulated that the origin of the magneticmineral(s) was linked to magnetotactic bacteria oriron-reducing non-magnetotactic bacteria, ratherthan detrital in origin. It is also thought that themain remanence carrier was greigite. The greysediment of unit 3 has been classified as bisul-phuric, where sediment depositional and post-depositional processes took place in oxygen-poorconditions. These conditions were suitable for theformation of monosulphide minerals and pyriteframboids.

(3) The remanent magnetization of all the units wasvery stable and, despite the rather complex post-depositional processes envisaged for units 3 and 4(sections 3 and 10 and 1, 2, 4, 7, 9 and 11respectively), all the units produced statisticallyacceptable remanence data and most were assignedcalendar dates which were in general agreementwith other evidence of palaeochannel age. How-ever, the processes of magnetic mineral productionand the verification of the remanence carryingmineral(s) require clarification. It has been foundthat weak magnetic fabrics, whether depositionalor post-depositional in origin, did not mask ordistort the NRM.

(4) The data from Hemington indicate that palaeo-channels and ox-bow lakes, which are often closelyassociated with settlement, are ideal environmentsfor successful archaeomagnetic dating, especiallywhen the sediment has remained waterlogged sincedeposition. Sediment fabric and geochemicalanalyses suggest why this environment is so suit-able for archaeomagnetic studies. Such sedimentsare typically found on and near lowland floodplain

sites (Brown & Keough, 1992) where they havefrequently been utilized for a variety of purposesfrom flax retting to refuse disposal. Such sites inconjunction with adjacent buried land surfaces andstructures can provide high quality environmentaldata if adequately dated.

AcknowledgementsThe authors would like to thank Chris Salisbury for hishelp and enthusiasm and P. Clay and other members ofthe Leicestershire Archaeological Field Unit as well asthe land owners Ennemix Ltd. The work was donewhile one of us (CE) was in receipt of a Science BasedArchaeology/NERC research award.

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Documents

Archaeomagnetic Dating and Palaeochannels Sediments