Presented in the session, “Chert Sourcing, Techniques, Methods and
Applications,” 77th Annual Meeting of the Society for American
Archaeology, Memphis, Tennessee, April 19, 2012.
Ft. Payne Chert: Infrared Laser Spectroscopy (ILS) Dating of
Palaeo-American Artifacts and Trace Element Characterization
Richard Michael Gramly, PhD*
David Hunter Walley
American Society for Amateur Archaeology
North Andover, Massachusetts
(gramlyasaa@verizon.net)
Ft. Payne chert, which has been mapped variously as 1) part of the
Mississippian Tullahoma Formation (Jenkins 1915), 2) part of the
Tuscumbia Limestone (compiled by Osborne et al. 1989), and 3) in a
group with the New Providence Shale and Maury Formation (Marcher
1962), has its type locality at Ft. Payne, DeKalb county, northeastern
Alabama. Unfortunately the exposure of chert at the type location is
poor.
For the purpose of this presentation and future investigations we
recognize a new type locality of Ft. Payne chert – one where exposure is
good and outcrops are easily accessible for sampling (Figures 1 and 2).
This locality lies near a major highway passing through Leoma,
Lawrence County, south-central Tennessee. Ft. Payne chert was an
important raw material for populations throughout prehistory in this
region. Sites with Ft. Payne chert artifacts are abundant along the West
Fork of Sugar Creek in the vicinity of the type locality.
The description of Ft. Payne chert given by Marcher for Stewart
county, northern Tennessee (1962: 13-14) and quoted here also applies
to fresh raw material at the Leoma type locality:
Chert, which occurs as persistent beds or irregular nodules in the
limestone, is nearly everywhere brownish black to olive black. Because
of its dense and brittle character the chert fractures along irregular
planes, producing angular pieces of all sizes. Under magnification the chert
is very fine grained and fairly uniform in texture, much like the limestone.
The silica is mostly cryptocrystalline, but small amounts of chalcedonic
silica are present. Most of the chalcedonic silica occurs as irregularly-shaped
spherulites randomly scattered through the rock. Brownish organic matter
and finely disseminated iron oxides are abundant, much more so than in the
limestone. Small carbonate rhombs are present but are rare. Small cubes and
irregular masses of unaltered pyrite also occur, but most of them have been
more or less oxidized.
At the Leoma type locality fresh Ft. Payne chert is “medium dark
gray (N4)” in color (Figure 3); while, chert that has been exposed to the
elements for a few seasons lightens in color to “olive gray (5Y4/1)” or
even “pale brown (5YR5/2)” – see USGS Rock-Color Chart (1975).
After more than 13,000 years of weathering in the soils of Tennessee and
northern Alabama, nearly all Palaeo-American flaked stone artifacts
made of Ft. Payne chert have achieved a “light olive gray (5Y6/1)” or
“yellowish gray (5Y7/2)" color sometimes with white mottling. Iron
staining may partially mask old, color-bleached surfaces.
The most abundant 12 elements in unweathered Ft. Payne chert from
the Leoma type locality as determined by X-ray fluorescence are in
descending order: Si, Ca, K, Fe, P, Sr, Ti, Zr, Cl, S, Ba and Sn. The range
in per cent abundance is from 13.01 +/- 02 (Si) to .000043 +/- 2.4 or 43
+/- 2.4 parts per million (Sn). Apart from silicon, the chert is dominated
by calcium, potassium and iron. It is interesting to note that the average
abundance of barium in fresh, unweathered Ft. Payne chert is only 34-38
parts per million. This low value stands in marked contrast to the
abundance of barium within the weathered rind of some ancient stone
tools. We have measured concentrations 60-90 times the amount present
in Ft. Payne chert fresh from the outcrop.
Ft. Payne chert is well exposed in northern Alabama and centralwestern Tennessee. Without doubt it is the most important tool material
in the archaeological record of that region (Miser 1921: 133) and
adjacent northeastern Mississippi. To the south of the Ft. Payne zone and
stratigraphically overlying it is another Mississippian-age formation with
abundant chert – the Bangor Limestone (see Osbourne et al. 1989).
Bangor chert, which is gray and translucent at thin edges when fresh,
retains its color even after prolonged weathering. In that respect it
resembles chert in the St. Louis and Ste. Genevieve formations, which
are well exposed across Kentucky and northern Tennessee beyond the
Ft. Payne zone (see for example, Shawe 1966). Discriminating
visually between artifacts made of Ft. Payne chert versus artifacts
fashioned of Bangor, St. Louis and Ste. Genevieve cherts does not
appear to be a problem. However, our recent work with Middle Stage
Cumberland artifacts from the Phil Stratton site that were assumed to be
Ft. Payne chert has revealed significant variations (Figure 4). In short,
quick visual inspection of weathered artifacts may confuse Ft. Payne
chert with other raw material varieties.
We emphasize the abundance of organic matter that has been
observed in Ft. Payne chert by Marcher (1962). It reminds us of
lignin-rich Knife River flint from the Dakotas. Both Ft. Payne and Knife
River exhibit marked color change when weathered. We hypothesize that
microscopic biogenic organisms find these raw materials to be nutritious
and live within their surfaces. Further, we believe that these organisms
may be responsible for some chemical and physical transformations
during weathering.
What Is Weathering?
The characters of weathering rinds on cherts and other siliceous rocks
have often been discussed; however, the processes that create them
remain “complex and not well understood” (Wagner 1998: 301). In a
general way, the net result is thought to be subtractive, that is, elements
on the outside of a weathered rock become depleted leaving behind
relatively higher amounts of less soluble components. Organic acids
generated by decaying plants and carbonic acid in rain-water are thought
to be responsible for dissolving rocks and liberating their ions. In certain
soils and climates, chert may be leached throughout leaving few traces
of its original composition and rendering it friable – good examples
being Clovis-age artifacts from the Topper and Crosby Bay sites, South
Carolina that are made of Coastal Plains chert (Charles McNutt and
Christopher Moore, personal communication).
In the case of Ft. Payne chert this elementary view of weathering
cannot be the full story, as we have determined that some elements are
enriched at the surface of ancient artifacts. Further, enrichment seems to
increase over time – the most ancient specimens exhibiting the highest
concentrations.. In Figure 5 are shown graphical outputs of elemental
abundance for 1) raw Ft. Payne chert, 2) a weathered surface of a Clovis
point made of Ft. Payne chert, and 3) a weathered surface of a Middle
Stage Cumberland point of Ft. Payne chert. As we have observed for
other samples, the raw Ft. Payne chert has low barium (50 ppm). On the
other hand, the Clovis point was found to have 1,593 +/- 19 ppm; while,
the Cumberland point yielded the result 3,157 +/- 22 ppm. In other
words, the surface of the Clovis point was 32 times richer in barium than
unweathered rock to be found within it. The Cumberland point’s surface
was 63 times richer. Such impressive changes because of weathering call
for an explanation.
Irradiation of the barium-rich surfaces of Palaeo-American artifacts
with sources of strong blue-light in the 440 nanometer range causes
fluorescence in the yellow wavelength spectrum, as seen with the aid of
suitable filters. This zone of fluorescence, 100-150 microns thick, is
obvious at 500 magnifications (Figure 6). The substance(s) causing this
type of fluorescence is unknown; likewise, its genesis can only be
guessed. One speculation is that fluorescence is connected with
anaerobic, microscopic, biogenic organisms living within the chert and
nourished by abundant fossil organic remains. Their respiration requires
the uptake of oxygen-rich salts – especially barium sulphate (aka barite
or BaSO ). Barium ions excreted by these organisms during respiration
might be fixed temporarily as barium sulphide; however, in the long run
the action of carbonic acid in the environment would bind barium as a
carbonate (witherite or BaCO ). One or another of these barium salts,
perhaps in combination with other trace elements, may account for the
yellow fluorescence of weathered Ft. Payne chert when it is stimulated
by a 440 nanometer laser or other intense light source.
That certain bacteria have the ability to “attack” relatively insoluble
barite and liberate barium ions has practical applications when dealing
with toxic by-products of drilling for petroleum (Krumholz 1999). Also,
the bacterium Desulfovibrio desulfuricans has been shown to affect
barite in sewage sludge (Baldi et al. 1996). Uptake of barium, perhaps
resulting from bacterial action, has also been observed in decaying bones
at Amboseli, Kenya (Trueman et al.2004). Therefore, it is not asking too
much of our credulity to accept the idea that high levels of barium in
weathered tools of Ft. Payne chert are caused by bacterial growth.
Apart from barium enrichment, there are few other surprises when
we compare elemental abundances in raw Ft. Payne chert with
abundances in the rind of Cumberland fluted points. The amount of
calcium within weathered Cumberland artifacts has decreased to as little
as 1/20th the value in raw Ft. Payne chert; likewise, silica (Si) may be
reduced by as much as 80%. Tin, zircon and sulphur disappear in
weathered chert. Such changes may be understood as normal,
subtractive, chemical weathering. On the other hand, there is a marked
increase in molybdenum between raw Ft. Payne (zero – none present)
and its weathered facies (271 +/- 22 parts per million). This change
suggests that molybdenum, too, has been concentrated by bacteria. We
hypothesize that the tetra-oxide, PbMoO , also may play a role in their
respiration; we would expect a derivative salt to be deposited within the
weathering rind.
Thus, insofar as Ft. Payne chert is concerned, weathering is both
subtractive and additive with biogenic organisms playing an important
role. The build-up of dead organisms and their complex, barium-rich
(and molybdenum-rich?) excreta within the weathering rind, we
hypothesize, might provide some “traps” or “roosts” for atomic-sized
particles to accumulate. If we could devise a means of measuring the
amount of accumulated particles within ancient chert artifacts, we would
have a means of dating them relatively.
Radon in the Archaeological Record
The element radon, atomic number 86, was discovered in 1900. At
last count 36 isotopes were known – the number having increased by 10
since 1990 (Handbook of Chemistry and Physics, 1990 edition)! Of
these many isotopes only Radon-222 is of practical concern as it has the
longest half-life (3.8 days) and is a strong alpha-particle emitter. Radon222 originates with the radioactive decay of Uranium-238 via Thorium230 and Radium-226. Uranium and, thus, radon have an uneven
distribution across North America. Alluvial soils, as a general rule, have
small amounts of it; while, residual soils (upland soils) in un-glaciated
regions can have a great deal. Alabama north of the Tennessee River,
central Tennessee and central Kentucky exhibit relatively high radon
values (EPA Map of Radon Zones) in both ground water and soil.
Perhaps not coincidentally, this radon-rich region is underlain by
Mississippian-age limestones and Ft. Payne chert.
Buried chert artifacts come in close contact with radioactive radon
dissolved in rain water, exposing them to energetic (5.6 MeV) alpha
particles. Electrons in the outermost shells of atoms, we hypothesize, are
“dislodged” by collisions with alpha particles and find temporary
residences in traps within the chert. As we might expect, the upper
surface of a buried artifact, which is fully exposed to radon in rain water,
should have more dislodged electrons at an high-energy state than the
downward-facing side of an artifact. This predicted difference or
“sidedness” has, in fact, been observed repeatedly by us. The absence of
such “sidedness” is a strong argument against the authenticity of a bogus
archaeological specimen.
In addition to the effects of radon, electrons may be boosted to
higher-energy traps by atomic particles and gamma radiation from
decaying radioactive elements that pre-exist within the chert itself and
the environment surrounding it. The techniques of thermo-luminescent
(TL) dating and optically stimulated luminescent-dating (OSL) are
predicated upon this assumption. However, radon in water, we
hypothesize, is primarily responsible for the accumulation of highenergy electrons within the weathered rind of cherts – and particularly
Ft. Payne chert. Unlike other researchers who view radon damage as a
nuisance and an impediment to their dating techniques (e.g., Richter and
Krbetscher 2006: 695), we embrace it.
Raman Fluorescence, Stokes Scattering, and Measuring Age
In 1928 C. V. Raman and K. S. Krishman, as well as other scholars
working independently, observed that some monochromatic light
reflected back from a target had shifted frequency. Eventually it was
established that many elemental bonds fluoresced when bathed in strong
monochromatic light. Further, the wavelength “signatures” of
fluorescing bonds were a means of testing for the presence of specific
elements and gauging purity – as in the case of diamonds. Graphs of
fluorescing light yielded by carbon-carbon bonds of pure diamonds are
strong and sharp (see, for example Figure 7); while impure diamonds
yield a less focused output, that is to say, a light of greater band width.
These days lasers are the source of narrow band-width energy used
to stimulate fluorescence. Infrared lasers generate Raman fluorescence
equally as well as or better than lasers that emit shorter wavelengths of
the visible spectrum; however, analysts have observed an unwelcome
side-effect known as Stokes scattering. An infrared laser is particularly
effective in causing high-energy electrons, which have accumulated in
traps within rocks and minerals by exposure to radon, to fall to lower
energy states. When they fall, photons are emitted.. This scattering –
Stokes scattering – may even be powerful enough to over-ride and
obscure actual Raman signatures. Rocks and minerals targeted by an
infrared laser (We have employed a laser with a wavelength of 785-786
nanometers) will begin to glow and then fade as their high-energy
electrons are dislodged and fall back to more tightly bound positions
within atomic arrays. Using a detector (charge-coupled device or CCD),
the glow may be measured and recorded graphically. A typical output is
shown by Figure 8.
The accumulation of high-energy electrons within the weathered zone
of ancient artifacts made of Ft. Payne chert is greater than we have
observed for many other chert varieties. In other words, electrons
boosted from low-energy states by the radioactive decay of radon find
many traps to occupy in Ft. Payne chert as compared to some other raw
materials. Since the measured light output from weathered Ft. Payne
chert is strong enough to override most Raman emissions, it becomes an
ideal substance for relative dating.
Stokes scattering may also be observed throughout the unweathered
interior of artifacts; however, light output is much reduced compared to
the chemically and physically altered outer rind. The contrast in intensity
between light emitted from a rock’s interior versus exterior when
stimulated by an infrared laser is a recently patented means of
authenticating artifacts (Walley, n.d.; also US patent Number 8134133,
March 13, 2012). If there is no contrast in light values for the exterior vs.
interior, one must conclude that the object is unweathered and has been
manufactured modernly. Also, by employing a laser beam with a “virtual
focus,” it is possible to measure Stokes scattering within an artifact
without destructive cutting or drilling. Such capability should be a boon
to museum curators and scientists everywhere.
Applications
Like any other method of dating archaeological remains,
investigators must select samples for infrared laser spectroscopy (ILS)
with care if good results are to be achieved. Heavily abraded specimens
from an active beach or a sand bar in a river channel are poor candidates
for ILS dating, as their weathered rinds with high-energy electrons may
be truncated. Likewise, an artifact on the ground surface that has been
subjected to brush fires cannot be expected to generate a light reading
bearing upon its age of manufacture.
Something must also be known about the soils where artifacts for
dating are collected. Were they found within alluvium, glacial debris, or
at un-glaciated, upland locations? Ft. Payne chert artifacts of the same
type but from different soils, we have observed, yield significantly
different light values. Also, perhaps it goes without saying, artifacts
thought to be the same age or belonging to the same archaeological
culture but made of different raw materials may produce different light
values. Armed with these understandings, we may discriminate among
unburned artifacts from equivalent soils that are assumed to be of
identical raw material. At the Phil Stratton Cumberland site in Logan
county, western Kentucky, “look-alike” flaked tools – all thought
initially to have been made of Ft. Payne chert with predictably high
light values – were determined to represent two or more lithic
populations. The small group of artifacts shown in Figure 4 yielded light
values of 37,000 - 52,000 units and 24,000 - 30,000 units – with an
outlier specimen (modernly burned?) showing only 14,000 units. Upon
closer inspection, the specimens with a lower light value appear to have
a similar fabric, are more gray in color, and are more translucent than
what we might expect for Ft. Payne chert within its region of outcrop.
A simple demonstration of the utility of ILS relative dating is
provided by three artifacts that were collected in 2011 from the plowed
surface of a prehistoric site at Bates Pond, near Leighton, Colbert
county, northern Alabama (Figure 9). All three were fashioned of Ft.
Payne chert, weathered similarly, were unburned and exhibited slight
iron staining. Two of these specimens – a trianguloid endscraper and a
basal fragment of a fluted point preform – are Clovis in age; while, the
third is a medial fragment of an Early Archaic projectile point of the
Lost Lake type. Averaged measurements of light intensity for both sides
of the three specimens are tabulated below:
Table. ILS Measurements of Prehistoric Flaked Tools of Ft. Payne Chert
From the Bates Pond Beach Site, Colbert County, Alabama
Clovis Preform Base
Side A
Side B
66,250
109,500
76,250
51,250
87,750
54,000
84,500
75,000
71,500
91,500
Clovis Endscraper
Side A
Side B
77,250
94,500
80,750
52,000
51,500
102,750
89,750
70,750
77,250
74,250
Lost Lake Point
Side A
Side B
50,000
61,500
50,000
64,500
57,250
50,700
66,000
48,750
50,000
59,000
77,250
76,250
Both sides = 76,750
75,300
78,850
Both sides = 76,175
54,650
56,800
Both sides = 55,725
Assuming that the Clovis artifacts are 13,000 calendar years old and
solving the equation 55,725/76,460 = 13,000/X for X, we arrive at an
estimated calendar age for the Lost Lake point of 9,474 calendar years.
This result is in keeping with the range of radiocarbon ages for all
manner of Lost Lake points (9,000-7,500 RCYBP); see Perino 1986:
232), having a median age of 8,250 RCYBP. Such a median age is
equivalent to 9,240 calendar years – an excellent fit with the
experimental ILS date.
Another application of ILS dating to an archaeological assemblage
involves a series of 22 Ft. Payne chert projectile points or knives known
to be from the surface of the Heaven’s Half Acre site (aka, 6th Street
site) near Leighton, Colbert county, northern Alabama. This extensive
site, which encompasses a cluster of discrete and sometimes overlapping
habitation loci, has yielded many thousands of Palaeo-American and
Very Early Archaic flaked stone tools since the 1960s (King 2007). It
occupies upland, karstic topography that is well away from the modern
floodplain of the Tennessee River or any of its tributaries.
After typing the assemblage, an averaged light value was obtained
for each specimen. Five readings on both faces were made. Four Late
Stage Cumberland points with no evidence of heat damage yielded the
group mean 69,075 light units (range 65,275 - 72,250); while, the mean
light unit value for 12 Clovis points was found to be 71,982 (range
59,525 - 94,950). Since these means are very close, there appears to be
no demonstrable age difference between Clovis and Late Stage
Cumberland at the Heaven’s Half Acre locality.
Interesting to note, two Very Early Archaic points among the
assemblage were found to have va1ues of only 58,050 light units
(Greenbrier point) and 41,800 (Beaver Lake point). These results
suggest a more recent age for the points, as one might expect.
An adequate test of the utility of ILS dating requires large samples
of Ft. Payne chert artifacts falling within recognizable types. In practice,
it is challenging to obtain artifacts with sound, detailed provenience
from private collectors and even most museums. Yet, the quest for large,
trustworthy samples must not cease as some fresh truths might be
revealed.
We end our presentation with the outcome of ILS testing upon a
group of 19 Cumberland (N = 11) and Clovis (N = 8) fluted points made
of Ft. Payne chert from upland findspots in Lawrence county, Tennessee
– the very same county where our new type locality for Ft. Payne chert
is located. These Cumberland points belong to the earliest two stages of
a long-lived Cumberland Tradition (Gramly 2010, 2012) and are easily
segregated by their attributes from Late Stage Cumberland points – such
as those at the Heaven’s Half Acre site. We found that the group of
Clovis points yielded only 75% of the light value recorded for the 11
Cumberland specimens (also see McNutt, Cherry, and Walley 2010 for a
discussion of this outcome). Relative to Clovis, the earliest stage of the
Cumberland Tradition may be as much as 3,000 years older. In a general
way, this finding is in harmony with the higher barium abundance for
Cumberland points of Ft. Payne chert compared to Clovis (Figure 5).
On these grounds alone, yet not forgetting the small size of our
measured sample, we suggest that Cumberland has older roots than
Clovis. It is remarkable to think that the long-sought evidence for a “preClovis” phase has always been before us – hiding in plain sight as a
Cumberland point!
Acknowledgments
We thank Doug Puckett of Muscle Shoals, AL and other amateur
archaeologists in Lawrence county, TN who furnished Ft. Payne chert
artifacts for ILS relative dating.
*Address all enquiries to the senior author who bears all responsibility
for any shortcomings of this paper.
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Captions for Figures:
Figure 1. Seam of Ft. Payne chert in the bed of the West Fork of Sugar
Creek, on the farm of Mr. and Mrs. Charles Edmunds, 276
Old Florence-Pulaski Road, southeast of Leoma, Lawrence
county, Tennessee. R. M. Gramly photograph.
Figure 2. Map of east-central Lawrence county, Tennessee showing
location of new Ft. Payne chert type locality.
Figure 3. Colors of Ft. Payne chert. A, freshly-broken, dark gray; B,
slightly weathered, olive gray; C, slightly weathered, pale
brown; D, severely weathered, light olive gray; E, severely
weathered, yellowish gray and partially iron-stained.
A-C, Leoma type locality; D, Palaeo-American burin,
from “Brambly Bottom,” Shoal Creek, Lawrence county, TN;
E, Palaeo-American unifacial tool, Heaven’s Half Acre site,
Colbert county, AL. R. M. Gramly photograph.
Figure 4. Series of nine Palaeo-American flaked stone tools from the
Phil Stratton site, Logan county, Kentucky. By visual inspection they were thought to be Ft. Payne chert. Examination
with an infrared laser spectrometer, however, revealed that
stored light values varied and that other chert(s) in addition to
Ft. Payne may be represented. A, sidescraper and graver (P.S.1952), average 37, 124 light units; B, hollow scraper/beak on
core tablet (P.S.-136), avg. 40,125 l.u.; C, beak on fragmentary
sidescraper (P.S.-2051), avg. 13,583 l.u.; D, combination tool
(P.S.-1327/1906), avg. 38,166 l.u.; E, tool fragment (P.S.1465), avg. 30,416 l.u.; F, tool fragment (P.S.-3), avg. 51,875
l.u.; G, beak (P.S.-1683), avg. 27,958 l.u.; H, utilized flake/
graver (P.S.-1653), avg. 20,333 l.u.; I, possible graver (P.S.171), avg. 24,208 l.u. R. M. Gramly photograph.
Figure 5. Relative abundance of barium (peaks marked with an arrow)
as determined by X-ray fluorescence for A) raw Ft. Payne
chert, B) weathered rind of a Clovis point of Ft. Payne chert,
and C) weathered rind of a Cumberland point. Operator: David
Walley using a SpectroXepos (TM). polarized tube device.
Figure 6. View at 500X of the edge of a cross-sectioned Middle Stage
Cumberland point of Ft. Payne chert showing yellowfluorescing, weathered rind 100-150 microns thick and
unweathered interior that fluoresces orange (caused by
calcite). David Walley photograph.
Figure 7. Raman spectrum generated for gem-quality diamond excited
at room temperature by a laser with a wavelength of 228.9
nanometers (nanometer = 1 X 10 meter). Arrow indicates
primary peak. From rsta.royalsocietypublishing.org, 3/5/2009.
Figure 8. A: Output of incidental energy or Stokes scattering (areas
under curved lines) for both sides of an authentic Texas chert
knife that has been stimulated by a 785 nm laser. Note similar
shape of curves but the marked difference in amplitude. The
curves for a modern reproduction will be nearly identical in
amplitude, sometimes flatter, and may exhibit peaks of Raman
fluorescence for specific atomic bonds. B, Generalized
portrayal of Raman signals only – showing no Stokes
scattering.
Figure 9. Relatively dated artifacts of weathered Ft. Payne chert from the
Bates Pond Beach site, near Leighton, Colbert county,
Alabama. A, Clovis preform base and trianguloid endscraper;
B, medial section of a Lost Lake point showing resharpening
by beveling. R. M. Gramly photograph.