To the field archaeologist the most obvious-and often the most abundant - constitu-

. ent o f a site is d ir t. . . Dirt, properly called soil or sediment, is the subject matter of

sedimentology. „. * . .

S C H IF F E R 1 9 8 7 : 2 0 0

ediments are com posed variously o f particles o f disaggregated rock, dust from

hatever source, bits o f dead anim als and plants, and chem ical precipitates. T heir

position on the surface o f the Earth or the bottom o f lakes and seas creates three-

raensional sedim entary bodies (deposits) w hich are subsequently m odified in

aracteristic ways by the five spheres o f the clim ate system. In com p an y with

irock, sedim ents underlie the landform s on w hich life processes occur. For

^archaeologists, sedim ents are the enclosing m edium and-the environm ent for the

physical and chem ical rem ains that com prise archaeological sites.

I N T R O D U C T O R Y C O N C E P T S

7n contrast to the readiness with which archaeologists borrow geom orphological

techniques for identification and description o f landform s, developm ents in petro-

graphic techniques seem to be adopted slowly and reluctantly by them. M ethods for

¿the technical description and interpretation o f sedim ents and soils, particularly, need

further developm ent and m ore intensive application in archaeology. As w ith fish that

cannot be expected to be aware o f water, archaeologists often take for,granted the

É A S I C P R I N C I P L E S O Fj k

s e d i m e n t o l o g y a n d s o i l s

Sc i e n c e ... *

materials w ith in w hich their sites occur, rather than seeing them as problems and

- interpretive opportunities. Skilled geoarchaeological w ork rem ains, regrettably, a

specialist dom ain instead o f being incorporated as a m atter o f course in all field wori

' M in erals are in organ ic chem ical com poun ds in crystalline, form ; rocks are com. j

posed prim arily o f m inerals, som etim es accom panied b y o rgan ic detritus and chem. 1

ical precipitates. M in eral m atter o f the regolith recirculates through cycles of j

exposure, erosion, deposition, and bu rial at the surface o f the Earth. Rocks and sedi- 1

ments are disaggregated b y w eathering; the resulting particles áre eroded and trans- J

ported by com ponents o f the atm osphere, hydrosphere, cryosphere, an d biosphere "1

that rearrange the surface o f the Earth and rem odel landform s. Sedim ents form |

when particles in transit are deposited, as transport m edia lose energy. The condi- |

tions o f tran sp ort and deposition can often be inferred from attributes o f the sedi- 1

ments, w hich are im p ortan t sources o f proxy data on form er environm ental states. 3

These potentials are discussed below un d er the headings o f “ Lithology,” the study of

rocks and m inerals, “Sedim ents as archaeological contexts,” the study o f sediments in sites, and “ Pedology,” the study o f soils.

258 S E D I M E N T S A N D S O I L S «

L ith o lo g y

T he prim eval rock o f the planet cooled and crystallized from a m olten state to form

the continental and ocean ic crusts o f early geological tim e. Igneous rocks, cooled

from the m olten state called “ m agm a,” include such m aterials as granite, diorite,

basalt, and obsidian. Igneous rock that flow ed on the surface o f the Earth as lava is

“extrusive”; that w hich infiltrated un der pressure into o r betw een bodies o f rock is

called "intrusive.” Igneous rock bodies m ay be very extensive; the massive “shield

rocks” that form thestable ancient cores o f the continents belon g to this class. In our

own tim e volcanic eruptions are co m m o n enough to rem ind us that m olten material

continues to em place new rock on the crustal plates. The extrusion o f lava a lo n g rift

zones beneath the oceans is probably nearly constant if the rifts are considered as a whole.

S edim en tary rocks are com posed o f m ineral m atter transported by w ind, water,

o r ice and consolidated after deposition. T h e consolidated m aterial m ay be m inerals

weathered from older rocks, chem ical precipitates from water, or detrital organic

materials. Sedim ents are consolidated and lithified (transform ed into rock) by

cem entation o r com p action o r both, resulting in such rocks as sandstones, lime­

stones, and siltstones, all reflecting in their names their sedim entary origins.

M etam o rp h ic rocks are m odification s o f lithic material from the other two

groups that has been subjected to great heat, as from contact w ith m agm a, or both

B A S I C P R I N C I P L E S | 2 5 9*

Ijjjat a n d pressure, as occurs during m ountain building when rocks are folded and

vaulted. In such extrem e conditions, both igneous and sedim entary rocks are

|^¿dified by form ation o f new crystal states that are stable at high tem peratures and

'-pressures. por exam ple, granite can be changed to gneiss, lim estone to m arble, silt-

¡¡¡¡jope to slate. ' . • - _

Ü Before there can be sedim entary rocks there must be sediments, masses o f uncon-

“ ¿[¡dated particles o f m ineral o r biogenic materials. Sediments are always deposited

l l n geosphere surfaces, w hich m ay lie beneath air, water, or ice, w ith obviously

d iffe re n t consequences for their archaeological relevance. Sediments aré fundam en-

f t l l y im portant to archaeologists because they constitute the m atrix for archaeologi-

|v¿al remains, the enclosing m edium which keeps the remains in place and defines

P iieir im m ediate physical and chem ical environm ent. Sediments constitute also the

pS{»ntext o f archaeological rem ains and sites, locking them into stratigraphic and

t~locational relationships w ith other classes o f data. The attention paid by environ ­

m en tal archaeologists to this second characteristic o f sediments is w hat prim arily

distinguishes them from their non-environm ental archaeologist colleagues, w ho

;> may consider sedim ents no m ore than expendable “dirt.” The am ount o f archaeolog-

..ically relevant inform ation that can be w rung from the analysis o f sedim ents is

. 'limited only by o u r skill, im agination, and funding (Fritz and M oore 1988).

Sedim en ts as arch aeolo g ical contexts

_;In the absence o f sedim ents there can be no typical archaeological sites. Piles o f

worked stones on bare bedrock can qualify by definition as archaeological sites but,

:■ by providing the investigator with the utter m inim um o f inform ation about the

piling event o r accom panying conditions, they lim it investigation to elem ents o f

form; they are artifact aggregations, rather than sites. Sediments provide context and

structure. Sedim ents enclose artifacts and features, maintain relationships am ong

objects, and protect buried materials from a range o f disturbances. In their role as

burial environm ents, sedim ents may also disturb and dam age archaeological m a­

terials. Additionally, it is always worth asking whether or not the original deposi-

tional surface is preserved in the sediments.

Archaeological sites occur in most kinds o f sedim ents. Sites in p rim ary context

may be buried by aeolian, alluvial, colluvial, volcanic, m arine, or lacustrine sedi­

ments. Redeposited artifacts, provided they survive transportation, also o c c u r in a

; great variety o f sedim ents, although once m oved from their prim ary location s they

.} J°se m uch o f their strictly archaeological in form ation and becom e a special class of.

geological phenom ena. Each class presents its particular physical and chem ical

B A S I C P R I N C I P L E S .

characteristics and in terp retation al p roblem s, each in turn requiring a special set of

analytical m ethods. - ■_

In order to understand the crucial relationships betw een a site o r deposit o f

facts and its enclosing m edium , archaeologists need to k n o w as m uch as possible

about five characteristics o f sedim ents:

1 the source o f the m aterial, w hether it is residual o r derived, the nature o f thepai*¿f-

rock, and, i f derived, from w h ich d irection and w hat distance;

2 the transport m edium w hich m o ved and deposited it;

3 the depositional en viro n m en t in w hich it fa m e to rest; . _ —,

4 any subsequent n atural transform ations o f the deposit, in clu d in g mechanical or

biological disturbances and chem ical or physical changes such as soil fonnatioi

cem entation, or com p action ; and

5 any subsequent cu ltural transform ations. . ....................... ...

H ay’s study o f the F o o tp rin t T u ff in th e East A frican Laetolil Beds (R. L. Hay 1981

Part IV Case Study) exem plifies th e k in ds o f data and lo g ic involved in determining these five characteristics. T h e fact that he w as w orkin g in Iithified sedim ents changes som e o f the techniques, b ut n o t th e basic research strategy. T h e source (1) o f the vol­

canic ash that com prised the tu f f w as traced to th e extin ct volcan o Sadiman 6

observing the slope o f the beds an d b y m atch in g the chem ical com p osition o f the asL—

in the two locations. T h e tran sp ort m edium (2) was the force o f the volcanic explo­

sion, w ith subsequent fall throu gh air; after the initial d eposition, the ash was deter­

m ined to have been disturbed o n ly b y rainwater. These con clusions were reached by

observing that the bed d in g o f the dry-season tuffs was abou t the sam e thickness

across the m in or relief o f the surface, in dicatin g that, unlike other ash-falls in the

series, it had been only ligh tly d isturbed by w in d . T h e up p er tu ff show ed som e redis­

tribution from topograph ic highs to lo w poin ts, and w ater transp ort was suggested

for that. T he depositional en viro n m en t (3) for m ost o f the Laetolil Beds was inferred

to be a dry savanna, on the basis o f the included fossils and som e w ind movement of

the ash which indicated little vegetation to offer obstruction; the rainfall that dis­

turbed the upper layers o f th e F o o tp rin t T u ff was interpreted as-seasonal. Natural

transform ations Í4) subsequent to deposition included the footprints, subsurface

disturbances by term ite colonies and rodents, the form ation o f a carbonate crust, the

cracking o f the crust by the m ovem en t o f burrow in g anim als, and, ultimately,

lithification. N one o f the investigators w h o have craw led over the F o o t p r in t Tuff in

the years since its recognition have fo u n d any evidence o f cultural t r a n s f o r m a t i o n s

(5). W hile disappointing, this is n ot surp risin g given the great age o f the tuff, which

antedates by over a m illion years any cu ltural rem ains or behavior kn o w n anywhere.

P ed o lo gy

lolog)’ is the science o f soils - chem ically and m echanically altered terrestrial sedi-

jjyjts. Soils fo rm on and beneath the subaerial surfaces o f sediments that are stable

¿only slow ly aggrading. T h e form ation o f a soil requires above all else tim e; there-

•, a soil represents a p eriod in w hich deposition occurred only slow ly i f at all - a

^¿positional hiatus and a tim e o f relative stability. A surface that is rap id ly bu ild in g

lirrapidly erodin g w ill not support the form ation o f a soil. W hile the d eposition o f a

is o f sedim ent m ay be thought o f as an event, w ith a beginning and an end, the

nation o f a soil is always a process, and soils must be understood in processual

juiis. The process has a beginning, which is usually coincident w ith the form ation

f a stable sedim en tary surface. However, it is not know n that soil form ation as a

cess has an inherent endpoint; the process typically ceases w ith an environm ental

nge that leads to burial o r rem oval o f the sedim ent supporting the soil,

irchaeological m aterials, even entire sites, occur within soils, but the relationship o f

oü processes to a site o r an artifact m ust be independently determ ined in each case,

fhe soil m ay have been form ed before irchaeological materials were deposited on or

anit; it may have form ed after the creation o f the site, or it may have been disturbed by

Imman activity and then continued to develop with appropriate adjustm ent to the

vironm ental change. -¿¡■II The p rim ary literature o n soils was developed w ithin agronom y and is directed

^ ^ to w a r d im p ro vin g grow ing conditions for econom ically im portant plants.

¿G eologists and geom orphologists, w ho also w ork in and around soils, have devel-

«sroped a literature that meets their special needs. The soils literature directed to the

needs o f archaeologists is limited (Cornw all 1958; H olliday 1990,1992; L im brey 1975;eIs-f Waters 1992) and is not sufficiently detailed to be o f substantive help in local situa-

J ¿ -tions. A rchaeologists, therefore, need to use the soils literature as it exists, and build

jj on it in consultation w ith local experts (C h a p te r s ) .

There are at least three w orking definitions o f the term “soil," w hich can co m p li­

cate co m m u n ication . To agronom ists, soils are surficial materials that support plant

growth. A gronom ists ignore buried soils, even to the point o f not recognizing them ,

and tend to evaluate archaeological deposits (anthropogenic soils) in terms o f their

horticultural potential. Nevertheless, agronom ists can be helpful, provided full

com m unication about analytical goals is established before analyses are undertaken.

To construction engineers, “ soil” is all unconsolidated materials that can be “d u g”

rather than “ blasted.” T h eir soils are o u r sedim ents and regolith; it is helpful for an

archaeologist to k n o w this before tryin g to interpret engineering drilling logs that

may constitute the o n ly prelim inary glim pse o f subsurface sedim ents in urban areas.

262 S E D I M E N T S A N D S O I L S

To geologists, “ soil” is surficial sedim ent altered by w eathering, w hether buried 0r

not. This defin ition is closest to the archaeological usage, and Q uaternary geologists

share w ith archaeologists an enthusiasm for buried soils and their information

content (H olliday et al. 1993). H ow ever, geologists m ay include-archaeological

deposits in their concept o f “soil,” failin g to distinguish them because geologists are

trained to n otice and interpret p hen om en a at scales larger than those ty p ica l of"

archaeology (Stein and Linse 1993). A rch aeologists, w h o m ust com m unicate within •

three groups o f specialists, sh o u ld be alert to the need for clear understanding o f the~

language in use in p articu lar situations. • ;

A lth ough sedim ents o ccu r across the extent o f the g lobe itself, and soils cover th?^

. continents, both sedim ents an d soils are h igh ly variable at the m icro- and m e P S

scales. G eneralization is d ifficult, since archaeologists need to apply soils analyses.^

principally at the local and m icro-scale. T h e text that follow s here and in Chapter»^'

- constitutes the barest in tro d u ctio n to the co m p lex w orld o f surficial sediments and: "

soils. Further reading could w ell begin w ith Soils in archaeology (H olliday 1992) and=¿

Reconstructing Quaternary environm ents (Low e and W alker 1984: Ch. 3), progressing—

to chapters in specialized texts such as Soils and geomorphology ( Birkeland 1984). T h e lj

archaeological literature on soils analysis can be q uite un forgiving to neophytes, pre- 2

sum ing as it does a technical basic vocabulary. Ultim ately, jh e descriptive soil surveys^

published by m ost national governm en ts p rovid e the m ost detailed information that

is available outside o f the analytical laborato ry for th esite and local scales.

S T U D Y T E C H N I Q U E S I N S E D IM E N T O L O G Y

Sediments constitute the context in w hich archaeological m aterials are deposited

and retained; their identification and analysis can inform about the history o f the

materials and the site itself, abou t agents o f site burial, about the environment in

which hum an behaviors that defined the site took place, and about chemical and

physical conditions that determ ined the preservation o f remains. U n d e rsta n d in g of

these processes properly begins w ith an understanding o f sedim ent source and

history.

Interpretation o f sedim ents and soils requires an inform ed com bination o f field

and laboratory techniques. Field observation s are crucial; sedim entological consul­

tants w ho are sim ply given bags o f m aterials for laboratory analysis cannot be as fully

supportive o f archaeological investigations as their m ethods and expertise ideally

allow. Sediments and soils are three-dim ensional bodies, w hose horizontal extents

usually far exceed their vertical extent; they typically display variation in all direc­

tions. Variation necessarily raises issues o f sam pling adequacy that can only be

^ l^ lved by field investigations. T he discussion in this section is directed principally

Kjrtiiard the investigation o f sedim ents exclusive o f soils.

Sources o f sed im en ts

Ith respect to origins, regolith (m ineral aggregates) is either “ residual” or

¿rived .” Residual regolith (technically “saproiite” ) form ed in place by disintegra-

T -0n o f underlying bedrock, while derived regolith (sediment) has been deposited

¿after transport. W eathering products m ay be fragments o f rock, grains o f m ineral

-alter, chem ical solutions, o r all three. T hey are strongly determ ined by the mineral

“im p o s it io n o f the parent m aterial and by the clim atic conditions and w eathering

Shanism s that obtained during disintegration. For instance, granites subjected to

m es o f heat and cold in very d ry environm ents break up into their constituent

^s¿neral grains - quartz, feldspars, and micas - and undergo no further disruption.

3 m o is t clim ates, however, granites are reduced to grains o f quartz and m ica, while

- ^e feldspars w eather into clays. If these residues are subjected to water transport, the

ins o f m ica and to a lesser extent those o f quartz will be further reduced by a ttri­

tion; the clay w ill be carried o ff in suspension. The original three m inerals, after

nsport, m ay be deposited in very different locations under distinct hydrodynam ic

¡imes, because o f their distinct specific gravities. W hile the m inerals are all

igether, identifying them as the w eathering products o f a particular granite is fairly

raightforward; once transported and separated, they cannot be traced to their

íbrigins. Rocks o f m ore elaborate m ineral com position, particularly those containing

’ minerals less com m on than quartz and clay, may be recognized as the sources o f

mineral associations even after displacem ent over long distances.

• Residua! deposits form in place as bedrock is disaggregated by w eathering agents

(sun, water, ice, w ind, salts, acids) attacking the inter-crystal bonds. If the loose

’material is n ot m oved by w ind, water, or ice, plants will colonize it and soils w ill begin

toform. W ithout disturbance this process can continue, perhaps at slow in g rates, to

great depths. O rganic detritus form s sedim ents in place, norm ally settling from

water.

Derived sedim ents constitute the largest class o f regolith, since the most likely tale

of unconsolidated material is m ovem ent by wind, water, ice, and the force ol gravity.

Gravity, w hether or not aided by ice and water, draws loose matter from cliff faces to

form talus slopes; from cave roofs to floors; dow nhill to form colluvium ; and

tro u g h water to form subaqueous sedim ents in lakes and oceans. M ovin g ice carries

materials on, in, and beneath it and deposits them as till. M oving w ater carries sed i­

ments away from ice sheets and other sources; as water slows o r becom es otherw ise

B A S I C P R I N C I P L E S

S E D I M E N T S A N D S O I L S ■ m B A S I C P R I N C I P L E S 2 65

overloaded, it drops its load to fo rm any o f a series o f diverse fluvial deposits

currents m o v in g along the shores o f lakes an d ocean s sort and deposit coarse m a £ p i

als along beaches. Carbonate-rich w ater b u b b lin g up in springs or flowing t h f o i^ S

lim estone caves deposits calcareous “ tufa” or travertine that hardens into

m o vin g w in d lifts and carries sand and silt-sized particles, som etim es to great <j§pr

tances, depositin g them in dunes an d sand sheets o r in beds o f loess.. Wind?

disperses v o lca n ic ash after it has been exp loded in to the air. -

A rch aeolo g ical sites o ccu r in a ll these kinds o f deposit. Incorporation ofarchae¿S

'■ logical m aterials into residual deposits occurs In the course o f soil formation and chu rn ing o f sedim ents b y organism s and ice. T h e m echanism s o f incorporation

various derived sedim ents are research problem s in each case; the challenge is tof

learn w h ich o f a large but n ot infinite set o f agents a n d processes was involved.

K n ow in g th e source o f a sedim ent is a long step tow ard k n ow in g its history. As

granite exam ple above shows, tracing sedim ents to their sources is n ot a simpleiU

w ith a guarantee o f success. In cases where h igh ly characteristic or unique s u ite w ^

m inerals survive transportation and deposition, their identification in a sediment can

indicate the probable source or source area. It is self-evident that the source areaftÜt

any sedim ents w ill be found in the direction from w hich the transporting agent came

upstream for w ind, water, and ice. T his rule o f thum b can greatly sim plify the taskasL

long as n on -h um an agents are at issue. Identifying th e source o f ro ck fragments; which

by definition constitute a suite o f m inerals, is m uch m ore straightforward than is

sourcing disaggregated minerals. Petrographic m ethods such as thin-section micros­

copy, X -ray diffraction o f clay m inerals, neutron activation for trace elements, and the

set o f spectroscopic m ethods all give good results w hen properly chosen. Many of

. these techniques are used by archaeom etricians tracing the raw materials o f lithic arti­

facts and clays; the principles and m ethods are the sam e for rocks in natural deposits.

O rgan ic sedim ents com posed o f m acrofossils, su ch as peats, generally accumu­

late w here the p lants grew, or in depositional basins very close by; vegetative detritus

does not travel well. However, with the intervention o f hum an agents, such materials

(e.g., peat, lignite, and coal) m ay be transported for use over great distances. In such

cases, sourcing is possible only when the m aterial is clearly exotic and the original

plant association is obvious, as m ight be the case fo r m ontane forest plants carried

into a desert. Subaqueous sedim ents com posed o f m icrofossils such as diatomites

and foram iniferal oozes can be traced o n ly by iden tify in g the characteristics o f water

bodies in w hich they form ed, and in the case o f ancient rocks, the kn ow n ages of the

fossils. W hile such inform ation m ay o n occasion co n tribu te to a r c h a e o lo g ic a l inves­

tigations, especially in subm erged sites, it is m ainly lim n ologists and p a l e o c l i m a t o l o -

gists w ho need to kn ow about the tem perature and salinity o f w ater bodies. Such

_ ,

| l l 1 Velocity o f transporting agents and sizes of particles moved (selected)

^transport¡¡¡velocity (miles per hour) Sedimentary particles moved

3 transport

i velocity (meters per second)

clay

fine sand

gravel up to pea size

gravel up to thumb size

gravel up to size of hen’s egg

Diameter of grains suspended (m m )

0 . 0 4

0 . 0 8

0 . 41

0.81

WSource: H unt 1974:141.

ninations are m ade on the basis o f assum ed analogies with the m odern habi-

s o f organism s closely related to the fossil form s.

T ran sp ort m ed ia

Ifeter, w ind, and ice transport sedim entary m aterials m ore or less parallel to the

face w ithin characteristic ranges o f distance. These several media sort materials

size and shape, according to the energy in the system (Table 11.1). T he coarsest

Ipnaterials are m oved by high-energy systems such as glacial ice, fast-m oving water as

jiñriversin flo od and storm waves a lon g a beach, w ind in tornados and other cyclonic

¡ptorm s. The finest grades o f m aterials, silt and clay-sized particles, m ay be carried by

-||wind andw ater for great distances, from the centers o f continents to the deep oceans.

^ G ravity can be the agent o f vertical transportation through characteristically short

.¿distances in volvin g the settling o f particles in place, or falling from a c liff or cave roof.

■^Gravity acting alone does not sort particles, affecting all indiscrim inately as G alileo

I demonstrated at the Leaning Tower.

¡g:- Transport m edia are reflected prim arily in the texture (particle size and sorting)

|and secondarily in the structure (bedding) o f sedim ents, because o f the close rela-

jtionship betw een the energy in the system and its capacity for sorting the particles

266

carried. In high-energy systems, in addition to sorting there m ay also be sign ified

attrition o f particles during transport, further reducing their size. The m ill-l^

action o f glaciers, w hich can reduce rock to fine “flour,” is the extrem e example, but

given enough tim e m edia such as particle-loaded w ater and w in d cari w ork similar

transform ations. Unsorted and unstratified m ixtures o f coarse and fine particles may

be difficult to interpret in terms o f transport m edia. T h ey are classed as diamicton

unless there is some additional evidence indicating their origin as glacial till, land-

slides, o r m udflows - all involving a significant com ponent o f gravity. Although no

m em bers o f this confusing class o f sedim ents are good preservers o f archaeological

sites, enough claims have been m ade for sites in and under such deposits that archae­

ologists need to be alerted to their characteristics and the potential confusions they

brin g (e.g., Shlem on and Budinger [1990]; see selected references in D incauze [1984]).

A llu viu m , sedim ent m oved by and deposited from rivers, is characteristically

“graded,” which m eans that the sedim ents deposited are sorted according to the speed

o f the water. Slowing water drops coarse m aterials first. Coarse materials typically ini­

tiate a graded depositional sequence, the characteristic “ fining upw ard” signature of

overbank floodwaters with finer materials toward the top. Such graded sequences

m ay be repeatedly deposited as stratified sedim ents. A eolian deposits (sand particles

in dunes and silts in loesses) are usually well sorted by w ind speed (Pye 1987; Pye and

Tsoar 1990). D une sands may show the characteristic cross-bedded structure.

Both the topography and internal structure o f sedim entary bodies provide evi­

dence o f the transport media that deposited them. Term inal m oraines, river terraces,

and dunes norm ally testily by their scale and form to deposition from ice, water, and

w in d, respectively. T he sorting and grading that are typical o f variable speeds of

transport by w ind and water result in bedded (stratified) sedim ents, the individual

beds o f w hich may be internally com plex, thus p rovid ing a num ber o f analytical cri­

teria for discrim ination o f the transport m edium (Fig. 11.1). A lignm ent o f particles

w ithin a sedim ent body is also indicative, being best developed in w a t e r - l a i d materi*

als w here elongated particles tend to align w ith the axis o f m ovem ent. Magnetic,,

a lignm ents are best developed in particles settling from still water, as in deep lakes.

U nconsolidated sedim ents m oved dow nslop e under the force of gravity (colluvia-

tion, solifluction, gelifluction) are p o orly stratified, if at all, blit may be roughly

aligned.

D ep ositio n al en viron m en ts

I S E D I M E N T S A N D S O I L S

W hen sedim entary bodies enclose or bracket archaeological materials, i n f e r e n c e s -

m ade abou t the paleoenvironm ental con dition s at their deposition can be used to

FEATURES

Parallel bedding planes

Non-parallel bedding planes

Marker horizon (layer of ash)

TYPE

Horizontal

Massive

Cross

Cross-laminated

Graded

Horizontally laminated

Massive, with lens of gravel

Deformed (folding, faulting, loading)

Undulating bedding plane

4 cm 2 .

0

figure 11.1 Terminology for stratification and bedding structures. The cross-bedding struc­

ture forms as wind or water deposits sediments over ripples and down the farther

slopes, with the ripples moving downstream. (After Waters 199;: Fig. 2.12.)

understand the environm ents o f the m aterials o r sites. In this section, we address the

environments o f deposition, related essentially to sedim ents themselves. For archaeo­

logical interpretations, depositional environm ents are only part o f the story, since we

' also need to understand th e environments o f incorporation - how the archaeological

^materials cam e to be associated w ith the sedim ents and the environm ents o f burial.

Those issues are developed in Chapter 12.

M aterials m obilized by ice, water, or w ind are carried as long as the transporting

agent m aintains sufficient energy to m ove them . D eposition, therefore, represents a

change o f environm en t for both the m aterials and the transporting m edium . T his

change is recorded, m ore o r less clearly, in the structure o f the deposit itself, w hether

stratified, graded, sorted, o r not. Variation in physical characteristics w ithin sedi­

ment bodies, in e ither the vertical or horizontal dim ension, indicates variation in the

mimediate environm ents o f deposition. Vertical differences reveal the stratification

°f a sedim ent mass, expressing change in tim e. H orizontal differences in sedim ent

bodies (fa d es) reveal environm ental differences in space.

B A S I C P R I N C I P L E S 2 0 7

S E D I M E N T S A N D S O I L S B A S I C P R I N C I P L E S

on> W ; - ' l

- D epositional environm ents are interpreted prim arily fro m structural evi<j

secondarily from textural characteristics. Structural evidence includes both sedT

m en tary structures and landform s. Sedim entological concepts im portant here art

bedding, the vertical contrasts in sedim en t bodies that define stratificati ' ' ~

face, the conform able contact surface betw een two different beds; and unco:,JU

m ity, an erosional surface betw een tw o beds. Particle surface textures that inf0¿.

ab ou t transp ort agents are also relevant for depositional contexts, as are

qualities o f sedim ents such as sortin g a n d grading. T h e degree o f particle r

m easured as the frequency o f interstitial air spaces in a deposit, can also be infbntp;

tive abou t sedim ent history. - .. -^ ¿ a j É a

Sedim entary structures (bedding, lenses) incorporate evidence about the sca¿ •;

frequency, and rates o f depositional an d erosional events. T h e m ajor determinant of

sedim entary accum ulation rates everyw here is climate: the scale and seasonal d ü b jp f

bution o f precipitation and the frequen cy o f storm s determ ine the prevailing deposit’l l

tio n al agents, the stability o f sedim entary systems, and the frequency and amplitude' ?

o f interruptions. For these reasons, sedim en tary landform s are em ployed as climate 4 proxies. It is crucial to realize, however, th at the stability o f a landform o r sediment ^

b o d y is a fun ction not o n ly o f the strengths and frequency o f external perturbations

b u t also o f the internal state and con dition o f the system. T he internal characteristics "

con trol response rates and the energy available for response to stim uli. Even when s

pertu rb in g factors are similar, responses in one system (e.g., a fluvial basin) maybe-

different from those in other systems.

Subaerial deposits

Sedim ents deposited in glacial and periglacial.environm ents dom inate m ajor por­

tions o f the northern hem isphere, but their direct relevance to a r c h a e o lo g ic a l sites is

lim ited. O bviously, no archaeological sites are contem porary w ith the deposition of

till o r glaciofluvial (outwash) deposits. Sites associated w ith such deposits are centu­

ries o r m ore older or younger than the form ation s themselves, and by their very exis­

tence reflect less extrem e environm ents.

Periglacial deposits, on the other hand, m ay be intim ately involved w ith human

settlem ent and activities, n ow as in the p ast, but they have never been en viro n m en ts

con ducive to large concentrations o f hum an beings. M odern periglacial areas, where

large-scale freeze-thaw processes are studied , are m ainly far rem oved in time and

space from the ice age periglacial environm ents occupied by Paleolithic commu­

nities in the O ld and N ew W orlds and are likely to be significantly different in the

details o f their seasonality and tem perature ranges. Therefore, paleoclim atic and

p aleoenvironm ental studies based on periglacial characteristics in s e d im e n ts must

K.fiBidertaken w ith in form ed caution (C lark 1988; W ashburn 1980).- A ctive peri-

gal environm ents are characterized by gelifluction and subsurface disturbances

¡ted to perm afrost, b y w in d erosion and deposition, and by seasonal fluvial action

^typically overloaded, braided streams; all o f these have characteristic sedim ento

f s t r u c tures (C h o rle y et al. 1984; Siim m erfield 1991; W ashburn 1980). D ram atic

¿ndary sedim en tary structures such as ice and sand wedges, soil involutions, and

¡Auction lobes (Fig. 11.2) m ay be encountered b y archaeologists w orking far from

; Arctic. Interpretation o f such features requires the involvem ent o f experts w ho

^scrupulously evaluate their relevance for any associated archaeological rem ains

outside o f the A rctic, m ay postdate them by large spans o f time.

Strong periglacial w inds that lift the finer grades o f sedim ent from exposed

ih and till deposits carry silt-sized m aterials far dow nw ind from the periglacial

onm ents them selves, to deposit them as loess in the cool, usually dry, steppe

fd grassland environm ents o f continental interiors. Loesses are typically massive

isits w ith strongly vertical structure and w ith bedding rare or only subtly devel

ed. Because pollen is unreliably preserved in calcareous loesses, terrestrial gastro-

constitute the m ajor source o f incorporated evidence about clim ate and

;etation. B uried soils m ark depositional interruptions and provide evidence for

perature and precipitation cycles. Sequences o f buried soils in m id-continental

es have been show n to correlate well w ith the glaciaW nterglacial cycles

ntified in deep-sea sedim ent cores (Kukla and A n 1989).

f Temperate sedim entary environm ents, on the other hand, are dom inated by

ifluvial erosion and deposition by perennial streams (Table 11.2). Terrestrial fluvial

deposits are typically local in scale and therefore both highly sensitive to local co n d i­

tions and variable in tim e and space. Unless disturbed, abundant vegetation in tem-

|perate environm ents reduces the effectiveness o f erosional forces, m aintaining

©relatively stable sedim ent bodies. W hen surfaces o r slopes are destabilized, either by

"•’“denudation o f vegetation o r by w ater saturation, sedim ents m ay move quickly,

§ t forming colluvial deposits or alluvial fans, m aking their particles available for

* further fluvial transport. O therw ise, periodic deposition on floodplains is the sedi•S*£r :mentary environm ent most typical o f tem perate zones. A rchaeological sites oriented

toward rivers and stream s m aybe preserved within o r un derfloodplain deposits.

The com p lexity characteristic o f fluvial deposits challenges generalization.

Responses to changed conditions vary according to the state o f the system involved;

- they may be com p lex or sim ple, m assive o r very lim ited in area. A change in either

climate or base level can evoke cyclical erosional and depositional responses in a

¿i stream netw ork, varying in space and tim e (Chapter 9). Since archaeological scales o f

observation are essentially finer than geological scales, the com plexity o f fluvial

If

2/0 S E D I M E N T S A N D S O I L S

after thawing

stones sliding from domed centre to marginal furrow

Figure 11.2 Schematic of the formation and form o f selected periglacial sedimentological

features: ice wedges, sorted polygons, and involutions. The ice wedges contribute»

the polygonal cracking patterns that may result in polygons. In volutions sliding down-

slope complicate solifluction structures. Note that, to the unwary, each structure can

resemble archaeological features such as postholes, hearth circles, and stratigraphy. (AfterCatt 1988: Figs. 2.49,2.51, and2.52.)

systems c a n seriously mislead archaeological efforts at understanding e n v i r o n m e n t s

o f deposition and the triggers o f change.. E xtrapolation from a r c h a e o l o g i c a l - s c a l e

observations o f fluvial deposits to regional processes is not recom m ended.

Sediments in subtropical arid environm ents reflect the special processes and

landform s o f those areas. Precipitation is stron gly seasonal and episodic; vegetation

cover is consequently sparse. T h ese circum stances are con ducive to episodic floods

frost crack

J— or pocket downward inclined ice injection

B A S I C P R I N C I P L E S

j j 2 Flow words

•-following related words are based on -luv-, a root meaning “flowing,” derived from Latin fluere,

“%w ■phe prefixes change the meaning in obvious ways. The adjectival form is given for regularity.

luvial ~ fluvial: flowing in rivers, or pertaining to a river

S s luvial alluvial: of sediments deposited by rivers (flowed to)

luvial colluvial: o f sediments sliding down hill, usually lubricated by water (hill flow)

luvial eluvial: o f solutes and fines removed by water in suspension or solution. The A

r or E zone of a soil (flowed out)

luvial ¡lluvial: o f solutes and fines precipitated or deposited by groundwater in the B

zone o f a soil (flowed in)

luvial pluvial: o f abundant rain; a rainy season or period

large erosional and depositional events, as well as to large-scale aeolian activity

—- g and depositing sedim ents. A rid subtropical fluvial sedim ents are typically

"rly sorted, as in alluvial fans and gravel spreads on pediments and in arroyos

pter 9). Fine-grained sedim ents at the downstream limits o f fans and arroyos

'de m aterial to winds, that carry away sands, silts, and dust. W inds rem oving

and silts leave behind lag deposits form ing gravelly desert pavements,

emeral shallow bodies o f water dissolve salts and carbonates from the dust

sited in them and, on drying, leave m ineral-rich crusts on the surface o f playas

lake beds).

-Jn rainy tropical environm ents, dense vegetation stabilizes slopes and retards

sion o f sedim ents. In such climates, chem ical weathering dom inates and chem i-

erosion and eluviation o f the finest sedim entary particles are characteristic.'

nsequently, slopes and elevated surfaces are deeply mantled with residual regol-

Episodically, the loose material becom es unstable, usually when saturated, and

nps or slides dow nslope where it becom es accessible to m obilization by rivers,

•pical rivers carry m ainly clay-sized particles in suspension and colloids and ions

ÍP solution. Broad tloodplains are seasonally inundated and typically swam py

ifetween floods. A rchaeological sites in the tropics tend to be situated on the more ■tfable, raised landform s, but they are subject to burial under colluvium and dense,

r-rich alluvium .

% Deposits in caves and rockshelters vary with the specific con dition s and cli-

®tes o f their host landform s. Stringently localized, speleological deposits are

^vertheless an especially diverse group. T h e open fronts o f caves and c liff shelters

ive particulates from runn in g w ater that ponds inside and from w inds that are

kd there. A llu vial and aeolian sedim ents create a m ixed record o f very local

S E D I M E N T S A N D S OI L S

sequences o f ch an gin g depositional environm en ts related to external c o n d itio n

Caves and shelters fo rm by the loss o f rock m ass fro m cliffs an d roofs; their f]00r5

-accum ulate ro ck shatter (clasts) and finer p articles fro m those sources. Deposition o f shattered ro c k is episodic, p artly reflecting cyclic extrem es o f atmospheric tem-

* perature, o r m oisture, p artly respon din g to co n d itio n s on th e surface o f the rock m ass itself. F or a lon g tim e the degrees o f so rtin g and o f an g u larity o f rock waste in lim eston e caves w as interpreted as d irect p ro xies o f clim ates, especially of glacia l-in terglacia l cycles. T h e h isto ry o f such deposits is n o w reco gn ized as being

far m ore co m p lex (B u tzer 1981b; Straus 1990). L im eston e caves, the surficial areas o f u n d ergro u n d d rain age system s, are characterized b y carb o n ate deposits called speleothem s (stalactites, stalagm ites, an d travertine), w h ich c o n trib u te essentially to the special strangeness o f these in terior spaces as w ell as to the preservation of

diverse m aterials deposited on the floors and subsequ en tly sealed u n d er crystal--

lized crusts. Because th e y lie un dern eath a stretch o f surface, caves an d shelters

m ay also receive gro u n d w ater from cracks o r op en in gs in th eir roofs and back

w alls. W ater co m in g thus from outside m ay either erode or d ep o sit sedim ent on the floors o f enclosures.

Subaqueous deposits _______ -

, Lake and p o n d deposits constitute special classes o f o rgan ic sedim ents. T h eir impor­

tance in paleoenvironm ental reconstruction (see B erglund 1986) gives them a role

far in excess o f their frequen cy on the face o f the Earth. B eing subaqueous, lake and

p o n d sedim ents do not form soils and do not support archaeological sites, although

they m ay incorporate archaeological materials. T heir im portan ce to archaeologists

lies in their centrality for paleoenvironm ental and palepclim atic reconstructions uti­

lizing the organic particles such as pollen, diatom s, and m acrofossils included in

them. Ponds and lakes o f tem perate zones collect heavily biogenic sedim ents in

typical postglacial sequences running upw ard from abiotic clay to gy ttja (a lg a l-r ic h

organ ic detrital m ud) and/or m ud, and finally peat, as the w a te rb o d y is filled b y sed­

im ents and plants. In basins so deep that they lack oxygen at the base o f the water

co lum n , and thus do n ot support life in their depths, sedim ents m ay s h o w strongly

seasonal variation in texture o r color, fo rm in g sequences o f annual deposits (v a rv e s

or rhythm ites) that m ay b e counted like tree rings (C hapter 5). Large deep lakes in

tectonically active areas are p rized for their thick, stratified, clim atically informative

sedim ent bodies. Shallow saline lakes in a r id lands form less diverse paleoclim atic

records, and have less archaeological relevance.

N ear-shore m arine sedim ents are o f interest to archaeologists as the locations o f

inundated terrestrial landscapes o r as platform s for shipwrecks. T h e y m erit special

B A S I C P S ! N r i i ' L E S

e rtise , but the principles fo r their study are novel only in scale. T h e floors o f

jjg o o n s, estuaries, and coastal ponds are m ixtures o f alluvial, organic, and m arine

Isjirrent deposits, reflecting the energy levels o f the local aquatic systems. O ffshore,

Sjnundated terrestrial and coastal landform s m ay com plicate matters at the landward

Idge o f continental shelves. Postglacial m arine transgression oyer the shelves typi-

«jfy planed o ff preexisting sedim entary landform s and deposited a broad sand sheet

¿ th e surf zon e m oved landw ard (Belknap and Kraft 1981).

Organic inclusions -w¿Plant and anim al rem ains included in sedim ents are am ong the richest evidence for

^positional environm en ts p rovid ed they are interpreted w ith care and inform ed

im a g in a t io n . Inclusions can range in size from the bones o f whales or m am m oths to

grains o f pollen. N ot all organ ic m aterials relate to depositional environm ents. T h e

bulk o f the in clusions m ay derive fro m anim als and plants living in or on the sed i­

ments subsequent to deposition (rem nant), w hile others may be elem ents o f com -

^ munities that were carried along w ith the sedim ents, finally com ing to rest far from

their native habitats (redeposited). T h e distinction is, o f course, crucial to the inter-

•s pretation o f environm ents relevant to any archaeological materials involved, w hich

^ m u st them selves be evaluated for their status as rem nant or redeposited m aterials.

2 Remnant (au to ch th o n o u s) fossils are problem atic as to their time o f introduction

into a body o f sedim ent, belon gin g n orm ally to tim es follow ing the subaerial depo-

sitiona) event itself. T heir environm en tal signals m ust be evaluated for their ch ron ­

ological relationships to the depositional event and to the archaeological events

under investigation. N aturally redeposited (allochthon ous) m aterials belon g to

earlier tim es and distant space in relation to any deposit that contains them . As e le­

ments o f sedim entary history, they represent environm ental con dition s at their

source. T h e y may, consequently, either com plem ent or contradict the au to ch th o n ­

ous evidence. H ow m uch tim e and space separates them from the deposit itself is to

be determ ined in each case; it is never irrelevant to interpretation o f the deposit.

O rganic m aterials introduced to a deposit by people may be exceptions to both these

relational rules.Bogs, fens, m arshes, and swam ps, as depositional environm ents, are interm ediate

between subaerial and subaqueous environm ents - more am biguous even than

fluvial deposits. A globally- useful typ o lo gy o f wetlands is provided by Retallack

(1990:213-214). W etlands attract people, not as com fortable places to be, but because

o f their rich b iotic resources. A rchaeological sites were rarely form ed o n such su r­

faces; m ore often they are underneath them, having been incorporated by wetlands

expanding beyond their m argins. O ccasionally sites were built above them , as in the

2 7 4 S E D I M E N T S A N D S O I L S B A S I C P R I N C I P L E S

cases o f trackways, refuge villages, and ritual sites, fam ous for their degree o f 0

preservation. Because the sedim ents enclosing such m aterials are composed*35*"

dom inantly o f vegetation, th ey are discussed in Part V I.

Field m é th o d s ' ' «

O bservation o f sedim ents in the field depends u p o n visu al access, w h ich is aciuevW-'

b y seeking o u t n atural exposures o f subsurface m aterials (scarps, eroded surfacgp l

b y creating exposures (usually vertical) excavated fo r the purp ose, or by u ^ g li

special equipm ent to pull cores fro m beneath a surface that is n o t m ore direclfrv

accessible. For geological a n d archaeological investigations, the vertical exposur e ^

“section” o r “profile,” is preferred sin ce it m in im izes d isto rtio n and provides good tw o-dim ensional access. T h e larger the exposure, the better the" investigatory

observe spatial variation in the sedim en t body. .. ..

Particles that com prise a sedim ent m ay vary in com p osition , size, shape.orient;

tion, sorting, grading, p acking, and cem entation. In com bin ation , these characte

tics create the texture and structu re o f the sedim ent. Texture is defined by

com bined attributes o f p article size, shape, and sorting, w hich together determiné

w hether a sedim ent is fine o r coarse grained, hom o gen eo u s or heterogeneous

Structure, on the other hand, is the result o f the m anner o f transport and deposition _

o f the particles constituting a sedim ent and its stratification, and m ay also reflect 1

subsequent transform ation processes. Materials deposited from w ind, moving^

water, still water, ice, o r other agents v a ry in characteristic arrangem ents o f partides 4

w ithin the sedim ent in their orientation , packing, and size grading. Postdepositional 3

disturbance o f sedim ents by ice, plants, or anim als w ill be recorded in structural 5?

attributes. Structural and stratigraphic observations in the field a re essential precon- 7

d itions to adequate sam pling, since sam ples must be obtained from each discrete stratigraphic unit. 4

D escription ofsedim ents in thefield must be undertaken w ith a w a re n e s s that sed­

im ents are both m atrix and context. T herefore, description o f observed materials

and structures should be accom panied by active question ing and hypothesis formu-

lation, to collect data for alternative interpretations. O n ce the num ber and boundar­

ies o f lithostratigraphic units in a study section have been defined, each unit m aybe

sam pled. Sam pling locations should be selected to represent the full d i v e r s i t y o f the

sedim ents, and each sam ple should norm ally include material from o n ly one unit. If

the full diversity cannot be sam pled alon g a single transect, additional s a m p le

locations should be selected as com plem ents (Fig. 5.2). Sedim entary samples from j

exposures m ay be taken in bags, tubes, o r sam pling boxes; the choice o f container

É isample sizes depends on the research questions to be addressed and therefore the

SCjs o f analyses that are anticipated (C att and W eir 1976; C o u rty et al. 1989; Stein

S a m p l i n g o f inaccessible sedim ents by coring presents a special range o f tech-

K jp ro b lem s and challenges. Sedim ent sam pling and analysis is distinct from sam -

|g fo r particular kinds o f archaeological data; it should be pursued by lithological

Üiods (e.g., A a b y and D igérfeldt 1986; Gale and Hoare 1991; Reineck and Singh

^ Stein 1987)-

SÉ'§L Lithostratigraphy -- ~

stratigraphic section, interruptions in texture, structure, or m in eralogy o f the

„ je n t colum n are the criteria for defining sedim entary bedding units,

mparisons w ith sedim entary sequences beyond the im m ediate lo cality are

*d by reference to a form ally defined hierarchy o f lithostratigraphic units,

^stratigraphic units (“ form ations,” “members,” and “beds" in"declining order o f

j) are defined and described at type localities that are standards for com parison

¿formal nam ing. T h e local scale o f archaeological sites places them m ost directly

¿mem ber o r bed o f a regional lithostratigráphical sequence. Deposits at archaeo-

jical sites o f H olocene age are rarely classified into formal lithostratigraphic units,

being m erely im plied by the surficial deposits involved (e.g., alluvium , loess),

¡¡^assignment o f older sites to their correct lithostratigraphic m em ber o r form a-

can be a difficult endeavor but rewarding for chronological clarity (e.g., the

tolil case study). Hence, archaeologists should be aware o f the form alities o f

üostratigraphic classification. Although not needed everyday, the rules o f strati

pphic n om enclature and classification are im portant for com parison and correla-

>n of deposits at regional and larger scales. T he intricacies o f the form al system

ire well defined and accessible to non-experts in sources such as Ager (1993), Catt

.1986: Ch. 4), Farrand (1984), Stein (1987), and Waters (1991: 60-88). T h e official

iternational standards and nom enclature are set out in the periodically revised

'orts o f the N orth A m erican Com m ission on Stratigraphic N om enclature

1983).i p f Lithostratigraphic analysis and interpretation require scrupulous distinction

Wjietween description and interpretation o f observed phenom ena (Stein 1990; see also

¡ lin k e r !993J. In the field, description should be as precise and interpretation-neutral

¡IP p o ssib le. Such care will keep the description useful as interpretive hypotheses are

Subsequently tested against analytical data. W hen interpretation is kept separate

Jrom description, m ultiple analytical techniques and their different kinds o f data can

j|ftore effectively be brought to bear on problem s. O nly the sim plest o f historical

‘estions is likely to be answered definitively by a single analytical technique. T h e

¿ 7 6 S E D I M E N T S A N D S O I L S

distinction betw een field description and interpretation is clearly demonstrated in

Figure 11.3, a d iagram from a com plex English site o f M id d le P le isto cen e age.

L a b o ra to ry m e th o d s fo r d eterm in in g co m p o sit io n a n d s tru ctu re

T here is n o ideal o r required list o f laborato ry analyses that ca n be me.

invoked to satisfy the dem ands o f a scientific approach. T h e m e th o d s select®

the results obtain ed should have direct relevance for research qu estio n s, es]

those that arise du rin g field exam ination o f sedim ents. N o analysis ca n b e any

than the q ú ality and appropriateness o f the sam ples available an d th e research qu élp

tions asked. A rchaeologists m ust, therefore, ensure that there is e ffective communi-

cation and coordin ation betw een personnel in the field and in th e. laboral

the best situation is for sedim entological consultants to be in vo lved in b oth p.

(Rapp 1975).

T h e physical and chem ical properties o f sedim ents and p artic les are info]

abou t the sources and transport m edia o f particles and the depositional.envir

m ents and subsequent histories o f the sedim ents. T h e m eth o d s c ited here are;

illustrative purp oses only; th ey are definitely n ot exhaustive. A n y o n e planning to Z 5."

undertake or to utilize such analyses should obtain the n ecessary inform ation-

instruction from qualified practitioners. There is a w ide range o f ch o ice ami

m ethods, even for a p articular class o f in form ation; the choice w ill va ry w ith

^inform ation required, the nature o f the sam ple, the resolution desired , and the7"

equipm ent available (H olliday and Stein 1989). A pplications o f these m eth ods will

referenced in discussions that fo llow in this and later chaptérs. F uller discussiorLof

these m ethods and their lim itations, and references to the p rim a ry literature, maybe

foun d in G ale and H oare (1991). ..... .

Sedim en tary particles and cem enting m inerals can be identified by a variety of

chem ical and m ineralogical techniques, starting w ith sim ple chem ical tests for a key

elem ent. T h e choice am ong m ineralogical techniques, such as exam in atio n o f thin

sections un der polarized light, atom ic absorption spectrom etry, e lectro n micro-

probe, X -ray fluorescence, and X -ray diffraction, w ill d epen d on w hether heavy

m inerals o r clay m inerals aré involved, and w hether in form ation on m in eral concen­

trations is needed. Analysis o f the organic content o f sedim ents is trad ition ally done . g . ^

by com bustion and rew eighing, but w et m ethods m ay be better cho ices in some

instances (e.g., Low e and W alker 1984: 93; Waters 1992: Ch. 2). Q u an titative descrip­

tion o f sedim ents in terms o f particle size (gran u lom etry), im p o rtan t fo r a variety of

interpretive issues, is done by dry o r wet sieving, hydrom eter, pipette, sedim entation

colum n (G ale and H oare 1991; G oudie 1981: Pt. 3), o r one o f the n ew er electronic

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¡ s ° iS o ti°n° •cV.oV °° =0 M

0 0/ « 0 O •> O '- ' 15

°°1 ¿ S o f o c o ' . ?

s

o o t e c

Figu

re

11.3

Sche

mat

ic

sect

ion

thro

ugh

depo

sits

at

Hig

h Lo

dge,

al

. 19

92:

Fig.

3.1,

with

pe

rmis

sion

; or

igin

al c

apti

on,

© th

e Tr

uste

es

of

the

with

th

e fie

ld

desc

ript

ion

, lit

hos

trat

igra

phic

cl

assi

fica

tion

, Br

itish

M

useu

m,

Briti

sh

Mus

eum

P

ress

.)

and

inte

rpre

tati

on

of

the

unit

s.

(Rep

rodu

ced

from

Ash

ton

et »

Table 11.'3 Particle size classes: the Wentworth scale and phi (<f>) units

2 /8 S E D I M E N T S A N D S O I L S

Wentworth scale phi (<f>) units - mm equivalents ~~

Boulder

Cobble

Pebble

Granule

Very coarse sand

Coarse sand

Medium sand

Fine sand

Very fine sand

Coarse silt

Medium silt

Fine silt

Very fine silt

Clay

Colloid

Atore:The Wentworth scale is quantified in mm units (3rd column); the categories granule to cobble are ■ ■ i-subdivisions o f gravel. The phi scale is logarithmic, based on 0 = I mm. The conversion is —logjof the -_-diameter in mm; the advantage i-S that the scale is then based on whole numbers. Negative numbers repre­sent particle classes larger than coarse sand; positive num bers represent the progressively finer categories. Sources: Adapted from Lincoln fetal. 1982, Lowe and Walker 1984, and Waters 1992.

sensing devices em ploying laser beam s. Particle size definition can be accomplished

by direct m easurem ent or by reticules used w ith m icroscopes, except for the small

size ranges (Table u.3).

T he analysis o f particle shape is m ore problem atic than that o f size, since methods

for rapid and Objective determ ination o f shape classes are still being developed. In

cases where particle shape is a critical elem ent for interpretation o f sediments

(uncom m on in archaeology), it is best to consult som eone w orking actively on this

subject. T he surface textures o f sedim entary particles themselves contain useful

inform ation about the transport and depositional environm ents o f particles. Viewed

under high m agnification in a scanning electron m icroscope (SHiVl), quartz grains,

especially, display a wide range o f surface textures and m ic ro to p o g ra p h y developed in

different environm ents (Krinsley and D oornkam p 1973). W hile the interpretation of

these form s is less than direct and o bvio u s (Brow n 1973), th ere are o ccasion s when this

m ethod in the hands o f an experienced researcher can distinguish between otherwise

am biguous possibilities: e.g., the d istinction between beach and dune sands.

— 8.0 and larger— 6 .Qto — 8,0

- 2.0 t o - 6.0 - 1.0 t o - 2.0

0.0 t o - 1.01.0 to 0.02 .0 to 1.03.0 to 2.04.0 to 3.05.0 to 4.06.0 to 5.07.0 to 6.08.0 to 7.08.0 to 12 .0

■ >256.0,

64.0 tó 256.0 5 3 |4.0 to 64.02.0 to 4.0

1.0 to 2.0

.0.5 to 1.0

0.25 to 0.5 ,¡¿2

0.125 to 0.25 0.0625 to 0 .175; g g 0.0312 toO.Q625j30.0156 to O.OiUzr 0.0078 to0.015í¿ 0.0039 to 0.007g ~ 0.00024 to 0.0039

B A S I C P R I N C I P L E S

samples o f sedim ents retain the fragile three-dim ensional relationships

-g the m ineral, air, and w ater contents o f a sediment. M easurements o f mass,

■ 'ty, porosity, and m oisture content, quickly and reliably achieved, m ay refine

O p tio n and interpretation o f the sedim ent. Analyses o f the “fabric” o f a sedi-

®tinvolve m easurem ents o f the preferred orientation and dip o f m ajor particle

and o f the p ro p o rtio n and distribution o f air spaces (“packing” ) a m o n g the

iles. O rientation studies, tradition ally im portant in the analysis o f glacial

~~ts, are also relevant to the study o f alluvial material, in w hich context these

-es m ay have archaeological applications. W idely applicable in sedim entology

underutilized in archaeology, is X -radiography (Butler 1992). Radiographs

tte even sm all structures in aqueous and aeolian deposits, such as ripples that

te the direction and velo city o f the transporting water or w ind, as well as a

ro f inclusions. -Thin sections o f im pregnated sediments studied under

ication reveal fine details o f structure and content not otherw ise visible

“ Jlock et al. 1985; C att 1986:180-181; C o u rty et al. 1989).

P E D O G E N E S I S A N D D I A G E N E S I S

. Soil is never truly in equilibrium with its environment although we often assume an

"equilibrium state in order to develop an understanding of processes.w i l d 1 9 9 3 : 9 0

’ehave seen that rocks at the ju n ction o f geosphere and atmosphere are m odified by

.cathering. Sim ilar chem ical and physical changes m odify sediments after deposi-

”;n, during periods o f relative stability. G roundw ater acidified by carbon dioxide

dissolves and redeposits salts and oxides to begin the lo n g process o f diagenesis -

toning sedim ents into rock. Physical churning and chem ical changes induced by

‘ lánt and anim al life on and in deposits, aided by m oisture and tem perature changes,

‘ gin almost im m ediately to form soils, a process called pedogenesis. Hum ans, also,

lltevebeen active agents in soil form ation and degradation for a very long tim e. T he

iJrSverriding difference between diagenesis and pedogenesis is that the latter process is

dependent upon living organism s; organic m atter and biological activity are essen­

tial to the transform ation o f m ineral deposits into soil.

N atural tra n sfo rm a tio n near the surface

edogenesis is a continuous, reversible, and interactive process. It works progressively

om the surface dow n into underlying sediments. O rganic matter collecting on the

rface releases acid com pounds that, carried dow n in water, begin the chem ical

-o cn~§ O CO

oM V3E SQ_ C/3

o !h= 10 2 ó Q_ GO

< co

£ ¡d j= o O c o

ai mto g Q 0 0

S E D I M E N T S A N D S O IL S

transform ation o f m ineral m atter through differential leaching and de

Classic definitions o f pedogenesis recognize five “soil-form ing factors” (J en n yi^ ,^ Í

• clim ate

• biota

• topograph y ' >

• parent m aterial

• tim e.

W ith in each factor m any different processes react w ith sedim ents to produeesojjpl

that va ry stron gly but system atically in space and tim e (Brady 1990; Catt i

Johnson et al. 1990). Because the five factors cann ot b e quantified, áre n o t inde

dent, m ay va ry through tim e, and operate at different scales, soils scientists^

rather em barrassed b y them (Birkeland 1984:162-168). However, they comprist-p!

useful m n em on ic for the m ajor con tributors to pedogenesis and the environmental

constituents that m ay be studied throu gh soils.

T h e clim ate o f any particular place is a p ro d u ct in itially o f the m oisture and tem­

perature ranges defined by atm ospheric circulation interacting w ith the elevation*:

slope, and aspect o f landform s (Fig. 11.4). Extrem e clim atic com ponents interrupt or_

delay pedogenesis. For exam ple, w et sedim ents slide dow nslope, interrupting pedo­

genesis. Sedim ents below the w ater table resist the norm al oxidizing reactions that

typ ify active soils. Perm anently frozen sedim ents do not form soils, although they

develop characteristic structural features that record environm ental conditions

(W ashburn 1980). Very arid climates delay soil developm ent and produce typical „

desert soil characteristics, such as subsurface carbonate concentrations. In temper­

ate clim ates, on level to m oderately slopin g groun d, vegetation strongly influences

both tem perature and m oisture at the gro u n d surface. H igh temperatures in the

tropics accelerate oxidation and leaching, causing soils to m ature rapidly.

Biota in life and death con tribute the essential organ ic m atter from w hich pedo- ■

genic chem icals are derived. T h e physical ch u rn in g o f sedim ents and soils caused

by anim als an d plants is collectively term ed b io tu rb a tio n (disturbance by living

thin gs). “ T h e soil is clicking, turnin g, an d ch an gin g w ith the energies o f a fantastic

variety o f o ccu p an ts” (J. Hay 1981:38). Bacteria, and anim als on a scale from minute

m ollusks throu gh ants, earthw orm s, and larvae to insectivores, rodents, and lago-

m orphs (rabbits, hares), live w ith in o r actu ally derive their food from the sofl-

Som e p rey upon each other, entirely un d ergro u n d . In th e course o f th eir daily rou­

tines, som e o f these anim als d isplace o r digest sign ifican t masses o f sedim ent, som e

o f w hich is then redeposited on the surface. Plants, w h ich w e tend to think o f a5

static creatures, d isru pt the soil that su p p o rts and nourishes them as they expandm

Figu

re

,M D

iagr

amm

atic

tr

anse

ct

thro

ugh

Nor

th

Am

eric

a fro

m so

uthw

est

to n

orth

east

, sh

owin

g pe

doge

nesi

s va

ryin

g wi

th

vege

tati

on

and

dim

ate.

(A

fter

H

unt

1974

; F«

g- 6

6.)

282 S E D I M E N T S A N D S O I L S B A S I C P R I N C I P L E S 283'

size and increase in num bers, p ry in g ap art the sedim en ts to m ake space for their

roots! W h en roots die and rot, sedim ent drops into the h o llo w spaces left. Trees,

w h en m atu re .or aged, m ay be topp led b y storm s o r their o w n failing strength; root

m asses tear o u t sedim ent w h ich then grad u ally falls back, destroying the origina]

structure. ' ___ -_ j s

T opography’s influence on soil form ation derives from the effects o f slope, relief

élevation, and aspect. A n y slope at all perm its m ovem ent o f unconsolidated materi- ’

als to low er elevations. T he steeper the slope, the th in ner is soil likely to be because of

the shorter residence tim e o f particles in any one place. Elevation and aspect —

influence m icroclim ate, an d thus the bio logical activ ity in sedim ents. Even adjacent slopes on the sam e parent m aterial w ill sup port soil differences according to the' direction faced and solar energy received. --£££

“ Parent m aterial” is sim ply the orig inal deposit o f sedim ent, w hich determines:::

som e o f the soil’s potential fo r developm ent in term s o f chem istry, grain sizes, com­

paction o r perm eability, and o th er characteristics that set param eters for interac­

tions w ith groundw ater and air. O ver tim e, so il-form in g processes w ill m odify all

such characteristics b y the cum ulative effects o f chem ical action and physical churn­

ing (W ild ing et al. 1983).

D espite the diversity o f factors and com ponents, som e basic processes character- 4 ize all soil developm ent. Rainwater is acidified by carbonic acid and hum ic acid, the

latter from the decom position o f organic matter. A cidic water leaches salts, oxides,

and clays from the upper range o f sedim ent, the elu vial zone, and carries the solutes

and fine particles deeper into the sedim ent colum n where they may be deposited in

the ¡llu vial zon e (Table 11.2). T h e subtraction o f fine m aterials from the top and their

deposition below create soil ho rizo n s w ith in the sedim ent body: contrastive zones

parallel to the surface. In vertical section, such bands constitute a soil profile (Table

11.4; Fig. 11.5). T h e color, texture, and properties o f successive zones are changed by

the pedogenic processes acting in place, progressively deeper w ith time.

Soil horizons are form ed by change in situ, subsequent to the deposition o f sedi­

m ent. T h ey are not lithostratigraphic units, not layers, beds, or strata. In fact, the j

co lor and textural changes o f horizonation eventually destroy original stratification

by hom ogenization and chem ical m odification . T herefore, soil h o r i z o n s are not strat-

igraphic markers. A lthough they are occasionally so used in archaeological fie ld

work, such usage seriously distorts the con cept o f stratification and m ay p r e c lu d e

any clear understanding o f depositional events at a site.

All the soil factors play out their roles, h arm o n io u sly o r com petitively, as long as a

sedim ent b o d y remains in place. T h e relative influence o f any factor varies with con­

ditions, and the others adjust in turn. Soils are ecosystems; pedogenesis is nothing if

Jable 11 -4 Soil horizon nomenclature0

I f {aster horizons recorded in field studies

| p horizon

(.horizon

Organic material accumulated on the surface. :Humified organic m atter mixed with mineral substrate near surface; typically

dark-colored.Light-colored mineral horizon from which oxides, days, and organic matter have

been chemically leached (eluviationzone).Mineral horizon underlying O, A, or E horizon with little evidence of original

sediment structure. May be zone of accumulation (illuviation) o f sesquioxides, carbonates, and/or clays. Typically red in color.

Subsurface horizon impregnated with carbonate so that carbonate dominates the

structure. Typical o f soils in arid climates.Parent material of the soil, only minimally transformed by pedogenesis, underlying

A and B horizons. May show some evidence of weathering.

Hard bedrock.

Note'" Soils nomenclature, even for horizons, is more detailed than this, and varies geographically. Source: Adapted and simplified from Birkeland (1984:7), which see for details.

fnot dynam ic. O ver tim e, barring interference, a soil develops from rawness to a

pm aturity,that supports a richly diverse biota w ithin and above the soil.

Soils are considered im m ature or mature, according to the extent to which they

'i$L inhibit or perm it the full clim atic potential o f vegetation. However, m aturity is not

Éxtasis; soils con tin ue to change until they are destroyed. Pedogenesis is reversible,

«¿¡¿usually by burial or clim ate change, but soils can also be exhausted by vegetation

;J demands. Eventually, the natural perm eability o f any soil is com prom ised by

I# increasing clay concentration, com paction at depth, or developm ent o f carbon -

aceous or m ineral hardpans, depending 011 the type o f soil. The am ount o f tim e

required for such degradation varies w ith everything that influences pedogenesis,

and so cannot be predicted closely ( Johnson et al. 1990). Birkeland (1984: 204-220)

suggests that an organic A horizon develops to a steady state within a century or so,

- whereas a B horizon takes thousands o f years. Local factors will override these gener­

alities, and it is w orth noting that soils utilized for farm ing or grazing do not have

natural histories. O ld unburied soils, called relict soils, continue to evolve and

g,- change as long as they rem ain at the surface; their histories are partly recorded in

I" their chem ical and physical com positions.

The B horizons o f soils are the m ost dynam ic areas in profiles. Soils taxonom ies

I offer a large set o f labels for different kinds o f B horizons (e.g., H olliday 1990), the

S E D I M E N T S A N D S O I L S

Typical acid profile of humid region

: Soluble constituentslost

Figure 11.5 Soil horizons under tw o different clim atic regimes. (After H unt 1974: Fig. 6.4.)

m ost used o f w hich are Bt for clay accum u lations and Bk for calcareous concentra­

tions. T h e form er are typical o f soils in m oist environm ents; the latter, in arid cli­

mates. Both clays and carbonates accum u late in soils from two sources: chemical

changes within sedim ents du e to w eathering, o r the addition o f fine particles from

airborne dust. In both cases, the fine particles accum ulate in the B zone where they

m ay ultim ately dom inate com p lete ly over o rig in al constituents. Soils w ith very high

clay contents, from either orig inal deposition o r illuviation, shrink w hen drying and

swell on w etting (Vertisols). T h e deep cracks that can form when such materials dry

out cause slum ping and the in tro d u ctio n o f surficial m aterials at depths. The intri-

B A S I C ' P R I N C I P L E S

3 eking o f sun-dried m udflats is a fam iliar small-scale example o f such activity.

i>nate concentrations in soils range from fine thread-like features to m assive,

ot-like secondary deposits called caliche. Discussions o f B horizons particu-

§|§!ijseful for archaeologists are foun d in the volum e by Retallack (1990:264-276).

[ Cultural tra n sfo rm a tio n s o f soils

1 activities strongly influence soil developm ent by disturbing superficial sed i -

sa n d by changing their chem ical com position. Humans are m ajor bioturbators

sand agents o f deposition. T heir dom inance today relates to sheer num bers as

s to the efficacy o f their tools for digging, transporting, and depositing (Schiffer

). Plow zones are such ubiquitous phenom ena that they have a soil horizon des-

iion o f their ow n (Ap). In the U S soil taxonom y, surficial horizons chem ically

bed by hum an wastes and fo od debris have been given a special taxon:

iropic epipedon. E pipedon is the technical nam e for horizons near the surface

¡A ,E ,B ); the m eaning o f “ anthropic” should be self-evident. Archaeologists speak

ganthrosols,” but this term has n ot established itself in soil science. It is on ly a

iter o f tim e before it m ust be recognized as another class o f azonal soils (C hapter

lA nthrosols are characterized by physical disturbance, high organic content, and

Sposphate enrichm ent typically due to high concentrations o f animal wastes (Eidt

5). Exotic sedim entary particles (e.g., artifacts) are diagnostic o f anthrosols only

g|icompany w ith chem ical and structural changes.

^Analysis o f ancient agricultural soils is a developing m ethodology. Chem ical

|||nialyses and m icrom orphological (thin-section) m ethods have been m ost success-

lin identifying the changes in soil chem istry and structure attendant upon long-

i p low ing, fertilization, and irrigation (A rtzy and Hillel 1988; G roenm an-van

pfeateringe and Robinson 1988; Sandor 1992). Less attention has been paid to hoe

;horticulture and agriculture w ithout anim al wastes as those affect soils, but interest

rjand a literature are grow ing (e.g., Denevan et al. 1987). Paleolim nology also has pro-

¿vided insights into the scale o f soils transform ation and disruption due to hand cult i

l;#ation (Binford et al. 1987; Deevey et al. 1979; O ’ Hara et al. 1993).

T ran sfo rm atio n s at depth

tí In the C horizon , below the surface levels at w hich soils are forming, sedim ents m ay

be com pacted, cem ented, churned, and chem ically changed by processes involving

¡¿acids and ions carried in groundwater. Such subsurface processes collectively are

I termed diagenesis, and although they resemble som e o f the processes that form soils,

S E D I M E N T S A N D S O I L S B A S I C P R I N C I P L E S

their distinctiveness is im portan t to archaeologists because th ey indicate diff¡ P :'aspects o f subsurface environm ents in w hich archaeological r • * * * ^ ™ dÍSCUSSÍOn- A s with the Phi' scale o f Particle size

B ecause m a n y diagenetic processes take place at the water table o r b T ™ * * » l rab,e n '3)’ p H “ 3 “ S3* ™ IoSan th m such that the h iSher the hydrogen concentra-

be usefta] indicators o f the w ater table’s dynam ics O x id e , a T ' 0313 * e “ " ° n 3 fr° m ° *° M’ 7 ¡S neU trá1, num bersgro u n d w ater tend to be deposited at the water table Iron n Carried j j g g g g & in geasingly acidic, and higher num bers are increasingly alkaline. In highly acid

sin gly o r in sets, as w avy red lines that superficially resemble m3Cr° foSSÍ!S are ^ de^ ° Y ed> however> the inhibition o f oxygen-

gen u in e soil horizons, but are very different from both. This red c d o r n S S

con fused w ith burnin g; Hydrated m anganese and iron co m p o u n d s m ay

tinctive gray colors to sedim ents at depth, creating m o ttlin g o r the m o ^ p e ^

g ty in g that can p uzzle the un inform ed b y suggestive resem blance to buried soil

xides deposited from groundw ater have been involved in m an y disputed a r c h á É

logical interpretations. M assive accum ulations o f oxides an d c a r b o n a ^ f

“hardpans,” cem ented rock-like layers that have no h isto ry related“ to~sur

exposure. - • —

bacteria in such soils results in go o d preservation conditions for pollen grains.

,3s with high pH are p o o r environm ents for the preservation o f pollen, but are

'ent for preserving bone and shell. Consequently, carbonate crusts in archaeo-

gjcal sedim ents are w orth a close look; they m ay contain, im portant organic

terials..Water-saturated soils effectively exclude oxygen and thus preserve from decom -

_ :rs otherw ise fragile organic matter, including artifact classes rare in other envi-

gnments (C oles 1984). D am p or wet soils have another property significant for

archaeological m aterials: they support the transfer o f ions between buried organic

Clays leached from surficial zones o f soils and redeposited at depth resemble hñitS i É f l S ^ tprials and their soils matrices. The resultant chemical changes, tending

o f stratification. N odules o f clay or oxides resem bling small stones m ay fc'mfwiffimr "i i i lfeíñward hom ogenization, com plicate chem ical analyses o f archaeological materials

sedim ents at depths determ ined by groundw ater; their presence m ust n ot be i n t ^ S ^ ^ L m b e r t et al. 1984; W hite and Hannus 1983). Chem ical analyses can be useful com -

preted as a stony layer indicative o f stratification. The gradual settling and com^zi^^^^^^enients to other environm ental study o f soils form ation and history, helping to test

paction under pressure o f overburden experienced by water-saturated"sedinien t ^ ^ ^ ^ B lternative interpretations (M cBride 1994).

such as pond m uds and peat deposits is an aspect o f diagenesis w ith special implica­

tions fo r any included archaeological materials.. Pedogenesis a n d the arch aeolog ical record

Soils form ed orig inally at the surface m ay be later buried. A fter burial these paleo- .v®3t e

sols cease to develop as soils and becom e subject to subsurface m odifications by any ¿K i^ T h e dynam ism o f pedogenesis has im plications tor archaeology and

o f the processes active at their location. T h ey m ay undergo eluviation, illuviation, '3 j l§ p k > n o f archaeological sites that are only recently being appropriately exp oite

cem entation, com paction , or other processes that change their characteristics

(Retallack 1990). Buried soils retain for som e tim e the attributes a n d inclusions they

acquired w hile at the surface and, as a class, are am ong the m ost valued repositories

o f p aleoenvironm ental data. At the tim e o f burial they cease to be o r g a n i c a l l y active

and thus, unless truncated, m ay be m ore readily dated by radiocarbon than their

equivalents at the surface, w hich actively incorporate new organ ic m atter through­

out the zone o f bioturbation. In tim e, organic materials and the A zon e are lost to

diagenetic changes that transform soils properties ( Brady 1990).

Soils ch e m istry

T he destructiveness o f soils acids has long been recognized as lim itin g preservation

w ithin the archaeological record. Such recognition has established the notation of

soil pH (hydrogen ion concentration) as a standard item ¡11 archaeological fie^

^ P ed ogen esis as a process affecting the environm ent o f burial is progressive, continu-

|bus, variable, and reversible. Because soil, as m atrix and environm ent o f burial,

Sects what is available for archaeological study and the integrity o f observed associ-

f ations, as well as the condition in which materials are recovered, a basic awareness o f

Ü pedogenesis is essential to successful field work.

Postdepositional disturbance o f sedim ents and soils by the churning actions o f

organisins and/or ice has dism ayed m any observant archaeologists. A rtifact associa­

tions and features underground can be disaggregated or rearranged by such mecha-

I j msms, elim inating parts o f the record and creating false associations ( Johnson and

I Watson-Stegner 1990; W ood and Johnson 1978). Such destructiveness is inherent in

1*5 ; Pedogenesis; it is nearly ubiquitous and must be anticipated by responsible archaeol-

£°g*sts fBarker 1993).Regrettably for archaeologists, archaeological sites are not exem pt from the

f t ' natural processes o f erosion and deposition that ceaselessly reshape the surface o f the

288 S E D I M E N T S A N D S O I L S

' .:r."d- - iM p

» Earth. Erosional processes destroy landform s and d isplace everything on them. \vjjj?

each loss, the fabric o f ancient landscapes is torn. D ep osition al processes bury s ^ f

faces, subjecting them to com pression, deform ation , and diagenesis underground

O verburden n o t on ly changes the environm ent o f sedim ents, it also introduced

m echanical stresses that result in com paction, displacem ent, and particulate sortin

w ith in them. Pedogenesis an d diagenesis, erosion a n d deposition, all restructure

archaeological an d paleoenvironm ental records, req u irin g o f interpreters precis^

thoughtfu l observation and application o f in fo rm ed im agin ation (Schiffer 1987). r s :

S O I L S C I E N C E

T here are m an y reasons w h y archaeologists m ust k n o w som ethin g about soil science;

in fact, the m o re the better. However, it is a d iscipline o f its o w n , n o t som ething one"

can p ick up as a sideline. Here, w ith the em phasis o n paleoenvironm ental recon­struction, a few m atters are presented w ith the ho p e that they can ease readers into”

the som etim es arcane literature o f an im portan t discip line (Fanning and Fanning 1989). .

T h e analytical language o f soil science is p robably the m ajor obstacle to its usé in

archaeology. C o n tem p latin g the form al soil taxon om y used in the United’States and

increasingly elsewhere, Birkeland notes (1984:42) that it “carries such exotic combi­

nations o f Latin and G reek as C ryaqueptic H aplaquoli, A q u ic Ustochrept, and

N atraqualfic M azaquerts.” N o m atter; for the archaeologist the real problem with the

US soil taxon om y is that the criteria for classifying soils do n ot include historical

(genetic) concepts (G uthrie and W itty 1982; H allberg 1985; Soil Survey Staff 1975).

Furtherm ore, and perhaps for this reason, in practice the soil units show n on local-

scale soils m aps bear o n ly a tenuous relationship w ith the geological sedim ents o f the

parent materials. T h e Canadian system includes m ore genetic inform ation and is

therefore m ore im m ediately applicable to archaeological and paleoenvironmental

uses (C anada Soil Survey Com m ittee 1978). M ost industrialized countries have a

system o f their o w n (e.g., A very (1980) for Britain, Stace et al. [1968 J for Australia).

U N E SC O is developing international conventions for soils m aps ( F i t z p a t r i c k 1980),

but in the short run, archaeologists must fam iliarize them selves w ith the system in

use locally, and learn the lim its o f its reliability and applicability (e.g., Catt 1986;

further discussion in C h a p t e r s ) . Beyond that, active collaboration w ith a so il scien­tist is the best approach.

Because o f the w ay that they form, soils on con tiguous la n d f o r m surfaces vary

w ith the topography, vegetation, parent m aterial, and m icroclim ates. Soils, there­

fore, provide in form ation 011 the developm ent and environm en tal h istory o f land-

jjjs and surfaces. C atenas are sequences o f soils profiles varying dow nslope. A t the

j o f a slope soils are typically w ell drained, but subject to erosion and therefore

jtively thin. A t interm ediate elevations slopes m ay be m ore gentle and soils

¡¡opment deeper. A t the fo ot o f a slope, sedim ents accum ulate; soils m ay be deep,

any drainage lim itations w ill d irecdy affect the soils on such landform s.

jjnoiphologists exploit catenas to stu d y the subtle diversity o f landform histories

[die com plex p lay o f the so ils-form ing factors (Birkeland 1984: 238-254; Daniels

[Hammer 1992; G errard 1992; K nuepfer and M cFadden 1990).

Stan d ard analytical m ethods that provide clues to soils’ histories include the con-

ltional chem ical spectra, grain-size analysis (granulom etry), and percentage o f

anics (e.g., H olliday 1990; M acphail 1987: 361-363), w hich provide inform ation

out the p aren t m aterial and the transform ations it has experienced. T he transfor-

itions áre som etim es interpretable in term s o f the soil-form ing factors that dom i-

ited in the past. A nalysis o f the relative degree o f eluviation and illuviation am ongst

jrizons provides inform ation abou t relative ages o f soils and the contributions o f

e five factors o f soil form ation. Soil thin-sections reveal evidence for disturbances

jd m icrostructures in the soil that are d irecdy relevant to environm ental history

¡ullock et al. 1985; G oldberg 1992; L im brey 1992).

-Paleoenvironm entalinform ation is derivable from soils once one learns to elicit it

lalytically. In a volu m e addressed to paleopedology at geological scales, Retallack

~u99o) presents p articularly clear discussions o f the potentials and limits o f soils for

.-^paleoenvironmental inferences. Fine resolution is rarely possible, but if, say, the

■redímate d u rin g soil form ation d iffered significantly from that at the tim e o f observa-

;tion, som e evidence o f that difference m ay survive. Changes in the height o f the

Itw ater table can be read in som e subsoils. Resolving the differences in terms o f tim e,

gjhowever, is difficult. Soils contain biological residues such as pollen, charcoal, and

g? microorganisms that serve as clim ate proxies when they can be dated. Buried soils

p are the best sources o f paleoenvironm ental data; w hen the tim e o f burial can be■ • ' j----------- :--------

B A S I C P R I N C I P L E S | 28<

K specified, they can be eloquent. A p rom ising technique for reading past en viron m en ­

tal states d irectly from stable carbon isotopes in soils is based on the differences in

j?-carbon m etabolism between tropical grasses and o ther vegetation (C 4/C , ratios). In

. § suitable areas, vegetation co m m u n ity successions, interpreted as changes in m icro-

•j climates, have been tracked by carbon -isotope ratios in organic residues in soils

(Ambrose and Sikes 1991). Biological data derived from soils are considered further

as in Parts VI and VII.There is a grow in g, tantalizing literature on direct dating o f soils (Andersen 1986;

13» Matthews 1993; Scharpenseel and Becker-H eidm ann 1992). Archaeologists cannot

£ help but be attracted, even tem pted, by the prelim inary results and claims.

2 9 0 S E D I M E N T S A N D S O I L S

N evertheless, this subject m ust be approached w ith critical awareness. O re^ P .

m atter is certainly available in soils and can be extracted for dating. The d ifficu lt?!

in kn o w in g w h ich organic m atter is equivalent in age to the archaeological evejjtjEsI

be dated. Soils developm ent is a tim e-transgressive process. O rgan ic matter c y S J I

an d o u t o f soil throu ghou t the active life o f the soil; roots and burrow ing a n i|á í|l

penetrate deeply into the B zone. B ulk sam pling o f soil carbon in the A zone wiUíJIIÉj

ages averaged over the duration o f the soil w ith bias toward the youthful side; te<3jɧl¡

rally, w hat is dated is the “average residence tim e” o f the organ ic m atter sarnp||j|§

N o w that A M S dating p erm its the selection o f definable com ponents o f soil (c3 ¡ ¡

coal fragm ents, hum ates), sam ple selection w ill define the age determ ined. SoIú¿|¡¡

hum ates tend to be younger than b u lk charcoal, but they m ay som etim es be o ld ¿ ‘|

D ating o f close-interval sam ples th rou gh a sequence o f soil horizons often demon.-;

strates that soils are frequently churned, although the progressive translocation'ojB

older carbon com p oun ds in to the B zon e m ay im p art som e sem blance o f strati-1

grap hic order to the sam ple ages. Soils w ith archaeological m aterials include organic!

m atter both youn ger and o lder than the anthropogenic materials. Soils processes are, J

continuous; archaeological deposition is episodic. ¿ ¿ g

C O D A

Sedim ents and soils are the essential contexts o f field archaeology; on their appropri­

ate interpretation rests all un derstanding o f the relationships am ong artifacts and

aspects o f environm ents, past and present, as w ell as understanding o f relative ages.

T he results o f pedogenesis m ust be correctly distinguished from variation in sedi­

ments. T he structural p riority o f sedim ents over soils developed in them is a funda­

m ental tenet o f analysis. T h e field relationships o f sedim ents and soils must be

dem onstrated convincingly before interpretation o f included archaeological rem ains can begin.

C H A E O L O G I C A L M A T R I C E S

excavator’s aim should be to explain the origin of every layer and feature he encounters whether it be structural or natural; made by man, animal or insect,

accidental or purposeful.b a r k e r 1982: 68

surface on w hich hum ans lay foot or artifact is a potential archaeological site,

o n ly that subsequent processes not dislodge and transport the surficial

O f course, disturbance o f surficial sedim ents o f every kind is the norm al

This vulnerability ensures that archaeological sites are neither ubiquitous nor

The focus o f this chapter is on sedim ents and soils as matrices o f archaeological

at local and m icro-scales. We occasionally lift our eyes to regional-scale phe-

______as in considering the inform ation potentials o f widespread deposits o f loess

or volcanic ash, but we pay no attention here to the mega- and m acro-scales o f phe­

nomena o r to regional-scale interpretations.

M E S S A G E S I N T H E M A T R I X

Sedimentological analyses are undertaken to learn about the sources, transportation

agents, depositional and transform ational history o f the materials com prising

deposits (C hapter 11). A lthough archaeologists typically treat that inform ation as

background, environm ental archaeology must begin with the environm ents in

which materials, w hether cultural or natural sedim entary particles, were brought to

a site, deposited, and affected by postdepositional processes including pedogenesis

and diagenesis. T h e enclosing m atrix is the fundam ental source o f in form ation

about all the processes essential to understanding the context o f hum an behavior at a

site. Not all evidence is visible, and not all is extractable by techniques currently

291