Problems in the interpretation of radiocarbon dates: the Pastoral Neolithic of East Africa

Problems in the interpretation of radiocarbon dates: the Pastoral Neolithic of East Africa D A V I D C O L L E T T and P E T E R R O B E R T S H A W

Abstract

For the Pastoral Neolithic of East Africa radiocarbon dates suggest two apparent anomalies in the archaeological record: pastoralism in the Central Rift at perhaps 7000 bp, and the very long duration of particular pottery traditions. This paper examines the dating evidence closely, in particular the assertion that apatite gives more reliable dates than collagen. Both the assertion and the anomalies appear to be false. It is shown that there is no firm evidence for pastoralism in East Africa before about 4000 bp. Similarly, the Narosura tradition is of shorter duration and more recent than previously supposed. The meagre dating evidence for the Namoratunga sites and its implications are also assessed. Examination of the dates for the Pastoral Iron Age shows that the Turkwel tradition is dated only by a single date and that the appearance of twisted cord rouletted pottery in the present millennium post-dates the end of stone tool manufacture by several centuries. The paper concludes with a plea for more critical assessment of radiocarbon dates and for greater rigour in their presentation.

R6sum6

La datation par radiocarbone sugg~re pour le N~olithique pastoral de l'Afrique de l'est deux anomalies apparentes dans la s~quence archfiologique: l'existence d'une ficonomie pastorale remontant peut-~tre ~t 7000 ans bp dans le Rift central, et la longue dur~e de certaines traditions c~ramiques. La pr~sente contribution examine de pros les circonstances de la datation et en particulier, l'affirmation selon laquelle les fichantillons d'apatite donnent des dates plus fiables que cetles obtenues du collagkne. Cette assertion et les anomalies qui en d~coulent semblent ~tre erron~es. I1 est d~montrfi qu'il n'y a aucune preuve absolue signalant un Ndolithique pastoral dans cette zone avant environ 4000 ans bp. De m~me, la tradition cfiramique dite 'Narosura' a une durfie plus courte et plus rficente qu'il fitait auparavant suppos& Les maigres preuves pour la datation des sites Namoratunga sont ~galement examinees. Quant h l'Age du Fer pastoral, il semble que la tradition Turkwell n'est effectivement dat~e que par une seule dfiterminante, et que l'apparition au cours du mill~naire actuel d 'une c6ramique d~cor~e par roulette de fil tordu date de plusieurs si~cles aprks la fin de ta fabrication d'outils en pierre tailt~e. L'article se termine avec un appel pour

58 David Collett and Peter Robertshaw

des ~valuations plus critiques des datations par radiocarbone et pour plus de rigueur dans leur prfisentation.

A Neolithic stage in East African prehistory was first posited by Louis Leakey in 1931. Leakey (1931) thought that the presence of stone bowls, mortars and grinding stones indicated some form of agriculture. However Sutton (]966:38) abandoned the term 'Neolithic' as 'cumbersome and subjective', preferring the designation 'Later Stone Age' (LSA) which carried fewer implicit assumptions. Though not favoured by Sutton, the term 'stone bowl culture' gained popularity for several years (e.g. Cohen 1970). The mid-1970s saw" the return of'Neolithic' (Onyango-Abuje 1976) and since 1977 'Pastoral Neolithic' has become the norm. Pastoral Neolithic (PN) is used to refer to societies with an LSA technology, including pottery, and a pastoral economic base relying heavily on domestic cattle and/or ovicaprids (Bower, Nelson, Waibel and Wandibba 1977). There is no direct evidence for agriculture during the PN; however historical linguistics suggest that with improvements in archaeological recovery techniques such evidence should be forthcoming (Ehret 1974:8).

Recent research has added considerable detail to our knowledge of the PN of East Africa. This is reflected in the increasing number of radiocarbon dates for this period, with more than fifty new dates listed in a recent paper by Mgomezulu (1981). Some of this work has wider implications for both African prehistory and our understanding of the Neolithic generally. In particular Bower and Nelson (1978) report evidence for pottery in the Early Holocene (9000-5000 bp), for pastoralism by about 5500 bp and possibly as early as 7255 bp (see also Bower and Gogan-Porter 1981). In addition they suggest that several ceramic 'wares'l coexisted in the same geographic area over several centuries or even for millennia. West of Lake Turkana (Rudolf) the identification of what appears to be an archaeoastronomical observatory dating to 300 Bc (Lynch and Robbins 1978) has also aroused considerable interest.

The present paper examines in detail the dating evidence for early pastoralism, the long coexistence of'wares', and the archaeoastronomical site. It also discusses briefly the dating of the succeeding Pastoral Iron Age (PIA), a term which has been used to refer to societies with pastoral economies incorporating iron into a technology that often still includes flaked stone tools (Bower et al. 1977:119). We do not consider the evidence for pottery, apparently not associated with domestic stock, dating to the Early Holocene; in many cases this pottery is associated with bone harpoons and evidence for fishing (Sutton 1974). We should also point out that in some instances the evidence for early pastoralism discussed below is based not on the identification of faunal remains but of particular pottery 'wares' that, where bone is adequately preserved, are known to be associated with domestic stock.

The presence of pastoralism in the Central Rift valley of East Africa by 5500 bp and the long coexistence of pottery 'wares' can be considered as anomalies in the archaeological record since they contradict conventional wisdom about the spread ofpastoralism through Africa and the nature of ceramic traditions. The apparent anomalies may not in fact be such; however it is possible that they result from errors in the interpretation of the archaeological record. Problems of interpretation can occur at two points in the construction of regional sequences. Material may be incorrectly assigned to a taxon, and we have pointed out

Problems in the interpretation of radiocarbon dates 59

elsewhere that this is probably true of pottery from the Central Rift attributed to 'Kansyore ware' (Collett and Robertshaw 1980). Alternatively, some radiocarbon dates may be grossly

in error as a result of contamination or other poorly understood factors. The construction of a scheme of relationships in space and time between archaeological

assemblages requires cross-correlation between assemblages. Radiocarbon dating is essential for correlation because it is an independent technique tbr chronological ordering. This is particularly important for the PN of East Africa at the present state of research because many of the assemblages come from widely dispersed single component sites. Therefore evaluation of the radiocarbon chronology is a necessary first step in assessing the validity of the anomalies in the PN sequence.

Apatite, collagen and charcoal

There are a large number of possible sources of error in carbon dates. Three sources are of particular importance: long term fluctuations in atmospheric ~C, contamination of the sample by either modern or old carbonates, and faulty association between the artefacts and the material on which the date is obtained. Fluctuations in atmospheric radiocarbon can be corrected by the use of calibration curves, of which the 'best' on statistical grounds is probably that produced by Clark (1975), and do not pose a serious problem in the PN. The remaining two errors are not so easily dealt with and raise serious questions about the validity of some PN dates in East Africa. This is particularly true for contamination because it has been argued by researchers in Eastern Africa that bone apatite dates are more reliable than collagen or gelatin dates (Bower and Nelson 1978:559-60). Collagen and gelatin are essentially the same, gelatin being a refined form of collagen. Consequently much of the PN chronology is based upon apatite dates. However, it is known that apatite content of bone can be affected by soil chemistry, while mechanisms for changing the ~4C content of collagen do not exist (Michels 1973:161). Clearly, since apatite dates may potentially reflect contaminated rather than 'true' age estimates, one would expect that apatite dates would be treated with more caution than collagen dates. Dates obtained from burnt bone can be treated in the same way as charcoal since burning of bone leads to the formation of free carbon that is inert and not subject to alteration. However, reduction may not have been complete and some of the datable material in bone may not be chemically inert. I f this happens then burnt bone dates may also be affected by post-depositional chemical

factors. The argument for accepting apatite dates put forward by Bower and Nelson is based on

personal experience; the presentation of supporting data is not attempted, despite the existence of a suitable data base, much of which is presented by Bower and Nelson (1978:Table 1). Two statements form the basis of the argument: apatite dates are normally older than collagen dates, and apatite dates are in close agreement with associated charcoal dates while collagen dates are not. In order to test these statements objectively it is necessary to see whether dates obtained from the two bone fractions are in statistical agreement and whether apatite or collagen dates are more frequently in agreement with charcoal dates.

A number of techniques have been suggested for evaluating the compatability of radiocarbon dates, some of which have been reviewed by Ward and Wilson (1978) whose criticisms can be generalized to other techniques which they did not consider. Ward and


Wilson point out that any statistical test makes assumptions about the source of variance. In

radiocarbon dating there are two normally accepted sources of variation, one associated with

the statistical nature of radioactive decay and the second associated with laboratory errors in sample preparation. The former source of variation is quantified in the error term associated

with the date and the latter is unquantifiable. A third potential source of variation is

associated with the calibration curve. A real age can be associated with a number of possible radiocarbon ages depending on the distribution of variation around the line fitted to the data

points used to construct the calibration curve. The variance has been estimated by Clark (1975) as 502 when the date is less than 2700 bp and 602 when it is greater.

I f calibration is a source of variance, then one must incorporate different estimates of

variance under different conditions. Ward and Wilson (1978) outline two cases. In a Case I

comparison the radiocarbon dates are derived from the same object and must have the same 'true age'. Calibration variance is therefore not relevant and only the counting error need be

taken into consideration. However, if the dates are derived from different objects, for

example different hearths, then one cannot be sure that the objects have the same real age and calibration variance must be incorporated in the statistic. This is a Case I I comparison.

Comparison proceeds in two stages. Initially a pooled mean is calculated which is a

weighted average. The deviation of individual dates from the pooled mean is calculated and compared with the error term associated with the date. The formulae for a Case I comparison

are Ap (pooled mean) = (ZAi/E~2)/(Z 1/E~ 2) and the test statistic T = (A~-Ap)2/E~ 2. A~ is the

individual radiocarbon date and E i is the associated error term. In a Case I I comparison an error term (of502 years i fAp is less than 2700 bp or 602 years if greater) is added to Ei 2. The

values obtained from use of the formulae are summed up and the resultant statistic, T, is distributed as ;(~ with n - 1 degrees of ti~eedom.

In order to test the assertion that East African apatite dates are usually older than collagen

dates, pairs ofapat i te and collagen dates from the same bone sample were examined. Each

pair of dates was compared using the Case I formula. Three outcomes were possible: (1) the

pair could be in agreement, or (2) the apatite date could be significantly older or (3) significantly younger than the coIlagen date. A total of nineteen pairs of dates were examined

(Table 1). An analysis of the resultant table of outcomes (Table 2) by a ;(2 test (Bailey 1959:52-66) showed no significant difference between the frequency of observations in each

cell. It would, therefore seem that the direction of difference between apatite and collagen dates, when they are not in agreement, is a chance event.

I f dates in agreement are ignored, a cursory examination of the data might suggest that apatite dates may normally be older than collagen dates. A binomial test (Meddis

1975:58-9) was used to examine whether or not there was a significant difference between the frequency of older and younger apatite dates. A significant difference was not found. For the test to be significant the smaller group has to contain a maximum of only two observations. Therefore it seems that the assertion that apatite dates are normally older then collagen dates is not supported and the differences in observed frequencies could be due to sampling.

The second problem, the relationship between apatite, collagen and charcoal dates, can be

examined in the same way, although the sample size is very small. There are four instances where charcoal and apatite dates (Table 3a) can be compared and three cases for collagen and charcoal dates (Table 3b). Agreement was determined by the Case I I formula. An examination of the results (Table 4) shows that the relationship between charcoal, apatite


Table 1 Case I comparisons of apatite and collagen/gelatin dates from East Africa. Date pairs derived fi~om Bower and Nelson (t978) and Ambrose (in press). Pairs with modern apatite dates were not considered.

Apatite Collagen/Gelatin Lab. No. Date (bp) Date (bp) T Significance

GX-4585 2660+ 120 2030+ 140 11.67 P<0.01 GX-4586 2215+120 2535+140 3.01 NS (Not Significant) GX-4589 2030+ 140 2660+ 160 8.78 P<0.01 GX-4319 2045+125 895+105 49.62 P~0.01 GX-4463 1665+ 145 1255_ + 125 4.59 P<0.05 GX-4464 410_+110 1415+150 29.19 P~0.01 GX-4468 2990+170 1110+115 83.90 P~0.01 GX-4421 2680+150 1315-+135 45.75 P~0.01 GX-4160 2085-+135 1710+135 3.86 P<0.05 GX-5347 1600+130 1810+135 1.31 NS GX-5692 1855+ 186 2050+ 115 0.80 NS GX-4385 1965+140 1775+-115 1.i0 NS GX-4386 1440-+120 1965_+140 8.11 P<0.01 GX-4507 2030+ 125 1775+ 110 2.86 NS GX-5368 1820-+120 2070-+155 1.63 NS GX-5135 1965_+150 2155+140 0.86 NS GX-4465 1815+120 2225+155 4.37 P<0.05 GX-4506 1240+ 145 1250+ 115 0.005 NS GX-4420 1185+ 140 Modern - - P< 0.01

Table 2 Comparison of the frequency with which collagen and apatite dates are in agreement or significantly different t?om each other. Apatite dates with a modern reading are not included.

Apatite older Apatite and Apatite younger than Collagen Collagen agree than Collagen Frequencies

7 8 4 Observed 6.3 6.3 6.3 Expected if Random 0.07 0.44 0.86 (Obs. x Exp.)2/Exp.

X 2 = 1.37; d r = 2; Not significant

and collagen dates is not as simple as has been suggested and both apatite and collagen dates

may be in agreement with charcoal.

I t should be pointed out that in only three instances were charcoal, apatite and collagen

dates all obtained from the same level. In two cases the apatite date was modern and the

charcoal date agreed with the collagen date, but in the third case the apatite date agreed with

the charcoal one and the collagen date was too young. This hardly constitutes an adequate

sample for a valid generalization. O u r analysis implies that the assumptions underlying the interpretation of many PN dates

are not supported. Where more than one bone date is associated with an horizon statistical

techniques should be used to compare the dates. I f there is no agreement, part icularly if they


Table 3 a Case I I comparisons of apatite and charcoal dates from the same archaeological horizons.

Apatite Charcoal Lab. No. Date (bp) Lab. No. Date (bp) T Significance

GX-4383 1090+150 GX-4384 1255+140 0.44 NS GX-4160 2085_+135 GX-5138 2415_+155 2.3t NS GX-431 t Modern GX-4312 1545_+135 - - P<0.0t GX-4382 Modern GX-4381 445___ 120 - - P<0.01

b Case I I comparisons of collagen/gelatin and charcoal dates from the same archaeological horizons.

Collagen/gelatin Charcoal Lab. No. Date (bp) Lab. No. Date (bp) T Significance

GX-4160 1710+135 GX-5138 2415+155 3.85 P<0.05 GX-4311 1560+135 GX-4312 1545+135 0.005 NS GX-4382 485_+135 GX-4381 445_+120 0.005 NS

Table 4 Comparison of charcoal, apatite and collagen dates. Apatite dates with a modern reading are starred.

Apatite Collagen Apatite Collagen Collagen Apatite older older agrees agrees younger younger than than Charcoal Charcoal than than

Charcoal Charcoal Charcoal Charcoal

Obs. - - 2 2 1 2* Freq.

are paired dates (Case I comparison), then the horizon should be treated as undated as there

is no objective basis for choosing one date in preference to the other.

I t might be argued that where two dates are compared using the Case I I formula and

found to be not in statistical agreement they could nevertheless both be correct estimates o f

age and together provide a reasonable estimate of the durat ion of a level that was

accumula t ing over a long period. This is unlikely given the evidence provided by recent

experiments on site formation processes (e.g. Cahen and Moeyersons t 977), and can almost

certainly be rejected for the PN of East Africa, where potsherds can often be joined

irrespective of stratigraphic ordering and where we are, in the vast majority" o f cases, undoubtedly dealing with short- term occupations.

Furthermore, the use of single apatite dates as estimates for the time of occupat ion should

be treated with extreme caution as apatite is known to be affected by soil chemistry. While it might be thought that altered apatite dates would always be too recent as a result of

contaminat ion by modern carbonates in groundwater , this does not take into account the fact

that old carbonates may be dissolving in lower levels and may move into the apatite lattice of


bones in higher levels. A further problem is that post-depositional isotopic change can also affect the 13C/~2C ratios ofapatite (Land, Lundelius and Valastro 1980) by at least as much as 12%o (Schoeninger and De Niro 1982); thus the i513C correction values applied to dates run on

apatite may be in error. So far only the assumptions employed by Bower and Nelson (1978) and Lynch (1978)

regarding the relative reliability of dates on different materials have been examined. However other assertions concerning the consistency of dates produced by different laboratories occur in the literature. These too appear to rest on an inadequate data base. An example is the assertion that dates from the Geochron laboratory (GX) tend to be younger than dates from other laboratories (Phillipson 1977:10). This is based on a series of dates run by GX and the University of California at Los Angeles (UCLA) from a single site GvJm 22 (Gramly 1976). The dates from the two laboratories come from different levels and there is an inconsistency between the stratigraphy and the dates (Table 5). Clearly, the sample size is small and it is equally possible that the UCLA dates may be too old. Indeed UCLA has produced several dates for sites in the Central Rift Valley which are generally regarded as too old (see Ambrose in press). In general it can be noted that there appear to be no systematic

differences between radiocarbon laboratories, although very significant between- laboratories variability does exist (Clark 1975:252). Thus, in this instance rejection of GX dates solely because they are too recent is unwarranted. Consequently there is no firm basis for the assertion regarding the GX dates.

Table 5 Stratigraphic context of Los Angeles and Geochron dates from GvJm 22, Lukenya Hill. *There were problems in extracting collagen from the Helsinki sample.

Level Lab. No. Date (bp)

1.35-1.4m UCLA-1709A 17,670+800 1.4-1.45m UCLA-1709B 17,700+760 1.8-1.85m GX- 3699 15,320 + 450 1.9-1.95m GX-3698 13,730+430

HEL-535* 9910+300

Early pastoralism

I f the assumptions underlying the preferential acceptance of apatite dates are rejected then chronologies based on these assumptions should be re-examined. The crucial site for the acceptance of an anomalously early date for pastoralism in East Africa is Salasun (Bower and Nelson 1978). There are eight dates from five levels at Salasun. Six of the dates are paired apatite/collagen determinations and each of the three pairs comes from a different level. The remaining two dates are on apatite alone and come from the levels below those with paired dates. Bower and Nelson construct a sedimentary history for the site based on the acceptance of four of the apatite readings. Using the inferred sedimentary rates, they argue for the presence of pottery at about 6845 bp and perhaps as early as 7255 bp, and for the presence of pastoralism from about 5500 bp. The identification ofpastoralism is based upon Bower and


Nelson's ascription of the pottery from this level to Narosura 'ware' , elsewhere persistently associated with domestic cattle and ovicaprids. A comparison of the paired dates from

Salusun shows that none are in statistical agreement (Table 6). There is no objective basis for preferring apatite to collagen, and either series could be

correct or both could be wrong. The appeal to cultural similarity made by Bower and Nelson

to support the acceptance of a particular date should be treated with caution as assemblages

may be incorrectly assigned to particular 'wares', especially where the size of the assemblage

is small (ct: Bower and Nelson 1978:fn. 3). The two apatite readings from the older strata at Salasun may be affected by geochemistry. Consequently the only chronology which can be

objectively accepted from Salasun is a relative chronology based on a stratigraphic ordering.

I f the Salasun sequence is treated as a succession of archaeological horizons without a chronometric scale then the question of pastoralism and ceramics prior to about 3000 bp

from the Central Rift Valley remains unanswered. However, pastoralism and pottery are

associated with early dates in the Lake Turkana area as welt (Bower and Nelson 1978:562;

Barthelme 1977, n.d.; Robbins 1972). The dates for these assemblages should be examined to see whether the evidence from this region is more acceptable.

Table 6 Comparison of paired dates from Salasun (GuJj 13).

Dates (bp) Lab. No. Ap T Significance

1185-+ 140 GX-4420/A None - - different by Modern GX-4420/C observation

2990+ 170 GX-4468/A 1700 83.90 P~ 0.01 1110+ 115 GX-4468/C

2680+150 GX-4421/A 1926 45.75 P~0.01 1315+135 GX-4421/C

In the Turkana region Nderit pottery, a supposedly PN 'ware' , has been dated at three

sites, Aipa, Bb- 14 and Kangatotha, on the western side of Lake Turkana (Robbins 1972). In

all cases the pottery occurred on the surface and was not shown to be in direct association with the material on which the date was obtained. The Aipa site is a wind deflated holtov) containing a mixture of pottery and remains of domestic stock, all exposed on the surface.

The date was obtained from burnt soil in a hearth that was also exposed. When a site is deflated, material from a number of periods can co-occur on the surface. I t is impossible to

prove that a surface feature belongs to the same period as any of the material and the date must therefore refer only to the feature. The date for Bb-14 is similar. The pottery was in an erosional context on the surface. A burnt earth feature was found eroding out and was dated. There is no proof that the ceramics and feature came from the same level. Finally at Kangatotha the Nderit pottery was found on the surface of a lake sediment which had been dated to 4800 + 100 bp (no laboratory number given). Once more there is no convincing evidence for associating the pottery with the dated sediment. All of the dates could be valid but the absence of any proof of association means that the assemblages must be treated as undated.

The best dates for early PN sites have been obtained from the area to the east of Lake


Turkana (Barthelme n.d.). Three sites have been excavated, two of which contain Nderit pottery. At one of these sites, GaJi 4, the pottery is associated with ovicaprids. The third site, FwJj 5, has associated stone bowls and a single bone apatite date (GX-4643A) of 4000 + 140

bp. The excavator notes the problems associated with bone apatite and regards the date as tentative; it will not be considered in the following discussion.

Three dates were obtained from GaJi 2, two charcoal dates from a split sample and a single date on mollusc shell. The two charcoal dates are in close agreement (P-2609 3970 + 60 bp; SUA-634 4160 + 110 bp; T -- 2.3; NS) and have a pooled mean of 4014 + 53 bp. The shell date is significantly different (Ap = 4014 + 53 bp; SUA-635(S) 8915 + 140 bp; T = 8I t.23; P--~0.01) and can be rejected because the shell is probably reworked (Barthelme n.d.). There were six dates from the second site, GaJi 4. Three were obtained from a split sample of charcoal, and all these dates are in close agreement (SUA-637(B) 4100 _+ 125 bp; P-2610 3860 + 60 bp; SUA-637 3945 + 135 bp; T = 3.076; NS), with a pooled mean of 3931 + 50 bp. The two bone apatite dates are not in agreement with each other (GX-4642/IA 3405 + t30 bp; GX-4642/IIA 4580 +_ 170 bp; T = 30.14; P<0.01) or with the charcoal dates (Ap with GX-4642/IA: T = 13.81; P<0.01; Ap with GX-4642/IIA: T = 8.26; P<0.01). As charcoal is less susceptible to contamination than bone apatite the bone dates should be rejected. Similarly the shell date is considerably older and not in agreement with the charcoal date (SUA-638(S) 10,320 + 150 bp; T = t223.57; P<0.0001) and should be

rejected. The pooled means from the charcoal dates at GaJi 2 and 4 are both about 4000 bp, and

these represent the earliest dated PN occurrences in Kenya. The large discrepancy between this date and the date of 7250 bp from Salasun accepted by a number of workers (e.g. Mgomezutu 1981) is an illustration of how uncritical acceptance of dates may result in misinterpretation of the archaeological record. There is no indisputable evidence for

pastoralist sites with ceramics before about 4000 bp.

Duration of 'wares'

One of the apparent anomalies noted by Bower and Nelson (1978) was the persistence of ceramic 'wares' for long periods in the PN. Obviously this observation depends on adequate definition and correct assignment of pottery to ~wares'. Another critical factor is that individual assemblages should be welt dated. In view of the preceding examination of the assumptions underlying the interpretation of dates it cannot be assumed that this is true, and it is possible that the anomaly is a product of poor dating. A critical examination of the dates for a single ~ware', Narosura, leads to a contraction in its duration.

Dates for Narosura ~ware' range between 2990 and 1880 bp, with some suggestion that the lull range may extend from 5500 to 1410 bp. The latter range is based on the Salasun sequence and as such should be treated with extreme caution. Nine dated Narosura sites are known from the Central Rift and adjacent areas. The sites are Crescent Island (GtJj 2), Causeway (GtJj 3), Ndabibi (GtJi 3), Salasun (GuJj 13), Lukenya (GvJm 22, GvJm 44, GvJm 48, GvJm 52) and Narosura. All the dates li~om these sites were examined. Possible ancestral assemblages in northern Kenya were not considered. Paired dates (run on the same sample even if different fractions were measured) were analysed using a Case I formula. I f paired dates were in statistical agreement and more than one sample was run the Case II

Tab

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70

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7


formula was used to compare pooled means. In addition, the relationship between the dates and finds in any level, arbitrary or natural, was considered so that possible mixing of material could be assessed.

The results of the analysis are presented in Table 7. Only four of the nine sites have paired dates which are in agreement. Three sites, Salasun, Ndabibi and Causeway must be treated as undated. Although the paired apatite and collagen dates from GvJm 44, Lukenya Hill, are not in agreement, the apatite and charcoal samples could be estimates of the same event. The pooled mean of these dates can therefore be used as an estimate for the date of the assemblage. The dates from Crescent Island are more complex. There are four paired dates, but one of these (GX-4587/A) has to be rejected because the sample was too small tbr 13C correction (Onyango-Abuje 1977:372-5). Of the remaining pairs only one (GX-4586/A and G) is in agreement. A comparison of the pooled mean with the two unpaired bone dates shows agreement in one case (GX-4588/A). The other dated sample has no vertical provenience. The pooled mean and associated error term is 2367 + 87 bp. There are two more dates from Crescent Island, GX-3540 and N-1924 (Onyango-Abuje 1977:372). No provenience could be found for these samples and they are not in statistical agreement with the pooled mean. They have therefore been rejected.

The single pair of linked dates from Crescent Island was from the 40-50 cm level. It is possible that the upper levels have a more recent date. However, this would assume a gradual accumulation of deposits. Yet the site of Narosura has four dates between 30 cm and 90 cm below the surface (Odner 1972). All four dates are in agreement (T = 5.91; NS) with a pooled mean of 2599 + 57 bp. Although it could be argued that deposition at Narosura was not rapid and that the agreement between the dates is evidence that at least one of them is wrong, this would be fallacious since sherds can be refitted from all levels and no breaks in the stratigraphy were discernible. Thus it is clear that deposits may be accumulated very rapidly. The absence of any natural stratigraphic units at Crescent Island, coupled with the homogeneity of the ceramics through all levels, probably indicates that this is a single component site.

The dates from Narosura have already been considered. The final dated assemblage comes from GvJm 22 (Gramly 1975) and the single collagen date of 2250 + 50 bp, which is not ~3C corrected, must be rejected. If all the dated assemblages are plotted (Fig. 1 ), it can be seen that Narosura 'ware' dates between about 2700 bp and 1600 bp. In addition, the sequence appears to be more recent than the 'firm' chronology for Narosura 'ware' indicated by Bower and Nelson (1978). When compared with their long chronology it is considerably shorter. We should also add that our preliminary examination of the ceramics from some of the sites indicates that there is a great deal of variation within Narosura 'ware'. In view of this and the revised dating evidence we feel that the conservatism of this pottery tradition has probably been exaggerated.

O f s tones and stars

The discovery to the west of Lake Turkana of what may be the first archaeoastronomical site to be found in sub-Saharan Africa, together with stone cemeteries and rock art sites that may relate to the prehistory of Eastern Cushitic speakers, is of considerable interest (Lynch and Robbins 1978, 1979). Namoratunga I is a rock art and cemetery site while Namoratunga II is


YEARS bp

1 5 0 0 -

2000 -

0 J O

2 5 O O - z °

i U

3000-

Ld

m !

~n a. O ~L

3 3 O O - NAR. G t J i GvJm GvJm GvJm ~ J

2 4 4 5 2 4 8 BOWER & NELSON

Figure 1 Dates for sites with Narosura pottery and comparisons with the 'possible' and 'firm' chronologies of Bower and Nelson.

in addition claimed as an archaeoastronomical observatory. The Namoratunga sites are believed to date to about 300 Be. The date is an important component of Lynch and Robbins's arguments that the sites can be linked with the Eastern Cushites and that one of them is an archaeoastronomical observatory. However a carethl examination of the dating evidence reveals a host of problems.

There are actually two dates from Namoratunga I, one on apatite and one on collagen from different burials (GX-5042/A 2235 + 165 bp; UCLA-21240(C) 1200 + I00 bp). Lynch (1978) argues that the site was not used for very long, and thus that the dates ought to be more or less in agreement. The younger (collagen) date is rejected by Lynch ( 1978:222 fn.) on the grounds that 'in paired collagen and apatite s a m p l e s . . , collagen usually produced dates 100 to 1000 years more recent than the apatite' and that 'apatite consistently yields dates which closely approximate those obtained from wood charcoal'. No indication is given of whether the dates are 13C corrected. As we have already shown, these statements are not supportable. The younger date is also considered 'much too recent given the available linguistic and ethnographic evidence' (Ibid.:222 fn.) while the older date is 'firmly supported by the archaeoastronomical evidence . . . . . . as well as [by] the historical linguistic data' (Lynch and Robbins 1979:323). However, once the assertions about apatite and collagen dates are rejected, the argument becomes circular. The older date is accepted because it


agrees with Ehret 's (1974) glottochronologicat estimate of when Eastern Cushites should

have been in the western Turkana Basin; yet Lynch and Robbins (1979:323) point out the

value of cross-checking linguistic and archaeological data in historical reconstructions. Given the existence of the younger date, the older date on its own can hardly be claimed as

'verifying previous conclusions based on linguistic data alone'. Clearly Namoratunga I must

be deemed undated until more radiocarbon dates have been processed and scrutinized. There are further problems with the dating of the archaeoastronomical site, Namoratunga

II . Lynch and Robbins argue that it should be of the same antiquity as Namoratunga I, over

200 km distant, on the basis of similar graves and rock art. Namoratunga I I itself is not dated by radiocarbon. The archaeoastronomical ascription is based on the correlation between stone alignments and the azimuths at 300 BC of certain star constellations which are used in

calendrical calculations by present-day Eastern Cushites. However, even if we accept the

argument that the Namoratunga sites are of the same antiquity, there is still little reason why

we should accept the older rather than the younger date. I f we choose the younger date then the stones and the stars are no longer in alignment. Further, aside from the fact that the older

date may not be ~3C corrected, if it is calibrated using the dendrochronological calibration curve (Clark 1975) the date becomes 418 BC; yet in all the astronomical calculations the uncorrected date of 300 BC is used.

However support for the date favoured by Lynch and Robbins for Namoratunga I I is

provided by an astronomer. Paul (1979) demonstrates that, since 1000 BC, only in the period

from 430 BC to 150 BC have the relevant seven stars and star groupings matched the alignments of the stones. This evidence can be regarded as an independent test of the dating

hypothesis, though it is not conclusive. I t must be remembered that the identification of

Namora tunga I I as an astronomical observatory is a hypothesis, not a proven fact. Thus one cannot use Paul's work as both proof of the dating and proof of the observatory hypothesis. I f

the alignments of the stones have been measured correctly and the stones themselves have not moved since their erection, then the case for an astronomical observatory dating to about

300 BC would be strengthened beyond reasonable doubt by a reliable radiocarbon date,

i.e. one on well-associated charcoal or a pair of bone dates which are in agreement, from Namoratunga I I itself. Let us hope that this radiocarbon date wilt be obtained soon; it is a

sad truth that in archaeological and historical syntheses qualifying adjectives from more detailed reports tend to be lost and tentative ascriptions become historical facts (e.g, Lynch and Robbins fail to mention the younger date in their t978 paper).

Pastoral Iron Age dates

PIA traditions are known from the region west of Lake Turkana, the Central Rift Valley and surrounding areas. The Turkwel tradition is found in the same region as the Namoratunga sites. The available dates are said to place it between about AD 450 and AD 1100 (Lynch and Robbins 1979:325; Robbins 1980). However all the dates except two must be rejected on the grounds of poor association between the material dated and Turkwel pottery. Lynch and Robbins (1979) mention the uncertain association with the Turkwel ofGX-5041 and the fact that the pottery from Apeget I may not belong in the Turkwel tradition. N-909 can be rejected as the pottery is in erosional context and there is no evidence to identify the charcoal stain dated as an archaeological feature or to link it with the pottery. One of the remaining


two dates cannot be accepted because bone fragments and burned midden soil yield unreliable results (Gilet-Blein, Marien and Evin 1980). Thus only one date Of AD 1080 -t- 90 (UCLA-2124G) applies to the Turkwel tradition with any certainty. It is perhaps interesting to speculate that if the Namoratunga sites are more recent than previously assumed then the sites of the Turkwel tradition may be the settlement sites of the people buried at Namoratunga. I f not, then where are the settlements that belong with the Namoratunga cemeteries?

Bower and Nelson (1978:564) suggest that the transition to the Pastoral Iron Age in the Central Rift Valley began about 1400 bp and is indicated archaeologically by a tendency towards deformalization of lithic technologies and the adoption of twisted cord rouletted techniques for decorating pottery. However there are only four dates presented by them which are earlier than 485 bp. One of these is an apatite date (GX-4420/A) from Salasun which is not in agreement with its paired collagen date and must be rejected. Another two dates (GX-4506/A 1240 + t45 bp; GX-4506/C 1250 + 115 bp; NS) are a paired set of bone dates from a human burial with iron objects at Lukenya Hill. Although the dates are in agreement there seems no reason to attribute the iron objects (and hence the burial) to the PIA rather than the Early Iron Age or to trade between PN and Early Iron Age peoples. The remaining date (GX-4310 1025 +- 130 bp) from Masai Gorge rock shelter is not firmly associated with cord rouletted pottery but dates an occupation horizon when stone usage at the site had already ceased (Ambrose pers. comm.). Thus the available evidence suggests that although the use of stone artefacts in the Central Rift may have ended in the first millennium AD, twisted cord rouletting may only have appeared around AD 1500.

The Deloraine Farm site, located in the Rift Valley west of Nakuru, has also produced an assemblage with few stone artefacts that would fall under the definition of the PIA, though the finds include seeds of finger millet (Chittick 1979), and the pottery is not decorated by cord rouletting. There are seven dates from this site, three on charcoal and two pairs of bone dates (Cohen 1972; Chittick 1979; Chittick and Ambrose pers. comm.). Both pairs of bone dates are in agreement (GX-5541/A 1000 + 115 bp; GX-5541/G 1110 + 120 bp; T = 0.437; NS. GX-5542/A 1150 + 130 bp; GX-5542/G 985 + 130 bp; T = 0.775; NS) both in their collagen and apatite readings and with each other (Ap = 1058 +- 62 bp). The bone dates also agree with two of the three charcoal dates (Ap = 1058 + 62; GX-5543 1300 + 140 bp; T = 2.498; NS. Ap = 1058 _+ 62; N-652 1070 _+ 100 bp; NS by inspection) and taken together they suggest an age of about 1100 bp. The charcoal sample for the remaining date of 1990 -+ 80 bp (N-2313) was taken from a road-cut beside the site (Chittick and Ambrose pers. comm.) and clearly does not date the occupation. Therefore it seems necessary to distinguish between the appearance of the PIA in the Central Rift around 1100 bp and the

spread of twisted cord rouletting some 600 years later.

Conclusions

We have shown how an uncritical acceptance of radiocarbon dates may lead to apparent anomalies in the archaeological record. The implications are far reaching, particularly in relation to the claims for early pastoralist settlement in East Africa. The analysis of the radiocarbon dates has shown that there is no objective basis for accepting the early dates from Satasun. This means that there is at present no firm evidence for early pastoralism


before 4000 bp in East Africa. The question of earlier dates must remain open until further

dates are obtained and shown to be valid. The clarification of the PN chronology is obviously a major problem in African archaeology. I f very early pastoralism occurs in Kenya this

would be extremely important for our understanding of the origins of domestication. I t is

therefore imperative that the chronological framework be properly constructed and above criticism. Future research should be directed towards this end.

The question of the duration of ceramic 'wares' is more complex because two sets of observations must be examined, the assignment to a 'ware ' category and the date of the

assemblage. The examination of Narosura 'ware ' dates has shown that a contraction in the

duration of this 'ware ' occurs when dates are critically evaluated. Furthermore, a number of

Narosura assemblages are to all intents and purposes undated. An even more marked contraction occurs in the duration of Nderit 'ware' and only three securely dated sites are

known. An example of the effect of re-examining the placing of the assemblages into 'wares '

can be seen in the recent reassessment of Kansyore 'ware ' (Collett and Robertshaw 1980). Material assigned to this taxon from Lukenya and the Central Rift was found to belong to a different entity.

The discussion of the results has so far focused on archaeological questions. Perhaps one of the more interesting fields of speculation is the relationship between archaeological remains

and historical linguistic reconstructions and ethnohistorical data. In the interpretation of the

archaeoastronomicaI site of Namoratunga I I all these fields are combined. However, not only is Namoratunga I I dated by analogy to a site 200 km away, but there is little reason for

accepting the 300 BC date in preference to the date of about AD 750. O f course if one accepts on the basis of the ethnohistorical evidence that the Namoratunga sites were built by Eastern

Cushitic speakers then the historical linguistic evidence may predispose one to accept a

particular date. But because the archaeological evidence is weak, it cannot be used in this instance as confirmation of the historical linguistic reconstruction.

I t should be clear that bone may not be a particularly useful dating material. Certainly the

large number of anomalous apatite/collagen dates would indicate that bone dates should only be accepted with reservations. Charcoal is ideal for radiocarbon dating although even

with this material anomalous results may be obtained (Michels 1973). There are no guaranteed ways of ensuring that radiocarbon dates are valid. I t behoves the excavator and the synthesizer to interpret dates critically and to be prepared to reject or hold dates in a

suspense account until further work clarifies the chronology. We would further urge that more rigour be shown in the publication of details of radiocarbon dates. The format for the presentation of dates used by the journal Radiocarbon should be followed.

A number of statistical techniques, some of which we have used here, are available to assist the archaeologist and the historian to evaluate radiocarbon dates. Familiarity with these techniques may well breed contempt for some prehistoric chronologies.

Endnotes

1. The use of the term 'ware ' is a source of some confusion because it does not refer to a single type, an assemblage, or a technological aspect of pottery manufacture. Definition of types and comparisons of assemblages to define phases, facies and traditions would be a more


explicit and fruitful approach. Furthermore it would reduce the variability in the attributes

used to define PN taxa. In this article we place the term 'ware' in inverted commas to denote

its meaning as used by those working in East Africa.

Acknowledgements

We thank Harry Merrick for many helpful comments on an earlier version of this paper.

Naomi Mariwa typed and corrected numerous drafts.

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Documents

Problems in the interpretation of radiocarbon dates: the Pastoral Neolithic of East Africa