The main reason for our work up in this area was it is the geological origin of the Chivay / Cotallaulli obsidian type. Some of the best nodules we saw came from the an exposure that was eroding out of a gully exposing a dome in the northern part of the volcanic depression. This obsidian contains subparallel vertical fractures, so perhaps this precise exposure in the source area doesn't contain the best possible material. However we did find nodules measuring up to 23 cm in this area.