Cation-ratio varnish dating and petroglyph chronology in southeastern Colorado
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Cation-ratio varnish dating and
petroglyph chronology in southeastern
Colorado
LAWRENCE L. LOENDORF"
Pecked figures - seeming to depict humans, animals, objects and 'abstract' shapes - are
an important and recalcitrant aspect to the archaeology of the desert USA, in the Great
Basin and the Southwest. Where they are covered by desert varnish, they provide an
opportunity for a n absolute dating by cation-ratio method. Here - as they did not for a
similar study in South Australia reported in an earlier ANTIQUITY
- the cation-ratio dates
do seem to run alongside the chronological pattern inferred by conventional means.
During the past few years, a new method for canyon. Hundreds of petroglyph sites are found
dating petroglyphs, cation-ratio dating of rock on the walls of these tributary canyons and
varnish, has been developed and tested (Dorn along the walls of the main Purgatoire canyon.
1983; Whitley & Dorn 1987; Dorn et al. 1988).In Petroglyph sites are also numerous on a basalt
order to date petroglyphs by the cation-ratio dike that parallels the southern boundary of the
method, over the past three years 46 varnish PCMS. At present 175 rock art sites are recorded
samples have been collected from sites on the in the PCMS; dozens more are not yet recorded
Pinon Canyon Maneuver Site (PCMS) in south- (Loendorf et ul. 1988). The majority of the
eastern Colorado. Under the direction of an varnish samples reported in this paper were
archaeologist, more than half of the samples collected from 10 sites that were intensively
were collected to test the validity of the cation- recorded in 1988 and 1989 (Loendorf 1989;
ratio dating method. Most of these test samples Loendorf & Kuehn 1991). Each sample was
were selected to measure the numerical age of a collected and analysed by Dr Ronald Dorn of
petroglyph for which there was good relative Arizona State University, who developed the
evidence as to its age. The results were remark- cation-ratio dating method and applied it to
ably consistent; every sample that was used as a archaeological materials. The CRs were calcu-
test of the method produced dates that matched lated by using (K + Ca)iTi as determined through
the relative dates. PIXE and subsequently checked by ICP and
electron microprobe (Dorn et al. 1990). The
The Pinon Canyon Site and its petroglyphs cation-leaching curve was established by 14C
The Pinon Canyon Maneuver Site (PCMS), (TAMS) for seven varnish samples in southeas-
immediately west of the Purgatoire River, about tern Colorado (Loendorf 1989: 128-37). Kock
80 km north of the New MexicoiColorado varnish is well developed in southeastern Colo-
border in the southwestern USA (FIGURE I ) , is rado, with characteristics conducive to cation-
used by the US Army primarily for tank-warfare ratio dating (FIGURE 2).
training manoeuvres. The area is dominated by In the initial research at the PCMS, a rock art
rolling prairie dissected by streams that are typology was established. Not unlike those of
relatively shallow in the west and progressively previous researchers, the typology included
deeper as they approach the Purgatoire River petroglyph attributes that were grouped into
*
I l e p x t m c n t of Anthropology, University of North Dakota, Box 8254, University Station, Grand Forks ND 5 8 2 0 2 , USA
A.\'l'lcjllIl> fj3 [1991):2415-55CATION-RATIO VARNISH DATING AND PETROGLYPH CHRONOLOGY IN SOUTHEASTERN COLORADO 247
FIGURE 1 . The
location of the Pinon
Canyon Maneuver Site
(PCMS) in
sou theastern
Colorado.
types (Loendorf 1989: 75-118). The technique (Loendorf 1989: 351-62). This general chrono-
bv which the petroglyphs were made is an logy is based, in part, on the subject-matter in
important variable in the typology, with totally the rock art panels. Bows and arrows, not found
pecked figures differing in type from outline- with totally pecked figures, are common with
pecked figures and incised figures. In general, the pecked outline figures, while horses and
these three techniques are believed to be in guns are found with the incised figures. The
chronological order: the totally pecked figures, bow was in use in the region c. AD 200, and
the oldest are followed by pecked outline horses by AD 1550.
figures, and later in time by incised figures Six of the intensively recorded sites exhibit
F ~ G L I2.
K ~ :Buckscatter
electron microscope image
of a polished cross-section
of rock varnish formed on
the oldest dated
pc,troglj,ph a t the PCMS,
site 5LA5598. The white
scale bar at upper left is
10 niicrons long. The white
1ine separates the va rn is h
from the underlying rock.
This image of layered
rock varnish illustrates
h8:wthe varnish in the
ptroglpphs forms
continuous layers that are
good for varnish dating.
The pores (dark holes) are
the places where the
cution leaching takes
place.248 LAWRENCE L. LOENDORF
primarily the totally pecked variety of petro- (CR) dates on the rock varnish that has
glyphs. The petroglyphs at several sites are developed since the petroglyphs were made
dominated by abstract forms, with a small (TABLE 1).
number of quadruped or human forms. Other
sites exhibit the opposite pattern; a majority of Abstruct forms
the petroglyphs are quadrupeds and lesser The sites with large numbers of abstract forms
numbers are abstract oqes. The abstract forms are those with the oldest CR age estimates. The
were divided into four types: curvilinear, recti- oldest estimate for a petroglyph at the PCMS i s
linear, amoeba-like, and groups of dots. 4675k200 BP; it overlaps another of 4425k450
Together with the quadruped and anthropo- BP.* Both dates are for curving abstract petro-
morphic types, the abstract types were plotted glyphs on 5LA5598, the former for an amor--
in a seriation for the six sites (Loendorf & Kuehn phous pecked area adjacent to a dot and wavy
1991: 254). The seriation suggests a decline in line, and the latter for a tailed circle.
the abstract forms through time, with quad- These curving forms continue in the tradi-
ruped forms nearly dominating the sites at the tion, with a wavy-line petroglyph (site
recent end of the tradition (FIGURE 3). At no 5LA3212), dated 3550k350 BP (FIGURE 4A), and
point in the sequence, however, is a particular a similar wavy-line figure on 5LA5602 dated
type completely replaced by another. Among, 3350rt350 BP. Other curving and meandering
say, the quadrupeds, there may be an old figure forms on the six sites used in the seriation range
even though the majority are more recent. Using in date from 2700k300 to 450k125 BP. This
the seriation as a guide, however, a quadruped series indicating that abstract pecked cur-
is likely to be more recent than an abstract form. vilinear petroglyphs were made over several
Campbell (1969) developed a similar relative millennia, matches the relative sequence well.
chronology in the region with abstract forms
pre-dating the quadruped and human forms. Rectangular forms
Schaafsma (1980) discovered abstract forms Rectangular forms were also dated by CR. One
superimposed by rock-art of Pueblo cultures petroglyph with a rectangular form, on a frag-
with a known age. The specific age estimate for mented boulder, was dated 1400k150 BP;
the abstract forms is prior to AD 1,at which time another petroglyph, several intersecting lines,
the Basketmaker I1 sequence is dated in the dated 1100klOO BP. Varnishes on two other
American southwest (Schaafsma 1980: 47). rectangular petroglyphs with a very distinctive
form were also dated. Classed as bisected rect-
Ages by CR determination angular grids, they have an almost anthropo-
Petroglyph sites in the PCMS fall into a chrono- morphic appearance (FIGURE 4B, 4C), as each
logical sequence that duplicates other research has a single line dividing it vertically and
in the region, and this offers some confidence in extending beyond the perimeter of the grid on
its validity. More important are the cation-ratio both ends. At one end is attached a rounded
B
Site Ant phs Other
5LA5830
5LA5993
5LA5602
5LA5598
5LA3212
a
5LA3211
FIGURE 3. Seriation of rock art forms occurring on six sites in southeastern Colorado. Numbers within
the black bars are percentages.
* All cation-ratio ages are calibrated by radiocarbon dates corrected by Stuiver & Bccker (1986);all errors arc oiie sigma and
based on the standard error of three separate agcs assigned to three different cation-ratio measuremonts bv PIXE.CATION-RATIO VARNISH DATING AND PETKOGLYPH CHRONOLO(;Y IN SOUTHEASTERN COLORADO 249
site panel element mean lab # cation ratio petroglyph motif
date (BP)*
5LA5602 28 d 2300k200 ASU-PCT3A 7.07+0.08 pecked quadruped
!;LA5 599 2 C 330Ok 250 ASU-PC1 6.741-0.07 pecked quadruped
5LA5602 28 i 1900k125 ASU-PC2 7.241-0.06 peckedquadruped
5LA5602 7 b 3350k350 ASU-PC3 7.06k0.56 wavy line
5 LA3 2 12 4 e 3550k200 ASU-PC4 6.67k0.05 parallel series
5LA5598 220 a 675k100 ASU-PC5 8.18k0.09 anthropomorph
5LA5598 220 2700k300 ASU-PC6 6.92k0.10 amorphous shape
5LA5598 221 a 650k100 ASU-PC7 8.21k0.08 anthropomorph
5LA5 598 246 a 4675k200 ASU-PC8 6.42k0.03 amorphous shape
EiLA5598 143 a 1400k150 ASU-PC10 7.52k0.11 rectangular form
:;LA5 598 143 850k100 ASU-PC11 7.99k0.07 amorphous shape
5LA5598 172 a 2350k200 ASU-PC12 7.05k0.08 rectangular form
5LA5598 174 a 2300It_200 ASU-PC13 7.08k0.07 rectangular form
5LA5598 123 a 2000k200 ASU-PC14 7.19k0.07 rayed circle
5LA5598 101 b 4425k450 ASU-PC15 6.47k0.10 tailed circle
5LA5598 101 a 1300k150 ASU-PC16 7.59k0.09 curvilinear form
5LA5598 58 b 110Ok100 ASU-PC19 7.75k0.08 intersecting lines
5LA5598 7 a 1350k150 ASU-PC2O 7.56f0.11 curvilinear form
5LA5598 9 a 870k100 ASU-PC21 7.96t0.10 arrow point
5LA5555 700k100 ASU-PCZZ 8.16k0.10 anthropomorph
5LA5549 9 b 700k100 ASU-PC23 8.15k0.07 anthropomorph
5LA5549 1 C 725'1 50 ASU-PC24 8.14k0.16 anthropomorph
5LA5549 6 350k100 ASU-PC25 8.80k0.19 anthropomorph
5LA5255 2 a 375k75 ASU-PC26 8.75k0.15 outlined quadruped
5LA5255 5 b 350k100 ASU-PC27 8.85to.27 horned head
5LA5255 12 e250 LAWRENCE L. LOENDORF
E F
-
0 10 20 30cmCATION-RATIO VARNISH DATING AND PETROGLYPH CHRONOLOGY IN SOUTHEASTERN COLORADO 251
loop giving the appearance of a head. At the ces in the quadrupeds may also be related to
other, the line simply extends straight beyond temporal changes.
the border of the grid. In one of the figures, extra
lines attached to the outer border of the grid and Anthropomorphs
nearly parallel to the vertical line give the The oldest anthropomorphs dated by the
appearance of legs. One figure was dated cation-ratio method on the PCMS are about 700
2300+200 BP and the other, 2350+200 BP. to 800 years old. Some very crude stick-like
The age estimates for rectangular petroglyphs figures on sites 5LA3211 and 5LA3212 may be
suggest they may have started somewhat later older anthropomorphs, but the figures are as
than the curving forms. This is probably most easily labelled as intersecting lines. The rect-
correct for the well-made rectangular grid angular grid forms, discussed above, may also
forms. Even though these well-made grid forms be anthropomorphs. Well-made anthropo-
may have been made for a short time-span, other morphs with digits at the ends of their limbs
abstract rectangular forms appear to continue (FIGURE 4G-J) are dated 725+150 and 700k100
until at least 1000 years ago. BP. Some of these figures are in action scenes
with other human figures. It appears that well-
Quadrupeds made human figures are a late addition to the
The CR dates for the quadrupeds are very overall sequence, increasing in popularity
instructive. Two well-made quadrupeds, on site through time. As with the quadrupeds, it should
5LA5599 (FIGURE 4D) and on site 5LA5602 be possible to discover variation in anthropo-
(FIGURE4E), were both dated 3300k250 BP. morph form through time. The only dated site
Other quadruped animals on site 5LA5602, in a with recent human figures in the totally pecked
group of seven quadrupeds that encircle a single tradition, site 5LA5830, has small stick-like
boulder (FIGURE 4F), were dated 2300+200 and human figures surrounding a large herd of
1900k125 BP. All the quadrupeds on this boul- animals (FIGURE7). It is not known if these
der, labelled Carousel Rock, appear to have figures represent a second tradition, or if the
been made about 2000 years ago. A half dozen better-made anthropomorphs deteriorated
more quadrupeds are found on near-by boul- through time.
ders. At site 5LA5993, a single human figure is
surrounded by 36 quadrupeds (FIGURES 5 & 6), CR dates and cultural indications of date
with CR dates between 900 and 1000 BP. Finally, In sum, the cation-ratio age estimates match the
site 5LA5830 with 24 quadrupeds associated relative sequence developed for petroglyph
with 1 8 human figures (FIGURE 7), has a CR date sites on the PCMS, and the fidelity between the
450k75 BP. The CR dates match the relative relative and CR dates supports the validity of
curve established in the seriation. With the CR method. Equally important is the match
additional research it may be possible to find between the PCMS dates and the research of
changes in quadruped form through time. At previous investigators. No CR ages have been
present, the figures appear to increase in varia- obtained for petroglyphs in the PCMS research
bility through time; early figures have straight or that are out of the expected chronological order.
curved head appendages, and later ones have CR dating at PCMS is also internally consis-
branching antlers. Other more refined differen- tent where petroglyphs are superimposed. A
Selected cation-ratio dated petroglyphs from the Pinon Canyon Maneuver Site, southeastern
k k X J R E 4.
Co lora d o.
A parallel series, site 5LA5602, wavy line a t right was dated.
B & C bisected rectangular grids, 5LA5598.
D quadruped, 5LA.5599.
E quadruped, 5LA5602.
F' quadrupeds, 5LA5602, elements d and i were dated.
I;pecked outline anthropomorph, 5LA5255.
€1 anthropomorph superimposed over amoeba, 5LA5598, both were dated.
I scene with anthropomorphs and quadrupeds, 5LA5555.
J anthropomorphs in action scene. 5LA5549.252 LAWRENCE L. LOENDORF
FIGURE 5. The main panel at site 5LA5993 contains 36 quadrupeds. The single human figure is a t the
right. (Photograph by Peter Halter.)
Fl(;irKt.: 6 . Detail of the human figure and the quadrupeds immediately surrounding it, on the main panel
at site 5 1 ~ 5 9 9 3(Photograph
. by Peter Halter.)CATION-RATIO VARNlSH DATING AND PETROGLYPH CIIRONOLOGY IN SOUTHEASTERN COLOIIADO 253
i
7 . Human figures surround a herd of animals in a petroglyph panel at site 5LA5830.
FIGURE
pecked amorphous blob, dated 2700k300 BP, dates, 1750f300 and 1900k250 BP. Covering
was superimposed by an anthropomorph that this petroglyph were cultural deposits contain-
was dated 675k100 BP (FIGURE 4H). Another ing chipped stone tools and flakes, ground stone
pecked blob, dated 850+100 BP, was added to a fragments, burned and unburned bone, fresh-
rectangular form that was dated 1400k150 BP. water shell, and charcoal. A I4C determination
Both sets of CR follow the order of imposition. on charcoal for the deposit of 1220+130 b.p.,
CR dates for figures with bows and arrows or consistent with the cultural debris, is in a
other petroglyphs in the pecked outline tradi- correct relative order to the CR date.
tion are also correct. A figure at site 5LA5549, Another radiocarbon date was obtained for
with horned headgear and a bow in its hand, has soil covering a petroglyph panel at the Clay
a CR date of 350+100 BP. Horned heads on Creek site near Lamar, Colorado, 60 km east of
human figures and heartlines in bison figures the PCMS. According to Buckles (1989: 1 2 8 ) ,
are the dominant petroglyphs at another site, this petroglyph panel, exposed in a flash flood,
5LA5255. These elements, in particular the exhibits ‘pecked abstract forms. The majority of
heartline, are thought to have been introduced the glyphs are ovals and grid forms of intersec-
into the region some time after AD 1500 by the ting vertical and horizontal lines’. A radio-
Athapaskan-speaking Apachean peoples (Loen- carbon date indicates these petroglyphs were
dorf 1989: 299-302). Other petroglyphs in the covered by soil at AD 100+_100(Buckles 1989:
pecked-outline tradition resemble Apache Gan 128). These petroglyphs are very similar to the
or mountain spirits (Loendorf & Kuehn 1991: rectangular grid forms on site 5LA5598 (FIGIJRE
259). The CR dates for all of these figures range4B, C) CR-dated 2350k200 and 2300k200 B P .
between 400 and 300 BP. Thus even though CR Allowing some time for the petroglyphs on the
is not a good method for recent varnish, the Clay Creek site to have been covered, the CR
dates support the relative chronological dates and I4C stratigraphic age are consistent.
sequence. Of further interest is a radiocarbon date for a
circular pit, about 2 m in diameter and 1m deep,
CR dates and radiocarbon dates adjacent to the rectangular petroglyphs on site
CR dates also fall into line with those obtained 5LA5598. Test excavations did not reveal its
by radiocarbon. A meandering line petroglyph function, but a carbon sample from the lowest
on the wall of a small rockshelter has two CR levels was determined at 2290+140 b.p. (Beta-254 LAWRENCE L. LOENDORF
26793); the calibrated age of this sample, glyphs in the PCMS support the rock-art
23405140 BP (Stuiver & Becker 1986: 897), is chronology established for the region. These
within the statistical overlap of the cation- two samples, from site 5LA5255 and from site
ratios, which were measured before the radio- 5LA5569, are younger than expected. The
carbon age of the charcoal was determined. sampie from 5LA5255, selected to date a petro-
In sum, the radiocarbon dates from Clay glyph that was immediately above an exca-
Creek and from site 5LA5598 suggest the rect- vation unit in the rockshelter, was near the
angular bisected grid petroglyphs were made ground surface and slightly more eroded than
around 2300 BP, exactly the age established by other petroglyphs in the shelter. Cattle or other
CR. animals treading near the petroglyph may have
Other radiocarbon dates for cultural deposits influenced the varnish. At site 5LA5569, a small
near petroglyph sites also confirm the CR dates. cave, a sandstone shelf is located immediately
At site 5LA5255 the radiocarbon age estimate below the dated petroglyph. Persons who visit
for the upper cultural level, including ceramics the cave tend to sit on the shelf, where they rub
that throughout the region are the product of against the petroglyph and probably influence
Apachean groups, is 370k60 BP. The I4C date the varnish developing on it.
overlaps the CR age of the petroglyphs at the The dates for these two petroglyphs may be
site; furthermore, as discussed above, these accurate, but because they appear too recent,
petroglyphs are suggested, through indepen- more thought will go towards selection of petro-
dent means, to be made by the Apache. glyphs for future samples, to ensure erosion will
Another important site, on the Purgatoire not be a disturbing factor.
River canyon rim, is the Zookeeper site,
5LA5993, where a single human figure is sur- Conclusion
rounded by 36 animal figures (FIGURE 5). Four CR dating worked very well for PCMS petro-
CR dates in the main panel range from 900k150 glyphs. The CR dates are as consistent as any
to 10002250 BP, and one CR date for a second group of radiocarbon dates would be in a similar
panel is 1200k150 BP. The Zookeeper site is situation, and followed correct chronological
within 100 m of the Point site, 5LA6028, situ- sequence. More recent research suggests that
ated on a protruding canyon rim remnant that is 14C analysis of tiny amounts of carbon in the
isolated from the remainder of the canyon wall. first-formed varnish over petroglyphs is also an
At least seven house rooms were constructed on accurate dating method for petroglyphs.
the site by stacking slabs of sandstone in vertical Accelerator 14C dates of petroglyphs with ade-
and horizontal tiers. A test excavation into one quate carbon in their varnish may be the most
of the rooms produced chipped stone debitage accurate method for future research. However,
and charcoal, uncalibrated radiocarbon deter- the cost of CR dating is about one quarter that of
mination of 1 0 3 0 t 9 0 b.p. (Beta-37703). The 14Cby the accelerator method. For those trying
series of CR dates overlap with the I4C date, and to date petroglyphs or other rock engravings, the
the dates are again consistent with the chrono- success of CR in the PCMS is encouraging. The
logy suggested by seriation. method should be employed more widely,
Prior to obtaining the 14C date for the Point especially where there is opportunity for a
site, no two archaeologists offered the same secondary cross check of its accuracy. The best
guess as to its age; estimates ranged from 600 to approach to dating petroglyphs is a combin-
1750 BP. All the CR dates were reported before ation of accelerator 14C dates and CR dates
any of the 14Cdates reported above were known, together with all the relative schemes that can
except for the date at the Clay Creek site. (There be devised.
the radiocarbon date had been obtained before
the CR samples were collected, but it was not
published until afterwards.) The 14Cresults did Acknowledgements. The strong support of Tom Warrer
not and could not have influenced the results of and Mary Barber, US Army, and Steve Chomko and Jake
Hoffman, National Park Service; and the assistance of Kerry
the CR dates. Hackett, is appreciated.
All the varnish dates except two for petro-CATION-RATIO VARNISH DATING AND PETROC-LYPH CHKONOLOGY IN SO1JTHEASTERN COLORADO 255
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