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.

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.