When volcanic ash falls onto the surface of a peat-forming swamp and is then covered by peat, it forms a horlzontal parting that can be recognized in the subsequent coal bed. As a consequence of the acid leaching environment in the swamp, the original glassy component and some of the less resistant pyrogenic minerals in the ash alter to clay minerals, usually kaolinite.
United States Geological Survey, "Altered Volcanic Ash Partings in the C Coal Bed, Ferron Sandstone Member of the Mancos Shale, Emery County, Utah" (1981). All U.S. Government Documents (Utah Regional Depository). Paper 30.