osl and radiocarbon dating of a pre-angkorian canal in the ... · pdf fileosl and radiocarbon...

OSL and radiocarbon dating of a pre-Angkorian canal in theMekong delta, southern Cambodia

Paul Bishopa*, David C.W. Sandersonb, Miriam T. Starkc

aDepartment of Geography and Geomatics, University of Glasgow, Glasgow G12 8QQ, UKbScottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Ave, East Kilbride G75 0QF, UK

cDepartment of Anthropology, 2424 Maile Way, Saunders 346, University of Hawai‘i, Honolulu, HI 96822, USA

Received 9 May 2003; received in revised form 28 July 2003; accepted 2 September 2003

Abstract

This study presents preliminary results of research on pre-Angkorian canals near the ancient settlement of Angkor Borei in thesouthern Mekong delta of southern Cambodia. The canals have been mapped by aerial photograph interpretation and investigatedin the field by hand auger drilling of two canal traces and trenching of one of these. Luminescence profiling through the canal infillsuccessfully identified the base of the canal as well as revealing disturbance and mixing of the canal infill that was not apparent fromvisual inspection or sedimentological analyses of the infill sediments. OSL dating of the canal bed indicates excavation (orre-excavation) of the canal bed between the first millennium BC and the middle of the first millennium AD. This date is consistentwith the time of initial occupation of Angkor Borei in the fourth century BC. Multiple charcoal samples with a pooled age of earlyfourth to early fifth century AD probably signal the onset of canal infilling. The apparent demise of the canal coincides with a majorchange in land-use signalled in pollen and diatom data from Angkor Borei, but this change cannot be taken to indicatede-population of the region. This tentative chronology will be refined when more canals are investigated and greater precision isachieved in OSL dating of the canals. The latter will necessitate clearer identification and separation of unbleached (older,pre-archaeological) components and bleached (younger and dating the canal digging and/or operation) components in the OSLstored dose.� 2003 Elsevier Ltd. All rights reserved.

Keywords: Cambodia; Angkor Borei; Funan; Pre-Angkorian; Canal; OSL; Luminescence; Radiocarbon

1. Introduction

Cambodia’s remarkable cultural heritage is bestembodied in the spectacular monuments, the art andiconography of Angkorian material culture, and the richdocumentary record of Angkor Wat. Angkor Wat,however, represents only the endpoint of a deep histori-cal record. The Angkor Khmers may have had theirorigins in the Mekong Delta in southern Cambodia (Fig.1 ), which contains a rich yet poorly understood recordof about 1000 years of early historic occupation begin-ning at approximately 500 BC. Third century ADChinese travellers to this region described the “Kingdomof Funan,” which consisted of walled and moated cities

that housed rulers, elites, and artisans of fine goods suchas precious metals, jewellery, and other crafts (e.g.,[7,21–23,30]). Archaeological and documentary evidencefrom the region suggest that the Mekong Delta was athriving economic node in the maritime trade routes thatlinked India west to Rome and east to to China viamainland Southeast Asia (e.g., [2,6,32,33]).

Understanding the catalysts that transformed thisregion by the mid-first millennium AD is critical toexplaining emergent complexity in the Mekong delta.Thus, historians have argued that populations in theMekong delta participated in regional and internationalmaritime trade networks ([7,15,16,30]; see review by [17]:238–254). Today, as in the past, the Mekong delta islargely inundated during half the year, and transporta-tion requires canals and boats rather than roads. Inthe past, the delta’s centres (in both Vietnam and

* Corresponding author. Tel.: +44-141-330-6654E-mail address: [email protected] (P. Bishop).

Journal of Archaeological Science 31 (2004) 319–336

SCIENCE

Journal of

Archaeological

http://www.elsevier.com/locate/jas

SCIENCE

Journal of

Archaeological

http://www.elsevier.com/locate/jas

0305-4403/04/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.jas.2003.09.002

Cambodia) lay as many as 90 km inland from the SouthChina Sea coast [43]. Canal systems, therefore, may havebeen essential for transportation, communication andagriculture during the first millennium AD, and animportant step towards understanding this archaeologi-cal environment involves documenting and dating theancient canals that apparently linked the delta’s settle-ments into a regional network. To our knowledge, therehas been no field investigation of these canals sinceParis’s [28,29] pioneering aerial observations. Work re-ported here is a first step in assessing the nature,functions and ages of canal traces south of AngkorBorei. The aims of this work are: to map the canal traceson aerial photographs and on the ground; to identify thebase of the canal in the sub-surface and thereby to

determine the canal dimensions; and to evaluate theage(s) and infill stratigraphy of the canals.

2. Background

The prehistory and early history of Cambodia’sMekong Delta is hardly known. Louis Malleret’s fieldresearch at the Vietnamese site of Oc Eo produced theonly monograph-length archaeological reports from theFrench colonial period [21–23], and Vietnamese archae-ologists have worked continuously in their section of theMekong Delta only since 1975 (e.g., [8,14,24,44]). Onlyone archaeological project, the Lower Mekong Archaeo-logical Project, has undertaken systematic field researchin southern Cambodia (e.g., [39,41,42]).

100 km

PHNOMPENH

THAILANDLAOS

VIETNAM

CAMBODIA

HO CHIMINH CITY

ANGKORBOREI

PhnomPhnomDaDa

Oc Eo

KampongCham

Kratie

KohKer

SamborPrei Kuk

PrasatAndet

KompongPreah

KampongChhnang

AngkorPre

Kmeng

Sisophon

Battambang

Siem Reap

StungTreng

Mekong

MekongDelta

TonleSap

SouthChina

Sea

CardamonMountains

Elephant

Mountains Phnom

Da

Gulfof

Thailand

Fig.2

Fig. 1. Map of Cambodia, with box indicating Fig. 2. Inset shows Cambodia’s location in SE Asia.

P. Bishop et al. / Journal of Archaeological Science 31 (2004) 319–336320

Pierre Paris, a French colonial fonctionnaire, usedaerial observations and photography in the 1930s toinvestigate prominent linear traces radiating southwardsfrom Angkor Borei across the delta surface (Fig. 2).Paris [28,29] identified five ancient canals south ofAngkor Borei, four of them linear traces crossing thedelta surface (canals 1–4 in Fig. 2 and Fig. 3), and thefifth coinciding with the very straight Chau Doc River(Fig. 2). Paris suggested that these canals dated to thefirst to sixth centuries AD, served trade and communi-cation functions, and linked the capital of Angkor Boreito other major ancient settlements to the south andsoutheast in the delta (Fig. 2; see also [19]). Malleret’s[23] excavations at the port settlement of Oc Eo (Fig. 2)suggested to him that canals between Oc Eo and AngkorBorei were of late Funan period age, thereby datingthem to the late fifth and early sixth centuries AD. LikeParis, Malleret argued that canals connected Oc Eo toits hinterland, and that this major network of navigationcanals served as well to drain the low-lying delta area viaa secondary network of drainage channels.

Assessing the nature, ages and functions of the canalsin the vicinity of Angkor Borei and southwards to Oc Eoprovides important evidence in understanding the evo-lution of occupation of the southern delta region andthereby in evaluating trade-based models of state forma-tion in this region. An understanding of the age(s) of thecanals around Angkor Borei relative to the longerdistance canals to Oc Eo is likely to be a key toevaluating competing models of complex society forma-tion in the delta (cf. Stargardt’s [37]) survey of canalsand other hydraulic resources at Satingpra and hercomments on their importance of the resources inSatingpra’s development). If the canal connectingAngkor Borei and Oc Eo (Paris canal 4; Fig. 2 and Fig.3) is older than the canal system internal to the AngkorBorei area (Paris canals 1–3; Fig. 2), then the timing ofconstruction of the latter canals either indicates expan-sion of settlement in the northern delta and the develop-ment of communication links between settlements, or adecline in the pan-delta trade network and a retractionto more localised economic systems. If canal 4 isyounger than canals 1–3, then the local canals mightindicate the development of a strongly integratedregional system before links to international maritimetrade became important.

Canals are notoriously difficult to date [27,34]. Lumi-nescence data have the potential to date canal digging(or re-digging) and sedimentary infilling during and aftercanal use. Such luminescence approaches avoid poten-tial problems associated with using charcoal, wood orother artefacts recovered from the canal infill to date thecanal and its infilling. Such charcoal, wood or artefactsmay have been resident in the environment for anunknown time period before deposition in the infill,especially in quasi-fluvial and/or anthropogenic settings

such as canals. This environmental residence time is aform of the familiar ‘old wood’ problem which maycompromise, for example, the use of radiocarbondating of detrital organic material when dating environ-mental ‘events’, especially events such as canal excava-tion ([36]; see also [5]). Luminescence approaches,particularly those using the relatively easily bleachedOSL component [1], offer the possibility of datingexcavation events, because such excavation (or re-excavation) should ‘bleach’ the sediments at the exca-vated surface as it is dug. We report on the applicationof these OSL approaches here, providing an archaeo-logical context for our preliminary report [35] andcomparing the OSL results systematically with radiocar-bon results that we report here. The OSL approach wasbroadly successful, but a major challenge for suchluminescence-based approaches is sampling on the smallscales necessary to identify (and to date) excavatedsurfaces.

3. General setting

The setting and environmental history of the AngkorBorei area in the Mekong delta have been described byBishop et al. [4]. The delta surface is an extensive lowrelief plain of the Mekong River, less than 10 m abovesea level. Angkor Borei and the other settlements standon a terrace, slightly above the lower-lying and swampydelta surface, which is inundated annually (Fig. 3). Thislower, active delta surface consists of silty muds andclays and organic (peaty) muds and clays; acid sulphatesoils occur locally. A deeper sedimentary unit that iswidespread in the area is a pale bluish-grey clayey sand,locally with pinkish-grey and olive mottles. Muscovitemica flakes are common and locally abundant in thisunit. This sedimentary unit is encountered in our augerholes in and around Angkor Borei, and it is tentativelyinterpreted as representing a regional sedimentarysubstrate.

A regular grid of straight Pol Pot-era (1970s) canalscriss-crosses the delta surface at 1 km spacing, coincid-ing with the national UTM map grid. We are interestedin the straight traces of partially infilled canals that areoblique to the Pol Pot-era canals and were first describedby Paris. These ancient canals radiate from severalcentres, including Angkor Borei, Phrey Phdau Khnongand Kampong Youl (Fig. 3). Our mapping of thesetraces using recent aerial photographs confirms Paris’smapping and also reveals what appear to be other canaltraces (compare Fig. 2 and Fig. 3). The canal traces cutacross the modern canal grid to make direct connectionsbetween ancient sites. They are strikingly linear, exceptwhere the canals exploit sinuous palaeochannel systems,as in canals 2 and 3 from Kampong Youl (GR 935048)to Angkor Borei via the sinuous ‘oxbow’ lake at Phnom

P. Bishop et al. / Journal of Archaeological Science 31 (2004) 319–336 321

CHACHAUDOCUDOC

20 km

Mekong

RiverBassac R.

Chau D

oc R.

TAKEO

LONG XUYEN

RACHGIA

HATIEN

Phnom Bati

Banam

KdeiAng

WatKandal

BanteaiCha-Krey

WatChhocuteal

Phnom ChisorPrey

Lovea

Baiede

Cay Duong

Ba Phnom

Modern Vinh

The

Canal

4

4

4

5

1 2

3

CHAUDOC

PreyPhdau

Kas PreyPhdau

Prey Phky Phkoam

PreyKrabas

W. Romlok

AngkAngkorBoreiBorei

AngkorBorei

Vat Ang

W. Thnot Chum W. KrKrangangThomThomPrei Mien

WatPo

Montrey

Xg.YoulBanteai Samrong

Phnom Kleang

WatSosay

WatBatheatBatheat

Nui Sam

HonChang

TinhBien

PhnomPhnomBaBayangang

Nui Ba The

Hon Me

XomMha Thd

Oc Eo

Prey Phkoam

W. KrangThom

WatBatheat

PhnomBayang

Ancient canal,with numbering system of Paris (1941)

Topographic form lines onbedrock hill rising above the delta surface

Terrace top (never flooded)Limit of annual flooding

Archaeological localityModern locality

WatSpau

AnLungLungTienTien

AnLungTien

Fig.3

Fig. 2. Map of the setting and canal traces in southern Cambodia and southern Vietnam based on Pierre Paris’s [29] Figure 5. Paris [29] argued thatthe trace of Canal 4 extends south to Oc Eo, whereas canals 1, 2 and 3 seem to serve only the Angkor Borei area itself. The box indicates the areacovered by Fig. 3.


Angkor Borei (Fig. 3; GR 960100). Similar exploitationof palaeochannel systems has been observed in northcentral Thailand [3,13].

4. Field methods and results

The traces of Paris canals 1 and 2 were investigatedon the ground in May 2001. These canals exhibit clear,

linear, water-filled traces, which were expected to bereadily identifiable in the field, and both canals alsoexhibit dry linear traces crossing uninundated land,thereby facilitating access for drilling and trenching.Forty-seven drill holes to depths of up to 3 m werehand-augered on five transects across two canal traces,two transects on canal 1 and three on canal 2 (Fig. 3).The auger transects were a precursor to trenching the

PhKampongKampong

Youl

Ph PhrePh PhreyPhdauPhdau

KhnongKhnong

PhnomDa

PhnomAngkorBorei

2 km

Takeo

Takeo

River

River

Edge of highterrace

PARIS C

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3

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ANGKANGKORORBOREIBOREI

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PhKampong

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2m

3m

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2m

2m

3m

3m

4m

3m

Fig.4 Fig.5

6m

Palaeochannels

High terrace

Canals

Village or town

Spot height

Bedrock hill risingabove delta surface

Standing water andmodern channels asmapped on Takeo1:50,000 sheet

Edge of highterrace

EastBarayCoreAB2

Fig. 3. Map showing linear canal traces and selection of sinuous palaeochannel remnants on the delta surface southwest of Angkor Borei. Canaltraces and palaeochannels were interpreted from 1:25,000 stereo aerial photographs flown by Finnmap Oy in December 1992 and January 1993. Theaerial photograph interpretation was transferred to the ‘Takeo’ 1:50,000 topographic sheet base map using a Bausch & Lomb Zoom Transfer Scope,and with approximate photogrammetric control provided by prominent features identifiable on the aerial photographs and the topographic sheet,as well as by the intersections of the canals dug by forced labour during the Pol Pot era at roughly 1 km-spacings on the national map grid. Boxon canal 1 indicates locations of auger transects 4 and 5 (Fig. 4); the box on canal 2 gives the locations of the auger transects 1, 2 and 3 and trench01/01 (Fig. 5). Bishop et al. [4] discussed core AB2 in Angkor Borei’s east baray (NE corner of map).


canal traces, with the objective of reconnaissanceassessment of whether the sub-surface stratigraphy isconsistent with the linear trace being an infilled canal(in which case an infilled U-shaped channel wasexpected).

Canal 1, apparently much clearer on aerial photo-graphs than canal 2, makes an almost continuous lineartrace from Phrey Phdau Khnong to Angkor Borei (Fig.3). Southwest of the intersection of grid lines 94 and 12,the canal is a broad and deep, water-filled channel whichis said by locals to have been enlarged by a European aidagency in recent years. It was not investigated further atthis locality because of this disturbance. Northeast ofgrid line 94, canal trace 1 remains very clear and hereconsists of a shallow straight channel, about 1 m wide,flanked by narrow linear fields either side of, andparalleling, the central channel (Fig. 4 A). The bundsmarking the outer boundaries of these fields are between5 and 10 m from the central channel. This surfacemorphology of parallel bunds demarcating an innerlinear depression is very suggestive of an infilled canal,and an infilled canal morphology was expected in thesub-surface on transects 4 and 5, the two hand augertransects in canal 1 (Fig. 4).

Three transects were augered across the trace of canal2 where it is dry and consists of two narrow bunded ricefields on a collinear extension of a water-filled canal(Fig. 3 and Fig. 5). The linear trace is lost to thenortheast in an irregular swampy depression that mayinclude a natural palaeochannel remnant. Transects 1and 2 investigated a section of narrow linear canal trace,whereas transect 3 was located in the transition betweenthis linear trace and the irregular swampy area to thenortheast (Fig. 5A).

The auger holes in canal 2 penetrated either dark greyloams, silts, clays and silty organic (peaty) clays andloams, or mottled clays. The stratigraphy of augertransects 1 and 2 clearly exhibits a broad U-shapedmorphology of organic clays and loams inset withinreddish-orange and white mottled clays which are inturn underlain by grey–blue and buff–pink sandy clays(Fig. 5B). As noted above, the latter are likely torepresent the regional substrate. Auger transect 3 doesnot exhibit the clear U-shaped morphology in the sub-surface, presumably reflecting the transect’s location atthe transition from canal to the swampy area to thenorth-east.

The stratigraphy on transects 1 and 2 confirms thatthe trace is an infilled canal and canal 2 was thereforeinvestigated further by trenching. Trench 01/01 washand-excavated across canal 2 adjacent to auger holetransect 2 (Fig. 5A). The trench stratigraphy wasdescribed and logged and sampled for sedimentologicalanalyses, radiocarbon dating and luminescence analyses;reduced pottery sherds were also recovered from thecanal infill.

5. Laboratory analyses

Two bark samples recovered from the stratigraphicauger holes in the canal 1 transects (Fig. 4B) and fivecharcoal samples from Trench 01/01 in canal 2 (Fig. 6)were dated using AMS radiocarbon analysis. Prior tothis dating, taxonomic identifications of all samples wereattempted by the Glasgow University ArchaeologicalResearch Division using microscopic examination ofstandard transverse, radial longitudinal and transverselongitudinal sections of the samples, and the PROSEA[31] guide. The PROSEA guide is concerned mainly with“useful” SE Asian plants and so many of the charcoalsamples were not illustrated in the guide [25]. None theless, the five trench samples that were dated here wereidentified to family level.

A range of laboratory analyses was undertaken onsamples from trench 01/01 to characterise the substrateand infill sediments of canal 2 in terms of their sedimen-tology and luminescence contents. The sedimentological,luminescence and geochemical analyses provide inde-pendent lines of evidence to assess our field identificationof the bed and banks of canal 2. Correct identificationsof canal bed and banks are essential, firstly, in confirm-ing that the linear trace is a canal, and, secondly, inmeasuring canal dimensions, such as width and depth,which may be a guide to canal function.

6. Canal 1

The sub-surface stratigraphy revealed by the augerhole transects across canal 1 provides only hints of aninfilled canal (Fig. 4B). Transect 4A suggests the pres-ence of the north-western canal bank, which is alsohinted at in transect 5, whereas transect 4 suggests thesouth-eastern bank of a canal. Radiocarbon ages about6 ka BP from only 1.6 m to 1.8 m below the presentground surface (Fig. 4B and Table 1) are very mucholder than the likely ages of the canal and the datedmaterials are likely to represent pre-canal substrate. Theradiocarbon ages therefore demonstrate that if this traceis indeed a canal, the canal must have been very shallow.This issue will be clarified by future sub-surface investi-gation, and no further data are presented here forcanal 1.

7. Morphology and stratigraphy of canal 2

The stratigraphy of the trench wall in canal 2 ispresented in Fig. 6. Canal morphology is indicated bythe shallow U-shaped geometry of the boundarybetween unit VI and unit VII, this boundary cuttingmore deeply into unit VII at the centre of the trench, aswould be expected in a dug canal feature. The infilledcanal structure is clearly inset within mottled reddish-orange and, at depth, bluish-grey and olive–brown (or


locally buff–pink) clays and sandy clays. At least sixindependent lines of evidence confirm that the interfacebetween unit VII and units V/VI represents the baseof the canal. The overall lenticular geometry of the

sediments underlying the linear trace is as would beexpected in an infilled canal, noting especially the way inwhich the sediments thin toward the margins of thestructure (e.g., the thinning of units II, III and IV at

(a)

0

3641

42

43

46

4445

TRANSECT 4A

TRANSECT 4

Auger hole with number(e.g. "39" is AH01/39

TRANSECT 5

40

39

38

37

50m

Bund

BundChannel

(b)

Organic clays

Grey brown clay

Orange & red mottle clay

Charcoal sample

Light grey &/or olive &/or pink sandy clay to clay

Bun

d

Bun

d

Cha

nne l

Bun

d

Bun

d

Bun

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el

Cha

nnel

39 40 36 41 42

38 37

47 46 43 44 45

00

1

5 10 15 20

?

?

?

?

?25metres

met

re

TRANSECT 5

TRANSECT 4

TRANSECT 4a

Fig. 4. A. Map of locations of auger transects in canal 1 (see Fig. 3). B. Auger transects 4, 4A and 5 across canal 1. Note the sub-surface suggestionsof a very wide infilled canal morphology. The radiocarbon ages of the charcoal samples are given in Table 1.


both the north-western and south-eastern ends of thetrench stratigraphy). Secondly, the progressively lowerenergy conditions that would have prevailed with theinfilling of the abandoned canal are consistent with thegeneral fining-upwards of the infill sediments (Fig. 7).Thirdly, the recovery of ceramic sherds from the base ofunits V and VI is consistent with the discarding ordropping of sherds and their sinking to the base of thecanal (Fig. 5). The inferred base of the canal also marksa change from greater variability in sedimentologicalcharacteristics (density, moisture content, organic con-tent) in the infill sediments to greater uniformity of thesecharacteristics in the substrate (Fig. 7). The hardpan atthe base of the canal (Fig. 5) is consistent with precipi-tation from groundwater moving along the interfacebetween the substrate into which the canal was dug andthe canal infill sediments. Similar hardpans have beenreported from the beds of infilled irrigation canals in theAmerican southwest [18]. Finally, and perhaps mosttellingly, the contrast between the low levels of lumi-nescence stored dose in the canal infill and the abruptlyhigher stored doses at the top of Unit VII (the regionalsedimentary substrate), especially in profiles 2 and 3(Fig. 8), is consistent with the infill being youngermaterial inset within older material. The abrupt increasein stored dose marks the base of the canal. Sanderson etal. [35] have provided more detailed discussion of theseluminescence data.

The south-eastern end of the canal trench best exem-plifies the relatively simple ‘layer-cake’ stratigraphy tobe expected in a canal infilling through time (Fig. 6). Theclear ‘step-down’ of the canal bed (the base of unit V),15 m from the north-western end of the canal, is

interpreted to be an original excavation feature datingfrom when the canal was dug (or re-dug). The relativelysimple ‘layer-cake’ stratigraphy of the south-eastern sideof the canal is matched by luminescence profile 3 (Fig.8C). The ancient pre-archaeological substrate has rela-tively high stored doses corresponding to deposition tensof thousands of years ago, whereas the canal infill hasmuch lower stored doses consistent with depositionwithin the last two millennia [35].

Luminescence profile 2, in the middle of the canal, ismore complex (Fig. 8B), with an unexpected increase instored dose at the top of unit 4, half way up the canalinfill, which apparently is not matched by any complexi-ties in the stratigraphy (Fig. 6) or the sedimentologydata (Fig. 7). The increase in stored dose in this sampleindicates the admixing of older (unbleached) materialswith the younger (bleached) material and is thereforesuggestive of disturbance. The impact of the residual“unbleached” signal in this mixed-luminescence samplevaries depending on the mineral fraction and the lumi-nescence stimulation method. In the polymineral system,both the IRSL and post-IR blue OSL signals, which aredominated by feldspar, show 10–20% of the equivalentdose of the substrate (i.e., ‘old’ unbleached) layers(Fig. 8). By contrast, the TL results for the mixed-luminescence samples half way up the canal infill inluminescence profile 2 show equivalent doses of some80% of the substrate value. It is well established that therates of bleaching of TL are far lower than for OSL andIRSL signals. Nevertheless, given sufficient illuminationof sediments prior to their enclosure (i.e., their burialand shielding from light), it should be possible toreduce TL signal levels by approximately one order of

06

Auger hole with number(e.g. "39" is AH01/39

TRANSECT 3

Trench01/01

Bun

d

Bun

d

SwampyArea

TRANSECT 2

TRANSECT 10 20 40m

05

04

25

2627

2830

3101

3234

3508

15

16 18 1719 0223

2120

22

07

14

13

03

12

11

10

09

Fig. 5. A. Map of locations of auger transects and trench 01/01 in canal 2 (see Fig. 3).


magnitude. The retention of 80% of the initial TLsignal may therefore imply that the luminescenceanomaly within luminescence profile 2 is associated withmaterial that has only been partially exposed to light,as well as being related to the admixing of older ma-terial. The admixing of old material from the canalbanks, and only partial exposure to light of this materialas it was being admixed, may well explain the character-istics of the apparently anomalous mixed-luminescence

samples half way up the canal infill in luminescenceprofile 2.

The infill stratigraphy at the north-western end of thetrench is the most complex of the whole canal infill (Fig.6), perhaps reflecting episodes of infilling and re-digging,which also would explain the complexities in lumi-nescence profile 1 (Fig. 8A). The stratigraphy at thisnorth-western end and the canal infill luminescenceproperties at the centre and north-western end thus

Organic clays

Grey brown clay

Orange & red mottle clay

Light grey &/or olive &/or pink sandy clay to clay

04 25 26 27 2829

30 31 01 32 33 34 35 08

?

?

? ?

?

?

??

00

1

5 10 15 20 25metres

met

re

TRANSECT 2

CANAL

TRANSECT 1

TRANSECT 3

CANAL

05 15 16 1817 19 02 23

21 20 22 07

06 14 1303 12 11 10 09

Bun

dB

und

Bun

dB

und

Fig. 5. B. Auger transects 1, 2 and 3.


+++++++++++++++++++++++++++++++

++++

+

OSL 09OSL 09

OSL 08OSL 08

OSL 03OSL 03OSL 01OSL 01

OSL 02OSL 02

OSL 04OSL 04

OSL 05OSL 05

OSL 06OSL 06

OSL 07OSL 07

CHCH04

04

CHCH0303

02 & 0302 & 03

CHCH0101

01

CHCH0202 CHCH

1414

CHCH1515

CHCH1313

CHCH16

CHCH0909

CHCH0707CHCH

05

0505

CHCH12

CHCH1111

CHCH10

CHCH0808

CHCH0606

I

I

I

IIIIIIII

IIIIIIIIIIII

IIIIII

IIIIII

IVIVIVIV

IVIV

V &V1V &V1

V &V1V &V1

VIVI

VIVI

VIVI

VIVI

VIIVII

VIIVII

V

V

1 23

Charcoal samplewith number

Luminescence profileswith profile number

Sedimentology samples

OSL dating samplewith number

Sherd with number

+++

1LEGEND

+++++++++++++++++++++++++++++++

++++

+

OSL 09

OSL 08

OSL 03OSL 01

OSL 02

OSL 04

OSL 05

OSL 06

OSL 07

CH04

04

CH03

02 & 03

CH01

01

CH02 CH

14

CH15

CH13

CH16

CH09

CH07CH

05

05

CH12

CH11

CH10

CH08

CH06

I

I

I

IIII

IIIIII

III

III

IVIV

IV

V &V1

V &V1

VI

VI

VI

VI

VII

VII

V

V0

0

50

100

150

200

200

0 5 10 15 20

cent

imet

res

cent

imet

res

metres

0 5 10 15 20metres

Fieldbund

Base of canal

Ground surface

NORTHWEST

SOUTHEAST

Dark grey massive clay

Dark grey organic ('peaty') clay

Pale yellowish grey massive clay

Dull grey and dull yellow-brown mottled clay

Dull grey sandy clay loam to clayey sand

Mottled orange and gritty clay

HardpanHardpan

Fieldbund

Topsoil

Base of canal

Hardpan

Fig. 6. Stratigraphy in north face of trench 01/01 in canal 2. The upper part of the diagram gives the canal cross-section at true (natural) scale and the main diagram (lower) is at 5� verticalexaggeration.

P.

Bishop

etal.

/Journal

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Table 1Radiocarbon ages and selected OSL ages (in italics to help in distinguishing from the radiocarbon ages), from canal 1 samples and Trench 01/01 samples in canal 2. For trench 01/01, samplesare tabulated in stratigraphic order (deepest at the bottom of table, shallowest at the top—see Fig. 6). Calibrated ages were determined using OxCal3 and are given in terms of a calibrated agerange and that age range’s probablity of being the ‘true’ calibrated age range. For CH06 and CH12, the calibration curve yields two calibrated age intervals and the calibrated age intervalwith the higher probability (P) is tabulated first (in bold). The pooled age calculation followed the method of Ward and Wilson [47]. See Table 2 for more detail on the OSL ages listed here

Charcoal sample number(see Fig. 4b and Fig. 6)

OSL sample number(see Fig. 6)

Stratigraphicunit (Fig. 6)

Charcoal taxonomicidentification

Laboratorynumber

d13C ‰ Age(BP)

Error OSL or calibrated C14 age ranges

Min Max P (%) Min Max P (%)

Canal 1AH01/44, 160–180cm Indeterminate bark AA-54987 �25.3 6375 50 5480 BC 5260 BC 95.4AH01/46, 162–164cm Indeterminate bark AA-54988 �28.3 6190 55 5300 BC 4990 BC 95.4

Canal 2OSL03 Top II 2120 410 530 BC 290 BC 68.2

1850 240 90 BC 390 BCOSL01 Top III 1405 140 AD 445 AD 735 68.2OSL04 Top III 1370 130 500 AD 760 AD 68.2OSL05 Top IV 2015 240 �255 BC AD 225 68.2

CH07 IV cf. Rhizophoraceae AA-50726 �25.248 1550 40 AD 420 AD 610 95.4CH12 IV Rhizophoraceae AA-50729 �25.779 1600 37 AD 380 AD 570 94.0 340 370 1.4CH11 V & VI Euphorbiaceae AA-50728 �25.733 1555 50 AD 410 AD 620 95.4

OSL06 V & VI 3300 270 1570 BC 1030 BC 68.22050 260 310 BC AD 210

CH06 V & VI cf. Dipterocarpaceae AA-50725 �26.924 1640 40 AD 320 AD 540 91.5 260 280 3.9CH08 V & VI Aracaceae (Palmae) AA-50727 �26.995 1605 45 AD 340 AD 570 95.4Pooled age: CH06, 07, 08, 11, 12 1592 19 AD 423 AD 533 95.4

OSL07 1710 240 AD 50 AD 530 68.22720 650 1370 BC 70 BC1700 270 AD 30 AD 570

P.

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00

I

II

III

IV

V & VI

VII

Hardpan

Dep

th /

cm

Organic content (%)Mean grain size (µm)

Moisture content (%) OSL Profile 2 (Gy)

Density

50 100 1500 50 1000 5 10 150 15 30 450 50 100 150

20

40

60

80

100

120

140

160

180

200

Organic content (%)

Mean grain size (µm)

Moisture content (%)

OSL Profile 2 (Gy)

Density

Fig. 7. Sedimentology and luminescence profile 2 data. To ensure that the luminescence data are plotted in the correct stratigraphic unit, their depthsare plotted with respect to depth on the adjacent sedimentology profile. The luminescence samples are plotted at their true measured field depths inFig. 8.


imply a complex sequence of aggradation and re-cuttingprior to the final infilling. Such re-cutting and aggra-dation probably imply that the canal had an extendedoperational life prior to its demise.

The maximum depth of the canal prior to infilling wasbetween 1.20 m and 1.65 m, depending on the depths atwhich the ancient canal banks and ground surface aretaken to lie. If the base of unit VI marks this ancientground surface, the canal was only ever a maximum of

1.2 m deep (cf. the base of unit VI at the south-easternbank of the canal; Fig. 6). The width of the canal at thetrench location is at least 15.75 m, the north-westerncanal bank not having been exposed by trenching. Theless precise data of auger transects 1 and 2 (Fig. 5B)indicate the canal width to be up to 25 m. This widthwould give a width/depth ratio of about 21 for canal 2.

Canal 2 is therefore a broad, shallow channel thatmakes a direct connection between the ancient

Dep

th /

cm

Equivalent dose / Gy

0

20

40

60

80

100

120

0

20

40

60

80

100

160

140

120

0

20

10

40

30

50

60

70

80

90

25 50 75 25 50 7550 100 150 200 250 50 100 150 200 250

PROFILE 1Polymineral IRSL


Canal bed


Post IR Blue OSL

Post IR Blue OSL

Post IR Blue OSL

Polymineral TL

Polymineral TL

Polymineral TL

Quartz OSL

Quartz OSL

Quartz OSL

A

B

C

Canal bed

Canal bed

Fig. 8. Luminescence profiling results from A. profile 1 (north-western section); B. profile 2 (central section); and C. profile 3 (south-eastern section)of the canal trench plotted using actual sample depths (cf. Fig. 6 and Fig. 7.)


settlements of Angkor Borei and Kampong Youl. Thisdirect connection strongly implies that the canal func-tioned as a transport and/or communications link. Theshallowness of the canal means that it was presumablyable to accommodate shallow-draft vessels only, but thecanal width means that these vessels would not have hadto have been especially narrow.

8. Age of canal 2

OSL ages, radiocarbon ages and taxonomic identifi-cations of radiocarbon-dated charcoal samples are listedin stratigraphic order in Table 1; Table 2 providesfurther detail on the OSL determinations. In broadestsummary, basal units of the canal infill most probablydate from the 1st millennium BC to early 1st millenniumAD, based on the combination of small aliquot SARdata (OSL07: 720�650 BC to AD 290�240) and feld-spar data (OSL06: 50�260 BC and OSL07: AD300�270; P=68.2% in all cases). The five radiocarbondeterminations from the basal three infill units (Units IVand V/VI; Fig. 6) are statistically indistinguishable andgive a pooled age of 1592�19 BP (calibrated early fifthto early sixth centuries AD; P=95.4%) (assessment ofstatistical similarity and calculation of pooled ages bothfollowing the methods of Ward and Wilson [47]. Theseradiocarbon ages appear somewhat younger than thebasal OSL determinations, but using 2� uncertainties onthe OSL determinations results in overlap of the radio-carbon and some of the OSL determinations. The strati-graphically higher OSL01 and OSL04 determinations atthe top of Unit III (mid-fifth to mid-eighth centuriesAD; P=68.4%) are consistent with the radiocarbon ages.Sanderson et al. [35] have discussed the rationale foraccepting these two latter determinations as the leastlikely of all the trench 01/01 OSL determinations toinclude an older (i.e., unbleached) component (see alsoTable 2).

In detail, there are inconsistencies within and betweenthe luminescence and radiocarbon age data, presumably

reflecting the shortcomings and errors to which both ofthese types of age data are potentially subject. Thus, it isof concern that the five radiocarbon age determinations,spanning three stratigraphic units and approximately 50cm of sedimentation, are statistically indistinguishable.One or more of at least four explanations may acountfor this situation: sedimentation of Units V/VI and IVwas extremely rapid; and/or charcoal of the fortuitouslysame age has been deposited through time as Units V/VIand then IV progressively built up; and/or single-agedcharcoal has been redistributed throughout Units V/VIand IV by bioturbation (of the human or faunal kind);and/or some other mechanism has disturbed the integ-rity of the charcoal stratigraphic position. If the firstmillennium BC OSL age is accepted as the age of finalcanal excavation (or re-excavation) and abandonment,there was an extended interval of non-sedimentation inthe canal if the early fifth to early sixth centuries ADradiocarbon ages are accepted at face value.

The luminescence data from basal stratigraphic units(OSL samples OSL02, OSL06, OSL07, OSL08 andOSL09) are likewise problematical in detail, since SARdose-distributional analyses of these samples (and, tosome extent, of all nine quartz OSL samples), andsmall-aliquot analyses from two of these, show clearevidence of mixing of canal sediments and unbleachedsubstrate. Such mixing is entirely consistent with thesesamples’ basal stratigraphic settings, in which bottomcurrents and bottom disturbance are likely to be associ-ated with the erosion of an older (i.e., unbleached)pre-canal substrate material with a high stored dose, andadmixing this with younger bleached material that wasbleached during canal digging (or has settled to the canalbottom through the water column and been bleachedduring canal infilling). In an attempt to identify thisyounger component, Sanderson et al. [35] undertookselected small aliquot quartz OSL, feldpsar OSL andfine-grained feldspar OSL analyses, which are summa-rised in Table 2. The feldspar analyses on OSL03confirm the quartz SAR results for this sample, a finding

Table 2Summary of various OSL data from Trench 01/01. See fuller discussion by [35]

OSLsample

Quartz SAR results(BP)

Small aliquot OSL results (ages in years BP) Feldspar OSL results(ages in years BP)

Fine-grained feldspar OSL results(ages in years BP)

OSL03 2120�410 1850�240OSL01 1405�140 Only one component in stored dose; confirmed

as well-bleached at depositionOSL04 1370�130OSL05 2015�240OSL06 Mixed materials 3300�270 2050�260OSL02 Mixed materialsOSL07 Mixed materialsOSL08 Mixed materials Mixed material; low stored dose components

give ages of 1710�240 to 2720�6501700�270

OSL09 Mixed materials?


that we cannot yet interpret but which probably pointsto the need for single-grain analyses. The two forms offeldspar analysis on OSL06 yielded ages in the first tosecond millennium BC, which may be too old.

The OSL03 determination from the top of unit II (530BC to AD 290; P=68.4%) pre-dates the stratigraphicallydeeper unit III OSL ages (OSL01 and OSL04: AD 445 toAD 760; P=68.4%) and Unit IV calibrated radiocarbonages (CH07 and CH12: AD 380 to AD 610; P=w95%)at one sigma confidence intervals. However, the OSL03determination was based on very limited amounts ofmaterial (four quartz discs only), leaving large uncer-tainties in the OSL age, which at 2 standard deviationsare compatible with the stratigraphy and other datingevidence. Sample OSL05 from the top of unit IV (255BC to AD 225; P=68.4%) seems, however, to imply anearlier sequence than the radiocarbon dates would sug-gest. The internal consistency of the radiocarbon agesand the ages from OSL01 and OSL04 may therefore betaken to argue for a short sedimentation chronology ofthe canal, with the implication that the OSL03 andOSL05 determinations may be composed of two or morecomponents that require small aliquot or single-graindeterminations to be separated.

9. Discussion

Stratigraphic excavations across the site of AngkorBorei yield consistent evidence for the site’s occupationsince the fourth century BC and for intensive occupationbetween c. 200 BC and AD 200 [40–42]. The datareported here point broadly to the excavation (orre-excavation) of canal 2 at an as-yet-indeterminate timebetween the 1st millennium BC and the first half of thefirst millennium AD, and its operation until the earlyfifth to early sixth centuries AD, if the radiocarbon datesare taken at face value. Clear evidence thus exists forconstruction of canal 2 coevally with occupation atAngkor Borei.

The directness of the canal’s link between AngkorBorei and Kampong Youl (Fig. 3) suggests that thecanal most likely functioned as a local communication/transport canal. The canal would presumably haveserved also to drain adjacent fields: Mabbett’s andChandler’s ([20]: 68) remark about Oc Eo, that “whatwas needed was not irrigation but drainage”, is equallyapposite for the Angkor Borei area. Irrigation was anunlikely use for the canals in the delta, which remainswell-watered after the rainy season by drainage fromCambodia’s Great Lake (Fig. 1) [26]. Indeed, largeirrigation networks would probably have been unneces-sary in the Mekong delta if recession-rice agriculture wassufficient to feed large populations in the delta [12,45].Incidentally, if the canals discussed here were for irriga-tion (and we reiterate that this was probably unlikely),the extremely low gradient of the delta surface means

that the canals would have functioned by providing‘pathways’ for water to penetrate into cultivated areas,rather than for ‘shifting’ flowing water around thelandscape as is more usually the case in canals that areinvestigated archaeologically (e.g., [11,18,27,34]). Ouraerial photograph interpretation does not reveal a net-work of distribution canals for such irrigation water,and if the canals did fulfil an irrigation function, eitherthey served a relatively restricted area adjacent to thecanals, or water was distributed by simple drainage fromfield to field.

The stratigraphic and luminescence evidence forre-digging and disturbance of canal sediments along thenorth-western bank (Fig. 6 and Fig. 8A) probably pointsto ongoing problems with swamping and shallowing ofthe canal even when it was being used, and the conse-quent need to re-dig it. Disturbance might also havebeen associated with bank collapse, which is consistentwith the greater steepness of the north-western bank (seeauger transects 1 and 2, Fig. 4B). After ceasing opera-tion, canal 2 apparently became quite quickly swampyand shallow.

Accepting the early fifth to early sixth centuries ADradiocarbon dates at face value, canal abandonmentapparently coincided with a major change in land-useintensity that is evidenced in core AB2 from the AngkorBorei east baray (Fig. 3) [4]. Core AB2 tells of decreasedintensity of land-use in the region in the fifth to sixthcenturies, as signalled by an abrupt decrease in charcoalconcentrations in the core, a decrease in mineral sedi-mentation rates, as well as regional decline in grasslandsand the expansion of regrowth taxa. This apparentdecline in intensity of land-use is consistent with thedemise of the canals, but it is overly simplistic tointerpret these changes in terms of, for example, regionalde-population. Angkor Borei was an important centreuntil at least the mid-seventh century AD, as is evi-denced by: the recovery from the site of early seventhcentury inscriptions; the recovery of several pre-Angkorian sculptures that may date to as early as themid-sixth century (from the Phnom Da temples 3 kmeast of the canal 2 trench; Fig. 3) ([9,10]; and the factthat this region of southern Cambodia is a majorsource area of pre-Angkorian inscriptions ([46]:97and Maps 2 and 3). In other words, the site of AngkorBorei continued to be occupied after canal 2 ceasedoperation.

The relationship between the Angkor Borei canalsand those at Oc Eo is intriguing. The extensive canalnetwork associated with Oc Eo, both within Oc Eo itselfand linking the coastal entrepot to its rural agrarianhinterland (Fig. 2 and Fig. 3), dates to the late fifth andearly sixth centuries AD [23,24]. Canal 4 in the AngkorBorei area, which has not yet been studied on theground, was suggested by [29] to be the key communi-cation link between Angkor Borei and Oc Eo (Fig. 3). It


is unclear whether canal 2 was likewise part of this largerregional network, and testing of this notion must awaitfurther work. It is worth reiterating that we are suggest-ing the abandonment of canal 2 in the early fifth to earlysixth centuries AD. If Malleret’s [23] and Manguin’s andVo Si Khai’s [24] dating is correct, the Oc Eo canals wereconstructed as the Angkor Borei canal 2 was ceasingoperation. The recovery of Oc Eo trade goods at Sating-pra in peninsular Thailand and elsewhere in the regionpoints to Oc Eo’s importance as a trading port duringthe sixth century AD ([15,38]:137–139), at the very timethat Angkor Borei canal 2 was being abandoned. Thesetwo lines of evidence, if the dating of the Oc Eo canalsand of the Oc Eo trade good finds throughout the regionare sound, suggest a shift in economic dominance fromthe inland delta (where Angkor Borei is located) to thesouthern, more coastal part of the delta. Either canal 2near Angkor Borei was replaced by other canal systemsat this time, or internal links within the Angkor Boreiarea were becoming less important than region-widenetworks.

10. Conclusion

Pierre Paris’s [28,29] mapping of canal traces has beenconfirmed by both augering and trenching, at least in thecase of his canal 2; the sub-surface stratigraphy ofParis’s canal trace 1 remains enigmatic but it mayrepresent a very large canal, which would certainly beconsistent with its prominent landscape trace. Theapplication of luminescence profiling to confirm thestratigraphic position of the canal bed in canal 2 is asignificant advance, and has special potential in situa-tions in which the position of the canal bed is uncertain(for example, when there is little contrast between canalsubstrate and canal infill). Such luminescence profilingalso has the potential to be applied to continuous coredata, providing the core hole stays open after coreextraction (or is cased), to permit down-hole dosimetrymeasurements. Luminescence profiling has also pointedto disturbance and re-digging of the canal infill, which inthe case of luminescence profile 2 at the centre of thecanal, was not suspected from the stratigraphic andsedimentological data. Finally, the potential to useluminescence to date infilled surfaces has also beendemonstrated by the OSL dates from the infill. Ouroptimism that the dug surface of the canal bed could bedated using OSL was not completely justified, however.The canal bed samples incorporated mixed (bleachedand unbleached) material, presumably as a result ofdisturbance and churning of the bed during canalexcavation and re-excavation, and/or entrainment ofsubstrate material during canal operation.

The OSL data point to the excavation of canal 2between the first millennium BC and the first half of thefirst millennium AD, and both OSL and radiocarbon

dating point to its demise in the fifth to sixth centuriesAD, when it was abandoned and rapidly swamped up.The stratigraphy and luminescence properties of thecanal infill exposed in trench 01/01 imply a complexsequence of aggradation and re-cutting prior to the finalinfilling, especially on the canal’s north-western bank.Such re-cutting and aggradation probably imply that thecanal had an extended operational life prior to itsdemise.

A full regional chronology of canal construction, useand abandonment must await the excavation and datingof the other canal remnants, especially canal trace 4which apparently connected Angkor Borei to Oc Eo.Uncertainties in the dating of canal 2 will be addressedin further work including additional AMS determina-tions from closely spaced stratigraphic pollen concen-trate samples, plus extension of the work to other canalsequences with more single grain/small aliquot OSLanalyses. None the less, the data from canal 2 and coreAB2 and from archaeology at Angkor Borei provideclear and consistent evidence that supports models of aflourishing local economy early in the first millenniumAD, and suggest that the area experienced majorre-organisation or re-structuring in the early fifth toearly sixth centuries. This restructuring almost certainlydid not involve major de-population, however; thiswould come a century later when the seat of powermoved north in the seventh century AD. The mid-firstmillennium changes in the regional pollen and charcoalevidence signal a change away from land-use thatemployed extensive burning [4]. Such a change mighthave involved, for example, a move away from cultiva-tion of rice in bunded fields with dry season burning, toflood recession cultivation of rice (cf. [12,45]). In somerespects, our study supports historical reconstructionsthat envision Funan at its peak from the third to sixthcenturies. AD. This evidence, however, is counterbal-anced by dated inscriptions from the region that post-date the period historians conventionally associate withFunan. Ongoing research suggests that the settlement ofAngkor Borei continued to flourish amid a landscapethat supported a different and perhaps less intensiveform of land-use. Whatever this land-use, it is clear thatit was accompanied by the abandonment of canal 2.

Acknowledgements

Field work was supported by grants from theNational Geographic Society (grant no. 6087-97), theFoundation for Research and Exploration on CulturalOrigins (FERCO), and the National Endowment for theHumanities (grant no. RZ-20199-97). Our special thanksgo to Cambodian colleagues, including Minister ofCulture Her Royal Highness Princess Norodom BophaDevi for permission to undertake research, and to Under


Secretary of State Chuch Phoeurn for continued collab-oration in our research in Angkor Borei. Thanks also toour Cambodian archaeological colleagues from theRoyal University of Fine Arts (Ministry of Culture andFine Arts) for field assistance with canal research duringthe 2001 field season: Mr Thuy Chanthourn, Mr PhonKaseka, Ms Mam Vannary and Mr Ya Da. We alsothank the community of Angkor Borei for their assist-ance and cooperation throughout our fieldwork. Finally,we thank our University of Glasgow colleagues: MikeShand (diagrams), Peter Chung (sedimentologicalanalyses), Jennifer Miller and Susan Ramsay (woodidentifications), and Anne Dunlop and Jane Drummond(photogrammetry).

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