"At the outcrop the Cachimayoc pipe pierces one of the few limestones of the Huayllabamba formation of middle Cretaceous age. The pipe could have resulted from exploding igneous emanations, salt-plug piercement, or collapse into a cavity; the first possibility is favored. Faults marginal to the pipe reflect a joint pattern and could have formed by movement along and eventual coalescing of individual joints. The interior of the pipe was selectively mineralized by bleaching, calcite, pyrite, possibly radioactive minerals, and possibly selenium minerals. The outcrop is oxidized and leached. The pipe controls the distribution of closely associated and slightly anomalous radioactivity and apparently also selenium, the latter being strongly suggested by the distribution of the selenium-indicator plant Astragalus garvancillo Cav."