4.5. Pliocene (Barroso group) obsidian in the Colca valley
The obsidian that was widely used in the prehispanic central Andes occurs in Tertiary flows along the western cordillera. In the main part of the Colca valley, obsidian is found in the lower levels of Late Tertiary lava flows on both the north and south sides of the Colca river. East of the town of Chivay, the Chivay obsidian type has been observed to occur where Barroso flows contact the older Tacaza deposits (Burger, et al. 1998: 205). Barroso flows (TBa), clearly evident in photos shown in Figure 4-11 and Figure 4-15, extend atop Tacaza flows to the east of Chivay, and obsidian has been observed where layers described as "Tertiary intrusive, porphorytic" (T-po) extend atop Tacaza flows to the northwest of Coporaque (Figure 4-23). A fission track date on an obsidian sample collected east of Chivay confirms the Pliocene origins of these flows with a result of 3.52 ± 0.15 Ma (Poupeau and Labrin, 10 Oct 2006, pers. comm.). To date, research has shown that high quality obsidian flows in the Colca valley that occurred during the late Tertiary fall into two chemical groups: (1) the Chivay type found to the east of the town of the Chivay, and (2) the "Uyo Uyo" type that lies across the Colca River, west of the town of Coporaque. In the course of fieldwork in 2003 the Upper Colca project sampled obsidian from both locations and had results analyzed by the Missouri University Research Reactor, as will be described below.
4.5.1. Chivay obsidian source observations
Obsidian at the Chivay source was observed obsidian in natural contexts eroding from the base of what is possibly a collapsed rhyolitic dome (Cerro Hornillo), and from the south-west flank of Cerro Ancachita between the elevations of 4900 and 5000 masl. In the majority of locales, obsidian appears as concentrations of cobbles in a pumaceous rhyolite soil matrix where unconsolidated outcrops seem to occur as jointed and weathered flow bands strike the surface. A similar context is described by Healan (1997: 84) at Ucareo, a central Mexican obsidian source, where he notes that unconsolidated outcrops are not in-situ, but such outcrops are best considered as "primary features" because they have not undergone lateral movement. Cobbles from these outcrops often have a very thin cortex at the Chivay source.
The only consolidated obsidian flow to strike the surface in the Maymeja area is finely jointed in the vertical direction, offering fragmented primary material that is poorly suited for obsidian tool making. It is notable that this flow is exposed in a gully in the northern portion of the Maymeja depression where glacial erosion is most pronounced and the bed of a small glacial tarn, forming only during the wet season, is located nearby.
|
(a) |
(b) |
Figure 4-21(a). Small box in lower-right gully shows Q02-1, an obsidian flow eroding out of ashy-pumaceous soils below western arm of Cerro Ancachita. (b). This obsidian is of limited use for tool making because it contains vertical, subparallel fractures.
On the southern half of Maymeja, a heavily exploited unconsolidated outcrop strikes the surface at the principal quarry pit (Q02-2), but only small nodules (5-10 cm long nodules, and a rare piece up to 15 cm long) remain. In these areas several quarry pit features, one large pit measuring 4 x 5m and 1.5 m in depth (Section 7.4.1), and two shallow pits measuring approximately 1-2 m in diameter that are possibly modern were observed further down the ridge. The larger quarry pit is located on a slope and on the downslope side lies a debris pile made up of primarily small, non-cultural (unmodified) nodules of obsidian, but with the occasional flake or retouched flake. Quarry pits surrounded by discard piles have been termed "doughnut quarries" by Healan (1997: 86-87) describing the Ucareo source in central Mexico where such pits have been found in great abundance. Variously sized quarry pits have also been described by researchers at other obsidian sources in central Mexico (Darras 1999: 80-84;Pastrana 1998).
Nodules at the Q02-2 quarry pit are found in two principal forms: a long, narrow form and a spherical nodule form. It is possible that the nodule forms reflects differences in emplacement, with the long, narrow nodules resulting from relatively thin flows while spherical nodules are unconsolidated outcrop forms. As will be discussed in Ch. 7 (Section 7.4.1), these nodule forms appear to have influenced knapping strategies as narrow nodules offer more angles and a different flaking geometry as compared with spherical nodules. Pastrana and Hirth (2003) describe reduction strategies for biface production that exploit long, narrow nodules at the Sierra de las Nevajas (Pachuca) source in central Mexico (Figure 7-2).
Chivay type obsidian outside of the Maymeja area
Elsewhere around the base of the dome Cerro Hornillo obsidian was encountered eroding from the ground in smaller nodules (2-5 cm long). These obsidian exposures are pronounced on the eastern and south-eastern slopes of Cerro Hornillo around 4900 masl where scatters of subangular pebbles and cobbles, or angular shattered felsenmeer carpets of these small obsidian pieces, were encountered. In glacially eroded areas and along wind-scoured ridges these obsidian surfaces occur as lag gravels where finer soil has been transported away by Aeolian processes, leaving only obsidian nodules. These nodules appear to be weathering from rhyolitic flows and the tool-making quality of the raw material seems to be compromised of three characteristics: (1) Size- remaining nodules were typically quite small; (2) fracture quality - heterogeneities in the material caused the material to fracture unpredictably; (3) visual quality - the nodules were often occluded with bubbles and ash particles.
|
|
Figure 4-22. Obsidian gravels exposure in tephra soils east of Cerro Hornillo. |
In this study, obsidian containing heterogeneities due to the presence of bubbles or ash particles is termed "Ob2" obsidian, while homogeneous obsidian that was probably preferred for tool production in antiquity is referred to as "Ob1" obsidian.
Pulpera / Condorquiña flow
In the course of the Upper Colca Project research an obsidian source was encountered on the eastern toe of the Barroso lava flow where flows emanating from Cerro Ancachita and Cerro Hornillo terminate near the community of La Pulpera. According to the INGEMMET map (Ellison and Cruz 1985), this obsidian appears to have formed where silicic lava flows belonging to the Barroso group contacted Late Miocene ignimbrites from the Pichu formation and cooled rapidly leaving obsidian exposed by erosion in this area. This obsidian does not appear to be of tool-quality as the nodule size is small (<5cm) and it contains many heterogeneities that interfere with the fracture characteristics of the stone. Samples of this obsidian were sent to M. Glascock at the Missouri University Research Reactor in 2002 and the samples were determined to be of the same chemical group as Chivay (Glascock, pers. comm. 2002).
Obsidian Cortex
The cortex of nodules at the Chivay source is often remarkably thin. The spherical nodules, described above, seemed to be more closely affiliated with a very thin cortex that is under a millimeter in thickness where hydration appears only as a slight discoloration on an otherwise smooth external surface. In other cases, particularly on long and narrow nodules, a textured and raised, but sometimes rough and ropey, cortex is evident that was referred to as "tabular cortex".
This geologically derived variation in cortex is meaningful to archaeologists because when the cortex was thin it appears that it was sometimes left undisturbed on the faces of many preforms, but when the cortex was the rough or tabular type, it seems to have been a central obstacle to knapping. Cores partially covered with rough cortex were discarded after flakes were removed from the non-cortical face. One possible explanation for the intensified quarrying observed at the Q02-2 quarry pit is that nodules recovered in this area contained a high frequency of the thin type of cortex, a cortex type which would have represented less of an obstacle to knapping.
The cortex form can influence reduction strategies in important ways. First, if cortex is extremely thin then it does not pose a structural obstacle to knapping and the priority on reducing an item's weight by decorticating it close to the raw material source may be lessened. Second, the thick tabular cortex on one side of long, narrow nodules greatly limits the potential of these nodules unless the cortex can be removed effectively. We speculate that the origin of the very thin cortex is related to the glacial history of the obsidian source. These unconsolidated outcrops were likely to have been compressed and eroded by the presence of glaciers and the effect of this glaciation on obsidian outcrops may have served to further fragment and introduce water into the obsidian flow, which appears in the oldest specimens as a layer of perlite. As a consequence, the extremely thin cortex may have resulted from glacial erosion and moisture introduced during the Pleistocene, rather than from characteristics of the original quenching environment of the obsidian flow. Obsidian hydration dating may allow direct dating of the fracturing of the obsidian, however the unreliability of hydration dating in contexts of high temperature variation and unknown moisture levels (Ridings 1996) suggests that results from hydration rinds alone are probably of limited value.
Rare 6 Type distribution
In her dissertation, Brooks (1998: 443) notes that Glascock identified the Uyo Uyo samples as matching the "Rare 6 Type" obsidian that had been previously encountered in Burger's earlier work with Lawrence Berkeley Lab (Burger and Asaro 1977: 56). However, Glascock cautions that the calibrations are not perfect between the LBL and the MURR results, particularly with small sample sizes. In recent communications Glascock (2006, pers. comm.) is not confident that the Uyo Uyo and Rare 6 Type are the same type and he believes a re-analysis would be required to confirm it.
In Burger's earlier study he identified Rare 6 type from two projectile points in Cuzco and Puno (Burger, et al. 2000: 312-313). One sample was from the surface of the site of Chinchirmoqo that lies 2 km from Pomacanchi in the department of Cuzco. The other Rare 6 Type sample was a projectile point found on the surface of the site of Taraco on the north side of Lake Titicaca. These surface samples are difficult to assign to a specific time, though Burger et al. placed both samples in the "latter part of the Early Horizon and Early Intermediate Period", a period that roughly corresponds to the Middle to Late Formative using the chronology of the present project. Excavation work currently underway by Charles Stanish and colleagues at the site of Taraco may reveal additional Rare 6 Type obsidian artifacts.