,,how ,,old ,,is ,,it8 ,,the ,,story ,,of ,,dating ,,in ,,archaeology ,,museum ,,of ,,new ,,mexico ,,press ,,popular ,,series ,,pamphlet ,,no4 #b ,,how ,,old ,,is ,,it8 ,,dating ,,in ,,archaeology by ,james ,schoenwetter ,there is a whole field of science devoted to the invention and development of dating methods--or 8clocks0 as we may think of them4 ,it is called geochronology1 the science of dating events4 ,there are relatively few geochronologists1 scientists trained in the use of all kinds of dating methods and in the theories upon which these methods are based4 ,geochronologists tell us that there are two major types of clocks3 those that tick at an absolute rate of speed which can be measured1 and those which tick only once in a while4 ,a clock of the first type yields what is called an absolute date1 revealing the number of hours1 days1 years1 centuries or millennia since an event occurred4 ,a clock of the second type yields what is called a relative date1 placing an event as before or after another event1 but does not tell us exactly how far they are apart in time nor how long ago they occurred4 ,depending upon how accurate his date must be to solve the problem he has set for himself1 the archaeologist will select absolute or relative dating methods4 ,often1 of course1 the type of clock he wishes to use is not available1 and he must use the next best type4 ,probably he will try to use a number of clocks of different kinds on the problem since each clock will act as a check on the others4 ,the absolute clock utilized most widely in archaeology is the historical record4 ,men have used calendars for a long time1 and have often left records with written dates4 ,on tombstones at a site in old ,virginia1 on the pedestals of statues and other monuments from classical ,greece and ,rome1 on the walls of the tombs of ,egyptian kings1 dates are clearly inscribed which can be related to the sites dug into by the archaeologists4 ,these dates must often be recalculated in terms of the ,christian calendar which we use4 ,most calendars in use in the ,mediterranean1 the ,near ,east and ,china during classical antiquity have been successfully correlated with the one we use today1 and a date inscribed or noted on such sites can be considered in our own terms4 ,other calendars1 such as those developed in the ancient cultures of the ,maya on the ,yucatan ,peninsula1 have yet to be accurately correlated with our own4 ,such calendars can be used on their own terms of course1 and a site which has an inscription in the ,mayan calendar is known to be so many years older or younger than another one with a different date in that calendar4 ,we speak of such a situation as a floating chronology4 ,that is1 the sequence of events and the number of years which separate them are known1 but the dates of those events in absolute time are unknown4 ,illustration3 7uncaptioned7 ,tree rings afford another kind of absolute clock1 the dendrological method4 ,each year a tree adds a growth ring4 ,depending on the amount of water the tree has available to it for cell growth1 the ring will be wider or narrower4 ,certain trees whose water requirements are high live near streams or other places where their roots can tap a constant supply of water4 ,such trees1 referred to as complacent1 have annual rings which are all of about the same width4 ,other sensitive trees live in places where they must depend almost wholly on rainfall for their water supply1 as on the slopes of hills or in the clefts of rocks4 ,such trees have annual rings which vary in width depending upon the amount of rain they receive4 ,in any given area1 especially in arid and semiarid regions1 some years have more rainfall than others4 ,the sensitive trees will produce wider rings during years when there is more rainfall and narrower rings in years when there is less4 ,often there are periods of a decade or so when all of the sensitive trees will produce the same pattern of ring growth2 for example1 three years of narrow rings1 one year of wide1 two more of narrow and three more of wide4 ,such a pattern is called a signature4 ,signatures are the basis of tree ring chronologies4 ,all the trees in a region did not begin growing at the same time of course1 but every time there is a series of years which will produce a signature1 all the sensitive trees still alive will have that signature4 ,let us say we cut down a sensitive tree in #aifj and1 by counting back the rings1 find signatures at #aidj-de1 #aiaj-ad1 #ahhj-hi1 #ahba-bg1 #agie-ahjj and #agej-eh4 ,next we recover a beam from an abandoned ,spanish ,mission built in #ahaj4 ,it happens to be from a sensitive tree1 and we can spot the #agie-ahjj and the #agej-eh signatures near the outer rings4 ,now we have two records which can be said to be crossdated4 ,let us assume that the ,spanish ,mission log give us signatures as far back as #acej4 ,an abandoned ,indian pueblo produces a log with signatures crossdating those of the ,spanish ,mission log and continuing the record back to #abej4 ,older and older archeological sites will yield older and older signatures with each log crossdating some of the signatures of younger logs4 ,at present we have a tree ring calendar for certain species of trees extending back to about #ajj ,b4,c4 ,this is called a master tree ring chronology4 ,a log from an archaeological site may contain only one or two signatures and be1 in itself1 a floating chronology1 but by comparison with the master chronology one can determine the cutting date4 ,having a series of cutting dates for the construction timbers in an archaeological site yields the probable dates at which the site was built and occupied4 ,illustration3 ,diagram to ,illustrate ,tree ,ring ,dating4 ,reproduced by special permission from ,stallings1 8,dating ,prehistoric ,ruins by ,tree-,rings40 ,c ,,this ,,beam ,,came ,,from ,,an ,,old ,,house ,b ,,this ,,beam ,,came ,,from ,a ,,house ,a ,,this ,,was ,a ,,living ,,tree ,,when ,,cut ,,by ,,us ,,the ,,ring ,,patterns ,,match ,,and ,,overlap ,,back ,,into ,,time ,,specimens ,,taken ,,from ,,ruins1 ,,when ,,matched ,,and ,,overlapped ,,as ,,indicated1 ,,progressively ,,extend ,,the ,,dating ,,back ,,into ,,prehistoric ,,times4 ,not all kinds of trees can be used for dendrochronology4 ,pine1 fir and pinyon are the most useful2 juniper can sometimes be used4 ,oak1 cottonwood1 willow and others are very difficult to date and frequently cannot be used at all4 ,another widely used absolute clock is that based upon the orderly decomposition of carbon4 ,this is the radiocarbon or ,c-#ad method of dating4 ,molecules of various substances are made up of atoms4 ,we know now that not all atoms of a substance are precisely the same4 ,we speak of isotopes of an atom4 ,to clarify this let us think of the atoms of carbon as being made up of a mass of ping-pong balls4 ,some atoms will have more ping-pong balls than others1 but all will have enough and in the proper order to be carbon atoms4 ,each of the atoms with #ad ping-pong balls we shall refer to as the carbon-#ad isotope of carbon4 ,there will be other isotopes of carbon atoms too4 ,the carbon-#ad isotope is about average in life span4 ,it has been determined that in any group of ,c-#ad isotopes half of them will lose two of their ping-pong balls and become ,c-#ab isotopes in #e1gcj years1 plus or minus #dj4 ,this is known as the half-life of the ,c-#ad isotope4 ,the 8plus or minus #dj0 allows for laboratory error in terms of years4 ,now all living things contain carbon atoms1 and some of those atoms are ,c-#ad isotopes4 ,the amount of ,c-#ad isotopes in a living organism quickly reaches a stable percentage after which there is no increase or decrease while the organism is alive4 ,after it dies1 the ,c-#ad supply is not replenished1 and with the passing of #e1gcj plus or minus #dj years1 it has half the number of ,c-#ad isotopes it had when alive4 ,in #aa1dfj plus or minus #hj years1 it will have one quarter as many as it had when alive4 ,the geochronologist takes a certain weight of carbon-bearing matter from an organism which once lived4 ,with simple chemistry he can determine the number of carbon atoms in the material1 usually charcoal1 wood1 bone or shell4 ,he places the material in a chamber equipped with geiger counters and records the number of ,c-#ad isotopes converting to ,c-#ab isotopes within a certain number of hours or days4 ,since he knows how many carbon atoms there are in the specimen1 he knows how many ,c-#ad isotopes there would be if the specimen were alive4 ,he also knows that as the number of ,c-#ad atoms decreases the number of clicks on the geiger counter will decrease too4 ,for example1 if there are #b1jjj ,c-#ad isotopes1 the decomposition of half of these over a period of approximately #e1gcj years would register #a1jjj clicks on the counter4 ,in the next #e1gcj year period there would be #a1jjj isotopes left1 and only #ejj of them would decompose to register as geiger counter clicks4 ,the geochronologist does the counting and analysis of the results and sends the information back to the archaeologist in the form of the number of years that have elapsed since the carbon was part of a living creature2 for example1 #a1ejj plus or minus #aej years ,,bp 7before present74 ,the archaeologist1 converting this to the ,christian calendar in #aifd1 would come up with ,a4,d4 #dfd plus or minus #aej years4 ,when was the sample actually alive8 ,we don't know exactly1 but statistically we have a ninety-five percent chance of being right if we say sometime between ,a4,d4 #afd and #gfd4 ,this clock ticks in centuries4 ,but the radiocarbon clock doesn't tick very long1 even in centuries1 before running down4 ,by the time #cj1jjj to #dj1jjj years have gone by1 the ,c-#ad in any sample is almost gone1 and there is too little left to give enough geiger counter clicks unless one is willing to wait a lifetime to record two or three clicks4 ,the ,c-#ad dating technique measures time by radioactive decomposition of materials4 ,there are other clocks which depend on the chemical decomposition of materials4 ,the forgery of the ,piltdown ,man fossil was detected by a dating method which depends on the decomposition of bone protein and its replacement by fluorine4 ,fluorine is an element which occurs naturally as a gas1 but which combines readily with other elements to form compounds4 ,some of these elements are common in bones4 ,since fluorine is one of nature's most reactive elements1 it tends to escape from the compound it is in and to form other compounds4 ,as the protein in a bone decays1 it is often replaced by fluorine4 ,the amount of fluorine in old bones1 then1 is expected to be more than in young bones since it has had more time to accumulate4 ,since fluorine does not accumulate at a constant rate1 it affords only a relative measure of age4 ,in the case of ,piltdown1 a group of bones was discovered at a site1 and was said to contain those of one individual who lived about #fj1jjj years ago4 ,almost #ajj years after they were discovered1 these bones were put to the fluorine test4 ,it was found that 7#a7 some of the bones had less fluorine than others1 so not all were of the same antiquity and could not have belonged to the same individual1 7#b7 the younger bones had as much fluorine in them as modern bones1 and 7#c7 the older bones had more fluorine in them than bones known to be #fj1jjj years old4 ,illustration3 7uncaptioned7 ,another form of absolute dating of importance to archaeology is that called the varve method4 ,this can only be utilized as an absolute clock under very special circumstances however4 ,varves are like tree rings in a way4 ,in a lake which is sufficiently deep1 or at the edge of a glacier1 particles of sediment are being deposited continuously as a sort of fallout from the water4 ,during the winter1 when the glacier freezes or the density of the water in the lake increases because of the cooler temperature1 less particles are deposited1 and those which are deposited are usually of a characteristic color or texture4 ,during the summer1 when the glacier melts or the lake warms up1 more and different particles are deposited4 ,the bands of deposited sediment are called varves2 every year two varves are formed4 ,starting from the top1 one can count back the number of years in a varve series4 ,if the top varve is of known date such as the present year1 one has a calendar with each varve having a known date4 ,attempts are made to correlate one varve series with another in order to recover even longer series4 ,if the archaeologist is lucky1 and it is not rare in ,europe1 there will be materials from a site buried in the local varve sequence4 ,counting back gives an absolute age for the artifacts embedded in the site and thus an approximate age for the site4 ,the ,european varve chronology is believed to extend back to about #ifej ,b4,c4 ,most of the clocks which the archaeologist uses to produce relative dates1 the before-or-after kind1 have as their theoretical basis the principle of stratigraphy4 ,in effect the principle of stratigraphy assumes two things3 that the rocks of the earth are constantly wearing down by erosion1 and that things which appear to be alike actually are alike and are probably more or less the same age4 ,if rocks are constantly wearing down1 it follows that the surface of the ground is constantly building up4 ,thus the surface we walk on is a younger1 higher surface than that which our ancestors walked on4 ,when we dig below the surface1 those things we find which are at higher levels are younger than those which we find at lower levels4 ,the deeper we dig1 the older things get4 ,there is no reason to believe that the rate of deposition on the surface is the same everywhere4 ,if we dig two feet in one place we may be at a level which is now five feet below the surface in another location4 ,if we find a particular object1 say a type of pottery1 on the surface at site ,a and the same kind of pottery five feet below the surface at site ,b1 we can use the second of our assumptions and maintain that both pieces are of the same age4 ,then any objects found at higher levels than five feet at site ,b are younger than the piece of pottery and are younger than anything found at site ,a4 ,this is the principle of stratigraphy4 ,like tree rings1 objects in stratigraphic sequence can be crossdated4 ,these sequences may be of various kinds as any object will do4 ,distinctive bands of sediment1 distinctive artifacts1 types of fossils1 specific details of chemistry or any other phenomenon may be used with varying amounts of success4 ,suppose we have the following sequences of objects at sites ,a and ,b3 ,illustration3 7uncaptioned7 ,a ,b ,black earth ,brown dust ,caliche ,black earth ,eroded layer ,caliche ,cobbles ,yellow silt ,soil ,brown silt ,brown silt ,eroded layer ,cobbles ,now there are some things that are similar about these two profiles and other things that are different4 ,both profiles contain layers of black earth1 caliche1 an eroded layer and cobbles4 ,both profiles contain pottery1 and both contain arrow points4 ,the type of pottery in profile ,a is the same as that in profile ,b1 but in ,b there is brown dust above the black earth4 ,in profile ,b the brown silt is above the eroded layer1 while in ,a it is below the eroded layer4 ,in profile ,a the arrow points have different shapes than the ones in profile ,b4 ,what we need to correlate the sequences are horizon markers1 objects that are enough alike to be in the same time range4 ,the pots in both cases look the same and are embedded in the same kind of sediment1 the black earth4 ,they form one horizon4 ,the caliche may be a horizon marker1 but this is not positive since at profile ,a it is above the eroded layer1 while at profile ,b it is above the yellow silt4 ,the eroded layer sits above cobbles at both profiles1 which makes the eroded layer-cobbles complex a pretty good horizon marker4 ,the brown silt is not a horizon marker because it is above the eroded layer at one profile and below it at the other4 ,furthermore1 the arrow points which it contains are not all of the same kind4 ,the correlated sequence1 using the horizon markers1 must be as shown in the accompanying chart4 ,the stratigraphy proves that the arrowheads in the upper brown silt must be younger than those in the lower brown silt4 ,the next time we find arrowheads of the types recovered in the upper brown silt we will know1 regardless of the stratigraphic sequence in the new locality1 that they must be younger than the types found in the lower brown silt1 and they must also be older than pottery of the type found in the black earth4 ,we won't know how many years old they are1 but we have dated them in the sense that we know they are older than some things and younger than others4 ,the principle of stratigraphy is one of the archaeologist's most useful theoretical tools as it allows one site to be compared with others4 ,our example has shown how artifacts 7pots and arrowheads in this case7 and sediment types can be placed in stratigraphic order to provide a clock for archaeological dating4 ,other objects which are commonly used as horizon markers in stratigraphic sequences are fossils4 ,remains of extinct animals are often used to prove extreme antiquity4 ,remains of plants and animals will often indicate similarity of ecological conditions at two sites and allow them to be crossdated4 ,illustration3 7uncaptioned7 ,brown dust ,black earth ,caliche ,yellow silt ,brown silt ,eroded layer ,cobbles ,soil ,brown silt ,the geologic-climatic method of dating combines the principle of stratigraphy with an interpretation of the meaning of natural occurrences4 ,when geologists demonstrated that the earth had recently gone through a period when enormous glaciers advanced and retreated across the northern and southern hemispheres1 they began to speculate on the effects of such conditions on the landscape4 ,the surface would be scraped down to bare rock by the advancing ice sheet4 ,when the ice retreated1 its load of rocks and cobbles would tend to be left behind1 while the smaller particles of dirt would be flushed away in the rivers formed by the melting ice4 ,if the archaeologist found tools among the boulders and cobbles1 those tools should date to the period when the cobble and boulder stratum was formed1 the period when the glacier was retreating4 ,here the geology gave clues to the climate1 and if the age of the climatic event was known1 artifacts associated with the geology could be dated4 ,it is through the geologic-climatic method of dating that archaeologists discovered that human beings lived in southern ,europe at the margin of the last great glacier1 and therefore the date of their occupation was on the order of #bj1jjj years ago4 ,it is not necessary to use the geologic aspect of geologic-climatic dating if one has other clues to the climate4 ,plant and animal fossils are clues to ancient climates1 since living organisms have a tendency to live in climates to which they are best adapted4 ,if we find the fossil bones of a giraffe or an ostrich in the ,sahara ,desert1 we can conclude that at some time in the past the ,sahara was not a desert but a veldt1 since giraffes and ostriches live in the veldt today4 ,if stratigraphy allows us to relate artifacts to those fossils1 we can maintain that the makers of those artifacts lived at the time the veldt existed in what is now the ,sahara area4 ,now if we can relate the fossils to a date when such a climate could have obtained in the area1 we have a date for the artifacts4 ,animal fossils are relatively rare1 and fossil remains of plants which can be seen with the naked eye are even rarer4 ,but microscopic pollen grains are not particularly rare in the stratigraphic sequences of sediments1 and are directly associated with artifacts in many kinds of sediment4 ,the pollen of most plants is protected by a tough coat like that on many seeds4 ,millions of pollen grains are produced by local vegetation each year1 and a percentage of them are buried in the yearly accumulation of sediment4 ,once they are buried1 the tough outer covering is preserved4 ,illustration3 7uncaptioned7 ,when pollen grains are extracted from their sediment matrix1 the different types of plants which grew in the area at the time the sediment was laid down can be recognized from the distinctive characteristics of their pollen grains4 ,the pollen analyst has the job of interpreting this information to discover the nature of the vegetation patterns in the past and the climates associated with them4 ,through dating those climates1 the artifacts with which the pollen sample was associated are dated4 ,some archaeological sites do not allow dating by any of these methods4 ,the farmer plowing a field may pick up an arrowhead and wonder how old it is4 ,can the archaeologist date the arrowhead8 ,the answer is a qualified yes4 ,what the archaeologist will try to do is give an educated guess as to the age of the arrowhead4 ,he might1 for example1 know that arrowheads of that type are found at a site which is dated about ,a4,d4 #ajjj by various methods4 ,by correlation1 he could apply that date4 ,if the arrowhead is not of a type which has been reliably dated1 the archaeologist may rely upon the method of dating known as seriation4 ,this method depends on the second assumption basic to the principle of stratigraphy1 that similar objects tend to be about the same age4 ,the farmer's arrowhead may not be exactly like any which has ever been discovered1 but it probably will be more like some known ones than others4 ,observing the general style of the arrowhead and the way in which it is made1 the archaeologist can make a pretty good estimate of when it was made4 ,let us assume that the archaeologist goes into an unexplored area where no absolute or relative dating techniques are available4 ,there he finds a number of sites with potsherds of types new to archaeology4 ,can he date these sites8 ,again he uses seriation1 but this time he inverts the logical proposition4 ,if things which look alike tend to be of the same age1 things which do not look alike should tend to be of different ages4 ,his first task would be to separate all the different types of potsherds4 ,taking a specimen of each type1 he lays them out in a row4 ,if there is any difference in the sites through time1 the styles of pottery will change correspondingly4 ,but there will be some styles which change slowly and some which will influence others4 ,for example1 if we were to seriate the style of automobile rear fenders for the period #aief to #aifd1 we would observe that they began to sprout taller and taller fins2 then the size of the fins became reduced more and more4 ,some of the style aspects of pre-#aief rear fenders went along with the development of fins and others dropped out4 ,some of the style aspects which were developed with the fins were retained after the fins decreased in size4 ,seriation of the design styles on the potsherds will result in a series of developments in style which are probably in chronological order4 ,broad straight line designs may give way to mixed broad and narrow straight lines1 then to narrow straight lines1 then to narrow wavy lines4 ,the archaeologist could then maintain that sites which have potsherds with broad straight lines are separate in time from those with sherds having narrow wavy lines4 ,he won't know which is the older1 because the sequence could work either way4 ,stratigraphy or the cultural similarity between other artifacts at the sites will probably resolve this problem4 ,the reader will now have realized that many of the clocks used by archaeologists are interrelated4 ,relative dating clocks such as stratigraphy1 pollen dating and geologic-climatic dating are utilized together where possible1 and all are dependent upon the principle of stratigraphy4 ,crossdating is thus of vital importance and is constantly undertaken4 ,the archaeologist tries to employ both absolute and relative clocks to find out the age of a site4 ,stratigraphy yields a series of relative dates for artifacts within the site2 geologic-climatic and pollen dating yield a series of dates for types of sediment and samples collected in association with those artifacts2 tree ring dates yield the absolute age of the site which allows the pollen1 stratigraphic and geologic-climatic dates to be comprehended in terms of absolute age4 ,radiocarbon dates act as a check on the tree ring dates and1 if they agree1 lend support to the pollen and geologic-climatic dates4 ,the pollen and geologic-climatic dates from the site are compared with similar dates from other sites as additional checks4 ,since the clocks used by the archaeologist tick at different rates of speed1 and since not all of them are dating the same thing1 the archaeologist usually ends with a series of dates for any given site4 ,new geochronological techniques are always being invented and perfected4 ,here is a list of some that are expected to become available in the next few years3 #a4 ,the obsidian hydration method4 ,obsidian1 a naturally formed glass1 is so constituted chemically that it takes chemicals from its environment at a slow rate4 ,as it does so1 the outside layer changes from transparent to translucent or opaque4 ,the depth of the opaque layer1 it is hoped1 can be used as a measure of the amount of time since the surface was exposed4 ,as many artifacts were made by chipping obsidian to form a sharp edge1 this method may reveal the time since an obsidian artifact was made4 #b4 ,the thermoluminescence method4 ,the chemical properties of certain minerals change when the minerals are heated to high temperatures4 ,if they are heated again they will glow1 and the amount of time they glow upon reheating depends on the amount of time that has elapsed since they were heated originally4 ,as yet the rate has not been accurately calculated1 nor is it yet understood what effect different kinds of soils and atmospheres may have on this rate4 ,if the method is proven1 it will be invaluable to archaeology since it will afford a way of dating pottery directly on an absolute time scale4 #c4 ,the paleomagnetism method4 ,when an object containing particles which can be magnetized is heated1 the magnetic particles line up according to the earth's magnetic field4 ,when the object cools1 the particles are trapped in this position4 ,we know that the earth's magnetic field is changeable1 and that at different times it has been oriented in different directions4 ,work is now in progress to determine how the field has varied through time and how successfully one can date materials such as pottery or hearths which were exposed to high temperatures in the past by the difference between the present magnetic field and that trapped in the object4 ,museum of ,new ,mexico ,press ,santa ,fe #aife ,illustrations by ,mary ,spencer and ,phyllis ,hughes ,transcriber's ,notes --,silently corrected a few typos4 --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,in the text versions only1 text in italics is delimited by underscores4