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).