Large areas of concentrated coal balls (permineralized peat) up to 4 m thick obstructed longwall mining in the Herrin Coal at the Old Ben No. 24 mine. The largest coal‐ball area mapped contained >1500 m³; several areas contained >400 m³ of coal balls. In‐mine mapping established that there were two types of roof (freshwater and marine), and that the coal balls were spatially correlated with the marine roof units. Regional studies and local data revealed that the younger, freshwater Energy Shale (mud) originally covered all of the peat deposited at the mine, but the mud was locally removed during a period of erosion.

The great majority of coal balls are found within the coal seam, where they were permineralized in situ by carbonates. Some coal balls are found exposed on the eroded coal surface and others are within the channel fill associated with the erosion, which predates any marine sedimentation. Thus, the mineralization of the coal balls was synchronous with the erosion of the Energy Shale mud.

Data from C‐and O‐isotope, geochemical, and mineralogical analyses of coal balls and associated materials were used to refine a depositional model of coal‐ball formation. The concentrated coal‐ball areas were created by the triggered degassing of CO₂ from partially decomposed peat in the presence of cations from fresh waters; ¹³C/¹²C ratios in these coal balls average −23.9‰. ¹³C/¹²C ratios as low as −34‰ occurring in the most concentrated coal balls are consistent with CO₂ produced by anaerobic oxidation of methane. Top‐of‐seam coal balls were formed later and show slight to strong marine influence; their ¹³C/¹²C ratios average −10.8‰.