Ch4 Regional Geography

Regional Geography and Geology of the Upper Colca Project Area

The Chivay obsidian source is located on the margins of the Peruvian altiplano above the Colca Valley in southern Peru. The geographical position of the obsidian source area (71.5° S, 15.6° W) at on the western edge of the altiplano above the richly productive Colca valley had bearing on circulation of Chivay obsidian in regional prehistory. This chapter begins with an overview of the geographical relationships in the Upper Colca study area including climate, biotic zones, and resource availability. The chapter will then describe the ecological and geological context in the Colca Valley, and the influences that spatial relationships may have had in prehistory.


Figure 4-1. View of the volcanic Chivay source area above the town ofChivay in the Colca valley. The high point in the center-right of the frame, Cerro Ancachita, is above the Maymeja area. This view is from the Arequipa highway to the south-west of the Chivay source at 4720 masl.

Subsequently, further details on the geological structures in the area of the Chivay obsidian source will be presented, together with the geomorphological process that have influenced obsidian procurement in the area. Finally, this chapter concludes with a discussion the formation of silicic lavas and tool-quality obsidian, and the research that has been conducted to date documenting Tertiary obsidian flows in the Colca Valley.

4.1. The geography of the Colca Valley study area

A brief review the geography of Upper Colca study area better contextualizes the ancient lifeways and relationships that are the subject of this dissertation. The 2003 research project was organized into three distinct blocks of contiguous survey (1, 2, 3) with other adjacent areas numbered 4, 5, and 6. The three major survey blocks correspond with ecological zones in the Upper Colca area and the overall ecological variability in the project area will be reviewed here by a discussion of each block.

Ecological complementarity between highland herders and valley agriculturalists is a widespread feature of Andean economy. Research into the productive potential of "pure" pastoralism in the Andes has shown that the caloric return and efficiency is insufficient to sustain communities without inputs from non-pastoral communities (Thomas 1973;Webster 1973). The Upper Colca study area lies immediately above the altitude of intensive valley agriculture that begins at the village of Tuti (3840 masl), corresponding with the upper portion of the suniand with the punaecological zones, and reflecting the local precipitation and temperature gradient (ONERN 1973: 39;Pulgar Vidal 1946). The town of Tuti is at approximately the altitude of Lake Titicaca, and in some ways the land use practices observable upstream of Tuti in the Callalli area are comparable to those in the Lake Titicaca Basin, although this western slope location does not have the moderating effect of the large lake body itself.

Local elevation



Upper Colca Survey Blocks

4000 - 5000


Rich pasture lands and rugged volcanic terrain in the Chivay obsidian source area, the San Bartolomé puna near Chalhuanca, and below to the town of Pulpera.

1, 2, 4, 5

3600 - 4000


High elevation agricultural lands, lower quality pasture, upper river valley from Chivay to Sibayo / Callalli.

3, 6

3300 - 3600


Lower portion of the main Colca valley.


Table 4-1. Andean ecological zones with approximate local elevation values for each zone.

Annual rainfall ranges from 550 and 750 mm, depending on elevation, and the average annual temperature is 1° C in the puna (ONERN 1973). In this area, precipitation is higher with altitude, but temperature, especially night time temperature, drops with altitude. The result is a balance between the potential for dryland agriculture, altitude, and climate that has periodically allowed farming in the Upper Callalli area in prehispanic and Hispanic times.


Figure 4-2. Survey blocks in the Upper Colca study area are shown with modern production zones described in Table 4-1. These zones reflect changes in altitude and rainfall as the Colca river, descending westward, is surrounded by puna pasture lands.

A relationship of mutual benefit exists between residents and resources of the higher altitude ecological zones and the vegetatively-productive main Colca valley. A principal assumption of this research is that the procurement of resources like obsidian have long been structured by the human use of complementary ecological zones. Some form of regular articulation between the highlands and the valley therefore probably existed through much of prehistory without special recourse for obsidian provisioning. This interaction may have taken the form of regular visits to the Chivay obsidian source area by valley agriculturalists, who may have been hunting or herding in the adjacent highlands, or by way of visits by highland pastoralists traveling from the puna regions in order to visit the Colca valley to barter for agricultural goods.

4.1.1. Climate across the study area

The Colca valley is located in the western cordillera of Andes in a semiarid climate that is cool and unpredictable. As a high altitude region of tropical latitude, diurnal temperature variation is more prominent than annual temperature variation (Denevan, et al. 1987;Troll 1968). Precipitation is highly seasonal, however, and it is changes in rainfall and availability of pasture that strongly influence the scheduling and intensity of pastoralism and dry land agriculture throughout the study area. The south-central Andes is a region south of 15° S latitude, outside of the Intertropical Convergence Zone, and precipitation is relatively unpredictable with high interannual variability because it results largely from the convection of humid air from the Amazon Basin to the east (Johnson 1976). The western slope of the Andes thus lies in a rain-shadow, and warm, dry winds from the east result in low average annual precipitation as one descends the western flanks of the Andes. In the ecotone that is the Upper Colca study area the herding economy thrives, because these communities can seasonally exploit the rich pasture lands and greater rainfall of the high puna while simultaneously interacting with the communities and products of the main Colca valley, communities to which they may have family linkages.

Of the major obsidian sources in the Andes, the Chivay obsidian source is the highest elevation source and the environmental conditions in the area during prehispanic procurement visits to the source are the subject of some speculation. Annual climatic variability under modern conditions at the Chivay source can be inferred from the relationship between altitude and temperature, known as the lapse rate, and this rate may be calculated using records from nearby meteorological stations. A mass of rising air will cool at the dry adiabatic lapse rate that is often estimated as 0.98° C per 100m of ascent for dry conditions, while the saturated adiabatic lapse rate is typically 0.5° - 0.6° C per 100m at dew point when temperatures are around 10°C (Adiabatic lapse rate 1999). This theoretical lapse rate may be compared with empirical temperature data reported by the ONERN environmental investigation, a regional study of the entire Majes-Colca drainage that preceding the construction of the Majes hydroelectric project. These data include mean monthly temperature records from meteorological stations at Pañe near the Colca headwaters, Sibayo on the northern edge of our Upper Colca study area, and the mid-altitude stations of Arequipa and Pampacolca (ONERN 1973;WMO 2006).

/Figs_Ch4/weather_stations_map-2.jpg /Figs_Ch4/weather_stations_graph-2.jpg

Figure 4-3. Temperature by Altitude at mid- and high-altitudeArequipa meteorological stations. All data from ONERN (1973) except Arequipa values from W.M.O. (2006). These data represent a lapse rate of -0.56°C per 100m of ascent.

These empirically-derived mean annual temperature values from the Arequipa sierra show a lapse rate of -0.56°C per 100m, which is in the range of the standard saturated adiabatic lapse rate of -0.5 to -0.6 per 100m, discussed above, from which can be inferred a mean annual temperature of 0.8°C at the Chivay obsidian source. Given the aridity of this region much of the year, the relatively low-slope (saturated) lapse rate is unexpected and the raw tabular data in the ONERN report suggests that a more detailed examination of the data will better allow the temperature variation at the Chivay obsidian source to be inferred.

Models based on local meterological data

Monthly mean temperature data collected from 1952 - 1970 at local meteorological stations show highs, daily means, and lows, and the considerable differences from the lapse rate function that probably result from the dramatic diurnal temperature variations and the relatively thin atmosphere in this region. The seasonal effects on lapse rate can be considered by looking more closely at data from the nearest reliable weather stations, at Sibayo and Pañe.


Figure4-4. Comparison of Temperature highs, means, and lows for August and January from two meteorological stations with records kept between 1952-1970(ONERN 1973).

August (winter, dry)

January (summer, rainy)
































Temp. Change / 100m







Figure 4-5. Mean monthly temperatures (°C) from data in ONERN (1973). Inferred temperatures at the Maymeja area of the Chivay obsidian source shown in italics.

/misc/image014.jpg /misc/image015.jpg

Figure4-6. Precipitation by Altitude (left), precipitation in the study area (lines represent highs, averages, and lows) is highly seasonal as shown in 14 yr precipitation record from Sibayo on right. Data from ONERN (1973).

The local relationship between altitude, temperature, and precipitation on the dry western slopes of Arequipa are evident in these data derived from tables provided by ONERN (1973, Appendix II). Diurnal temperature variation, highest during the dry winter months of July, August, and September results in a steep lapse rate such that low temperatures in Sibayo, near the lowest part of the study area, are only 3° C warmer than those mean winter lows at the Chivay source. Curiously, the high temperature values cross over between summer and winter months (

Figure4-4) resulting in somewhat more of a temperature difference between Sibayo and Pañe during the rainy summer months than the winter months, probably reflecting the higher precipitation at altitude. As most precipitation, including snowfall, comes during the summer months of January and February, greater cloud cover and reduced solar insolation at altitude are expected during the summer months.

At the Chivay obsidian source the inferred temperature values are in keeping with observations made during the course of research at the source in August and September 2003, as will be discussed below. This is not to suggest that the modern climate regime existed during prehispanic times, but these relative contrasts in temperature and precipitation values throughout the Upper Colca study area probably have existed throughout the Holocene although actual precipitation and temperatures varied from those of the modern conditions. Paleoclimatic reconstructions from the Terminal Pleistocene and through the Holocene using data from cores collected in large lakes and glaciers were discussed in Chapter 3.

It is important to stress that adiabatic lapse rate temperature estimates do not account for other important factors affecting local climate such as local insolation, vegetative cover, and wind, as well as temperature contrasts due to solar heating where the atmosphere is thin, and temperature inversions in mountain valleys (uncommon at tropical latitudes). Further, orographic effects such as mountain and valley winds have specific and localized effects in the Upper Colca survey blocks. Seasonality in the upper Colca region includes rain and snowfall in the highest reaches during the wet summer months as well as a high incidence of lightning strikes (a major cause of death in the Peruvian highlands today). Modern herders in the Colca area adapt to these conditions by distributing their impacts and exploiting the high altitude resources primarily during the dry season.

The equatorial bulge in barometric pressure

An issue related to temperature variation is the presence of an equatorial bulge in atmospheric pressure as one ascends to high altitude (Ward, et al. 2000: 26-28;West 1996). In low latitude areas, barometric pressure at a given altitude is correspondingly higher than is pressure at that altitude in mid and high latitude areas of the world.


Figure4-7. Latitude against barometric pressure (West 1996). Lines show altitude in km. Annual temperature seasonality is minimal in the equatorial areas and thus seasonal effects at low latitudes are not shown in this graph.

Models of variance in barometric pressure with latitude at a given altitude based on data derived primarily from weather balloons explain why there is greater available oxygen at 5000m in low latitude areas than in high latitude areas. The differences in pressure are particularly notable during high latitude winters.


15° latitude

60° latitude, summer

60° latitude, winter

Equivalent Altitude
















Table 4-2. Equatorial bulge and effects on barometric pressure (in torr) at 15° and 60° latitude. From data in (West 1996: 1851). Equivalent altitude column shows altitude at 60° latitude with equivalent pressure to the value shown at 15° latitude.

As shown in Figure 4-7 and Table 4-2, the equatorial bulge in pressure means that the available oxygen at the Chivay source at 5000 masl is only close to the pressure that of a location nearly 1000m lower if the source were located at 60° latitude (in winter). In other words, during the winter at 60° latitude, at approximately the latitude of Anchorage Alaska, one would have to be at only 4400 masl altitude to find available oxygen at levels equivalent to that found at 5000 masl at the Chivay source. Barometric conditions similar to those found near the summit of Nevado Ampato at 6000 masl can be found in winter at 5310 masl near Anchorage, AK. The high altitude Chivay source is not as inhospitable as would be this zone at the equivalent altitude in higher latitude areas. Resource patches, such as rich bofedal grazing areas, are sufficient to draw seasonal or permanent residents to these altitude zones in the Andes. Raw material sources, particularly mining operations, are another significant draw to high altitude locations both in past and in modern times (Ward, et al. 2000: 336-344).

4.1.2. Lower elevation biotic zones: Study Area Blocks 3 and 6

Beginning with the lower elevation part of the survey area and moving upstream, the vegetation of the Colca valley above Tuti consists of low grasses dominated by the ichuvariety during the dry season. The Colca River is relatively low gradient in this area, and two or three levels of natural river terraces are evident along the valley margins. The geography of the zone is dominated by the fact that the confluence of two large river systems (Colca and Llapa) occurs here. This area holds the largest settlements in our study area, but both agriculture and pasture appear marginal in immediate area of these towns. Rather, the settlement of the upper river valley seems to reflect the importance of the articulation between the main Colca valley and the broad altiplano. The village of Sibayo rests precisely at the main confluence of the Colca River and the Llapa River, and as is evident in the Shippee-Johnson expedition, Sibayo has long served as a principal modern ingress to the Colca Valley until the Chivay-Arequipa highway was completed (Shippee 1932).


The bulk of the contemporary populations in the study area reside in towns in block 3 established during the colonial period that are distributed along the Colca river. The largest settlement inside the study area is Callalli, a town with a 1993 population of 1295 persons, and across the Colca River the population of the town of Sibayo is 508, and upstream in Block 5 the cooperative of Pulpera numbers 85 residents (I.N.E.I. 1993). These towns are primarily service centers and district seats for widely distributed populations with an economy based largely on pastoral products, and on extensive interaction through trade with their wealth in camelid herds, and have long resided in rural hamlets and herding outposts. Callalli and Sibayo first formed as part of the sixteenth century reducción of the Yanque Collagua, and in the colonial period the province of Collaguas (Caylloma) held three-quarters of the livestock in all of Arequipa (Cook 1982;Manrique 1985: 95-96). The dominance of the herding economy in these upper valley towns is evident in the 1961 census where both Callalli and Sibayo populations are reported as 93% "rural", while the average percentage for all nineteen towns in the Colca census, including large and dense agricultural communities downstream, was only 52% "rural" (Cook 1982: 44).

These early villages also formed an important source of labor for colonial mining ventures in the Cailloma region (Guillet 1992: 25-27). Mercury and copper were mined between Callalli and Tisco (Echeverría 1952 [1804]) and Lechtman (1976) reports a structure in Callalli known as " La Fundición" that is described as "stone metal smelter, probably colonial, said to be for copper smelting: mineral, scoria on surface". A location known as "Ccena" or "Qqena" is described as having "metal smelters near a pre-Spanish occupation site: mineral, scoria, surface sherds" (Lechtman 1976: 11). The toponym "Ccena" can be found close to the Llapa and Pulpera stream confluence upstream of Callalli. These historical smelters were not encountered during the course of our survey in this particular area.

Pyrotechnical installation

One structure ([A03-842], Figure 4-8) was identified in the course of the 2003 survey that appears to be of colonial period construction and it appears to have some kind of pyrotechnical function (B. Owen 2006, pers. comm.). The structure has two doors in the lower area, apparently providing access to the lower furnace. The internal lower construction is built of thermally altered stones and has a cracked lintel. A variety of pyrotechnical structures are known in the south-central Andes (Van Buren and Mills 2005) that were used to heat lead, silver, or copper, and other oven types (e.g., pottery, bread) are known in the region as well. Slag or other evidence of smelting was not encountered in the soils adjacent to the structure, however, although the building is immediately adjacent to a stream channel and such materials may be eroded or difficult to detect.



Figure 4-8. Exterior and interior of probable colonial pyrotechnical structure at Achacota near Pulpera, upstream and south of Callalli [A03-842]. One meter of exposed tape is visible in each image.

Vegetation and dryland agriculture in the study area

The Callalli area consists primarily of high elevation grassland punaecology (1973). Dry bunch grasses are available much of the year in this area, but during the wet season (austral summer) a greater variety of grasses become available and herds are brought to the valley to exploit the pasture of chilliwuaand llapagrasses.

Callalli and Sibayo lie in the upper reaches of agriculture at this latitude and evidence of abandoned fields are visible in the upper valleys. Plants like tubers, oca, and chenopodium were historically viable at this altitude (Echeverría 1952 [1804]), although microclimatic variability is influential in these conditions of marginal dryland agricultural production. Guillet (1992: 24) cites evidence for climate change from historical sources that describe the cultivation of maize above the current limits for this crop, and coca, membrillo, and peppers on terraces that lie at altitudes where these crops are not feasible today. Wernke (2003: 51-52) considers the significance of climate change evidence from ice cores in mountain ranges to the east for Colca valley culture history and agriculture.

In Markowitz's (1992) ethnographic study at Canaceta, a village at 4000 masl and approximately 12 km upstream of Callalli, villagers explained that they had formerly engaged in agriculture at this altitude, but that they had recently abandoned the practice due to lack of rainfall. She observes that practicing a mixed subsistence system of agriculture and pastoralism was an important cultural ideal in the Canaceta, but that in recent years due to changes in the climate and in economic circumstances they had increasingly become specialized on pastoral production complemented by exchange (Markowitz 1992: 48). Under modern circumstances there is probably not a very high return on labor invested in agriculture in this area as the increased economic integration, and an improved transportation infrastructure with intensive farming areas at lower elevations, has further induced residents towards specialized economic practices. The current distribution system emphasizes pastoral production in the highlands and higher yield agriculture on lands at lower elevations.


Figure 4-9. Tuber cultivation at 4200 masl surrounded by large tuff outcrops.

In the course of survey in 2003 cultivation was observed in a few high altitude locations, such as the upper reaches of Quebrada Taukamayo. The plots were at 4200 masl in a north-north-east facing (10°) aspect and a mild slope (8°). The area is relatively sheltered by the presence of lava tuff flows (see Figure 4-9) that may have had a temperature moderating effect acting like large terrace stones that are known to reduce diurnal variation by absorbing heat during the day and releasing heat at night in the immediate valley microclimate (Schreiber 1992: 131).

Flora and fauna

Important flora and fauna to residents of the Upper Colca region include the following (Gomez Rodríguez 1985;Guillet 1992: 130;Markowitz 1992: 42-44;Romaña 1987;Tapia Nuñez and Flores Ochoa 1984). Major flora comprise grasses such as Chilliwa( Calamagrostis rigescens), a frost-resistant perennial grass that thrives during the rainy season and in bofedales, grazed by a wide range of animals and also used for roof thatching. Other important puna pasture grasses include llapa, malva, sillo, and paco,though these are principally consumable by herbivores only during the rainy season. Ichu/ Paja( Stipa ichu) is a common grass used for thatching. In the higher elevation bofedales one can encounter parru, a grass preferred by alpacas. Wild fruits are gathered seasonally by locals including locoti(cactus fruit), q'ita uba(wild grapes), and sanquayo(a plant related to chirimoya) (Markowitz 1992: 43). In the high elevation area of the obsidian source yareta( Azorella compacta), a green, flowering cushion plant is one of the few flora that grow in the unirrigated areas of this harsh volcanic terrain. In addition to animal dung, dried yaretais the only dense, combustible fuel widely available above 4500 masl. As a local herder, T. Valdevia demonstrated, the cushion plant will burn when it is kicked over and allowed to dry out for several weeks. Drought and cold-resistant shrubs, including tolaand cangi, are valuable sources of firewood in the punatoday, though the shrubs are over harvested in many areas.

Fauna species include a number of birds that are hunted for their meat including the Grey Breasted Seedsnipe known as puko elquio( Thinocorus orbygnianus), partridges ( pishaq), and the guallata,the large white Andean Goose (Chloëphaga melanoptera)(Hughes 1987;Markowitz 1992: 43) .Several Andeancondors ( Vultur gryphus), for which the Colca is renowned, flew repeatedly near our work at the Chivay obsidian source in 2003. Wild mammals observed in the study area include viscacha( Lagidium peruana), tarucadeer ( Hipocamelus aticensis), and the wild camelid vicuña (Vicugna vicugnaor Lama vicugna).Thetrout found in the streams represent an important food source, but these fishes were introduced in the nineteenth century.

4.1.3. High Puna: Block 2 survey and adjacent Blocks 4 and 5

This high altitude zone of the study area is dominated by the Pliocene lavas and Quaternary moraines, as will be described in more detail below in the geology section. Much of these survey blocks consisted of porous lava rock and sandy soils covered by a mantle of ash, lapilli (rock fragments between 2 - 64 mm across) and the occasional volcanic bomb (molten rock between 64 - 1000 mm). During the dry season surface water was available only sporadically across the area and, unsurprisingly, reliable water sources frequently have archaeological sites nearby. The renowned "Ventanas del Colca" tuff formations occur on the edge of the San Bartolomé survey area (block 2) at the point where the highway connecting Callalli with Arequipa climbs out of the Pulpera drainage and straightens out for its run across the open puna grassland.

This area is also remarkably wet, the environment is productive, and faunal density is relatively high. Even during the dry season the Block 2 area has reliable water.


Figure 4-10. ASTER scenes with the terminus of volcanic breccia outcrop in green, photosynthesizing plant areas (bofedales) shown in red pixels, and ash in white. The black diagonal line is a seam between the two scenes.

Block 2 geography

Block 2 can be characterized as containing rich bofedales, reliable hunting opportunities, and access to lower Colca Valley resources only two-day's travel away. This area, known as San Bartolomé, can be considered a "puna rim" ecological area because animal and plant species are affected by the presence of lower elevation Colca Valley environments immediately downhill to the north and west, and warmer air rises from the lower valley affecting the local climate. The Pliocene eruption of the Barroso group vents of Huaracante, Hornillo, and Ancachita resulted in the predominantly silicic coulee flows of lava described as "Centro Huarancante" by the INGEMMET study (Palacios, et al. 1993: 139). On the eastern edge of the Huarancante Group the viscous lava flows terminate in breccia outcrops where they overlie crystalline ignimbrites (TBa-c) of older Pliocene age, also belonging to the Barroso formation. Below the toe of the lava the crystalline ignimbrites rich in dacite appear as light colored, sandy soils, where perennial surface water has created good grazing opportunities in this flat expanse to the east of the lava flows. Bofedales are found below each quebrada descending from the lava formations, a pattern that is perhaps the result of subsurface water moving through the lava flows and emerging at the contact zone with the ignimbrite formation.

A number of small rock shelters occur along the base of the upper lava flow. The rock shelters are typically dry, but the floors are sloping out onto talus slope below and so they offer little in the form of shelter inside the drip line. There are some notable exceptions, as will be reported in Chapter 7, and relatively dense occupations were found at few of the best rock shelters. On the whole, Block 2 follows along the eastern periphery of a lava flow where there is a concentration of resources. Water emerges from the lava onto the open pampa, grazing opportunities for wild and domestic herbivores on the bofedales and adjacent grasslands are good, and Andean geese and other puna bird species are seen in the greatest numbers. The lava flow provides topographic variability on the perimeter of a wide and often windy plain, providing shelter and occasional, small rock shelters.


Figure 4-11. View of San Bartolomé (Block 2) area during the dry season from atop a toe of Barroso lava looking northeastward. For scale, our white pickup truck is visible below. A rich bofedal is visible 3 km to the northeast.

Current environmental conditions suggest that the edge of the lava flow served as a rich ecotone with water, shelter, and numerous hunting opportunities for bird species, vicacha, tarucadeer, mammals, and probably the wild camelids guanacoand vicuñaat certain times in the past. We often observed a number of wildlife species in the course of research in this area including viscachaand geese, and perhaps the density of wildlife in this locale was due in part to lack of surface water elsewhere, on the adjacent lava beds and ashy soils. Even well into the dry season, parts of Block 2 area have reliable water and soft grasses.

Regional geography

Block 2 is a natural bottleneck for economic traffic moving between the Colca and the Titicaca Basin along what may be interpreted as the prehispanic trail system. Due to the steep descent to the Pulpera on the north side, and the glaciated volcano Nevado Huarancante on the south-west side, many travelers would probably have traveled through Block 2. The presence of a very large bofedal and rich hunting opportunities in the adjacent lava flows to the west probably made the Block 2 area an even greater attraction for travelers with caravan animals. This area lies on the periphery of the Colca valley and it is approximately one long day from the rich grazing areas of the Escalera access to the Colca valley, and about two days from the town of Yanque.

4.1.4. The Chivay Source: Block 1

The geological source of Chivay obsidian is above 4800 masl among the lava flows from two Barroso (Pliocene) volcanic vents named Cerro Ancachita and Cerro Hornillo. The discussion here focuses on the geographical context of Survey Block 1, and the geology of these volcanic features will be explored in more detail in Section 4.3.3, below.

In the high altitude portions of the Upper Colca survey, most notably in the Chivay obsidian source area above 4800 masl, the local temperature and climatic exposure is strongly affected by the lack of vegetative cover and the katabatic (mountain breeze) and anabatic (valley breeze) winds. The winds were a daily feature during the Block 1 fieldwork in the months of August and September 2003, and the winds are most notable in the mornings and evenings when the temperature differential between the high altitude areas and the warmer Colca Valley are greatest. Extrapolating the local lapse rate from Colca meteorological stations suggests that the obsidian source, at around 5000 masl, would have mean temperatures of 0.5 to 1.0° C year-round (

Figure4-4), not accounting for the effects of wind in this exposed area. In the course of fieldwork daytime high temperatures were approximately 5° C, and nighttime temperatures were commonly -9° C with the coldest night measured at -12° C. These data largely corroborate the estimates derived from the local adiabatic lapse rate discussed above.

4.2. Tectonic geology

The Colca Valley lies on the western margins of the broadest region of the Andean cordillera at 15° south. The Andes consist of several parallel mountain chains and in this region that includes areas of southern Peru, western Bolivia, and northern Chile the parallel chains separate and landforms between these mountain chains comprise the broad, high-altitude valleys and plateaus known collectively as the altiplano. The altiplano is a treeless region at an average altitude of 3700 masl extending from the Lake Titicaca Basin to Bolivia's Lake Poopó thatis second only to the Tibetan Plateau as the largest plateau in the world (Clapperton 1993: 45-47;Pearson 1951). The Quechua term punafor "elevated area" applies to the ecological band of the altiplano, but punacan also refer to a broader regions and life zones. The steep Andean cordillera includes volcanism over some of the thickest crusts in the world (Aramaki, et al. 1984: 217), and has resulted in the steep mountains and compressed ecological zonation that strongly influences human organization and subsistence strategies in the region (Troll 1968;Winterhalder and Thomas 1978).


Figure 4-12. Select raw material sources in the central Andes.




Other names





Titicaca Basin, Cotallalli

Burger, et al. 1998, Brooks, et al. 1997





Frye, Aldenderfer, Glascock 1998





Burger et al., 2000, 2001, 2002.





Yacobaccio et al, 2004

Sora Sora




Giesso, 2002, pers. comm.





Burger,et al. 1998.





Burger, et al. 2005




Umasca, Cusco Source

Burger 1998, Jennings, et al. 2002

Querimita Basalt




Giesso 2000, 2003

Uyo Uyo




Brooks 1998, Wernke 2003




Mistaken for Quispisisa

Glascock, et. al., in press.




Andahuaylas A

Glascock, et. al., in press.




Andahuaylas B

Glascock, et. al., in press.

Table 4-3. Coordinates and names of select raw material sources in the central Andes. Coordinates in WGS1984 datum.

It is volcanic processes at a continental scale that have resulted in the discrete pattern of obsidian sources in the central Andes in an arching line between 3000 and 5000 meters above sea level. As discussed by Clapperton (1993)and Thorpe et al. (1982;1981), the volcanic origins of these features involves a consideration of plate tectonics in western South America. Off of the coast of Peru the oceanic Nazca plate is subducting beneath the South American plate, resulting in the magmatism, seismicity, and tectonism characteristic of the central Andes. Geologists date the initial uplift and deformation of the Andean geosyncline to the Laramine Orogeny during the middle Cretaceous period, a period of worldwide mountain building. The Andean batholith, a massive igneous intrusion that underlies the western range of the central Andes (Cordillera Occidental), formed during this orogenic period. The eastern range of the central Andes (Cordillera Oriental) arose from laterally compressed geosynclinal rocks that emerged as folded stratigraphy. The trench between the western batholith and the folded eastern stratigraphy, known as the Titicaca trough, continued to accumulate sediments through the Miocene. A quiescent period followed when the central Andean region matured through erosive processes into a relatively level surface of the altiplano, moderating the terrain that lay above the volcanic, sedimentary, and metamorphic layers that make up the structure of the Andes. This surface undergoing erosion could not properly be called the altiplanoduring the Late Miocene because, interestingly, the surface was still under 1800 masl by 10 Ma (Gregory-Wodzicki 2000). Estimates of uplift rates on the order of 0.2 - 0.3 mm/yr have created the raised plateau known as the Altiplano from the Miocene to the present. Recent estimates by Thouret et al. based on incision dates in major canyons of Ocona below the Cotahuasi valley suggest that "downcutting may have taken place before 9 Ma but most likely before 3.8 Ma and again before 2.7 Ma, based on dated valley infillings" (Thouret, et al. 2005). They suggest that accelerated valley incision was due to increased runoff that resulted from glaciation of the high Andean peaks, and by implication the Chivay obsidian source was at already at high altitude, probably covered by glaciers, by some time in the Late Miocene and Pliocene. Given the

4.3. Formations in the Upper Colca Valley

The volcanic terrain and the temperate lower valley in the Chivay area have largely conditioned human activity patterns in the region for the past 10,000 years. The local geological sequence will be summarized from text and maps from Palacios et al. (1993) the primary INGEMMET source for the region, as well as the ONERN (1973) geological study associated with the Majes Irrigation project, and the discussion will be supplemented with evidence from more recent specific studies in the region.


Figure 4-13 Rock groups in the Colca region (based on Palacios et al., 1993).

The Colca region is dominated by late Cretaceous sedimentary rocks overlain by Tertiary flows and tuffs of basaltic to rhyolitic composition. Some Jurassic sedimentary strata are exposed consisting of sedimentary and metasedimentary rocks. The region has continued to be volcanically active during the Pleistocene and Holocene epochs in the form of stratovolcanoes that ring the Colca valley as well as exogeneous andesitic domes and flows occurring in the Colca River valley itself.

4.3.1. A geological descent of the upper Colca drainage

If one measures the river from its birthplace in Imata near the department of Puno, among a fan of tributaries that flow into Lake Jayuchaca at around 4500 masl, to its final exit into the Pacific Ocean at Camaná, the course of the Colca River is approximately 450 km (Parodi 1987: 31) with an average gradient of 1º. The river that begins in the low gradient marshlands of the Peruvian altiplano becomes increasingly steep as it descends into one of the world's deepest canyons, finally flattening out as it emerges on the littoral batholith only 48 km from the sea.

For the first 70 km from its origin the Colca River trends northwest at a low gradient, open channel descending towards the community of Huinco (3950 masl). This lower portion of this low gradient section of the Colca drainage is under the 250 million m3Condoroma reservoir, a product of the 1970s Majes Hydroengineering Project that, combined with water contributed by tunnel from the much larger Angostura reservoir in the Apurimac drainage, has resulted in a sustained year-around flow in the Colca between here and the Tuti diversion dam (Gelles 2000;Maos 1985). At Huinco, the river turns abruptly to the south-west where it continues to descend as a gentle channel, cutting through the Miocene lava formation known as Tacaza as well as wind-scoured river terraces. Immediately before entering the northern edge of Block 3 of our study area, the river cuts through strata of uplifted Cretaceous limestone striking west-north-west and forming outcrops and rock shelters on the edges of the upper river terraces (Figure 4-15). Downstream at the confluence with the Llapa River the Colca River returns again to the Tacaza formation for five more kilometers before entering the Inca formation, an andesitic exogenous dome dating to the Middle Pleistocene. This Quaternary formation fills the upper Colca valley from this point downstream to near the town of Coporaque and Yanque where fluvial conglomerates overlie it as a result of natural damming of the river by mudslides.

In its descent, 5 km below Sibayo, the incising of the Colca River begins immediately upon entering the recent exogenous dome of the Inca formation, an the river remains incised until it exits the Colca canyon approximately 100 km downstream. The Llapa River, which joins the Colca from river-left at Sibayo, emerges from the sandy tuff layers known as the Castillo de Callallituff formations (Noble, et al. 2003) and the Chalhuanca rhyolite dome fields to the south-east of Callalli. The entrenched Colca River descends more rapidly upon entering the Inca formation approximately 10 km downstream with perennial tributary streams entering primarily from glacierized stratovolcanoes that ring the Colca Valley. The first of these tributary streams are the creeks entering from the north upstream of Tuti draining the southern and eastern flanks of the Nevado Mismi volcano, while streams that form just on the other side of Mismi have recently been confirmed by a National Geographic expedition in 2000 to be the source of the Amazon. Immediately upstream of the town of Tuti a diversion dam was constructed across the Colca where the river is entrenched in a ravine 50m deep. At this point water collected by the Majes Project is diverted into a system of canals and tunnels along the south bank of the Colca that finally crosses into the fertile Pampa de Majes near the Pacific coast.

Below Tuti, the Colca river channel is deeply incised into Quaternary exogenous domes. Continuing downstream, the river turns west, again becomes entrenched, and begins dropping more steeply in the vicinity of Chivay. The geology, geomorphology and soils of the main Colca valley have been well-studied in the past several decades as a result of the "Colca Valley Terrace Project" organized by William Denevan (1986;1988;Denevan, et al. 1987) and more recent reviews of research in the main Colca valley can be found in dissertations by Wernke (2003: 34-66) and Brooks (1998: 57-84). In its steep westward drop to the Pacific littoral the river subsequently enters the 3,270m deep Colca canyon, the third deepest canyon in the world after the Yarlung Tsangpo (Tibet) and the Cotahuasi (Perú). Here the river cuts through folded sedimentary and metamorphosed layers predominantly belonging to the Yura group (Jurassic and Cretaceous) until it emerges as the Majes River on to the large alluvial plain that leads to the sea.

Figure 4-14. Legend showing geological map units in maps that follow.


Figure 4-15. Geological map units and 2003 Survey Block boundaries (in gray) for the Upper Colca project study area. Legend shown in preceding figure.


Figure 4-16. Geological map units shown on ASTER scene from 28 Sept 2000; legend is shown in Figure 4-14. In general, red pixels show areas of photosynthesizing vegetation (bofedales).

4.3.2. Yura and associated sedimentary strata

Starting with the oldest rock groups in the study region, the Yura group, the geological history of the Upper Colca region will be summarized through to the Holocene. The Jurassic and Cretaceous (> 66.4 Ma) sedimentary strata in the region include quartzite, shale to sandstone, dolomite, and limestone. The Yura sedimentary formation is exposed on both sides of the Chivay-Arequipa road around the Sumbay junction and to the 10 km to the east of the Llapa and Pulpera confluence. Quartzite outcroppings of the Yura formation appear to have provided material for the abundant artifacts made of quartzite observed in archaeological sites in the Callalli area. These quartzite Yura strata do not appear as cartographic units on the 1985 INGEMMET map (Ellison and Cruz 1985), but Parodi (1987: 47) notes that fine-grained quartzite outcrops occur west of Callalli and these features were encountered in recent fieldwork (JKYu west of Callalli on Figure 4-15 ). Quartzite outcrops form in metamorphic sandstone and occur in the oldest metamorphic strata in the Colca region. Similarly, chert, chalcedony, and quartz precipitate from diatoms in sedimentary contexts (Andrefsky 1998: 51-56;Luedtke 1994), and consequently exposures of these materials are found in the Mesozoic strata in the region and in cobbles form in many stream beds. Interestingly, cherts were noted in the Ichocollo creek in Block 6 of the survey, but an examination of the headwaters on the Cailloma (31-s) geology map (Davila M. 1988) reveals no layers older than the Tertiary in that watershed.

Steeply uplifted Cretaceous formations appear in two portions of the study area (Palacios, et al. 1993: 28-30, 36) and these formations, in addition to nodules found in riverbeds, may have provided the local sources of non-obsidian toolstone in the area. The thick calcareous Yura sedimentary exposures north of Sibayo are dramatic examples of uplift of these Cretaceous strata. Additionally, the slopes just east of Chivay below 4000m between the town of Canocota, close to Calera hot springs, and as far south as the Quebrada de los Molinos consist of Murco and Hualhuani formation sedimentary rocks that include siltstone, quartz-rich sandstone, and limestone layers. These areas, in addition to cobbles encountered in riverbeds, may have represented the local non-igneous sources of material for stone tool production that include chert and chalcedony.

4.3.3. Oligocene and Miocene lavas

Oligocene and Miocene lavas

Tacaza Group

Flows belonging to the Tacaza group are found throughout the south-central Andes, however the only portion that appears in the Colca region belong to the older Tacaza with dates in the range of 30.21±0.73 Ma and 26.51±0.6 Ma. At 1900m thickness these Colca lava flows are the thickest Tacaza layers in the larger region (Palacios, et al. 1993: 86).


Figure 4-17. Andesitic Tacaza deposits with breccias and tuff outcrops in the Quebrada de los Molinos drainage. The Chivay obsidian source in later Barroso deposits is found high above, on the right side of the photo.

In the Colca area, these deep deposits of lavas and breccias consist of andesites and trachybasalts (containing higher feldspar content) intercalated with tuff bands. The Tacaza layer appears predominantly on the western half of our survey zone.

Pichu Formation ash flows and the Castillo de Callalli

On the eastern side of the study area, the Tacaza formation is overlain with the Pichu formation consisting of sandy tuffs and white ignimbrites. Among these Miocene ignimbrites layers is the Castillo de Callalli formation, an ash-flow tuff that rises dramatically from Llapa river just upstream of Callalli and is a principal landmark in the Upper Colca region. In the INGEMMET study (Ellison and Cruz 1985) the Castillo de Callalli was assigned to the Pichu formation. This landmark is an approximately 400m hill of silicic ash-flow tuff ranging from densely welded to non-welded tuff. The layering in this formation has recently been subject to a more detailed study involving isotopic dating and phenocryst mineralogy (Noble, et al. 2003). The recent work shows that this formation is not a single stratigraphic unit, as presented in the INGEMMET study, but rather it consists of two layers separated by 16 Ma.


Figure 4-18. The lower section of the Castillo de Callalli is known as "Cabeza de León". Evidence of an LIP pukara was encountered on the summit [A03-935].

The lower part of Castillo de Callalli is adjacent to the main road to Callalli and follows the Llapa River. The Cueva de Quelkata, a rockshelter with a predominantly Terminal Archaic component (Chávez 1978) that was dynamited by the Majes Project road construction, is at the base of this formation. This lower section has well developed columnar jointing and is a densely welded, devitrified ash-flow tuff, and K-Ar dating on phenocrystic hornblende indicates that this lower flow is 20.7±0.6 Ma (Noble, et al. 2003: 33). The upper section is described as "partly welded vapor phase crystallized tuff with the physical characteristics of the distal part of an outflow sheet". Phenocrystic sanidine from this section yielded an40Ar-39Ar age determination of 4.72±0.02 Ma (Noble, et al. 2003: 35). The study suggests that the upper part of the Castillo de Callalli formation is associated with the Cailloma caldera to the north which erupted three times during the Pliocene. Further discussion of Miocene volcanism in the Orcopampa area of the Chila cordillera, between the Chivay and Alca obsidian sources, can be found in Swanson's (1998) geology dissertation.

4.3.4. Pliocene lavas - Barroso group

Barroso lavas in the Colca area include predominantly andesitic and trachyandesitic flows covering an area of approximately 320 km2. In the case of the Barroso group, the emplacements are contiguous lavas that occur as transversal flows, as andesitic domes, and occasionally as rhyolitic domes. At the Chivay source, two vents dating to the Barroso formation occur at Cerro Ancachita and Cerro Hornillo, and at the highest peak in the Centro Huarancante formation named Nevado Huarancante, to the south. Transversal flows and crests emanating from these flows created adjacent peaks such as Cerro Saylluta and Cerro Llallahue. Lavas, silicic coulee flows, and viscous volcanic breccias flowed from these vents and traveled up to 15km, into the Block 3 study area to the east. Curiously, in the Cailloma quadrangle study, immediately north of Chivay, Barroso group volcanism in the Cailloma caldera was dated to the Pleistocene rather than the Pliocene epoch (Davila M. 1988). The important distinction is that recent dating of Cailloma caldera deposits (Noble, et al. 2003: 35) appear to pre-date the Barroso group flows that are responsible for tool-quality obsidian formation in the western Cordillera.

At the Chivay obsidian source, Barroso group flows are superimposed on Tacaza levels, and both groups have been eroded and incised by later fluvial and glacial erosion. It is proposed by Burger et al. (1998: 205) that obsidian occurs at the Chivay obsidian source where silica rich magma from Barroso eruptions cooled rapidly when the flows contacted the older Tacaza group deposits. The emplacement of Barroso group obsidian flows will be discussed in more detail below.


Figure 4-19. Detail of Chivay source, Maymeja area with INGEMMET geological map units shown on ASTER scene from Sept 2000. Contact between TTa and TBa on the west appears offset and likely conforms to the horseshoe shaped valley. Yellow arrow shows direction of glacial striations.

Origins of the Maymeja volcanic depression

The Maymeja area is a depression surrounded by transversal flows and domes that appear to have been heavily glaciated in the Pleistocene epoch. The Maymeja depression contains certain features that resemble those of a volcanic caldera resulting from eruption-induced subsidence and collapse. However, further consultation with volcanologists indicates that the Maymeja depression is likely nota caldera. The characteristics that do suggest that Maymeja is a caldera include: a circular, steep-walled perimeter, occasional ignimbritic deposits, the Anchachita and Hornillo vents located along the margins, and remnant vent-like features in the center of the Maymeja area (Fisher and Schmincke 1984: 360;Karátson, et al. 1999;Szakács and Ort 2001). However, the small size (2 km diameter), an irregular southern and breached western margin, and overall paucity of ignimbritic materials in the region suggest, rather, that the margins of this area were defined by highly viscous rhyolitic lava flows from Ancachita and Hornillo that were subsequently eroded into the circular form of a cirque, particularly on the south-facing (heavily glaciated) slopes, as a result of abundant Pleistocene glaciation. An example of a large Pliocene caldera is the Cailloma caldera that dominates the Cailloma quadrangle immediately to the northwest of the Chivay area (Davila M. 1988). Rather, the Maymeja area can be more generally described as a volcanic depression that underwent significant glaciation during periods subsequent its Pliocene formation.

4.3.5. Pleistocene - Ampato group

Pleistocene - Ampato group

Pleistocene Inca Formation

Named for the site of the Inka bridge over the Colca adjacent to Chivay, the Inca formation consists of Andesite and Trachyandesites that occur in exogenous domes and flows that appear to emanate from the north side of the Colca river, just north of the town of Chivay. These domes are composed of andesites and trachyandesites marked by a high percentages of alkali feldspars (Palacios, et al. 1993). Two Potassium Argon dates from these flows by Sandor (1992: 232-235) indicate that they formed during the Middle to Late Pleistocene (64,000 ± 14,000 bp and 172,000 ± 14,000 bp).

4.3.6. Holocene stratovolcanoes

Arequipa is a volcanically active region with a number of stratovolcanoes that have erupted repeated over the past 10,000 years. These peaks include Huayna Putina, Misti, Sabancaya, and Ubinas (Gerbe and Thouret 2004;Thouret, et al. 2002;Thouret, et al. 2002;Thouret, et al. 2001;Thouret, et al. 2005). The regular deposition of tephra from these peaks provides consistent strata that may aid in archaeological excavation work. Archaeologists working in the western cordillera will benefit from the tephrachronology sequences currently being developed by volcanologists in the region.

4.3.7. Glaciation

In modern times glaciers generally occur above 5,000 - 5,200 masl in Arequipa (Clapperton 1993;Dornbusch 1998;Fox and Bloom 1994), with differences in precipitation being the single largest contributor to variation in snowline altitude between the eastern and western cordillera. During the Last Local Glacial Maximum (LLGM) remote sensing studies of glaciated landforms suggest that there was a regional snowline depression of 600 - 800m in the western cordillera region of Arequipa during the Late Pleistocene (Clapperton 1993;Klein, et al. 1999). However glaciological studies show that the response of snowline to aridity is not uniform across the region, and that "as snowline rises in response to increasing aridity, it becomes less sensitive to temperature perturbations" (Klein, et al. 1999: 81). Recent evidence from ice and lake core studies in central and southern Peru (Smith, et al. 2005) have shown that the LLGM occurred in the tropical Andes around 21,000 cal years ago or over 10,000 years before uncontested evidence of human presence in South America.

The extent of glaciation during the Terminal Pleistocene and Early Holocene in the Colca region is of direct interest to this study of the Chivay source because the Maymeja source area itself was potentially glaciated into the Holocene epoch, and glacial geomorphology appears to have eroded high altitude obsidian deposits like Chivay. Lake and glacial core studies, as well as radiocarbon dates on vegetative material in deglaciated areas, indicate that despite the evidence for glacial advance during the Late Glacial, aka the "Younger Dryas" (9550 - 10,850 cal BCE) in the northern hemisphere, the glaciers of the tropical Andes appear to have retreated during this period (Rodbell and Seltzer 2000: 335;Seltzer, et al. 2002). The evidence suggests that the cooler temperatures were associated with a decline in precipitation, and that this precipitation decline resulted in glacial retreat.

Glaciation of the Chivay source area

These regional data on snowlines are corroborated by evidence of terminal glacial moraines in the Quebrada de los Molinos at 4400 masl and in the adjacent Quebrada Escalera at 4300 masl. On the east side of our study area glacial moraines were observed just west of the Ventanas del Colca feature on the south and east end of a dramatic U-shaped valley at 4350 masl on Quebrada Porhuayo Mayo. The INGEMMET map series (Palacios, et al. 1993) indicates that morainal deposits of silt, sand, and gravel are evident elsewhere in the study area (Figure 4-15), typically at or above 4300 masl, corroborating the evidence from the regional model with a local snowline during the LLGM at 4300 - 4400 masl, or approximately 700 meters lower than conditions evident in 2003.

A team from the University of Maine including professors Daniel Sandweiss and Harold Borns explored the question of the Early Holocene deglaciation of the Chivay Source area in the late 1990s. As shown on Figure 4-15, a10Be date of circa 10,00014C BP (9450 cal BCE) was acquired from a quartzite erratic on a moraine at 4650 masl to the east of the Chivay source area (data courtesy of Daniel Sandweiss, 2006). This sample suggests that the terrain surrounding the source was glaciated at this elevation and higher as late as the Early Holocene. Establishing the rate of deglaciation and the exposed areas at a given time period will require further glaciological study.

The evidence of glaciation in the Maymeja area of the Chivay source area is pronounced. Glacial erosion is evident on the south-facing slopes of the northern part of the Maymeja depression, as is expected in the southern hemisphere. The south and south-western slopes of Ancachita peak are steep and unstable, and at the base of this slope is a recessed glacial tarn that appears to retain water during the wet season. A moraine blocks the exit of this tarn feature, but on the slopes below lateral moraines parallel the path of the glacial tongue descending from below Ancachita. In the most deeply eroded part of this northern area contains the only continuous surface flow of obsidian encountered in the entire study area: the Q02-1 source which contained vertical, subparallel fractures and was unsuitable for tool production. Other effects of glaciation on obsidian distributions include the presence of transported obsidian nodules in parallel and terminal moraines in the Quebrada de los Molinos.


Figure 4-20. Glacial polish and striations (aligned towards camera) on lava flows adjacent to Maymeja workshop on the southern end of the Maymeja area.

In the southern portion of Maymeja a lateral moraine is similarly visible, and the striated and polished benches of lava dramatically attest to the extent of glaciation in the area (see yellow arrow on Figure 4-19). The direction of striation on the these lava flows is consistently south-west or dropping towards the Molinos drainage, and striations persist on high exposed benches suggesting that the glaciers were large as they were striating rocks over twenty meters above the base of the Maymeja area.

During the Middle and Late Holocene glaciers in the south-central Andes, on the whole, have retreated. Glaciological studies conducted in southern Peru and western Bolivia show that retreat is most notable during the time range from circa 10,000 to 3,000 BP (circa 9,000 - 1,000 cal BCE) (Clapperton 1993: 464-466), as well as in last decades of the twentieth century. There is evidence for small advances in glacial extent since 1000 cal BCE, most notably the Little Ice Age circa AD1500-1850. Currently Ancachita is slightly below an altitude permitting glaciation to flow downslope into the Maymeja area, though 5100 masl elevation is glaciated in drainages on peaks like Nevado Sara Sara with large glaciated expanses in the high altitude accumulation zone, and areas of peaks on the eastern side of the Andes like Carabaya (northern Puno) are currently glaciated as low as 4900 masl (Dornbusch 1998). The possible effects of this glaciation on obsidian exposure and weathering in the Chivay area will be discussed in greater detail below.

4.4. How obsidian is formed

Even in volcanically active regions of the world, the geological formation of high quality obsidian is a relatively rare event in nature because a number of features must co-occur for volcanic magma to become tool-quality natural glass. The following description was developed largely from Shackley (2005: 10-15, 189) and Fink and Manley (1987). Obsidian can form when rhyolitic magmas are extruded and quenched in the course of a volcanic eruption. Rhyolitic magmas are silica-rich, acidic melts that are capable of flowing as viscous lavas. As rhyolitic magmas approach the earth's surface, the high water content (up to 10% H2O) begins to escape as vapor, changing the viscosity and the cooling rate of the flow, and resulting in a very low presence of water in obsidian. When water remains trapped in obsidian it sometimes forms bubbles of water vapor, reducing the homogeneity and fracture quality of obsidian for tool production. Fink (1983) found that obsidian emplacement tends to proceed along the following sequence associated with the eruption: (1) tephra fall-out from the initial explosive eruption, (2) basal lava breccia, (3) coarsely vesicular pumice, (4) the principal obsidian flow, (5) finely vesicular pumice, and (6) surface breccia. The best quality obsidian for tool-production often occurs not on the ground surface but slightly underground in subsurface emplacements around a volcanic vent where degassed magma squeezes into rock fractures free of dirt and ash particles. Obsidian over 20 million years old is rarely useable for tool production because, as a geologically unstable material, obsidian gradually devitrifies from a glass into a rock (Francis and Oppenheimer 2004: 163).



Compare with


Rhyolite (Felsic)

As an intrusive rock it is granite

Silica content

Rhyolite is usually >70% wt SiO2

Basalt: <52%, Andesite: 53-63% wt SiO2

Water content

Obsidian: 0.1 - 0.5 H20

Perlite: 3-4%, Pitchstone: 4-10% H20


Quality obsidian is usually <20 Ma

Obsidian >66.4 Ma (KT boundary) is devitrified.


Obsidian: 5.0 - 5.5

Quartz: 7.0

Specific gravity

Obsidian: 2.6 (2600 kg/m3)

Pumice: 0.64, Water: 1.0, Basalt (solid): 3.0

Compressive strength

Obsidian: 0.15

Chert: 100 - 300

Table 4-4. Characteristics of Obsidian.

During the extrusion of rhyolitic lavas it is the supercooling (instantaneous quenching) of the lava that creates obsidian, an atomically disordered natural glass with the structural properties of non-flowing liquid. This lack of crystalline structure in aphyric obsidian results in an isotropic lithic material with excellent flaking properties and the potential for extremely sharp edges because it has no prevailing fracture direction and it fractures at the molecular level. Obsidian has a low specific gravity as it is acidic, it also lacks crystalline structure, and it has relatively low hardness. Obsidian has high tensile strength but it has extremely low compressive strength and, combined with the non-crystalline structure, the result is implements with relatively brittle characteristics and fragile working edges (Hughes 1998: 367;Luedtke 1994: 93;Obsidian 2006;Speth 1972: 52). The cortex ofobsidian from primary deposits can visually vary widely depending on the context ofemplacement and weathering processes. Obsidian flows that cool where tephra is present can melt a thin layer of the adjacent ash and the fused material appears as a thicker cortex (Figure 4-21).

4.4.1. Chemical differentiation

An important attribute of obsidian for archaeological investigation is that obsidian flows are chemically distinctive allowing artifacts to be chemically linked to their geological source areas. These chemical differences in obsidian are the result of certain elements crystallizing to solids and being removed from the magma as per the Bowen reaction series, resulting in a distinctive geochemistry for lava from most magma chambers and sometimes for each extruded lava flow. Prior to, and during, a volcanic eruption, magma evolves as changes in temperature and pressure causes chemical differentiation and leads certain minerals to crystallize and settle out of the melt.

As magma evolves, further melting and crystallization change the nature of the solids, and crystals that accept the incompatible elements may form in the liquid. Some feldspars, for instance, are good hosts for strontium, as is mica for rubidium. "Evolved" obsidian magmas may contain these crystal "hosts," and the ratio of a given element between the liquid and solid phases will change dramatically. Changes of this kind issue a particular chemical character to a given obsidian…The result of these processes is that the incompatible-element mix of a given obsidian source varies from any other and becomes a sensitive indicator of origin (Shackley 2005: 10-11).

This process creates detectable chemical differences in obsidian that permits methods such as X-ray florescence (XRF), Instrumental Neutron Activation analysis (INAA), and various types of inductively coupled plasma mass spectrometry (ICP-MS), and Proton-induced X-ray emission-proton-induced gamma ray emission (PIXE-PIGME) to chemically characterize the material (Neff and Glascock 1995;Shackley 1998).

4.4.2. Obsidian color

The color of obsidian is most often black but it also occurs in red, brown, bronze, purple, blue, green, gray, silver, clear, as well as with banding that includes some of the colors listed above. Obsidian coloration results from the oxidation state of tiny crystals that occur in the melt (Volcano Hazards Program 2000). The black color that is common in obsidian is the result of tiny (< .005 mm) magnetite (iron oxide) crystals, red is usually from hematite present in highly oxidized obsidian, and green results from variations in iron oxidation. Microscopic crystals of various types of feldspars may yield the unique blue, purple, green or bronze colors associated with "rainbow obsidian". Banding results from the folding-in of an oxidized flow surface as the lava continues to move, with each colored streak perhaps reflecting the individual pulses in the obsidian eruption. Gold and silver sheen obsidian is argued to be caused by bubbles of water vapor trapped in the glass that are stretched nearly flat along flow layers (Obsidian 2006). Given the unusual visual qualities of obsidian, the color and banding in a particular nodule are characteristics likely to have influenced human use of the material.

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.



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.

4.6. Other Tertiary obsidian in the Colca valley

Other Barroso group deposits in the region include Pampa Finaya on the north side of the Colca River. In the course of his archaeological survey Steven Wernke (2003: 36, 39) surveyed Pampa Finaya and, unlike in the Chivay source area, obsidian was not found on the perimeter of this Barroso flow where it contacts Tacaza layers. However, Wernke identified an obsidian source to the west on Cerro Caracachi, a distinctive knob-shaped peak at the head of Quebrada Huancallpucy that is at a contact zone between a layer described by INGEMMET as an porphyritic intrusive material (T-po). This source was exposed in a distinct stratigraphic context and that the obsidian appeared to be of low quality for knapping as compared with the Chivay type.


Figure 4-23. Geological map units with Uyo Uyo sampling locations. See Figure 4-14 for legend. Selected archaeological sites in the main Colca Valley shown in blue.

On 30 Nov. 2003, I visited the area of Cerro Masita and Cerro Caracachi on the suggestion by Wernke (2003, pers. comm.) in order to collect geological samples, to determine whether higher quality obsidian is encountered on this dome, and to compare the deposits with what had been observed at the Chivay source (see Appendix B4). I approached Cerro Masita was approached from the road in the drainage north-west of Ichupampa and only observed small pieces of low quality obsidian near the summit of Caracachi (documented by Wernke) and on the eastern flanks of Masita. Analysis by at Missouri University Research Reactor (Glascock, 2005 pers. comm.) established that this material belongs to the "Uyo Uyo" chemical group first recognized in samples provided by Sarah Brooks (1998: 443-445). In 1993 Brooks collected modified and unmodified nodules from the site of "Uyo Uyo" near Coporaque that were analyzed at MURR in 1995. Brooks (1998: 443-445) writes that the geological origin of these nodules was not determined, but the presumption was that the source lay uphill from the site of Uyo Uyo, as there were many unmodified nodules at the site, an observation that was confirmed by these hikes. No large nodules were encountered, however, and it appears that the Chivay source east of Chivay remains the sole source of high quality obsidian in large nodule form in the Colca valley.

4.6.1. Tripcevich source sampling work

In the course of Upper Colca 2003 survey work geological samples of unmodified obsidian were collected from a variety of natural contexts throughout the Upper Colca study area as well as elsewhere in the Colca valley. In order to best characterize the elemental variability within a chemical type, Shackley (1998: 100-101) recommends collecting samples from throughout the primary and secondary deposition area in sufficiently high numbers to recognize sub-source variability and reduce the chances of mischaracterization.


Figure 4-24. Bivariate plot showing Dysprosium against Manganese for Tripcevich 2005 samples.


Figure 4-25. Map showing locations of Colca valley obsidian source samples analyzed by MURR in 2005.

On Figure 4-24 only samples NT0005, NT0008, and NT0015 fall within the Chivay ellipse giving the impression that there is chemical variability at the Chivay source. Bivariate plots across other elements, however, show that all of the samples fall squarely within the Chivay ellipse.













Condorquiña source near Pulpera









Condorquiña source near Pulpera









Condorquiña source near Pulpera









East of Cerro Hornillo









Maymeja SE rim along trail departing area at 140°









Quarry pit Q02-2, A03-219


















Chivay, A03-570. Cortex battered, poss. from glaciers.









Chivay, northern Maymeja









Chivay, northern Maymeja









On western shoulder of Ancachita with ash sample

















Quarry pit Q02-2, A03-219























Uyo Uyo


W. side of Cerro Caracachi







Uyo Uyo


East slope of Cerro Masita









Artifact from Yana Salla, Llalli









Near small rock shelter by top of landslide









From across valley near old canal









From across valley near old canal

















Above Ayawasi






Table 4-5. Peruvian obsidian source samples submitted to MURR by Tripcevich in 2002 and 2005. Coordinate datum is WGS84.

Three samples were provided from the low-quality Condorquiña source in the Pulpera area that were collected in 2002 in order to evaluate the chemical variability within the Chivay source. While the quality was poor, this characterization demonstrates further association between that the Chivay obsidian chemical type and the Pliocene Barroso lava flows evident in the Colca valley. One artifactual sample that turned out to be Chivay was from the town of Llalli close to Ayaviri, in Puno.

Additional samples were submitted from the Alca source collected during fieldwork in 2001 from an exposure described by Justin Jennings (Pers comm. 2001) to the west and south of Cerro Aycano. Kurt Rademaker and colleagues (2004) have since encountered much larger obsidian exposures to the east of this area, on moraines located on the northern slopes of Nevado Firura at approximately 4700 masl.

4.7. Conclusion

The geography and geology of Chivay obsidian deposits conditioned the human use of this source in prehistory. Spatial relationships around the source were reviewed here in terms of three primary contrasts. First, on a regional scale, the location of the Chivay source above the rich and productive Colca valley meant that the source was only a few hours from communities residing in the main valley, but that it was also accessible to puna residents, and herders and caravan drivers as the economy based on camelids expanded. Given the inefficiency of pure pastoralism, obsidian exchange was likely part of a larger pattern of sustained contact between herders and agriculturalists. Second, exploitation of particular sources of obsidian over others was probably limited by local conditions, as water is much more available in the Maymeja area than elsewhere around Cerro Hornillo. Finally, the quality of obsidian varies due to the formation and erosion contexts of Chivay obsidian in prehistory. The Q02-2 quarry pit appears to have been the source of obsidian that predominantly consisted of large nodules of homogeneous glass with a relatively thin and inobtrusive cortex, permitting efficient and predictable quarrying and production in prehistory. As will be discussed in the chapters that follow, these geographical factors were influential in the archaeological use of the Chivay obsidian source area as was documented in the course of the 2003 research project.