The age of Clovis—13,050 to 12,750 cal yr B.P.

New information for the Lange-Ferguson site provides a better understanding of its age. Charcoal dated to 11,140 ± 140 ¹⁴C yr B.P. (AA-905) and reportedly from the Clovis horizon (16) actually was from stratum C, 10 cm above the mammoth bones, and thus postdates the Clovis occupation (17). Only the two ages on XAD-purified collagen from two different mammoth bones found with artifacts in stratum B₃ date the Clovis activity at the site. These ages are 10,710 ± 130 ¹⁴C yr B.P. (UCIAMS-11344) and 11,110 ± 40 ¹⁴C yr B.P. (UCIAMS-11345). The age with the large SD (UCIAMS-11344) is disregarded in favor of the age with the lower SD. This equates to an age of 13,095 to 12,990 cal yr B.P. for Lange-Ferguson.

In 2007, two XAD-purified collagen ages on mammoth bone associated with Clovis artifacts provided the best age estimate for the Clovis activity at the Dent site (5). Since then, an XAD-purified collagen age of 10,960 ± 35 ¹⁴C yr B.P. (UCIAMS-116403) (18) and two ages on hydroxyproline of 11,055 ± 50 ¹⁴C yr B.P. (OxA-X-2736-11) and 11,155 ± 50 ¹⁴C yr B.P. (OxA-X-2736-12) (10) were obtained on the same bone. These five ages average 11,010 ± 15 ¹⁴C yr B.P. or 12,970 to 12,845 cal yr B.P.

Most radiocarbon ages from the Domebo site provide bracketing dates for this Clovis site. A single date of 10,960 ± 30 ¹⁴C yr B.P. (UCIAMS-11341) or 12,905 to 12,820 cal yr B.P. was obtained on XAD-purified collagen from mammoth bone directly associated with the Clovis artifacts from this site (5).

Previously, five radiocarbon ages on charred hawthorn (Crataegus sp.) seeds from two hearths associated with Clovis artifacts were reported for the Shawnee-Minisink site (5, 19). One additional date of 10,970 ± 50 ¹⁴C yr B.P. (OxA-1731) on seeds from a hearth close to a Clovis projectile point is reported (19). This new date overlaps the five other reported hawthorn seed ages at one sigma. The average of all six ages from the site is 10,940 ± 15 ¹⁴C yr B.P. or 12,885 to 12,770 cal yr B.P.

In 2007, there were no ages directly associated with the Clovis artifacts from Sheriden Cave (5). Bracketing ages placed the Clovis artifacts between five radiocarbon ages averaging 10,600 ± 30 ¹⁴C yr B.P. (12,685 to 12,620 cal yr B.P.) and two radiocarbon ages averaging 10,920 ± 50 ¹⁴C yr B.P. (12,880 to 12,760 cal yr B.P.) (5, 20). To directly date the Clovis assemblage, a ¹⁴C measurement on XAD collagen from one of the two bone projectile points associated with the Clovis lithic projectile point and other tools dated 10,915 ± 30 ¹⁴C yr B.P. (UCIAMS-38249) and calibrates to 12,830 to 12,770 cal yr B.P. (20).

Clovis artifacts are associated with mammoth bones at the La Prele (formerly named Fetterman) site, Wyoming (21). Fourteen radiocarbon ages have been obtained from the site, but only two of those dates provide an accurate age for the Clovis occupation. Contamination issues plague the other 10 dated samples—four dates on humins range from 10,323 ± 39 to 10,969 ± 52 ¹⁴C yr B.P. (D-AMS-4329; AA-105498), three dates on unpurified mammoth bone collagen range from 8890 ± 60 to 10,760 ± 30 ¹⁴C yr B.P. (CAMS-74661; UCIAMS-40174), two apatite ages on mammoth bone are 11,190 ± 130 and 13,997 ± 90 ¹⁴C yr B.P. (AA-107104; AA-109297), and three ages on ultrafiltered collagen from mammoth bone range from 10,654 ± 58 to 11,066 ± 61 ¹⁴C yr B.P. (AA-108894; AA-108893) (21). The ages we consider accurate are 10,965 ± 30 ¹⁴C yr B.P. (UCIAMS-206764) on XAD-purified bone collagen and 11,035 ± 50 ¹⁴C yr B.P. (OxA-X-2736-14) on hydroxyproline (10) derived from mammoth bone. These two ages average 10,985 ± 25 ¹⁴C yr B.P. or 12,915 to 12,835 cal yr B.P. and, finally, provide an accurate age for the La Prele Clovis site.

Two radiocarbon ages averaging 10,870 ± 20 ¹⁴C yr B.P. or 12,820 to 12,800 cal yr B.P. were obtained on XAD-purified collagen from mammoth bone associated with bone pile 2 and Clovis artifacts (5).

In addition to the three radiocarbon ages on XAD-purified collagen from bison bones associated with Clovis artifacts at the site reported in 2007 (5), two additional XAD-purified collagen dates on bison were published (22). These five ages average 10,820 ± 10 ¹⁴C yr B.P. or 12,755 to 12,745 cal yr B.P.

In 2007, the most reliable age for the ochre-stained Anzick-1 human cranium was an XAD-purified collagen date of 10,705 ± 35 ¹⁴C yr BP (CAMS-80538) (5). This age was younger than two dates on alkali-collagen from antler foreshafts associated with the human remains: 11,040 ± 60 ¹⁴C yr B.P. (Beta-163832) and 11,040 ± 40 ¹⁴C yr B.P. (Beta-168967) (23). These two dates should be considered minimum ages due to the low chemical specificity of alkali-collagen (12). In 2014, a new XAD-purified collagen age of 11,025 ± 30 ¹⁴C yr B.P. (UCIAMS-61661) was reported for one of the antler foreshafts (24). In 2018, hydroxyproline isolated from two antler foreshafts yielded ages of 10,900 ± 50 ¹⁴C yr B.P. (OxA-X-2739-55) and 11,050 ± 55 ¹⁴C yr B.P. (OxA-X-2739-56) (12). These two ages overlap the XAD-derived antler foreshaft age at two sigmas. Hydroxyproline was also isolated from the Anzick-1 cranium and yielded an age of 10,915 ± 50 ¹⁴C yr B.P. (OxA-X-2739-54) (12). This age is older than the XAD-derived age on the cranium by 210 radiocarbon years. The date on the hydroxyproline fraction should be considered the most accurate age for the human remains, and the XAD date should be disregarded. The reason for the failure of the XAD method to provide an accurate age for the human cranium is unknown, but could be due to the glue used to hold the cranial bones together or soaps used to remove ochre from the bones when first found. The new single–amino acid age on the human calvarium is now in agreement with the ages from the antler foreshafts and overlap by 1 SD. The average of the three foreshaft dates is 11,000 ± 25 ¹⁴C yr B.P., which calibrates as 12,960 to 12,840 cal yr B.P. The calibrated age for the Anzick-1 cranium is 12,880 to 12,760 cal yr B.P. and indicates that the human remains and Clovis artifacts are contemporaneous. Averaging all four ages that overlap at one sigma yields a radiocarbon age of 10,985 ± 20 ¹⁴C yr B.P., indicating that the Clovis burial occurred sometime between 12,905 and 12,840 cal yr B.P.

Only one age, 10,920 ± 250 ¹⁴C yr B.P. (Beta-81589) on hearth charcoal, was available from the Clovis horizon at the Cactus Hill site in 2007 (5, 25). The large SD associated with this age precluded its use to accurately date the Clovis occupation at this site. Five new radiocarbon ages on charcoal from hearths and other features associated with the Clovis horizon are reported for the site (25). These ages are 10,860 ± 60 ¹⁴C yr B.P. (Beta-266341), 10,840 ± 40 ¹⁴C yr B.P. (Beta-233642), 10,810 ± 40 ¹⁴C yr B.P. (Beta-233642), 10,890 ± 40 ¹⁴C yr B.P. (Beta-233643), and 10,910 ± 40 ¹⁴C yr B.P. (Beta-210651). The average of these five ages is 10,860 ± 20 ¹⁴C yr B.P. or 12,820 to 12,745 cal yr B.P.

Clovis artifacts along with the bones of mammoth, bison, and other extinct animals were recovered from sand filling a channel known as the Graveyard Gulch member of the Murray Springs formation (stratum F₁) (28). Artifacts and bones were also found on the eroded surfaces adjacent to the channel. Interpretation of the archaeological and faunal remains suggests that the site likely represents two or more episodes of use over a few years (28). However, eight radiocarbon ages on dispersed charcoal from stratum F₁ range from 11,150 ± 450 ¹⁴C yr B.P. (A-805) to 10,710 ± 160 ¹⁴C yr B.P. (TX-1459) (Table 2) (5, 28) and do not support a radiocarbon estimate of a short-duration event (see 14). These eight ages do not overlap at one sigma, and the date means range over the entire span of Clovis. Seven of the radiocarbon ages have one-sigma SDs of ±450 to ±140 radiocarbon years. Despite this, the eight radiocarbon dates were averaged to 10,900 ± 50 ¹⁴C yr B.P., a date commonly ascribed to the Clovis occupation at the site (16, 28). However, as expected, by averaging dates that span the range of Clovis, a date mean in the middle of the Clovis range is obtained. The averaging of the ages from the Clovis horizon does not provide an accurate date for the occupation of the Murray Springs site by Clovis foragers. An overlooked age from the site of 10,760 ± 100 ¹⁴C yr B.P. (A-1045; 12,825 to 12,675 cal yr B.P.) is reported on charcoal from a Clovis hearth (28). This age was rejected and considered to be contaminated by carbon from overlying deposits (28). This date, from a hearth, might not be contaminated and could actually represent the true age of the Clovis occupation at the Murray Springs site, thereby making Murray Springs a terminal Clovis occurrence. Given all these uncertainties, at present, we consider the Clovis occupation at the Murray Springs site to be insufficiently dated for inclusion in a Clovis chronology.

Clovis artifacts with mammoth bone were excavated from a sand and gravel-filled channel known as Mammoth Kill Creek (unit F₁) (16). Archaeological and faunal interpretations suggest that this is a single kill event or, perhaps, a site that was revisited with mammoth kills occurring in the same location over a short period of time. Twelve radiocarbon ages on dispersed charcoal from unit F₁ range from 11,470 ± 110 ¹⁴C yr B.P. (SMU-308) to 10,620 ± 300 ¹⁴C yr B.P. (SMU-347) (Table 2) (16). These 12 ages do not overlap at one sigma and again range across the entire span of credible Clovis ages; this is not the chronological signature of a single, short-lived hunting event (see 14). Despite these issues, the 12 ages were averaged to 10,930 ± 40 ¹⁴C yr B.P., the date ascribed to the Clovis occupation at the site (16). As at the Murray Springs site, by averaging dates that span the entire range of Clovis, a date mean in the middle of the Clovis range is generated. We doubt that this is an accurate age for Clovis activity at the Lehner site. Only two of the radiocarbon ages have SDs <100 radiocarbon years, and these do not overlap at two sigmas. Given all these uncertainties, we consider the existing radiocarbon dates too imprecise to accurately determine when the Clovis activity took place at the Lehner site.

Radiocarbon ages reported on charcoal from the Paleo Crossing site, Ohio, seemed to provide credible and reliable chronological control for the site (5, 29). However, recent excavations show that the reported radiocarbon ages do not accurately date the Clovis horizon (30). Most of the dated samples were collected from post-hole infillings that were described to be part of a prehistoric structure. Subsequent work at the site shows that the post molds are part of a historic structure (30). Furthermore, the original dates were reported to be on charcoal but were actually on bulk sediments that were a mixture of different strata of different ages. On the basis of this new understanding of the site, all previously reported ages for Paleo Crossing are disregarded and the site is defined as undated.

An ivory foreshaft from the Sloth Hole site yielded a radiocarbon age of 11,050 ± 50 ¹⁴C yr B.P. (SL-2850) (31). At least one Clovis point was found at the site, but it was collected out of context and the point’s relationship to the dated ivory artifact is unknown. Also uncertain is the exact geologic context of the dated ivory artifact. Last, because ancient ivory can be collected and used to make durable tools long after the death of the mammoth or mastodon (32), a radiocarbon age on ivory does not necessarily date the production of the artifact. Because of these uncertainties, the significance of the ivory foreshaft’s age is equivocal.

The evidence for an early Clovis occupation at the Aubrey site, Texas, is based on two radiocarbon dates on material reported to be charcoal, 11,540 ± 110 ¹⁴C yr B.P. (AA-5271) and 11,590 ± 90 ¹⁴C yr B.P. (AA-5274); the dates average 11,570 ± 70 ¹⁴C yr B.P. or 13,490 to 13,355 cal yr B.P. (33). This charcoal came from the Clovis occupation surface of camp B, which sits on the eroded surface of stratigraphic unit A2 and is overlain by unit G. The alluvial sediments underlying the Clovis surface are >17,000 years old, and the upper portion of the alluvium has been altered by pedogenesis and erosion. The age of the alluvium indicates that this erosional surface was exposed for thousands of years before the Clovis occupation. The age of the burial of the Clovis occupation surface is unknown, but a ¹⁴C measurement on humates from overlying unit G was 10,718 ± 90 ¹⁴C yr B.P. (SMU-2338; 12,755 to 12625 cal yr B.P.). The exposure of the geological surface before and after Clovis occupation would have created a situation where geologically ancient carbon could have become commingled with and contaminated charcoal samples associated with the Clovis occupation. In addition, the Clovis-age samples may have been contaminated by older, soluble humates derived from underlying deposits. The Late Quaternary sediments at the Aubrey site are predominately derived from Cretaceous bedrock that contains large amounts of ¹⁴C-free carbon. Large quantities of ¹⁴C-depleted carbon in the form of recycled Cretaceous palynomorphs are documented in the late Pleistocene deposits at the Aubrey site (34). Furthermore, the Aubrey site is downstream from and adjacent to outcrops of the Late Cretaceous Woodbine Formation, which contains lignite. Just 20-km downstream from the Aubrey site is the Lewisville site (35), where charcoal samples from hearths associated with Clovis points yielded ages of >37,000 ¹⁴C yr B.P. (O-235 and O-248). Reinvestigation of the Lewisville site in the 1980s revealed that small amounts of lignite contamination had skewed the radiocarbon dates to anomalously older ages (35). These chronological issues raise concern about the validity of the geologically early ages from the Aubrey site. Until more samples of chemically distinct and taxonomically identified charcoal and the soluble humate fractions from these charcoal samples are dated, the true age of the Aubrey site will remain unknown. If the current age estimates for the Aubrey site are confirmed, it would indicate that the oldest Clovis site occurs in the southern portion of the Clovis geographic range.

At El Fin del Mundo, Mexico, bones of a gomphothere (Cuvieronius sp.) are associated with Clovis lithics in the upper 15 cm of stratum 3B (36). A single sample identified as charcoal and associated with the gomphothere bones was dated as 11,560 ± 140 ¹⁴C yr B.P. (AA-100181) or 13,590 to 13,300 cal yr B.P. This date was statistically manipulated to yield a similar date mean, but with a SD reduced by over 50%. This “calculated” age is 11,550 ± 60 ¹⁴C yr B.P. (AA-100181A) or 13,470 to 13,350 cal yr B.P.

The dated “charcoal” from stratum 3B is described as “fragmented carbonaceous matter” that could not be taxonomically identified (36). Furthermore, the dated organics are geological; they are not from an archaeological feature such as a hearth. The organic matter that was dated was one of only two samples identified as charcoal with “woody tissue structure” from stratum 3B. Both were pretreated using standard acid-base-acid procedures. One sample dissolved completely during alkali treatment; its soluble organics yielded an age of 11,880 ± 200 ¹⁴C yr B.P. (AA-100182). The other sample (AA-100181), which produced the 11,560 ± 140 ¹⁴C yr B.P. age, yielded only 0.23 mg of carbon after pretreatment, a loss of 81% of its original mass (36). Three other samples of “fragmented carbonaceous matter” were collected from stratum 3B, but these lacked woody structure and were not dated. These laboratory observations—significant dissolution in alkali, and very small yields—are indicative of bacterially reduced organic matter, not combustion-derived charcoal. Alkaline springs deposited carbonates (marls) upslope of the site, and these marls are lateral facies of stratum 2 and 3 sediments (36). The dated sample from the site could have been derived from the spring deposits upslope of the site and could be any combination of terrestrial plants and emergent or submerged aquatic flora.

Furthermore, a carbonate reservoir effect may have made samples from the site date older than their true age. Dissolved inorganic carbon, (CO₃⁻²) in the site’s modern groundwater had a decreased ¹⁴C content relative to the modern atmosphere and yielded an apparent ¹⁴C age of 1180 ± 30 ¹⁴C yr B.P. (AA-92972; no Fm value was reported) (36). Similar or larger carbonate reservoir effects may have existed during the Late Pleistocene and Early Holocene at the site. Aquatic organic matter in the sediments containing the gomphothere bones and overlying diatomites would have had their ¹⁴C/¹²C values significantly depleted by carbonate reservoir effects. Consequently, the 11,560 ¹⁴C yr B.P. date on organic matter associated with the gomphothere may be 1000 ¹⁴C years too old. The uncertainty of using dispersed organic matter to date the site, combined with the unknown biological origin of the organic matter and the potential for carbonate reservoir effects, forced us to disregard this 11,560 ¹⁴C yr B.P. age.

We also question the recalculation of blanks and backgrounds used to modify the measured age to produce a revised age for the Clovis horizon with a smaller SD and a slightly younger date mean. The measured date mean and SD for AA-100181 is 11,560 ± 140 ¹⁴C yr B.P. This age was modified to 11,550 ± 60 ¹⁴C yr B.P. (AA-100181A).

For small samples, background values are inversely proportional to sample size. This is not the result of “machine-induced fractionation,” as claimed in (36), because micrograms of modern carbon contamination will affect a small sample more than a large one. Because blank values in table S7 in (36) are high [about three times the values quoted in (37) for blanks of the same size], blanks for the larger samples are probably subject to inverse-mass dependence, contrary to what is stated in (36).

Blank values typically vary in a quasi-random fashion, and this scatter (37) should be taken into account when assigning an uncertainty to the blank correction. On the basis of long-term evaluation of run-to-run scatter in blanks (and in similar variations observed in different blank aliquots measured in the same run), many laboratories assign an uncertainty of approximately ±30% of the actual blank value.

One or both of these effects can be seen in the raw data shown in table S7 in (36), where the small (0.1 mg of carbon) blank run on 8 August 2013 was about one-half the size of blanks run on 19 and 20 November 2012 (0.16 and 0.23 mg), but the August blank gave an Fm value of 0.035, which is almost four times as high. Even assuming an inverse size dependence, one would have expected a value of just 0.018 based on the 2012 values. This raises the possibility that the relatively good agreement between the November 19 and 20 blank results is simply the result of random chance and suggests that it would be prudent to adopt the ±30% uncertainty throughout.

The equation for the blank subtraction is F′ = F − (1 − F) * B, where F is the “raw” fraction modern value for the sample and B is the blank value. The 1 − F factor arises because the F values are ratios of carbon isotopes and the background adds stable carbon as well as ¹⁴C (e.g., adding modern carbon contamination to a modern sample with F = 1 has absolutely no effect).

The first sample in table S7 in (36) has F = 0.2443, and the one sigma uncertainty based on the scatter is ±0.0024 (one should always take the larger of the scatter and statistical uncertainties). If we take B = 0.009 ± 0.003 for samples of around 0.2 mg, then the background correction for a raw F value of 0.24 is (1 to 0.24)*(0.009 ± 0.003) or 0.0068 ± 0.0023. The uncertainty in the raw value and the correction should be added in quadrature (square, add, and take the square root), so the corrected value is 0.2375 ± 0.0033, and converting this to an age gives 11,550 ± 110 ¹⁴C yr B.P. The uncertainty is therefore almost twice the value of the ±60 years reported in (36). Because of all these issues, we conclude that the Clovis activity at El Fin del Mundo remains undated.

A Clovis component is suggested to be present at the Sheaman site, Wyoming, based on the occurrence of overshot and overface flakes, and bifaces with overface flake scars. The only diagnostic artifact from the site is a stone projectile point that has been identified as either Goshen or Agate Basin, but not Clovis (38).

Dating the Sheaman site has been difficult. Two dates on charcoal, 10,140 ± 500 ¹⁴C yr B.P. (RL-1241) and 10,100 ± 2800 ¹⁴C yr B.P. (RL-1000), are believed to postdate the artifact-bearing horizon (39). A bison long bone fragment from the artifact zone yielded an age on chemically unspecified collagen of 10,030 ± 280 ¹⁴C yr. B.P. (RL-1263) (39). To clarify the age of the site, two dates were measured on dispersed pieces of charcoal below the artifact zone and were 10,153 ± 90 ¹⁴C yr B.P. (AA-42979) and 1820 ± 170 ¹⁴C yr B.P. (AA-42535) (39). Another sample of dispersed charcoal was dated from the artifact-bearing zone and yielded an age of 10,250 ± 70 ¹⁴C yr B.P. (AA-42533) on the HCL-insoluble charcoal fraction and an age of 10,025 ± 85 ¹⁴C yr B.P. (AA-42534) on humates from the charcoal’s alkali-soluble fraction (39). Another date on dispersed charcoal above the artifact layer yielded a date of 9085 ± 70 ¹⁴C yr B.P. (AA-42532). A large volume of sediment was collected from the artifact-bearing zone and yielded charcoal and unidentified acid-insoluble organic matter that produced three dates: 11,040 ± 70 ¹⁴C yr B.P. (AA-40989), 11,379 ± 70 ¹⁴C yr B.P. (AA-40991), and 11,810 ± 300 ¹⁴C yr B.P. (AA-40988) (39). Despite these three ages not overlapping at one sigma and one having a SD of 300 ¹⁴C years, all three were averaged as 11,225 ± 50 ¹⁴C yr B.P.—the age assigned to the Sheaman site (39). To reassess the site’s age, we dated the osseous projectile point that was associated with the reported Clovis artifacts and the stone projectile point. The osseous point was made from cervid bone or antler, which had previously been identified as ivory. Three dates on XAD-purified collagen from this osseous artifact yielded an average age of 10,305 ± 15 ¹⁴C yr B.P. or 12,095 to 11,950 cal yr B.P. (5). Considering all the ages from Sheaman, we believe the most accurate ages are those derived from the osseous artifact because the dates were obtained on an artifact directly associated with the cultural horizon and the ages were derived from XAD purified collagen. The artifact technology and new ages suggest that the Sheaman site is either a physically mixed assemblage of Clovis and Goshen/Agate Basin artifacts or there is no Clovis presence at the site and all the artifacts are Late Paleoindian Goshen or Agate Basin. If the latter is true, many of the diagnostic flaking methods that we associate with Clovis bifaces, e.g., overface flaking, overshot flaking, and alternate opposed flaking, may not inherently define Clovis and may have been used in later Paleoindian times.

At the Casper site, Wyoming, a single Clovis point was found by two avocational archaeologists in loose, disturbed sand in an area where 8 m of dune sand had been removed by mechanical equipment. After recovery of the artifact, the discoverers dug into the sand and found bison bones. This discovery led to further investigations that uncovered numerous Hell Gap–style projectile points associated with the remains of over 100 bison (40). Two dates from the bone bed, one on charcoal of 9830 ± 350 ¹⁴C yr B.P. (RL-125) and the other on a bison bone of 10,060 ± 170 ¹⁴C yr B.P. (RL-208), place the Hell Gap bison kill sometime between 11,850 and 11,300 cal yr B.P. Below the bison remains, several long bones and other skeletal elements of camel (Camelops sp.) were found. XAD-purified collagen from the Camelops bone dated 11,190 ± 50 ¹⁴C yr B.P. (CAMS-61899) or 13,160 to 13,090 cal yr B.P. (40). No artifacts were associated with the Camelops, but the bones showed several green bone fractures (40). There is no stratigraphic association between the Camelops bone and the isolated Clovis point found above the layer with Hell Gap projectile points and bison bones. However, some believe the taphonomy of the camel skeleton confirms an early human presence at the site (26). They associate this early activity to Clovis based on the age of the camel bones and the presence of an out-of-context Clovis point. Because there is no association between the Clovis point found at the surface above the Hell Gap horizon and the Camelops bones stratigraphically below the Hell Gap horizon, the Clovis-age ¹⁴C date from the camel cannot be assigned to the unassociated Clovis projectile point. Consequently, the date on the camel provides no information for the age of Clovis.

In 2007, no credible age was available for the type locality of Clovis at Blackwater Draw, New Mexico. In 2012, “black charcoal–like plant remains” from the Clovis horizon, stratum B3, were dated (41). During chemical pretreatment, these samples dissolved 100% during alkali pretreatment. Upon adding HCl, a black precipitate formed, and upon drying and combustion, this black residue (“humic acids”) was dated in triplicate as 9920 ± 80 (AA-30453B2), 10,915 ± 70 (AA-30454B), and 10,935 ± 55 ¹⁴C yr B.P. (AA-87917). The 9920 ± 80 age was disregarded as too young, while the two oldest ages were accepted as accurately dating the Clovis horizon despite all three ages having been obtained on the same precipitate. None of the radiocarbon measurements on humic acids provide an unequivocal age for the Blackwater Draw Clovis horizon.