The preservation potential of a continual series of volcanic ash falls is slight. There must be a favorable depositional site downwind and within range of an area of pyroclastic activity. A favorable depositional site is a body of quiet or deep water, existing through a long period of time, with relatively rapid normal sedimentation that covers each ash fall before another fall occurs.
In a slowly subsiding basin, the early Paleocene Big Dirty coal swamp of central to eastern Montana provided an ideal depositional environment, and it was partially ringed by areas of volcanism 140-300 km north, northwest, and west of the western edge. As many as 65 ash falls, one by one, blanketed the swamp and were covered by organic debris. The resultant sedimentary package, the Big Dirty coal bed, is particularly well exposed in the Bull Mountain coalfield, northeast of Billings, Montana. Layers of altered volcanic ash and sanidine-rich crystal tuff average 1.5 cm thick and are separated by an average 7.6 cm of coal, tuffaceous coal, or carbonaceous tuff. The Big Dirty coal bed contains a rare continuous record of a period of frequent volcanic eruptions.