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.