Journal of Archaeological Science (2000) 27, 409–421doi:10.1006/jasc.1999.0465, available online at http://www.idealibrary.com on

Palaeomagnetic Studies of Burned Rocks

Wulf A. Gose

Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712, U.S.A.

(Received 15 March 1999, revised manuscript accepted 11 May 1999)

An assemblage of rocks, such as comprising a fireplace, will acquire a remanent magnetization upon cooling parallel tothe ambient geomagnetic field and thus will share a common direction of magnetization. Detailed palaeomagneticanalyses of oriented samples will reveal whether these rocks have remained in situ since the last heating event or whetherthese rocks have moved or represent discard material. Thermal demagnetization makes it possible in many cases toestimate the maximum temperature which a given rock has experienced. The direction of magnetization can be used toreposition rocks to their cooling position and the cooling history of boiling stones can be reconstructed for some rocks.This paper presents results obtained from burned rocks from various archaeological sites. � 2000 Academic Press

Keywords: ARCHAEOMAGNETISM, BURNED ROCKS, FIREPLACES, BOILING STONE.

Introduction

A rchaeologists have increasingly recognizedthe interpretive potential of burned rocksand of features composed of burned rocks

(e.g. Movius, 1966; Perles, 1977; Leroi-Gourhan &Brezillon, 1972; Isaac, 1982; Olive & Taborin, 1989;Thoms, 1989; Buckley, 1990a; Hodder & Barfield,1991; Hester, 1991; Wandsnider, 1997). Objectiveinterpretations of burned rock features and the humanbehaviour associated with burned rock features arecritical in archaeology. Archaeological investigationsinclude the study of the structuring of activities aroundhearths (e.g. Stevenson, 1991; Bellomo, 1993), rigorousdelineation of the behaviourally-relevant variablesassociated with burnt-rock features (e.g. Barber,1990a, b; Wandsnider, 1997), identification andinterpretation of lithological preferences in burnt-rockuse (Buckley, 1990b; Keeley, 1991; Brink & Dawe,1989), and experimentation to establish possiblerates of usage and formation of burnt-rock features(Witkind, 1977; Lawless, 1990; Williams, 1990; Leachet al., 1997). Most of these address explicitly orimplicitly possible behaviours by which rocks wereintentionally or unintentionally exposed to heat. How-ever, archaeologists rarely marshal direct evidence ofheating.

Magnetic studies of burned stones are a particularlypromising line of research for behavioural and contex-tual interpretations (Bucha, 1971; Barbetti et al., 1980;Schmidt, 1980; Clark & Barbetti, 1982; Ramseyer,1991; Gose, 1990, 1993, 1994; Takac & Gose, 1997;Leach & Bousman, 1998). This paper describes somebasic palaeomagnetic concepts as they apply toburned rocks and presents examples from differentarchaeological sites.

4090305–4403/00/050409+13 $35.00/0

Palaeomagnetic Methodological BackgroundArchaeomagnetism is the application of palaeo-magnetic techniques to archaeological samples. Themost common application makes use of the geo-magnetic field’s secular variation for dating purposes.Clay-lined hearths are the most popular archaeologicalfeature for this research, at least in North America(e.g. Wolfman, 1984; DuBois, 1989; Sternberg, 1989;Eighmy & Sternberg, 1990). The magnetic remanencein the clay is measured in the laboratory, and agesare estimated by correlating the mean direction ofmagnetization with the previously established secularvariation curve. However, the application of archaeo-magnetic analysis to issues of past human behaviourand site formation processes is relatively uncommon.

Abbott and Fredrick (1990) used a proton mag-netometer to measure the magnetic anomalies createdby burned rock middens and used the data to inferformation processes. The proton magnetometermeasures the total magnetic field, which is the sum ofthe remanent magnetization and the induced mag-netization of the rocks. Burned rocks usually havemagnetic susceptibilities much larger than non-burnedrocks (see Figure 11 and related discussion) and theinduced magnetization is directly proportional to thesusceptibility. Thus a pile of burned rocks will alwaysproduce a magnetic anomaly even if the rocks are adiscard pile, as indeed Abbott and Fredrick (1990)observed for their experimental midden. A more directapproach is needed to ascertain the integrity of afeature.

All rocks contain small amounts of magneticminerals such as magnetite or hematite. During theprocess of rock formation, these minerals will acquirea remanent magnetization parallel to the ambient

� 2000 Academic Press

410 W. A. Gose

Earth'smagnetic field

(b)(a)

Figure 1. Conceptual model of a fireplace. (a) A random assemblageof rocks will have a random distribution of directions of magnetiz-ation whether it is the original geological magnetization or a discardpile. (b) After heating, the rocks as well as the sedimentary liner willacquire a thermoremanent magnetization parallel to the Earth’smagnetic field.

PD(present dipolefield direction)

90270 +++++++++

++++++++

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

N

180

Inclination

Declination

D = 130°I = 40°

D = 150°I = –40°

Figure 2. Explanation of a Schmidt equal area projection(stereonet). The declination is measured along the perimeter of thecircle and the inclination increases from 0� at the perimeter to 90� atthe center. The graph shows the direction for the theoretical dipolefield as calculated for the sampling site (PD=present dipole). Bypalaeomagnetic convention, the stereonet represents both hemi-spheres; directions with a positive inclination are plotted with solidsymbols in the lower hemisphere (below the plane of the paper) andnegative inclinations are indicated by open symbols and lie in theupper hemisphere. The angular distance between the two data pointsat 130/40 and 150/�40 is 82�.

magnetic field, usually the geomagnetic field. Under awide range of conditions, this information is preservedover long periods of time. By collecting orientedsamples and measuring their magnetization, the direc-tions of magnetization at the time of rock formationcan be established. In geology, one of the most import-ant results of palaeomagnetic research is the docu-mentation of the motion of lithospheric plates (platetectonics, seafloor spreading).

In the archaeomagnetic investigation of burnedrocks, the magnetic signal of rocks is used in a differentway, namely to study human behaviour and site for-mation processes. For a random assemblage of rocksplaced in a hypothetical hearth before it is fired, thedirections of magnetization will be a random distri-bution as these rocks retain their original (i.e. geologic)vectors of magnetization (Figure 1(a)). If the samerocks are heated, they will acquire a common ther-moremanent magnetization (TRM) parallel to theambient field at the time of the heating (Figure 1(b)). Ifthe heating temperature does not exceed the Curietemperature (the temperature above which all magneticalignments are randomized, e.g. 580�C for magnetiteand 680�C for hematite), then the rocks will acquire apartial TRM. Progressive demagnetization makes itpossible to discriminate between these alternatives andyields the maximum temperature of heating.

Before proceeding with the discussion, some basicaspects of palaeomagnetic data acquisition, analysisand presentation are discussed. The procedures arestandard in palaeomagnetic research and are describedin detail by Butler (1992). After the natural remanentmagnetization (NRM) has been measured, a sample issubjected to progressive demagnetization. In thermaldemagnetization, the sample is heated to the desiredtemperature, allowed to cool in a zero magnetic field,and remeasured. For alternating field demagnetizationthe sample is placed inside a coil which produces amagnetic field, which then decays to zero. The numberof demagnetization steps is typically 10 or more. Themagnetic measurements are converted to declinationand inclination. These directions are displayed in a

Schmidt equal area stereographic projection (Figure 2).The declination is counted clockwise starting at northand the inclination increases from 0� at the peripheryto 90� in the center of the net. Positive inclinations areplotted in the lower hemisphere (below the plane of thepaper) and are shown with solid symbols, and negativevalues are plotted as open symbols in the upper hemi-sphere. As an example, two directions are plotted inFigure 2. Although they seem to lie close to each other,their angular distance is 82�. In archaeomagneticstudies it is helpful to plot the direction of the magneticdipole field (PD=present dipole) at the sampling siteas a reference direction because the expected directionof an undisturbed fireplace should fall within 20�–30� of that direction (Sternberg, 1989; Eighmy &Sternberg, 1990).

The directions for the hypothetical undisturbedfireplace of Figure 1(b) are depicted in Figure 3(a) Theslight scatter in the data is meant to reflect expectedsampling and analytical errors. If the samples experi-enced a temperature T1 of less than the Curie tempera-ture then the directions will scatter once thermaldemagnetization has exceeded T1 (Figure 3(b)). Thehigh-temperature directions indicate the geologic mag-netization of the randomly gathered rocks. The resultsfrom a fireplace constructed by students during anarchaeological field camp (Figure 3(c)) demonstrate

Palaeomagnetic Studies of Burned Rocks 411

N

(a)

PD

N

(b)

PD

N

(c)

PD

Experimental fireplace

Figure 3. (a) Using carefully oriented samples, the directions of magnetization, as measured in the laboratory, will plot as a tight cluster on astereographic net if the samples have remained undisturbed. Some scatter is expected due to experimental errors. (b) If the rocks were heatedto a temperature less than the Curie temperature of the magnetic minerals, then the directions will cluster at demagnetization temperaturesbelow the heating temperature and scatter when heated higher. (c) Results from an experimental fireplace built with limestones. In thestereonets, PD indicates the present dipole field direction at the sampling site. Solid dots are in the lower hemisphere, open symbols inthe upper.

that good data can indeed be obtained from rockfeatures.

Most critical to the magnetic data interpretation isthe application of the principal component analysis(PCA) which makes it possible to separate differentcomponents of magnetization (Kirschvink, 1980). Con-sider a rock sample which has cooled from above itsCurie temperature and remained undisturbed duringcooling. This rock will acquire a single component ofremanent magnetization parallel to the ambient mag-netic field. If this rock is subjected to either thermal oralternating field demagnetization in a series of steps,the observed directions of magnetization will plot asone point on a stereographic net (Figure 4(a)). Thesame data can be represented in a different way. If oneplots the north and east components for each demag-netization step in Cartesian coordinates, then there willbe a series of points heading towards the origin withincreasing levels of demagnetization due to the loss ofintensity of magnetization (Figure 4(b)). Because themagnetization is a three-dimensional vector, a secondgraph shows the same data in an orthogonal plane,either the up–down–east–west plane or, as we prefer,the north–south–horizontal plane. These two graphsare usually combined in one figure called a vectorcomponent or As-Zijderveld diagram. PCA then calcu-lates, in three dimensions, the best fitting mean vectorfor these data. Because the sample contains only onecomponent of magnetization the PCA will give thesame mean direction as will the statistics which arebased on the directions in the stereonet (Fisher, 1953).

The value of PCA becomes apparent when oneconsiders the slightly more complicated case of asample containing two components of magnetization.Based on the stereonet (Figure 4(c)), only one com-ponent of magnetization can be calculated usingdemagnetization steps 5 to 10. The As-Zijderveld

diagram (Figure 4(d)) clearly defines two components,one is revealed by the first four demagnetization stepsand the second over the range of steps 5 to 10. In thecase of burned rocks, the first component is usuallyinterpreted as the culturally created magnetization andthe high-stability component as the original geologicalmagnetization. If the sample in the figure wasthermally demagnetized then we interpret the resultsto indicate the sample was heated to a temperaturebetween steps 4 and 5. Thus the thermal history of eachrock can be reconstructed. However, caution needs tobe exerted because the same magnetic behavior wouldalso be observed if the sample was heated above theCurie point, allowed to cool to the intermediate tem-perature and moved, intentionally or unintentionally,into a new position in which it cooled to ambienttemperature. This alternate interpretation can gener-ally be resolved if the rock sample was fully orientedand if additional rocks from the same feature areavailable for magnetic analysis.

The merit of PCA is indisputable, but it comes witha price. Each sample has to be progressively demagnet-ized in steps small enough to allow a meaningfulinterpretation. We typically use 10 or more steps.Second, each sample has to be analysed individually.Therefore generating and analysing the data is alabor-intensive and time-consuming process.

For most archaeomagnetic studies of burned rocks itis important to collect oriented rock samples. Any rocktype can be used. Biased by my geological work, Iprefer to collect the samples by means of a portable,gasoline-powered drill. The 2·5 cm diameter cores areoriented with a special orientation device routinelyused in palaeomagnetic work (e.g. Butler, 1992). Thesize of the drill bit and the torque created by the drilllimit the size of collectible rocks to no smaller thanfist-size. Rocks of any size can be oriented by placing a

S

E, +H

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(c)

NRM

Step 5–10

Mean of steps 5–10Declination = 286°Inclination = 48°

U1 (NRM)

1 (NRM)

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4

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D

PCA component 1Steps 1–4Declination = 20°Inclination = –33°

PCA component 2Steps 5–10Declination = 290°Inclination = 43°

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Mean of steps 1–7Declination = 340°Inclination = 45°

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(b)2

3

4

7

D

PCA best fitting vectorDeclination = 340°Inclination = 45°

5

6

7

65

4

3

2

Figure 4. Stereonets and vector component diagrams for two samples. (a) This rock contains a single component of magnetization. Thedirections after each demagnetization step are identical, yielding a single point in the steronet. (b) In a vector component diagram the same dataplot as a straight-line decaying towards the origin due to loss of intensity during demagnetization. (c) The directions of a rock with twocomponents of magnetization. In the stereonet only steps 5–10 define a cluster of points amenable to calculating a mean direction. (d) Bycontrast, the vector component diagram and ensuing principal component analysis clearly define two components of magnetization. Inarchaeological settings, component 1 (steps 1–4) is usually the cultural magnetization. In the stereonet, crosses are in the lower hemisphere,open circles in the upper. In the vector component diagrams, crosses represent the projection onto the north–south–east–west plane, opensquares lie in the up–down–horizontal plane.

N U N

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(c)

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Hearth stonesLow temperature

component

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Dust Cave, Alabama

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Stone 1.1

575°C

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Stone 3.2

S

E, +HW, –H

D

NRM2.5 mT

5 mT10 mT20 mT30 mT40 mT60 mT

Clay 3

(a)

PD

Clay liner

Figure 5. Magnetic results from a fireplace at Dust Cave, Alabama. (a) Samples from the clay liner contain only one component ofmagnetization and the directions cluster well. (b) Vector component diagrams for the rock samples define two components in all rocks. (c) Thedirections of the low-temperature components cluster and agree with the directions obtained from the clay liner. The high-temperaturecomponents scatter implying that the rocks were not heated above the Curie point of its magnetic minerals. The clay samples weredemagnetized by the alternating field method and the peak field values are given in milliTesla (mT). The stones were thermally demagnetized.

PD is the present dipole field direction at Dust Cave.

414 W. A. Gose

cap of plaster of Paris on top of the sample. Aftercreating a level surface in the wet plaster, a north-pointing arrow is inscribed. In the laboratory, the handsample can be either drilled or cut by a rock saw.Cutting a sample with a saw can be particularlyadvantageous for small samples and for detailedstudies such as determining temperature gradients.Experience has shown that plaster of Paris is often asmagnetic as the rocks to be analysed and a sampleshould not be encased in plaster without first havingmeasured the magnetic remanence of a cured plastersample.

E

N

W

NRM

300

400

500

Rock B

NRM = 6.7 × 10–6 A m2/kg

0 3

500

Time (h)

Tem

pera

ture

(°C

)

321

500

400

300

200

100

Rock B

Matrix

Figure 6. Temperature profile of a limestone rock and the surrounding matrix. One thermocouple was placed inside a small hole drilled fromthe bottom into the rock to within 1 cm of its upper surface and the other thermocouple was wedged in the dirt matrix between adjoining rocks.The temperature on top of the burning oak logs reached 900�C. A sample from the rock was subjected to thermal demagnetization and theresults are shown in a vector component diagram which suggests that the rock reached a temperature of about 300�C.

T = 300

T = 400

T = 500

>500

>500

400 450 250

250

350>500>500

N

41TG307Feature 11

Figure 7. Palaeomagnetically estimated temperatures of heating (�C)plotted on a schematic location map. As emphasized by the iso-therms, it seems that the northern part of the feature is missing. Thiswas confirmed by later excavations.

Some ExamplesDust Cave is one of many small caves in the limestonebluffs along the Tennessee River in northern Alabamaused by prehistoric inhabitants from about 10,000 BPto 5000 BP (Goldman-Finn & Driskell, 1994). Exca-vation within this cave exposed a fireplace (Feature 87)delineated by a ring of nine small stones (Collins et al.,1994). This feature offered an excellent opportunity tocollect both the burned clay liner as well as the burnedrocks. The clay samples were subjected to alternating

field demagnetization because they were encased inplastic cubes. All five clay samples carried a singlecomponent of magnetization and thus the vector com-ponent diagram shows two straight lines (Figure 5(a)).The characteristic directions of magnetization, calcu-lated by principal component analyses, are depicted inthe adjoining stereonet. From the stones, cubicalsamples were cut and treated by thermal demagnetiz-ation. The data from stone 1·1 dramatically demon-strate the merit of vector component diagrams andprincipal component analyses. When only viewed inthe stereographic projection (Figure 5(b)), this samplewould be rejected because the directions never reach astable endpoint. But in the vector component diagram,two distinct components of magnetization are clearlyapparent. All nine stone samples contain two distinctcomponents of magnetization (Figure 5(b)). One com-ponent is stable up to about 300�C and a secondcomponent is revealed over the temperature rangefrom 350�C to 575�C. The low-temperature directionscluster whereas the high-temperature directions scatter(Figure 5(c)). The shared low-temperature componentis interpreted as being due to the last or only heatingevent and that the stones have remained undisturbedever since. The slight scatter in the directions reflectsexperimental errors or minor settling of the stones.This conclusion is fully supported by the results fromthe clay liner; the two data sets are statistically indis-tinguishable. The high-temperature magnetization isthe original geological magnetization which shouldexhibit a random directional distribution.

These data argue that the maximum temperature towhich the stones were heated was only about 300�C.The validity of this inference was tested in a simpleexperiment. A set of flat limestone clasts, about 10 cmthick, was placed in a circular depression and a fire ofoak logs was built on top. The temperatures weremonitored with a set of thermocouples. One thermo-couple was placed in the soil between two rocks andanother one was inserted into a small hole drilled from

Palaeomagnetic Studies of Burned Rocks 415

+H–H

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+H

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NRM

500

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+H–H

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475500550

46.0

500

400

375475

350

300

350

375

450

500

425 400

46.246.3

Camp Pearl Wheat, Texas

0 1

2

3

Figure 8. Vector component diagrams of four sister samples from the same rock. For clarity, only part of the data is shown. The externalsamples (46·0 and 46·1) saw temperatures of 500�C and 475�C while the interior samples were heated to only 400�C and 375�C. The observedtemperature gradient can be used to establish the direction of the heat source and the duration of the heating.

the underside of the rock to within 1 cm of the topsurface. The temperature readings were taken every10 min or after the temperature changed by 10�C. Thetemperature on top of the burning oak logs wasnear 900�C. After 2 h, the matrix reached 300�C andthe temperature inside the rock peaked at 335�C(Figure 6). A sample taken from this rock was sub-jected to thermal demagnetization in 50�C increments.The vector component diagram shows a distinctchange in direction at 300�C which would be inter-preted to be the maximum temperature of heating ifthe rock were an archaeological rock. This temperatureis quite close to the measured temperature, taking into

account that the demagnetization steps were in 50�increments and the measured direction agrees with thelocal magnetic field direction. In experiments designedto better understand the behavior of pottery samplesused for determining the intensity of the geo-magnetic field, Kitazawa and Kobayashi (1968)showed convincingly that the reheating temperature ofpottery shards could be accurately obtained by thermaldemagnetization.

The possibility of estimating the maximum tempera-ture which a burned rock attained can be useful inmany different ways (Wandsnider, 1997). Figure 7shows one example from a fireplace in Texas. Nine

416 W. A. Gose

N(b)

41HY209-MFeature 2

41HY209-MFeature 2

50 cm

(a)

Figure 9. (a) Plan map of burned rocks at Feature 2, MustangBranch site. Shading emphasizes the outer and inner rings of thefeature. (b) Stereographic projection of the directions of magnetiz-ation calculated by the principal component analysis for samplesfrom Feature 2. With very few exceptions, all samples have remainedin place since their last heating.

samples were thermally demagnetized and their heatingtemperatures are plotted on a site map with the rocksbeing represented by circles. As emphasized by thefree-hand drawn isotherms, the hotter rocks arerimmed by rocks which experienced lower tempera-tures, suggesting that the northern half of the featureis missing. Later excavations have proved thisinterpretation correct.

Camp Pearl Wheat is an early Archaic campsite nearKerrville in central Texas (Collins et al., 1990). Foursamples, 1 cm cubes cut from one rock out of a hearth,were progressively thermally demagnetized. The vectorcomponent diagrams clearly reveal two components ofmagnetization in each specimen (Figure 8). Specimens46·0 and 46·1 from near the outside of the rockexperienced temperatures of 475�C and 500�C, whilesample 46·2 saw 400–425�C and sample 46·3 reveals atemperature of 375�C. Such information on thermalgradients may allow inferences on the direction of the

heat source relative to each rock as well as the durationand/or number of heatings, details not previouslyachievable. The data have implications for determiningthe cooking strategies used by prehistoric peoples. Inaddition, calculation of the angular difference between,and thus axial rotation of, partial TRMs within asingle rock can allow modelling of the movement ofthat rock and, by extension, establish the patterned orrandom changes in feature structure.

The late Archaic Mustang Branch site (41HY209-M)is located on a bluff overlooking the Onion Creek ofCentral Texas. Within a large burned rock midden, aslab-lined pit was identified upon excavation (Feature2, Ricklis & Collins, 1994). The central pits are sur-rounded by a ring of vertical or near vertical slabs(Figure 9(a)). The small burned stones in the center ofthe pit seem to be the result of cracking of larger liningslabs. A total of 58 palaeomagnetic samples werecollected from all parts of the feature and subjected toprogressive thermal demagnetization. Principal com-ponent analysis reveals that all samples experiencedtemperatures in excess of 500�C, the mean estimatedtemperature being 530�C. The directions of magnetiz-ation from all parts of the feature agree and clusterreasonably well (Figure 9(b)). The few ‘‘stray’’ sampleswere identified in the field as having been moved byanimal burrowing. The data confirm that Feature 2 isindeed a fireplace that experienced little disturbancesince its last use. The observation that the small rockson the floor of the pit have the same direction ofmagnetization as the rocks from the encircling ringlends support to the interpretation that they arefire-cracked rocks which broke during use.

A few metres away from Feature 2, a group of ninelarge slabs of burned limestone form a second distinctfeature (Feature 3, Figure 10(a)) among the smallerburned rocks of the general midden. Their geometricarrangement suggests that they represent a fireplace.However, the directions of magnetization scatter sig-nificantly (Figure 10(b)). All but the low-temperaturecomponent of rock 58 (point 58L in Figure 10(b)) arereversely magnetized which unequivocally excludes thepossibility that they cooled in situ. The results fromstone 58 suggest that the rock was turned upside-downjust as the other rocks had been but that it was stillsufficiently warm to acquire a low-temperature mag-netization in its found position. Based on thermaldemagnetization, this temperature is estimated at200�C.

A possible interpretation is that these slabs were thecover stones of another fireplace, such as Feature 2,which were removed once the fire had died down. Theobserved directions of magnetization can be used torestore the rocks to the position in which they acquiredtheir magnetization, i.e. the position in the fireplace.This is shown schematically in Figure 10(c). The rocksare depicted as simple rectangles aligned in the north–south direction. In this reconstruction, the magneticdeclination is ignored only because it is rather difficult

Palaeomagnetic Studies of Burned Rocks 417

N

(b)

58 H

58 L

53

54

55

57

56

(a)

41HY209-MFeature 3

20 cm

55 56 54 53 5758 H

58 H

55 56 54

53

57

Position as found

Cooling position

Mag

netic

fiel

d lin

es

(c)

Figure 10. (a) Plan map of feature 3 at 41HY209-M. (b) The directions of magnetization indicate that these burned rocks do not constitute anin-situ fireplace. The negative inclinations imply that the rocks were turned upside-down after final cooling. (c) Interpretive reconstruction ofthe cooling position suggesting that the slabs were cover rocks of another fireplace.

to show the position of the rocks in three dimensions.The tilt of the rocks is not affected by this limitation.The inclination of the magnetic field lines representsthe mean inclination of the samples from feature 2. Thereconstruction supports the interpretation that theseslabs were cover stones.

In the course of measuring hundreds of burned rocksamples from different archaeological sites it becameapparent that many carried a very strong magnetiz-ation. For unheated rocks intensities typically fallwithin one or two orders of magnitude. The burned

rocks, by contrast, have intensities that are oftenspread over four or five orders of magnitude(Figure 11). In this graph, the intensity is plottedagainst the magnetic susceptibility. Susceptibility is ameasure of the total magnetic mineral content of asample. Usually, intensity is proportional to suscepti-bility. Figure 11(a) shows the results from sandstonesamples from an archeological site in Grimes County,Texas (Rogers, 1994). The intensities of magnetizationvary from about 10�6 to 10�2 Am2/kg as compared tovalues between 2�10�6 to 9�10�6 Am2/kg for four

418 W. A. Gose

0.110–2

1000

Intensity (A m2/kg)

(c)

100

10

1

10–310–410–510–610–710–8

41HY202N = 39

0.1

1000(b)

Su

scep

tibi

lity

(10

–6 c

gs)

100

10

141HY209-MN = 69

0.1

1000(a)

100

10

141GM224N = 52

Figure 11. Intensity of magnetization versus magnetic susceptibility.The results from the sandstones (a) and limestones (b) imply thatnew magnetic minerals were formed during heating. The open circlesin (a) and (b) are results from unheated rocks of the same lithology.(c) The data from site 41HY202 reflect the use of the great variety ofrock types readily available at this river terrace site. To convert thesusceptibility to SI units, the values need to be divided by 4�.

unheated rocks from the same area. The susceptibilitiesof the unheated rocks also span a very limited range. Asimilar result was obtained from the burned limestonerocks of the midden from the Mustang Branch sitedescribed previously (Figure 11(b)). The unheatedsamples were collected from the Austin Chalk whichunderlies the midden. In both settings, all burned rockswere of the same lithology collected from local rocks. Itis clear from these data that the increase in intensity isnot merely a reflection of the more efficient acquisitionof a thermoremanent magnetization as comparedto the original depositional magnetization, but theheated samples contain higher concentrations of mag-netic minerals which were created during the heating

process. Future research should investigate the exactmineralogical changes which must have occurred.

The Barton site (41HY202) lies on a gravelly terracein the flood plain at the base of the cliff hosting theMustang Branch site (Ricklis & Collins, 1994). Burnedrocks were collected from several features at thislocality. For these rocks, the susceptibility is notlinearly related to the intensity of the remanentmagnetization (Figure 11(c)). This is due to the factthat the burned rocks reflect the highly diverse litholo-gies of the river deposits which were derived from theTexas Hill Country. This observation suggests that,under certain conditions, these magnetic parameterscan be used to test whether the rocks in a midden werepreferentially selected. If a great variety of rock typeswere available for fireplace construction and the inten-sity increases linearly with susceptibility such as inFigures 11(a) and 11(b), then clearly one rock type waspreferred.

These type of analyses are not restricted to burnedrocks used in fireplaces but are applicable to any rockwhich was subjected to heating such as boiling stones.The magnetic signature of a boiling stone is expected tobe very complex. As the heated rock is submerged in avessel filled with water (and other ingredients), the rockcools and acquires a magnetic remanence parallel tothe ambient magnetic field. When the food is stirred therock will be moved into a different position and willacquire a magnetic remanence over the slightly lowertemperature range. Repeated stirring will impart aseries of partial thermoremanences, each with a differ-ent direction of magnetization. What is measured inthe laboratory is the sum of all these remanences.Progressive demagnetization sequentially removes onecomponent after the other, first the lowest temperaturecomponent or the component with the lowest coercivi-ties in the case of alternating field demagnetization,followed by the next temperature interval componentand so on. If the rock was in continuous motion, thenthe resulting magnetic remanence will be zero becausethe remanence acquired over each very small tempera-ture interval will be in random directions and thus thevector sum will be zero.

At a Late Archaic site in Lee County, Texas,archaeologists identified three well-rounded quartzitecobbles as boiling stones (Rogers, 1998). One samplefrom one of these rocks was subjected to alternatingfield demagnetization and a sister sample to thermaldemagnetization. The overall change in direction wassmall in either case but multiple components of mag-netization are readily resolved in the vector componentdiagrams. This is more clearly seen when the vectorcomponent diagram is enlarged. Upon AF demag-netization, five distinct directions are observed(Figure 12(a)), and a similar result is observed based onthermal demagnetization (Figure 12(b)). The stereonetsdisplay these directions. To get a better sense of themotions it is instructive to hold a pencil, visualize itemanating from the center of the stereonet and let the

Palaeomagnetic Studies of Burned Rocks 419

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Thermal demagnetization

U(b)

D

4

53 2

0.30.3

0.9

East

Nor

th

0 0.150.05

625

675

550

150NRM

0.8

0.7

0.6

0.5

0.4

0.1 0.2 0.25

400

NRM75

100150200250300350400450500550600625650675

NRM

675

675

NRM

550

Figure 12. Magnetic results from a boiling stone. The vector component diagrams reveal multiple components of magnetization, consistentwith frequent movement of the rock while it was cooling. One sample from the rock was AF demagnetized (a), another was subjected to thermaldemagnetization (b). Portions of the As-Zijderfeld diagrams are shown enlarged. The stereonets display the different directions calculated byprincipal component analysis. The direction 1 refers to the low-coercivity and low-temperature component, direction 5 to the high-stability orhigh-temperature magnetization.

420 W. A. Gose

tip of the pencil sequentially point in the directionindicated on the net.

A fundamentally different magnetic signature is tobe expected if the heated rock is placed in waterwithout subsequent movement. As the hot rock istransported from the fireplace to the food containerit will rapidly cool and acquire a high-temperaturethermoremanence. If the rock remained roughly in thesame orientation relative to north then the samplewill gain one component of magnetization. After sub-mergence in water the rock will cool and acquire asecond component of magnetization. Such rocks mayhave been encountered at site 41WB437 in WebbCounty, Texas (Gose, 1999).

SummaryThe results discussed above are intended to demon-strate different ways in which palaeomagnetic tech-niques can be applied towards archaeologicalproblems. The laboratory techniques are standard inpalaeomagnetic research. The merit and significance ofthese results are based on their ability to answerarchaeological problems. The magnetic analysis ofburned rocks is fundamentally different from archaeo-magnetic studies of clay-lined fireplaces (e.g. Sternberg,1989). In the latter case, the directions of magnetiz-ation of multiple specimens from one feature typicallyagree within a few degrees, making it possible to usethese data for archaeomagnetic dating. By contrast,an assemblage of burned rocks is expected to haveexperienced some movement due to collapse of thefireplace, animal activity or post-use settling, and thusthe directions of magnetization will exhibit somescatter. In general, these data do not have the resol-ution necessary for dating, but they provide uniqueinformation about the integrity of a feature and itsusage. Recently Wandsnider (1997) discussed thebehavioural implications of cooking technology.Archaeomagnetic analysis of burned rocks can providedetailed information that can be used to address someof these issues. For example, archaeomagnetic analysiscan provide data on the duration of heating and themaximum temperatures. Linking these data witharchaeobotanical and faunal analyses and capacity-planning models can provide a much more detailedunderstanding of cooking strategies. In other cases itwill be possible to use the magnetic data to reconstructfeatures, such as earth ovens. Site formation studies(Schiffer, 1987) are critical for understanding thearchaeological record and archaeomagnetic techniquesprovide firm evidence that can be used to determine thedetailed use and post-abandonment histories of a site.In architectural settings, this technique will show that astone building burned and cooled before it collapsedor that it collapsed while burning (Gose et al., 1994).As with most other techniques, the value of theinformation will depend on asking the right question.

AcknowledgementsThis work greatly benefitted from many discussionswith archaeologists, particularly Mike Collins from theTexas Archeological Research Laboratory. After I hadpresented a talk on archaeomagnetism at the localarchaeological society in late 1985, Mike approachedme with some suggestions of how palaeomagnetictechniques could be a useful tool in studying burnedrocks and we have collaborated on several projectssince. Britt Bousman, Steve Black, Darrell Creel,Robert Rogers, and Paul Takac helped me gain someunderstanding of the complexities of burned rockfeatures. The results depicted in Figures 3(c) and 6were obtained by graduate student Lori Douglas.Special thanks are due to Britt who encouraged me towrite this paper and provided me with constructivecomments. The thoughtful reviews by James Abbottand Rob Sternberg are appreciated.

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